Parker Chomerics EMI Shielding Windows Selection Guide

【选型】Parker Chomerics（派克固美丽）--Tecknit EMI 屏蔽材料选型指南

选型目录：

Company Profile and Products introduction

Electromagnetic Compatibility Overview

EMI SHIELDING DESIGN Special Applications

Knitted Wire Mesh

Metal Fibers and Screens

Oriented Wires

Conductive Elastomer

EMI Shielding Windows

Air Vent Panels

Conductive Systems Products

Shielding Components

Beryllium Copper Gaskets

Fabric-over-Foam Gaskets

EMI Shielding Products Glossary and Appendix

E. WINDOWS

RODUCT

PAGE

WINDOWS DESIGN GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E1 - E17

ECTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E19 - E20

TECKFILM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E21

TECKSHIELD F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E22

TECKSHIELD F: POLYCARBONATE WINDOWS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E23

TECKSHIELD F: ALLYCARBONATE WINDOWS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E24

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Design Guidelines to

EMI Shielding Windows

E. WINDOWS

INTRODUCTION

The DESIGN GUIDELINES TO SHIELDING WIN-

DOWS is intended to aid designers in understand-

ing the trade-offs associated with the selection

of specific materials against anticipated

performance.

One of the many requirements, which compro-

mise the shielding integrity of equipment enclo-

sures, is the need for large-area openings for

access to electronics, ventilation, and displays.

The displays may be panel meters, digital dis-

plays, oscilloscopes, status monitors, mechanical

indicators or other read-outs. The most critical

displays to shield against electronic noise are the

large area, high resolution monitors (CRT).

Shielding of these large apertures is generally

more difficult than those encountered for cover

plates, doors, ventilation panels and small aper

tur

es, such as connectors, switches and other

contr

ols in which the majority of the opening is

covered by a continuous homogeneous conduc-

tive (metal) plate. Ther

efor

e, when working with

window designs, which do not have a continuous

conductive cover, consideration must be given to

shielding as related to relative apertures and

scr

eens and suppor

ting substrates. These two

factors are inter-related and need to be treated as

a combined problem.

Shielding windows ar

e pr

esently manufactur

ed in

one of thr

ee ways: (1) laminating a conductive

screen between optically clear plastic and glass

sheets; (2) Casting a scr

een within a plastic

sheet; (3) applying an optically clear conductive

layer to a transparent substrate. Until recently,

the typical conductive scr

een was a knitted wir

mesh made fr

om Monel, tin-plated copper

-clad

iron core (Sn/Cu/Fe or Monel wire).

Knitted densities range from 30 openings per

inch for the 0.001-inch diameter tungsten wire

(94% open area) to 10 openings per inch for the

0.0045-inch diameter wire (90% open area).

These high open area meshes provide high opti-

cal transmission with average shielding effective-

ness (greater than 60 dB) below 10 MHz when

wire crossovers are adequately bonded.

Optically clear conductive coatings are produced

by depositing an electrically conductive transpar-

ent coating (ECTC) directly onto the surface of

various optical substrates. Typically, these coat-

ings can provide better than 50 dB shielding

effectiveness below 100 MHz with an optical

transmission of better than 70% over the visible

light spectrum. Increased shielding effectiveness

may be achieved by increasing the thickness of

the deposited coating material (decr

easing r

esist

ance) at the expense of loss in optical transmis-

sion and increase in optical reflection.

High-density woven wire screens have been

employed which have extended the useful high-

frequency response beyond 10GHz. These

screens have made use of silver-plated, stainless

steel wires; copper-plated, stainless steel wires;

and copper wires. In all cases these screens

make direct contact to a peripheral wire mesh

gasket, window frame or enclosure structure.

oven meshes have ranged from 80 mesh (wires

to the inch) to 150 mesh and wire diameters from

0.001 inch diameter to 0.0045 inch diameter

ypical per

for

mance for a 100 mesh scr

een will

provide almost 60% open area with shielding

fectiveness of up to 60 dB beyond 1 GHz.

Higher mesh densities and lar

ge wir

e diameters

usually result in higher shielding effectiveness

with lower optical per

formance.

In the following sections, various aspects of

shielding window design will be reviewed as relat-

ed to shielding performance, optical performance,

optical designs and methods for mounting win

dows to enclosures.

E-1

U.S. Customary

[SI Metric]

E. WINDOWS

SHIELDING PERFORMANCE

A great deal of information has been written and

published on total shielding effectiveness (SE) as

an aid in reducing electromagnetic interference

(electrical noise). Electromagnetic compatibility

(EMC) may be achieved by reducing the electro-

magnetic interference (EMI) below the threshold

level that disrupts the normal operation of an

electronic system. An electronic system can be

both an emitter and a susceptor. An EMI emitter

generates unwanted noise; a susceptor r

esponds

to unwanted noise. Military and governmental

specifications stipulate the allowable levels of

radiated and conducted emissions and the nec

essary circuit immunity to these emissions to

achieve electromagnetic compatibility (EMC).

Shielding requirements for shielding windows can

vary from moderate to severe. Any barrier placed

between an emitter and a susceptor that dimin-

ishes the field strength of the interference is an

EMI shield. The attenuation of the electromag-

netic field is referred to as its shielding effective

(SE). The standard unit of measurement for

shielding effectiveness is the decibel (dB). The

decibel is expressed as the ratio of electromag-

netic field strength on one side of a shielding bar-

rier to the field strength on the opposite side.

The losses in field strength (absorption and

reflection) from a shield are functions of the barri-

er material properties: permeability, conductivity,

and thickness, as well as the distance from the

mitter to the shield. Figure 2-1 depicts the rela-

tionship between decibels, attenuation ratio, and

percent attenuation.

In most shielding applications, shielding effective-

ness below 20 dB (10:1 reduction in EMI) is con-

sidered marginal due to long-term environmental

effects on the mating surfaces of enclosures and

shielding gaskets and barriers. Normally, accept-

able shielding performance covers the range from

30 dB to 80 dB. Above average shielding ranges

from 80d dB to 120 dB. Above 120 dB, shield-

ing effectiveness is difficult to achieve and diffi-

cult to confirm by measurement.

Figure 2-2 shows the range of shielding effective-

ness for the three primary barrier materials used

in shielding window: knitted wire mesh screens

(Band I), transparent conductive coatings (Band

II), and woven mesh screens (Band III).

Shielding performance is the primary considera-

tion in the design process and is, therefore, con-

sidered first.

E-2

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Figur

e 2.

Shielding Ef

fectiveness as a Function of Attenuation.

Figure 2-2.

Barrier Shielding Performance for Shielding

Windows.

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Design Guidelines to

EMI Shielding Windows cont.

E. WINDOWS

he shielding values presented in Figure 2-2 are

considered to be conservative based on measure-

ments in shielded room tests, which generally

show from 10 dB to 20 dB higher shielding effec-

tiveness. The origin of the data is based on the

theoretical relationship given by:

= 195-20 log

(df)

Where d is the mesh wire spacing in inches and f

is the threat frequency in Hertz.

Since most EMI pr

oblems are broadband (cover a

broad frequency range), the frequency of most

concer

n is generally the highest fr

equency within

that bandwidth envelope to which the equipment

is responsive and which may be a threat to elec-

omagnetic compatibility

. Ther

efore, the highest

threat frequency and the shielding requirements

at that frequency are both needed to determine

the type or types of windows, which ar

e suitable

for that application.

For example, assume the highest threat frequen-

cy is 10 MHz with a maximum required shielding

of 60 dB at that frequency. Figure 2-2 shows that

any of the three families of shielding materials

would be suitable to provide of shielding materials

would be suitable to provide adequate shielding.

n the other hand, changing the maximum threat

frequency from 10 MHz to 100 MHz would

eliminate the knitted wire mesh screens and the

transparent conductive coatings, leaving only the

high-performance woven screens as a suitable

solution.

Knowing which types of windows are available,

the next selection should be made on the basis of

the optical transmission that is attainable from the

scr

een materials or conductive coatings, plus the

optical substrate. Standard optical substrates

should cause only a minor reduction in optical

transmission should be less than 1% to up to

10%, depending upon the reflection and absorp-

tion fr

om coated and uncoated sur

faces of the

substrates. The following section will deal with

the evaluation of the windows from an optical

aspect of the specific materials to be r

efer

ed to

as per

cent open ar

ea. This characteristic is

important in determining optical contrast which

can af

fect operator fatigue in using devices such

as video display monitors.

Table 2-1 summarizes the general shielding effec-

tiveness ranges at specific frequencies for the

thr

ee shielding materials shown in Figur

e 2-2.

The three frequencies are 1 MHz (magnetic

field), 10MHz (electric field), and 1 GHz (plane

wave).

E-3

U.S. Customary

[SI Metric]

SUMMAR

Figure 2-1.

