Super Complete PCB Failure Analysis Technology

As the carrier of various components and the hub of circuit signal transmission, PCB has become an important and critical part of electronic information products. Its quality and reliability level determine the quality and reliability of the entire equipment.

PCB, as the carrier of various components and the hub of circuit signal transmission, has become an important and critical part of electronic information products, and its quality and reliability level determine the quality and reliability of the entire equipment. With the miniaturization of electronic information products and the environmental protection requirements of lead-free and halogen-free, PCBs are also developing in the direction of high density, high Tg and environmental protection. However, due to cost and technical reasons, a large number of failure problems have occurred in the production and application process of PCB, which has caused many quality disputes. In order to find out the cause of the failure in order to find the solution to the problem and to distinguish the responsibility, the failure analysis must be carried out on the failure cases that have occurred.

In order to obtain the exact cause or mechanism of PCB failure or defect, basic principles and analysis procedures must be followed, otherwise valuable failure information may be missed, resulting in failure to continue the analysis or may lead to wrong conclusions. The general basic process is that firstly, based on the failure phenomenon, through information collection, functional testing, electrical performance testing and simple visual inspection, the failure location and failure mode must be determined, that is, failure location or fault location. For a simple PCB or PCBA, the location of failure is easy to determine. However, for more complex BGA or MCM packaged devices or substrates, the defects are not easy to be observed through a microscope, and it is not easy to determine for a while. At this time, other means are needed to determine. Then it is necessary to analyze the failure mechanism, that is to use various physical and chemical means to analyze the mechanism that causes PCB failure or defects, such as virtual soldering, pollution, mechanical damage, moisture stress, medium corrosion, fatigue damage, CAF or ion migration, stress overload, etc. Then there is the failure cause analysis, that is, based on the analysis of the failure mechanism and the process process, to find the cause of the failure mechanism, and to carry out test verification if necessary. Generally, test verification should be carried out as much as possible. Through the test verification, the exact cause of the induced failure can be found. This provides a targeted basis for the next improvement. Finally, according to the test data, facts and conclusions obtained in the analysis process, a failure analysis report is prepared.

In the process of analysis, pay attention to the basic principle that the analysis method should be used from simple to complex, from outside to inside, never to destroy the sample and then to use the destruction. Only in this way can the loss of critical information and the introduction of new artificial failure mechanisms be avoided. Just like a traffic accident, if the party involved in the accident damages or flees the scene, it is difficult for a competent police officer to make an accurate determination of responsibility. At this time, traffic laws generally require the party who fled the scene or the party who destroyed the scene to bear full responsibility. The same is true for the failure analysis of PCB or PCBA. If an electric soldering iron is used to repair the failed solder joints or strong scissors are used to cut the PCB, then the analysis will be impossible, and the failure site has been destroyed. Especially in the case of few failure samples, once the environment of the failure site is destroyed or damaged, the real failure cause cannot be obtained.

Basic procedures for failure analysis

Light Microscope

The optical microscope is mainly used for the visual inspection of the PCB, to find the failure parts and related physical evidence, and to preliminarily judge the failure mode of the PCB. The visual inspection mainly checks the PCB contamination, corrosion, the position of the explosion board, the circuit wiring and the regularity of failure, such as batch or individual, whether it is always concentrated in a certain area and so on.

X-ray

For some parts that cannot be inspected by appearance, as well as the inside of through holes and other internal defects of PCB, X-ray fluoroscopy system has to be used to inspect. The X-ray fluoroscopy system uses the different principles of the moisture absorption or transmittance of X-rays by different material thicknesses or different material densities to image. This technique is more used to inspect the internal defects of PCBA solder joints, the internal defects of through-holes and the location of defective solder joints of high-density packaged BGA or CSP devices.

Slice Analysis

Slice analysis is the process of obtaining PCB cross-sectional structure through a series of means and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Through slice analysis, rich information on the microstructure reflecting the quality of the PCB (through holes, plating, etc.) can be obtained, which provides a good basis for the next step of quality improvement. However, this method is destructive, and once the sectioning is performed, the sample must be destroyed.

Scanning Acoustic Microscopy

Currently, the C-mode ultrasonic scanning acoustic microscope is mainly used for electronic packaging or assembly analysis. It uses the amplitude, phase and polarity changes generated by the reflection of high-frequency ultrasonic waves on the discontinuous interface of materials to image. The way is to scan the information in the X-Y plane along the Z axis. Therefore, scanning acoustic microscopy can be used to detect various defects within components, materials, and PCBs and PCBAs, including cracks, delaminations, inclusions, and voids. Internal defects in solder joints can also be detected directly if the frequency width of the scanning acoustics is sufficient. A typical scanned acoustic image is a red warning color to indicate the existence of defects. Since a large number of plastic packaged components are used in the SMT process, a large number of moisture and reflow sensitive problems are generated during the process of converting from lead to lead-free process. That is to say, the hygroscopic plastic package will have internal or substrate delamination and cracking when reflowed at a higher lead-free process temperature, and ordinary PCBs will often burst at the high temperature of the lead-free process. At this time, scanning acoustic microscopy highlights its special advantages in non-destructive testing of multilayer high-density PCBs. The general obvious burst board can be detected only by visual appearance.

Microscopic Infrared Analysis

Microscopic infrared analysis is an analysis method that combines infrared spectroscopy and microscopy. It uses the principle of different absorption of infrared spectroscopy by different materials (mainly organic substances) to analyze the compound composition of materials, and then combined with microscopy to make visible light With the same optical path as infrared light, as long as it is in the visible field of view, it can search for trace organic pollutants to be analyzed. Without the combination of a microscope, infrared spectroscopy can usually only analyze samples with larger sample volumes. In many cases in electronic technology, trace pollution can lead to poor solderability of PCB pads or lead pins. It is conceivable that it is difficult to solve process problems without infrared spectroscopy supporting a microscope. The main purpose of micro-infrared analysis is to analyze the organic contamination on the surface of the welded surface or the solder joint, and to analyze the cause of corrosion or poor solderability.

Scanning Electron Microscopy (SEM)

Scanning Electron Microscope (SEM) is one of the most useful large-scale electron microscopy imaging systems for failure analysis. Features can be magnified to hundreds of thousands of times for observation and analysis.

In terms of failure analysis of PCB or solder joints, SEM is mainly used to analyze the failure mechanism, specifically to observe the topography and structure of the pad surface, the metallographic structure of solder joints, measure intermetallic compounds, Solderability coating analysis and tin whisker analysis and measurement, etc. Unlike optical microscopes, scanning electron microscopes form electronic images, so there are only black and white colors, and the samples of scanning electron microscopes are required to be conductive, and non-conductors and some semiconductors need to be sprayed with gold or carbon, otherwise charges will accumulate on the surface of the sample. observation of the sample. In addition, the depth of field of SEM images is much larger than that of optical microscopes, and it is an important analysis method for uneven samples such as metallographic structure, microscopic fractures and tin whiskers.

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (Differential Scanning Calorim-etry) is a method of measuring the