Shielding Performance

Shielding Screen Material

Shielding Range (dB)

Magnetic

Electric

Plane

1 MHz 10MHz 1GHz

I Knitted Wire Mesh

30-40

60-70

20-25

(Monel-Cross over Bond)

10-30 CPI

II Transparent Conductive

40-50

70-80

30-40

Coating

8 to 24 OHM/Square

III W

oven WireMesh

65-75

95-110

60-70

(Copper Wire)

80-200 mesh

E. WINDOWS

OPTICAL PERFORMANCE

To deal with the material selection process an

understanding of optical properties of shielding

windows is imperative. These properties concern

the optical transmission of the finished window,

including optical substrate, shielding screen, lam-

inating material, coatings, and characteristics of

transmission color filters. This section discusses

the optical performance of the shielding screens.

Knitted mesh screens are produced on industrial

knitting machines that were originally developed

for the commercial, knitted fabric materials indus-

try. The machines have been adapted to handle

wire instead of yarn. In this process they produce

a continuous tube of material called a “stocking.”

The diameter of the stockings varies from 3/8

inch to 30 inches. V

arious sizes ar

e used to

make electrically conductive metal gaskets and

the conductive mesh screens for shielding win-

dows. The ir

egular shapes for

med in the knitting

process (see Figure 3-1) aid in minimizing any

obscuration of regular shapes as might be formed

in typed or printed information. The density of

the mesh is determined by the courses per inch

along the length of the stocking, the wire material

and the wir

e diameter

. To maintain a square pat-

ter

n of openings in both dir

ections, it is necessar

to call out the number of openings per inch

ound the stocking as well. This ef

fectively

determines the complete description of the knit-

ted mesh screen. Knitted screens are generally

limited to about 30 openings per inch when used

as a screen for shielding windows.

oven mesh using fine wires, generally much

smaller than 0.005 inch diameter, provide a sig-

nificant impr

ovement in shielding ef

fectiveness

over other shielding widow materials, even at

higher frequencies. These woven screens have

80 or more wires to the inch in both directions

(Figur

e 3-2). T

ypical mesh density is 100 mesh

(100 by 100 wires per inch), 120 mesh (120 by

120 wires per inch) and 150 mesh (150 by 150

ires per inch). Typical wire diameters vary from

0.001 inch to 0.0025 inch depending upon plat-

ing and blackening. Blackening of the screen

reduces reflections and improves image contrast.

Figur

e 3-2.

oven Mesh Scr

eens.

A thir

d shielding material is the transpar

ent conduc

tive coating. This material exhibits good shielding

properties at moderate optical transparency (refer-

ence Table 2-1 on shielding performance for knitted,

woven and transparent conductive coatings). Since

the shielding effectiveness is a function of the resis-

tivity of the transpar

ent coating which, in tur

n, is a

function of the optical transmission, ther

e ar

e trade-

offs in performance (see Figure 3-3 and Table 3-1).

An optimum r

elationship for this type of coating

occurs at approximately 10 to 14 ohms per surface

resistivity to obtain approximately 70% transmission

and greater than 50 dB shielding at 100 MHz.

Figur

e 3-3.

Light T

ransmission-Resistivity Relationship (Thin

Gold Coating).

E-4

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Figure 3.

Knitted Mesh Screens.

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Design Guidelines to

EMI Shielding Windows cont.

E. WINDOWS

igure 3-4 provides a ready reference for the optical

ransmission (percent open area) of the three types

of shielding materials for windows covering the most

commonly used knitted mesh screens, woven mesh

screens and transparent conductive coatings. The

commonly used materials are annotated by circle (O)

on the figure.

Figure 3-4.

Percent Open Area of Mesh Screen.

Section A of Figur

e 3-4 encompasses the useful

range of knitted materials. W

ire diameters from

0.001 inch to 0.0045 inch bound the upper and

lower limits while 10 to 25 CPI provide the limits of

mesh densities. These boundaries provide the high-

est optical open ar

ea ranging from about 80% to

eater than 95%. Bonding of wir

e cr

ossovers has

been assumed in all performance data shown in this

guideline.

Section B of Figure 3-4 depicts the useful range of

woven screen materials ranging from wire diameters

of 0.001 inch to 0.0045 inch and mesh densities

rom 80 to 200 mesh. The circles indicate common-

y used mesh materials that are generally readily

available. Performance for 100 mesh screen with

0.0045 inch diameter copper wire provides approxi-

mately 30% optical transparency and 70 dB shield-

ing, while 100 mesh with 0.002 inch Diameter cop-

per wire provides about twice the open area (64%)

while reducing the shielding effectiveness by only 10

to 12 dB.

Section C of Figure 3-4 (vertical coordinate) shows

the normal range of transparency for the transparent

conductive coating. These electrically conductive

transparent coatings (ECTC) have a distinct advan-

tage over screen materials when used with three

color CRT’s employing a color mask on the faceplate.

The color mask is used to delineate the specific

phosphor color to be displayed. The masks have a

color repetition pattern or pitch that varies from an

equivalent mesh density of about 60 mesh for br

oad

cast monitors to 130 mesh for the ver

y high-r

esolu

tion monitors. Whenever a repetitive pattern, such

as a shielding mesh scr

een, is placed in fr

ont of a

color CRT, patterns of dark and light bars are known

as moiré patterns. They occur as a result of the

mesh screen having nearly the same pitch as the

pattern of the CRT color mask. Rotating the mesh

will vary the number of bars. Changing the number

of wires per inch (mesh density) will also alter the

number of bars. Often ther

e is an optimum mesh

density, wire size and angular relationship to the

fixed CR

T color mask pattern that will minimize or

even eliminate the interference pattern.

These light and dark bars are the result of the pat-

ter

ns of two objects, either aligning up exactly with

each other to produce light areas or misaligning

completely and blocking all transmitted light to pro-

duce dark bars. Sometimes, it is dif

ficult to attain a

perfect match between the CRT mask and the

screen mesh. ECTC windows on the other hand do

not have a repetitive structure similar to the shielding

mesh screens. They are, therefore, ideal in some

applications as an EMI shield for color monitors.

The main limitations with the ECTC windows ar

e high

E-5

U.S. Customary

[SI Metric]

Shielding Screen Material

Shielding Range (dB)

Optical Open Area (%)

Magnetic

Electric

Plane

1 MHz

10MHz

1GHz

0.001” DIA.

0.002’ DIA.

0.0045” DIA.

I Knitted W

e Mesh

30-40

60-70 20-25 95-98% 90-96% 79-91%

(Monel-Cross over Bond)

10-30 CPI

II Transparent Conductive

40-50

70-80

30-40

60-80%

Coating (Molecular

8 to 24 OHM/Squar

e Structure)

III Woven WireMesh

65-75

95-110

60-70

64-86%

36-70%

30-41%

(Copper Wire) 80-200 mesh

Figur

e 3-1.

E. WINDOWS

cost, their tendency to be easily scratched, a notice-

able color tint for some coatings and a lower shield-

ing effectiveness than the woven mesh screens.

he TECKNIT EMI Shielding Design Guide is an

xcellent reference in determining the required

shielding for specific specifications (MIL-STD-461,

FCC, VDE and others) against equipment circuits

and

EMI generators. Tables 3-1 summarize the per-

formance capabilities of shielding windows from both

shielding and optical aspects.

OPTICALLY CLEAR WINDOW SUBSTRATES

Glass and clear plastic optical substrate materials are

the most common for covering large area apertures

for viewing windows. This section discusses the

basic properties of these materials for shielding

applications requiring both flat and curved windows.

GLASS SUBSTRATES

Glass substrate materials provide the hardest surface

for resistance to scratches and marring. Once fully

laminated, these windows closely match the proper-

ties of safety glass, with the added protection of an

embedded scr

een mesh.

oper

ties of the glass confor

to ASTM-C-1036 and

mirror to mirror select quality. Edges are cut and

trimmed to r

emove any sharp sur

faces. Edges may

be ground, ground and polished, beveled, or mitered

on special order as specified by customer drawings

or specifications. Standar

d glass window thickness

is 0.205 inch with a tolerance of plus or minus 0.020

inch. Other thickness may be furnished in the

ranges and tolerances shown in Table 4-1.

Maximum outside dimensions (length by width) ar

18 inches by 14 inches with a standard tolerance of

plus or minus 0.031 inch. Major defects such as

gaseous inclusions, which ar

e per

mitted by Federal

Specifications, ar

e culled before laminating. Glass,

in effect, when specified for shielding windows will

exceed the requirements as stipulated in federal

Specifications. Plate glass is specified to assure vir-

tually parallel and flat surfaces. See

TECKSHIELD-F

Data Sheet

for laminated glass windows.

PLASTIC SUBSTRATES

Not all-clear plastics ar

e of use in the manufactur

e of

shielding windows. Plastics ar

e divided into two gen-

eral classes: thermoplastic and thermosetting resins.

thermoplastic

material softens when heated and

har

dens on cooling. Since this action is reversible it

is possible for the material to be molded and remold-

ed without appreciable change in the material prop-

erties. The significant difference in

thermosetting

materials is the ir

eversible heating action. These

latter materials, once softened by heating, remain in

the shape formed during the original heating cycle.

Hence, the desired or final shape of the windows to

be made must be incorporated into the mold of the

part. Furthermore, with thermosetting plastics, the

desired color) other than clear) depends on the thor-

ough blending of the proper mixture of the coloring

agent with the plastic material before molding.

HERMOPLASTICS-Cellulose Derivatives:

he princi-

al cellulose derivatives are the nitrate, acetate,

cetate butyrate, and ethyl cellulose. The cellulose

plastics have a comparatively poor surface hardness

and poor abrasive resistance. They are readily

hygroscopic (absorb water) with a resultant change

in dimensions. Most do not possess the high optical

qualities of glass or some of the other plastic sub-

strate materials. Softening occurs at about 60*C for

these thermoplastic materials and, therefore must be

used in applications which will not exceed their soft-

ening temperature.

Cellulose acetate butyrate

(CAB)

is probably the best of the cellulose family of plas-

tics. It is especially suited to molding and possesses

lower water absorption than other cellulose derivates

and therefore, betters dimensional stability than cel-

lulose acetate.

THERMOPLASTICS-Synthetic Resins:

The principal

thermoplastic resin materials consist of polycarbon-

ates, polystyrenes and methyl methacrylates

(acrylic). In general these resins are characterized

by higher resistance to chemicals and lower water

absorption than the cellulose derivatives. They gen-

erally have optical characteristics ver

y close to glass

with a much lower tendency toward scratching, but

e still ver

y much softer than glass. Polycarbonate

is about 10 times easier to scratch or mar than the

methyl methacr

ylates (acr

ylic).

Polycarbonate

material is virtually unbreakable and

can withstand impacts gr

eater than 200 ft.-lbs. for a

one eighth inch thick sheet. Softening temperature

is about 125*C. The poorer than desirable scratch

performance makes polycarbonate a poor candidate

for viewing windows that r

equire periodic cleaning,

such as may be needed with cathode ray tubes

(CR

T). Some ar

omatic solvents (hydr

ocarbon) cause

surface stress cracking in this material.

Polystyrene

material is relatively hard and rigid, natu-

rally colorless and quite transpar

ent. The softening

range is about 20*C higher than the cellulose plas-

tics, but lower than that for acrylic resins. Most

other properties for this material are excellent except

for poor r

esistance to most or

ganic solvents.

Methyl methacrylate

(acrylic) material has high luster,

high transparency, and good surface hardness, is

comparatively inert chemically and is not toxic.

Essentially, acrylic possesses almost all the desirable

qualities of glass except for scratch r

esistance. Com

pared to other plastics, methyl methacrylate is hard-

er than most but still readily scratched by dust parti-

cles.

Methyl methacrylate is a very stable compound and

retains to a high degree its mechanical properties

under adverse environmental conditions. Impact

resistance when compared to some plastics is

poor

, although when compar

ed to glass it is much

superior.

E-6

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Design Guidelines to

EMI Shielding Windows cont.

E. WINDOWS

HERMOSETTING RESINS – ACP, CR-39 (PPG indus-

ries):

CP (Allyl Cast Plastic) is known as Columbia

Resin (CR-39). It is a transparent solid, cured from

the clear, colorless, water-insoluble liquid monomer

through the aid of a catalyst. It is strong, relatively

insoluble and inert. It is normally free of internal

haze, has a low water absorption and moderate coef-

ficient of thermal conductivity. Refractive index is

almost identical to that of crown glass, and yet, the

density is about one-half. The resin material is

uperior to acrylic and other plastics with respect to

oftening under heat, crazing, resistance to abrasion

and attach by chemicals. The continuous use tem-

perature is 100*C.

In summary, the three most likely candidates for

optical substrate materials in shielding window appli-

cation are glass, acrylic and CR-39, in that order.

Table 4-2 summarizes the performance characteris-

tics of these materials.

E-7

U.S. Customary

[SI Metric]

TABLE 4-1

STANDARD SIZES AND TOLERANCES

MATERIAL

MAXIMUM SIZE

TOLERANCE

THICKNESS (Overall)

REMARKS

Plate Glass

Standard

±0.031” Standard

(1)

Glass per ASTM-C-1036

(woven mesh)

32” x 56”

0.270

0.020 inch

32” x 32”

0.205

0.020 inch

Special

14” x 14”

0.145

0.020 inch

Plastic Standard

±0.031”

Standard

(acrylic)

Acrylic per L-P-391

(woven mesh)

24” x 24”

0.145 ± 0.020 inch

Special Special

(acrylic)

32” x 32”

0.205 ± 0.020 inch

32” x 56”

0.270 ± 0.020 inch

Plastic

18” x 22”

±0.031”

Standard

(cast)

(2)

Smooth or matte finish,

(knitted mesh

0.125±0.010 inch

Polycarbonate CR-39,

& ECTC)

(4)

Standard

(edge laminated)

(3)

Acrylics

0.135±0.015 inch

Special

(Cast)

0.060±0.010 inch

TABLE 4-2

PROPERTIES OF WINDOW SUBSTRATES (TYPICAL VALUES FOR CLEAR COLORLESS MATERIAL)

METHYL

PLA

METHACR

YLA

TE POLYCARBONATE

(1)

CR-39

PROPER

UNITS GLASS (ACRYLIC)

(1)

OPTICAL

Index of r

efraction –

1.529 1.48-1.51 1.59 1.50-1.57

ransmission

21-23

85-89

89-91

Haze

0.9 0.6

0.5-2.0 0.4

MECHANICAL

Flexure Strength

psi

12-14,000

12-13,000

5,000

Impact Strength (Izod Notch)

ft-lb./in.

0.4

12-16

0.2-0.4

Hardness

Rockwell

M80-M90 M68-M74 M95-M100

Specific Gravity

–

2.52

1.20

1.32

ELECTRICAL

Dielectric Strength

volt/mil

450-530

380-425

290

Dielectric Constnat

@1MHz

2.7-3.2

3.0-3.1

3.5-3.8

Volume Resistivity

ohm-cm

8x10

4x10

THERMAL

Ther

mal Conductivity

Btu-in/hr•ft

•

1.44

1.35-1.41

1.45

Specific Heat

Btu/lb

0.35

0.3

Coef

f. Ther

m. Expan.

in/in/

4.7X10

-6

45x10

-6

37.5x10

-6

60x10

-6

Continuous Use T

emp.

C/F

110/230

80/175

100/212

CHEMICAL/PHYSICAL

Water Absorbtion

% (24hrs.)

—

0.3-0.4

0.15

0.2

Abrasion Resistance

ASTM 1044

100

—

(1)

Connectors & Inter

connections Handbook V

olume 4, Materials, 1983.

(1)

TECKSHIELD-F

Specification Refer

ence, Appendix A-1

(2)

EMC-CAST

Specification Refer

ence, Appendix A-2

(3)

EMC-LAMlNA

TED

Specification Refer

ence, Appendix A-3

(4)

EMC-ECTC

Specification Refer

ence, Appendix A-4

(5)

Contact factor

y for lar

ger edge bonded windows.

E. WINDOWS

CONTRAST ENHANCEMENT

The optical performance of substrate materials may

be substantially improved by increasing the optical

contrast of the displayed image through glare reduc-

tion and optical filtering. Additionally, special surface

treatments for some plastics may increase the

scratch and mar resistance of surfaces subject to

frequent cleaning. Here special coatings can signifi-

cantly reduce the harsh effects of dust and dirt

scratches from cleaning materials, which cause

unwanted light scattering and image distortion or

obscuration.

Wherever high ambient lighting conditions are pres-

ent, loss in display contrast may occur from window

reflections unless these reflections are controlled by

means of antireflection coatings, matte finishes, opti-

cal color transmission filters, or special laminates

such as polarizers.

Antiglare or glare reduction techniques consist of

either an antireflection coating for glass windows or a

matte finish for glass or plastic windows.

Antireflection coatings utilize optical interference fil-

ters, while matte finishes are imprinted into the sur-

face of the substrate and scatter incident light to

reduce specular reflection (See Figure 5-1).

Color transmission filters transmit only specific color

hues within a comparatively nar

ow spectral band

reducing the amount of optical energy, which does

not contribute to the display image. Polarizers selec

tively block the passage of unwanted wide band

spectral energy such as is reflected from the internal

surface of a display.

ANTIREFLECTION COATINGS

Antir

eflection inter

fer

ence coatings are applied to

optical elements of shielding windows to r

educe

reflections. These coatings are applied by several

deposition methods, such as high vacuum evapora-

tion, sputtering thin film coating techniques. The

techniques to r

educe sur

face r

eflection fr

om glass

optical elements have been well known in the optical

industr

y for many years. V

irtually all lenses in mod-

ern cameras have a single or multilayer antireflection

coating. The amount and the rate of material

applied to the surface are controlled to obtain the

required film thickness. These specialized coatings

consist of several thin film layers of different materi-

als to obtain a par

ticular optical ef

fect.

The basic laws of optics determine the reflection that

occurs at a boundar

y between two transparent

media of different index of refraction (n). The index

of r

efraction is a measur

e of the speed of light in a

medium. For vacuum, the index is 1.00 and for all

practical purposes, it is 1.00 for air. Higher indices

indicate a slower propagation speed for light in that

media. The index for plate glass, such as used in

shielding windows, is 1.525. This higher index

means that the speed of light in plate glass is

approximately two-thirds the speed of light in air.

These indices are used to determine the percentage

of incident light, which will be reflected at the

boundar

The reflection (R) occurs at the boundary of interface

between two dif

ferent indices and can be calculated

from the equation:

For ng: the index for glass is 1.52

For na: the index for air, 1.00

For the indices given above, the ratio of reflected to

incident light is 0.04 or 4%. A similar r

eflection will

occur wherever a boundary between two different

indices exists, such as the boundary between glass

and air at the second surface. The front and back

surface reflections then may amount to a total of 8%

E-8

Mexico: 528-18-369-8610 • China: 86-10-67884650 • www.tecknit.com

Figure 5-1.

Glare Reduction Techniques.

– n

)

– n

)

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Design Guidelines to

EMI Shielding Windows cont.

E. WINDOWS

f the incident light being reflected back to the view-

r for plate glass with an index of 1.52. Figure 5-2

shows the relationship of reflection to indices from

1.0 to 2.0.

Figure 5-2.

Percent Reflection Against Index of Refraction.

Figur

e 5-3 represents schematically an air/antire-

flection-coating glass interface. The light wave-

ont represented by two electromagnetic rays (A

& B) impinge onto the front surface of the antire-

flection coating. Small portions (4%) of rays A &

B ar

e reflected while the larger portion of each

enters the coating. Ray A r

eflects another small

portion (4%) at the boundary between the antire-

flection coating and the glass substrate. The

thickness of the coating is exactly ? of the wave

length of the reflected light to be absorbed. The

reflected Ray A at the boundary between the

antireflection coating at Point 2 arrives at Point 3

exactly out of phase with Ray B (out of phase

occurs where Ray A is positive, Ray B is negative

and of equal amplitude). At point 3 the out of

phase condition results in destructive interference

between rays A & B with a complete cancellation

of the r

eflected wave fr

onts. The same cancella-

tion occurs at the back surface when it is also

subjected to the antir

eflection coating.

In reality, the number of materials that are avail-

able for antireflection coating are fairly limited,

equiring a high index of r

efraction for the lass

substrate and a low index for the coating. Under

exact conditions, it was shown in the paragraph

that the air

-to-coating boundary reflection may

esult in complete cancellation of the r

eflection

from the coating-to-glass boundary, thus produc-

ing a near zer

o r

eflection value at some selected

optical wave length. Unfortunately, in most appli-

cations, exact matching of indices and layer

thickness seldom occurs. Even for only slightly

mismatched conditions, the human eye is

extr

emely sensitive to low light levels. To the

untrained obser

ver

, a 1% to 2% reflectivity is still

very apparent and often difficult to distinguish

from an uncoated glass surface. To be effective

for glare reduction application, the coating must

educe a single sur

face r

eflection significantly

below 0.5% The transmission of TECKNIT high-

efficiency optical coating is greater than 99%

which is mor

e than 7% higher than that for

uncoated plate glass. Uncoated plate glass trans-

mits appr

oximately 90% of the incident light.

Surface reflections account for 8% and absorp-

tion accounts for approximately 2%. To avoid the

reflection of the second (back) glass-to-air bound-

, the back sur

face must be coated with a simi

lar coating.

The eight per

cent (8%) reflection of incident light

from the glass surface may be frequently as

intense as the optical ener

gy generated by many

displays. Cathode ray tubes (CR

T) monitors,

radar scopes for traffic controllers, digital LED and

LCD and electroluminescence are examples of

fairly low brightness displays. In some applica

tions where the ambient light is very high (out-

doors), the intensity of the reflected light may

exceed the light energy from most data displays.

E-9

U.S. Customary

[SI Metric]

Figur

e 5-3.

Air

-Antir

eflection Coating-Glass inter

face

E. WINDOWS

nder these conditions, it is often easier to see

the reflected image of the scene behind the view-

er than the display itself that has been completely

of almost completely washed-out (zero contrast)

by ambient light. In these cases, the use of light

dispersion (scattering surfaces as are provided by

matte finishes. Circular polarizers are useful for

eliminating reflections internal within the display

that can be reflected back toward the viewer

reducing image contrast.

MATTE FINISH

Matte finishes are used as an antireflection sur-

face treatment to effect a dispersion of specular

eflectance. These finishes for either glass (an

etch finish) or plastic (mold or cast finish) are

available as an alter

nate to thee antir

eflection

coating (HEOC for glass). Matte front surface fin-

ishes are used in applications where the shielding

windows may be used in close pr

oximity to the

display, such as flat (or nearly flat) CRT, plasma

display, LED, LCD, and electroluminescent and

monochrome or multicolor displays.

At or near normal incidence where ambient light

strikes the window straight on, light r

eflected is a

function of the indices as discussed earlier

. As

the angle of incidence incr

eases s measur

ed fr

the nor

mal (perpendicular) to the window sur

face, an abrupt increase in reflection occurs bout

∞

incident angle. These near grazing angles

e often coincident with the positioning of over-

head lighting. Reflections under these conditions

are best treated with a shading hood or by using

matte finish which dispense the reflected energy

(reference Figure 5-1).

POLARIZERS

Polarizers provide a third method of discrimina-

tion between optical signals and optical noise.

There are two basic types of polarizers, linear and

circular.

Electromagnetic radiation is generally conceived

of on the basis of field theory. An electric and

magnetic field are said to exist at right angles to

each other. In any random waveform, the orien-

tation of either field would be random in relation

to some fixed axis. Ther

efor

e, in a bundle of opti

cal waveforms or rays, there would be (statistical-

ly) a complete random orientation of the fields

(the electric field, for example) as shown in

Figure 5-4b. These waveforms would be unpolar-

ized; that is, there would be no preferential orien-

tation of either field. A polarized wave, then, is

one in which the fields ar

e specially oriented in

one direction, Figure 5-4A.

A linear polarizer selectively transmits an unpolar

ized waveform by resolving the field components

that ar

e aligned with the polarizing axis of the

polarizer. In this manner, the polarized waveform

consists of a single orientation of the electric field.

When viewed through another linear polarizer

(called an analyzer) with its polarization axis at

right angles (90º) to the polarized waveform, the

light will be completely blocked. When the axis is

aligned at other than a right angle to the polarized

wavefor

m, the wave is transmitted as a function

of the angle (COSINE

) between the axes of the

polarizer and the analyzer. For example, where

the axes are aligned at 45º, about 50% of the

polarized light will pass through the analyzer.

Linear polarizers are used to control light output.

These polarizers attenuate reflected light glare

form smooth objects where the reflected light has

been polarized in a known plane, such as hori

zontally. To minimize the reflected light, the lin-

ear polarizer acting as an analyzer is oriented with

its polarizing axis perpendicular to the reflecting

surface.

Circular polarizers provide an important additional

advantage. When viewing objects through a win-

dow, the objects on the inside of the enclosure

e generally oriented at various angles to the

window sur

face, such that the light that r

eflects

from those objects may be polarized in several

different planes.

E-10

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Figur

e 5-4.

Polarized and Unpolarized W

avefor

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Design Guidelines to

EMI Shielding Windows cont.

E. WINDOWS

he problem then becomes one of discriminating

etween light which enters the display from the

window side and light generated within the dis-

play. Generally, the acceptance angle of the light

entering the display will be fairly narrow (Figure

5-5). The farther away the display is located in

relation to the window, the narrower the accept-

ance angle of the interfering light and, therefore,

less chance that light will be retro-reflected back

to the viewer. Light, which originates outside the

acceptance angle will not contribute to the loss in

contrast with the image being emitted at the dis-

play (CRT, LED, annunciators - those displays that

generate their own illumination). Additionally, ori-

entation of the reflecting object within the display

plays an important part in determining what light

from the window will be reflected back out the

window toward the viewer.

CIRCULAR POLARIZER – HOW IT WORKS

A circular polarizer consists of linear polarizer in

series with a 1/4 wave-retarding element. It is

important that the linear polarizer precedes and is

oriented (aligned) cor

ectly to the ? wave-r

etar

ing element.

With reference to Figure 5-6, light passing

through the linear polarizer is polarized along its

polarizing axis and enters the 1/4 wave r

etarder.

The 1/4 wave retarder separates the polarized

rays into two equal rays that pass through the

retarder at different speeds (by virtue of two dif-

fer

ent indices of r

efraction). The thickness of the

retarder determines the phase relationship of the

two light rays and is selected to produce a 90º

phase shift (1/4 wavelength). After passing

through the 1/4 wavelength retarder, the phase

elationship of the rays remains constant. Upon

striking a highly reflective surface (specular), the

phase orientation of the two rays reverses with the

phase lagging ray preceding the previously phase

leading ray by 1/4 wavelength.

On r

eentr

y thr

ough the 1/4 wave element, the

retarder phase aligns the two rays and orients the

esultant wave at right angles to its original polar

ization. The 90º rotated polarized wave emerging

from the 1/4 wave retarder is then completely

blocked by the linear polarizer (the first element

of the cir

cular polarizer).

Cir

cular polarizers can not be used with LCD dis

play. LCD displays use linear polarizers in their

normal operation to effect selective filtering of the

external illumination. This type of display would

par

tially or completely block the incident light

from the circular polarizer, effectively defeating

the purpose of the various elements of the LCD.

OPTICAL COLOR TRANSMISSION FIL

TERS

Optical filters generally ar

e classified accor

ding to

their spectral pr

operties such as short wave cut-

off, long wave cut-off, bandpass, rejection, or

neutral density.

Shor

t wave cut-off filters are used to block the

ultraviolet while long wave cut-off filters may be

used to eliminate infrar

ed heating. Bandpass fil

ters are principally used to increase the signal-to-

noise ratio (contrast) of displays (or detectors).

Rejection filters ar

e usually employed to eliminate

specific spectral wavelength(s) or to minimize

their intensity, which might be harmful to the

operation of equipment, such as laser beam.

Neutral density filters reduce the average illumi-

nation across the visual spectrum.

E-11

U.S. Customary

[SI Metric]

Figur

e 5-5.

Glar

e Acceptance Angle.

Figure 5-6.

Circular Polarizer.

E. WINDOWS

n shielding window applications, transmission fil-

ters are used to provide various hue and shades

of transmitted light. To assist the designer in

selecting the proper filter for specific applications,

it becomes important to be able to calculate the

effect of material thickness and combinations of

elements that tend to alter the transmitted light

and the overall density of the filter.

Light transmitted through the filter material expe-

riences a first surface reflection, absorption within

the bulk of the material and losses due to the

second surface reflection. The transmitted light

(T) is a fraction of the incident light and the opti-

cal density of the filter is given by:

D = log

Wher

e ther

e ar

e several transmission factors

involved (multiple values of T), thee factors

should be included and multiplied together. For

example, if the transmission factor for a color fil-

ter at the peak wavelength is Tp and the optical

substrate transmission factor is Ts, the density

expr

ession would be:

= log

tandard colors are available for plastics which

broadly cover four hue classes (red, yellow, green,

blue) and neutral gray. Table 5-1 tabulates sug-

gested filters, which most nearly match the spec-

tral band for each of the emitters.

Figure 5-7 provides spectral transmission curves

for the more commonly used filters.

ABRASION RESISTANT COATINGS

The surfaces of most plastics are relatively soft in

comparison to glass. As a result, the front sur-

face of shielding windows are subjected to possi-

ble scratching and marring when periodically

cleaned to r

emove dust, dirt and grease in normal

handling during operation of the equipment.

These soft sur

faces can be tr

eated with specially

formulated coatings for use on thermoplastic and

thermosetting plastics.

Abrasion resistant coating not only provides

scratch and mar resistance, but is also resistant

to moisture and cleaning solvents. The coatings

re clear and non-yellowing and are resistant to

ultraviolet light. They can be applied to methyl

methacr

ylate (acrylic), polycarbonate or CR-39.

Polycarbonates ar

e not r

ecommended for normal

shielding window applications unless pr

otected

with an abrasion r

esistant coating.

E-12

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Figure 5-7.

Standard Spectral Transmision Filters.

TABLE 5-1

RECOMMENDED TRANSMITTING FIL

TERS FOR TYPICAL LED EMITTERS

EMITTER

FILTER

PEAK

PERCENT

NUMBER

WAVELINGTH

TRANSMISSION

TOTAL LUMINOUS

(

l

p

in nm)

l

p

TRANSMISSION

LED

Red

2423

650

Yellow

2422

580

Green 2092

530

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Design Guidelines to

EMI Shielding Windows cont.

E. WINDOWS

ASSEMBLY AND MOUNTING

The edge of shielding windows is prepared for

mounting to the enclosure by applying an inter-

face gasket, which conducts induced currents

from the shielding mesh or conductive surfaces to

the ground plane of the system.

There are essentially two basic barrier termina-

tions for shielding windows: (1) conductive bus-

bar; (2) conductive gasketing. The conductive

busbar I used to contact the shielding screen or

conductive coating. The busbar terminates the

edge of the window opening by contacting the

screen mesh while providing a flat surface on one

or both sides of the window (Figure 6-1) to make

electrical contact to the enclosure bezel.

Conductive gasketing is often used in combina

tion with conductive busbars to provide a resilient

inter

face for aid in absorbing hock and vibration.

CONDUCTIVE BUSBAR

A conductive busbar is an electrical conductor

that can be used as a common electrical connec-

tion around the perimeter of the shielding window

to the conductive shielding bar

rier of knitted wir

mesh screen, transparent conductive coating

(ECTC) or woven mesh screen.

Generally, the more economical way to manufac-

ture small shielding windows is to either laminate

or cast knitted wir

e mesh scr

een or woven mesh

screen into large area sheets and/or to dissect the

sheets into several smaller area windows. The

windows that ar

e cut to size fr

om the larger

sheets have the mesh screen emerging at the

four edges of the window as shown in Figure 6-1.

Contact is made to the screen by means of a con-

ductive busbar of either a highly conductive coat-

ing such as an organic-type paint which is highly

filled with conductive silver par

ticles or a deposit

ed metal film.

Silver is the pr

eferred filler for paint to attain max-

imum conductivity

. The liquid car

rier for the paint

is an acrylic base, which produces a hard, firm

usbar and is compatible with most optical sub-

trate materials. The busbar then provides a com-

paratively large contact area to which an electro-

chemically compatible, conductive, resilient

gasket may be attached for shock mount and

moisture barrier.

An alternate mounting method for these types of

windows, employing a peripheral busbar, is to

bond the window directly to the enclosure using a

conductive RTV (room temperature vulcanization)

adhesive or a conductive epoxy. This latter

mounting technique provides a comparatively

rigid mounting and should be backed up by sev-

eral mounting clips or fasteners to ensure proper

bonding and to reduce possible seam flexure.

CONDUCTIVE GASKETING

The termination of the shielding mesh screen to

attain maximum performance from the shielding

window is as impor

tant in the material and meth

ods selection as in the shielding screen itself.

Improper screen termination may severely reduce

the shielding effectiveness of a high performance

shielding window as may be required for perform-

ance shielding window as may be required for

NASCIM 5100A (T

empest) applications. There

e thr

ee recommended edge terminations for

woven mesh scr

eens in applications r

equiring the

maximum per

for

mance over any extended period.

The three methods are listed in order of perform-

ance.

Bond, Direct Contact, Self Gasketing: Shielding

effectiveness tests have shown that the most

consistent r

esults and highest per

formance are

E-13

U.S. Customary

[SI Metric]

Figur

e 6-1.

Busbar T

mination.

Figure 6-2.

Bond Direct Contact.

E. WINDOWS

ttained when the shielding screen is bonded

permanently to the enclosure by spot welding,

brazing or soldering, depending upon the

material used for the screen. Generally, this

method is not cost effective. A nearly identical

assembly may be attained by a mechanical

clamping of the screen as shown in Figure 6-2.

For both glass and plastic windows, the use of

elastomer gaskets (neoprene or silicone) as

moisture barriers and for shock mounting is

recommended.

2. Wrap-Around, Direct Contact, Self Gasketing:

The mesh screen is wrapped over a sponge or

hollow core elastomer gasket and secured to

the underside of the window (Figure 6-3). The

use of elastomer moistur

e barrier and shock

mounts to protect the window and screen from

possible adverse envir

onment is r

ecommended.

Inter

facial Gasket, Indir

ect Contact, Conductive

Gasketing: the mesh screen is extended along

the flat of the step formed in the lamination

ocess and secur

ed to the underside of the

window (Figure 6-4). A conductive metallic or

elastomer gasket I mounted and bonded to the

surface of the step. The gasket should be

resilient and compatible with the screen and

enclosure materials. Contact resistance must

be kept low by means of a low impedance

bond, such as a conductive RTV or conductive

epoxy. A recommended gasket for this type of

application, pr

oviding both EMC and moistur

barrier, is a knitted mesh bonded to a silicone

sponge (see T

ecknit DUOGASKET). The knit

ted mesh strip should utilize tin-plated phos-

phor bronze (TPPB). TPPB provides highest

shielding and environmental compatibility

between the shielding scr

een and the enclo

sure surface.

Many combinations of gaskets ar

e possible. The

three methods described have been successful in

specific applications. The gr

eatest number of

interfacing surfaces which must make low imped-

ance contact between each interface, the greater

will be induced electromagnetic noise current and

the lower the shielding effectiveness of the sys-

tem. As a rule of thumb, provide a 10:1 signal to

noise ratio margin (about 20 dB more shielding)

than may be actually r

equir

ed when all the mat-

ing surfaces are freshly cleaned and properly pro-

tected.

SURFACE PREPARATION

The primar

y function of an EMC gasket is to pr

vide impedance that matches or exceeds the con-

ductivity the enclosure and minimizes the cou-

pling ef

ficiency of the seam itself from becoming

a r

e-radiator

. Normally, the reflection and absorp-

tion functions of a conductive shielding gasket are

to a lar

ge extent masked by metal cover has been

eplaced by a quasi-continuous open mesh which

at best is equivalent to a very thin barrier. At high

equencies (about 100MHz) the screen does not

respond as a solid barrier. Special attention must

be paid to the method by which the induced EMI

cur

ents in the mesh screen are returned to the

system gr

ound. Any significant dif

fer

ence in

seam impedance, including that introduced by

the gasket materials, may produce nonuniform

cur

ent flow r

esulting in the generation of EMI

voltages. Such induced voltages can then

become sources of EMI radiated energy. To mini-

mize these effects, the seam design and prepara-

tion is important and the following features should

be incorporated into any new design:

1. Mating surfaces should be as flat and parallel

as practically possible.

2. Mating surfaces must be conductive and pro-

tected from oxidation by plating with a hard

conductive finish that is galvanically compati-

ble with each other and with interfacial gaskets

(tin, nickel, cadmium).

3. Protective coatings having less than half the

conductivity of the mating surfaces should be

avoided.

E-14

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Figur

e 6-3.

rap Ar

ound Scr

een, Dir

ect Contact (Most

Commonoly Used Configuration).

Figure 6-4.

Interfacial Gasket, Indirect Contact with Mesh

Screens (Most Economical)

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Design Guidesline to

EMI Shielding Windows cont.

E. WINDOWS

. Flange width should allow at least five times

he maximum expected separation between

mating conductive surfaces.

5. Mating surfaces should be cleaned to remove

dirt and oxide films just prior to assembly of

the shielding window to the enclosure and

bezel.

6. Bonded surfaces should be held under pres-

sure during adhesive curing to minimize sur-

face oxidation and to maximize conductivity

after cure.

CORROSION

Corrosion is one of the major factors, which influ-

ences specific design considerations. Generally,

the lightweight str

uctural materials, aluminum

and magnesium, are most highly active electro-

chemically when in contact with the more con-

ductive materials used for shielding. Selecting

suitable shielding materials and finishes, which

inhibit oxidation and corrosion and are compati-

ble with enclosure materials, becomes a major

tradeoff in the designing of shielding windows.

Cor

rosion occurs between dissimilar metals in the

esence of an electr

olyte. Dissimilar metals in

contact in the pr

esence of an electr

olyte cause

galvanic cor

osion. A single metal under str

ess in

the presence of an electrolyte may result in stress

cor

osion due to impurities embedded within the

conductor. Table 6-1, electrochemical compati-

bility gr

ouping, lists groups of common materials

used as structural, barrier and gasketing materi-

als. The rate of cor

osion (er

osion of the less

noble metal, anodic) depends upon the electro-

chemical potential difference between the dissim-

ilar metals and the strength of the electrolyte.

able 6-1.

ouping of Metals by Electrochemical

Compatibility.

(ANODIC)

Group I

Group II

Group III

Group IV

Magnesium Aluminum Cadmium Plating

Brass

Magnesium

Aluminum Alloys

Carbon Steel

Stainless Steel

Alloys

Beryllium

Iron

Copper & Copper

Aluminum

Zinc & Zinc Plsling

Nickel & Nickel Plating

Alloys

Aluminum

Chromium Plating

Tin & Tin Plating

Nickel / Copper

Alloys

Cadmium Plating

Tin / Lead Solder

Alloys

Beryllium Carbon Steel

Lead

Monel

Zinc & Zinc

Iron

Brass

Silver

Plating

Nickel & Nickel Plating

Stainless Steel

Graphite

Chromium

Tin & Tin Plating

Copper & Copper Alloys

Rhodium

Plating

in / Lead Solder

Nickel/Copper Alloys

Palladium

Lead Monel Titanium

Platinum

Gold

(CATHODIC)

election of materials from a common group pro-

ides the least chance for corrosion due to gal-

vanic action when materials are in contact for

extended periods of time in a normal office envi-

ronment. The materials are arranged in their

decreasing order of galvanic activity within each

group and from left to right. Materials at the top

of a group or in groups to the left erode under

galvanic action. Dissimilar metals, which are in

different groups, may be accommodated by plat-

ing one or both with a material that is common to

both the enclosure and the mating surface. For

example, aluminum and copper are not compati-

ble in most environmental situations since they

are not contained within one single group (alu-

minum is in groups I and II, while copper is in

groups III and IV. To make these materials com-

patible, either one or both, preferable the latter,

would have to be tin plated.

MOUNTING WINDOWS

Twist drills that are commonly used for metals

may normally be used on most plastics. Since,

when machining plastics, a scraping action pro-

duces better results than a cutting action; drills

may be repointed to provide zero rake angle.

Moderate speed and light pr

essures produce

best r

esults and minimize temperatur

e changes

at the cutting edge, which may r

esult in galling or

seizing.

Plastic windows may be pr

ovided with holes,

which are often used for mounting and access

holes for screwdriver adjustments for “zeroing” or

“scaling” digital readouts. These holes should be

drilled prior to the application of sur

face coating

or finishes whenever possible to prevent scratch-

ing or marring the surfaces of the window.

Holes or notches are not recommended for glass

windows.

Common mounting methods include pressure-

clips to secure windows under pressure during

curing and clamping bars for larger plastic or

glass windows. Bolt spacing © for windows,

especially those with resilient gasketing, should

follow the basic equation as given by:

E-15

U.S. Customary

[SI Metric]

C =

1/4

inches

480 (a/b) E t3

∆

13 P min + 2 P max

E. WINDOWS

here: a=width of clamping bar

b=width of resilent gasket

E=modulus of elasticity of cover plate

t=thickness of clamping bar

∆

H=H1–H2 (difference between max/min

gasket deflection)

P min=minimum pressure (at minimum

deflection)

P max=maximum pressure (at maximum

deflection)

The bolt spacing equation can be simplified by

making some assumptions:

1. The bar width (a) will always be equal or

greater than the gasket width (b); therefore,

the ratio a/b will usually be gr

eater than one

(1). The worst case, which requires the mini-

mum bolt spacing (C), occurs when a/b equals

one. Should the bar be twice the width of the

gasket, the bolt spacing could be increased by

about 20%.

2. The maximum closing pressure, as a rule of

thumb, should not exceed the minimum pres-

sure by more than a 3:1 ratio.

The minimum closing pr

essure with a solid

elastomer moistur

e seal should not be less

than 50 PSI (P min.).

4. Modulus of elasticity for most metals (clamping

bar) is greater than 10 PSI.

Assume a maximum deflection of 0.010 inch

(

∆

H).

Then, maximum bolt spacing, C, becomes:

C = 15(t)

3/4

xample:

luminum clamping bar 1/8 inch thick

(t) would require a center-to-center bolt spacing

of 3-1/8 or less.

SPECIFYING SHIELDING WINDOWS

Sections 1 through 6 have provided methods by

which the designer can establish minimum sys-

tem need from shielding and optical clarity

requirements.

Table 3-1

summarizes the shielding range in dB

and open area in percent (%) of three types of

shielding screen materials.

able 4-1

tabulates maximum sizes, thickness

and tolerances for standard glass and plastic opti-

cal substrates.

Table 4-2

tabulates optical, mechanical, electrical,

ther

mal and chemical/physical pr

oper

ties of stan

dard optical substrate materials: plate glass,

methyl methacrylate (acrylic), polycarbonate, and

CR-39.

Table 5-1

tabulates standard color transmission

filters for plastic substrates.

Table 7-1

summarizes standard features of the

TECKNIT TECKSHIELD-F

, and EMC-ECTC

windows.

able 7-1

ovides a suggested work sheet, which

will aid

TECKNIT

Application Engineers in handling

request for designing or ordering flat shielding

windows. For cur

ved shielding windows fully lam-

inated or edges bonded, contact factory. Usually

by consulting with the factory before the design

stage can result in cost savings and performance

enhancement for curved shielding windows.

E-16

Mexico: 528-18-369-8610 • China: 86-10-67884650 • www.tecknit.com

able 7-1

TECKSHIELD-F

(Fully Laminated)

EMC-ECTC

Maximum Size

32” x 54”

(813mm x 1372mm)

Shielding Material

oven Mesh or

ranspar

ent

Knitted Mesh

Conductive Coating

Shielding Effectiveness (1GHz)

>60 dB

>30 dB

Anti-Glar

e Finish (On Request)

Anti-Reflection Coating (On Request)

Yes

(HEOC)

(One Side Only)

(HEOC)

Color Transmission

Yes

Filters (On Request)

(Ref. Table 5-1)

Abrasive Resistant

Coating

(On Acrylic and Polycarbonate)

Circular Polarizers

Yes

(Fully Laminated)

(Edge Bond)

U.S.A.: 908-272-5500 • U.K.: 44-1476-590600 • Spain: 34-91-4810178

Design Guidelines to

EMI Shielding Windows cont.

E. WINDOWS

ENGINEERING SPECIFICATIONS ES-71-01,

TECKSHIELD WINDOWS – FLAT GLASS

I OPTICAL QUALITY

The finished window will meet the optical quality

criteria with respect to any imperfections and

defects as detailed below:

A. Minor Imperfections

1. Definition

– Any one of the following condi-

tions, exceeding 0.0001 square inches but

not exceeding 0.0025 square inch area

per defect and not exceeding 0.2 inch in

its longest dimension, in the viewing area:

embedded Particles

air bubbles

scratches

wir

e scr

een defects

2. Accept/Reject Criteria

The window shall not have mor

e than one

such “imperfection” per 40 sq. in. of view-

ing area.

B. Major Defects

Definition

– Any condition as described in

Section A, but exceeding 0.0025 squar

inch in area or exceeding 0.2 inch in its

longest dimension per defect in the view

ing area.

4. Accept/Reject Criteria

Any “Major Defect” shall be cause for

rejection.

II ANTI REFLECTION COATING (HEOC)

The multi-layer low-reflection coating will meet

the minimum acceptable requirements for optical

contrast enhancement when used for TECKNIT

EMI shielding windows.

A. Coated Area:

Unless otherwise specified, glass

elements shall be coated over their entire

effective aperture, except for an allowable

uncoated ar

ea with a maximum width of 0.060

inch around edges.

. Specular Reflectance:

hen applied to sub-

trate materials having indices of refraction of

1.5 ± 0.04, the specular reflectance from a

coated surface shall average less than 0.85%

for an angle of incidence of 10º over the wave-

length range of 450 to 650 nanometers.

C. Coating Quality:

The coating shall be uniform

in quality and condition, clean, smooth, and

free from foreign materials, and from physical

imperfections and optical imperfections as

follows:

The coating shall show no evidence of flak-

ing, peeling or blistering.

The coating shall not contain blemishes,

such as discoloration, stains, smears and

streaks or show evidence of a cloudy or

hazy appearance.

The coating shall show no evidence of

scratches, digs, or pinholes within a cen

tral area, which covers 60% of the overall

viewing area.

D. Abrasion Resistance:

There shall be no visible

damage to the coated surface when rubbed 15

times with a standar

d rubber-pumice eraser

under a for

ce of 2 to 2-1/2 pounds.

Humidity:

Continuous exposur

e to 100% r

ela

tive humidity at a temperature of 80º C.

F. Operating and Storage Temperature Range:

-55º

to + 80ºC continuous.

E-17

U.S. Customary

[SI Metric]

E. WINDOWS

E-18

Mexico: 528-18-369-8610 • China: 86-10-67884650 • www.tecknit.com

U.S.A.: 908-272-5500 • U.K.: 44-1476-590600 • Spain: 34-91-4810178

ECTC

™

Windows

ELECTRICALLY CONDUCTIVE TRANSPARENT COATING

E. WINDOWS

GENERAL DESCRIPTION

ECTC WINDOWS are custom designed optical

display panels produced by depositing a very thin

electrically conductive transparent coating directly

onto the surface of various optical substrate mate-

rials to provide high EMI shielding performance

coupled with good light transmission properties.

APPLICATION INFORMATION

Applications of ECTC WINDOWS are found in

equipment requiring visual displays where the

viewing panel must also serve to reduce the radi-

ated electromagnetic energy entering or leaving

the device.

SUBSTRATE MATERIALS

Most transparent plastic and glass sheet material

e suitable for ECTC coating. However

, even

those optical substrate materials with high quality

surfaces, have minute surface imperfections

which become mor

e appar

ent after coating. In

most applications these blemishes will not

degrade the appearance of the finished window

or the shielding per

for

mance.

CONDUCTIVE COATING

Standard ECTC coating has a nominal resistivity

of 14.0 ohms per square and a light transmission

of about 70 percent in the visible spectrum.

Applying ECTC coatings to both surfaces of the

optical substrate, incr

eases shielding effective-

ness by 6 to 10 dB, while reducing the optical

transmission from 70 percent to about 50 per-

cent.

ECTC coatings are easily damaged by abrasion

since “finger printing” from oils present in normal

skin moistur

e ar

e dif

ficult to r

emove. In normal

usage, the coating is applied to the inner surface

of the window substrate which permits cleaning of

the fr

ont sur

face with a commer

cial window

cleaner. NOTE: Inspection, handling and installa-

tion personnel should use clean, lint-free cotton

gloves when handling ECTC W

indows.

SPECIFICATIONS

MATERIAL DESCRIPTION

•

Optical Substrate

Acr

ylic:

Acr

ylic sheet per Federal Specification

L-P-391, Type 1, Grade C, clear (ASTM-D-4802).

Glass:

Glass sheet per Specification

ASTM-C-1036, clear.

Commercial Grade Polycarbonate

•

Conductive Coating:

TECKNIT ECTC vacuum deposited thin metal film.

Indium Tin Oxide coatings available upon request.

•

Busbar T

mination:

TECKNIT Silver Acr

ylic conductive

coating.

•

Mounting Frame (when specified):

Aluminum alloy

PERFORMANCE CHARACTERISTICS

•

Coating

Surface Resistivity:

14 ohms/square nominal

(±4 ohms/square).

•

Temperature Range

Acrylic:

-67°F to 150°F [-55°C to 65°C].

Glass:

-67°F to 167°F [-55°C to 85°C].

TERIAL

H-FIELD

E-FIELD

PLANE W

AVE

ECTC

100 kHz

10 MHz

1 GHz

20 dB

90 dB

30 dB

Tested in accordance with TECKNIT Test Method

TSETS-01, which is based upon modified MIL-

STD- 285. T

ypical values are based on a 5"

square window.

E-19

U.S. Customary

[SI Metric]

E. WINDOWS

BUSBAR TERMINATION AND

INTERFACE GASKETING

The edges of ECTC WINDOWS are terminated

with a border of highly conductive, silver busbar

material. This conductive band serves two pur-

poses:

1. Provides a uniform current distribution. The

busbar material has a very low surface resistivity

when compared to the ECTC coating.

2. Provides a more durable low impedance bear-

ing surface than the ECTC coating alone. An

interface gasket joins the ECTC window coating to

the enclosure panel.

The most widely used interface gasket is TECK-

NIT CONSIL, silver-filled silicone rubber gaskets.

These gaskets provide both environmental and

electromagnetic sealing without damage to the

busbar or coating.

FRAMING AND MOUNTING

Standard ECTC Windows can be mounted directly

to the equipment panel or enclosure without an

inter

face gasket using TECKNIT conductive

epoxy

. When using standar

d inter

face gasketing,

TECKNIT standar

d framing is available.

ANDARD OPTICAL SUBSTRATE MATERIAL

Table 1. STANDARD THICKNESS (T)

MATERIALS

THICKNESS (T)

TOLERANCE

in. [mm]

Acrylic

.062 [1.52]

±.016 [0.41]

.125 [3.18]

±.020 [0.51]

Glass

.090 [2.29]

±.020 [0.51]

.125 [3.18]

±.020 [0.51]

ANDARD WINDOW CONFIGURA

TION

Figure 1.

Window Dimensioning.

*Continuous Busbar around periphery

(TECKNIT Silver Acrylic Conductive Coating).

STANDARD FRAME STYLES

Figure 2.

Frame Cross Section

STANDARD FRAME DIMENSIONING

Figure 3.

Overall Frame Style

STANDARD TOLERANCES

Table 2.

WINDOW

SYMBOL DIMENSION TOLERANCE

A,B

18 in. [up to 457 mm]

±.031 [0.79]

FRAME

C,D,E,F

12 in. [up to 305 mm]

±.015 [0.38]

12-18 in. [305 to 457 mm]

±.020 [0.50]

K,L

12 in. [up to 102 mm]

±.015 [0.38]

4-24 in. [102.1 to 610 mm]

±.031 [0.79]

FRAME CROSS SECTION

W,X

0-.750 in. [up to 19 mm]

±.010 [0.25]

.750 -1.250 in. [19.1 to 31.8 mm]

±.012 [0.30]

S,T

.750 in. [up to 19 mm]

±.006 [0.15]

ORDERING INFORMA

TION

ECTC Windows are custom designed to customer

specifications and drawings. Customer drawings

should provide dimensional data as suggested in

Figure 3 such as overall size, viewing area, win-

dow size and thickness (dimensions AxB), type of

edge termination and interface gasket, type frame

by style number and special options. For assis-

tance, contact your TECKNIT representative or

factory engineer.

E-20

Mexico: 528-18-369-8610 • China: 86-10-67884650 • www.tecknit.com

U.S.A.: 908-272-5500 • U.K.: 44-1476-590600 • Spain: 34-91-4810178

Teckfilm

™

TRANSPARENT CONDUCTIVE COATING ON POLYESTER FILM

E. WINDOWS

GENERAL DESCRIPTION

TECKFILM is a highly conductive coating deposit-

ed on a transparent polyester film. It is available

in rolls 30" wide. Usable width is 28". The con-

ductive coating is overcoated with a ceramic type

film which serves to increase visible light trans-

mission and to provide a protective barrier that

exhibits electrical conductivity through the layer.

CONSIL

-II silver filled silicone elastomer material

is recommended between the TECKFILM and

conductive mating surface as an interface gasket

and an environmental seal between the enclosure

and TECKFILM window panel assembly.

APPLICATION INFORMATION

TECKFILM is designed for electric and planewave

shielding, grounding and static discharge applica-

tions. TECKFILM is used as a transparent, shield-

ing panel for visual displays in instrumentation

equipment, control panels, computer processing,

printers, peripheral equipment and large elec-

trode displays as a grounding shield.

MOUNTING TECHNIQUES

Various methods of mounting are as follows:

1. Affixed to conductive mating surface with

clamps or bonded with TECKNIT T

wo-Par

t R

Silver Silicone Adhesive Sealant (Part No. 72-

00036).

2. Mounted between a substrate and conductive

mounting surface with or without the aid of edge

bonding to the substrate.

NOTE: TECKFILM conductive sur

face can be

marred if handled excessively.

EMI SHIELDING PERFORMANCE

TECKNIT TECKFILM Shielding Effectiveness has

been tested in accor

dane with TECKNIT T

est

Method TSETS-01 which is based upon modified

MIL-STD- 285. Typical shielding effectiveness val-

ues ar

e based on a 5" squar

e window

MATERIAL

H-FIELD

E-FIELD

PLANE WAVE

100 kHz

10 MHz

1 GHz

TECKFILM

20 dB

90 dB

30 dB

SPECIFICATIONS

MATERIAL DESCRIPTION

•

Substrate:

Polyester film .005 in. [0.13mm] thick, clear and col

orless.

•

Conductive Coating:

Vacuum deposited thin metal film with

protective ceramic coating.

•

Standard Bulk Material

Part Number:

70-00117

PERFORMANCE CHARACTERISTICS

•

Substrate and Coating

Sur

face Resistivity:

14 ohms/squar

e (nominal)

(±4 ohms/square).

Visible Light Transmission:

70 to 80%.

emperatur

e Range:

-76

F to 300

F [-60

C to 150

°C].

ORDERING INFORMATION

Fabricated and rule die cut window shapes up

to 28" wide can be supplied. Contact your

TECKNIT area representative or factory engineer

for assistance.

E-21

U.S. Customary

[SI Metric]

E. WINDOWS

GENERAL DESCRIPTION

TECKSHIELD-F high-performance fully laminated

flat windows are specially designed to provide

optimum optical transmission and EMI shielding

in severe interference environments. TECK-

SHIELD-F windows have proven to be effective in

TEMPEST qualified Visual Display Units, as well

as in printers and enclosures requiring large view-

ing apertures. A special low-resistance mesh is

laminated between two layers of glass or acrylic.

The edge ter

mination between the window mesh

and the enclosure is designed to provide uniform

mesh-to-enclosure continuity around the entire

perimeter of the shielding aper

ture.

FEATURES

• Full lamination provides rugged construction,

prevents moisture intrusion or entrapment

between optical layers, enhances optical con-

trast by elimination of two optical media-to- air

interfaces.

•

High shielding per

for

mance of lar

ge viewing

aper

tur

es at a br

oad range of fr

equencies.

• Minimum optical distor

tion of viewed display.

• Design options include color filters and polariz-

ers for contrast enhancement, which permit

flexibility in matching optical and shielding

requirements to specific applications.

APPLICA

TION INFORMA

TION

TECKSHIELD-F high-performance flat windows

e designed for enclosur

es requiring superior

shielding against EMI radiation or susceptibility

They provide maximum EMI protection and high

optical clarity for teleprinters, digital, graphic, and

other flat displays. TECKSHIELD-F windows can

also be economically matched to most visual dis-

play units to minimize image distor

tion and to

maximize shielding ef

fectiveness.

EMI SHIELDING PERFORMANCE

MESH H-FIELD E-FIELD PLANE WAVE

SCREEN

100 KHZ

10 MHZ

1 GHZ

10 GHZ

100 OPI

55 dB

120 dB

60 dB

40 dB

145 OPI

55 dB

120 dB

80 dB

45 dB

Tested in accordance with TECKNIT Test Method

TSETS-01, which is based upon modified MIL-

STD-285. Typical Shielding Effectiveness values

are based on a 5" square window.

SPECIFICATIONS

MATERIAL DESCRIPTION

•

Standard Optical Media

Glass:

Per Specification ASTM-C-1036, Type 1, Class 1.

Acrylic:

Per Federal Specification L-P-391, Type 1, Grade C

(ASTM-D-4802).

•

Optical Media Options

Acr

ylic Colors:

See T

able 2.

Anti-Reflection Coatings:

Non-Glar

e Coating (Matte Finish).

High Efficiency Anti-Reflection Coating

(Less than 0.6% Reflection).

•

Mesh Scr

een

100 OPI:

Blackened Copper Mesh 0.0022" Wire Diameter,

60% Open Area.

145 OPI:

Blackened Copper Mesh 0.0022" W

ire Diameter,

45% Open Area.

Interface Gasket:

Copper Mesh Wrap-Around Termination.

See Figure 2.

Duogasket:

See Figure 3.

Busbar T

mination:

ecknit Silver Acr

ylic Conductive

Coating (Fig. 5)

PERFORMANCE CHARACTERISTICS

•

Operating & Storage Temperature

Glass:

-67

°F to 176°F [-55°C to 80°C]

Acrylic:

-67°F to 140°F [-55°C to 60°C]

E-22

Mexico: 528-18-369-8610 • China: 86-10-67884650 • www.tecknit.com

Teckshield

-F

HIGH PERFORMANCE EMC WINDOWS

STANDARD WINDOW CONSTRUCTION

Standard TECKSHIELD-F fully laminated window

onstruction consists of: (a) Standard mesh

screen, blackened and laminated between (b)

two layers of standard optical medium (clear and

colorless see Fig. 1), and with (c) an interfacial

asket (copper mesh wrap around or Duogasket)

o provide electrical continuity between the win-

dow mesh and equipment enclosure. The

Duogasket consists of an environmental seal

and an EMI gasket seal.

Standard window thicknesses are 0.205 in.

[5.2 mm] for glass substrates and 0.145 in.

[3.68 mm] for acrylic substrates.

FRAMING AND MOUNTING

Standard TECKSHIELD-F windows may be

mounted dir

ectly to the equipment enclosur

e uti

lizing the r

ecommended inter

face gasket ter

mina-

tion shown in Figs. 2, 3, 4 and 5. When specify

ing a finished mounting frame for the standar

window thickness shown in Fig. 1, provide a

drawing of the frame as shown in Fig. 6 using the

TECKNIT styles shown in Fig. 2-5.

In some instances, standard TECKSHIELD-F win-

dows may be mounted directly to the equipment

enclosur

e without an inter

face Duogasket by

using TECKNIT conductive epoxy to establish

an electrical bond to the enclosure. Additional

mechanical clips may be required to locate and

mechanically secure the window to the enclosure.

STANDARD TOLERANCES

Table 1.

WINDOW

SYMBOL DIMENSION TOLERANCE

A,B

18 in. [up to 457 mm]

±0.031 [0.79]

FRAME DIMENSION

C,D,E,F

up to 12 in. [305 mm]

±0.015 [0.38]

12 to 18 in. [305-457 mm]

±0.020 [0.50]

K,L

up to 4 in. [102 mm]

±0.015 [0.38]

4 to 24 in. [102.1-610 mm]

0.031 [0.79]

FRAME CROSS SECTION

W,X

up to 0 - .750 in. [19 mm]

±0.010 [0.25]

.750 to1.250 in. [19.1-31.8 mm]

±0.012 [0.30]

S,T

up to 0.750 in. [19 mm]

±0.006 [0.15]

ACR

YLIC COLOR TRANSMISSION FIL

TERS

able 2.

Red

Amber

Yellow Green Blue Gray

2423

2422

2208

2092 2069 2514

ORDERING INFORMA

TION

TECKSHIELD-F high-per

for

mance windows ar

custom designed to customer specifications.

Drawings should be provided that show dimen-

sional data such as overall dimensions, mounting

hole dimensions, desired viewing area, window

and frame thickness (when r

equir

ed), type of

edge terminations and interface gasket, type of

frame or bezel and special options. For assistance

contact your nearest TECKNIT area representative

or factor

y location.

U.S.A.: 908-272-5500 • U.K.: 44-1476-590600 • Spain: 34-91-4810178

eckshield

-F Polycarbonate W

indows

E. WINDOWS

E-23

U.S. Customary

[SI Metric]

FEATURES

•

80% open ar

ea-best light transmission of all

ecknit woven window meshes.

• Available as thin as .053" [1.35].

• -60°F to 158°F [-55°C to 70°C] operating

temperature.

• All standar

d T

ecknit EMI ter

minations available.

EMI SHIELDING PERFORMANCE

H-FIELD E-FIELD PLANE WAVE

100 kHz

MHz

1 GHz

10 GHz

80 OPI SS

35 dB

85dB

42 dB

30 dB

100 OPI SS

40 dB

105 dB

52 dB

35 dB

SPECIFICATIONS

MATERIAL DESCRIPTION

•

Mesh Screen:

Blackened 304 stainless steel, .001" dia.,

80 or 100 openings per inch.

•

Standar

d Substrate:

Polycarbonate, clear & colorless.

•

Available Upon Request

UL-94VO-rated polycarbonate

Abrasion resistant & anti-glare coatings

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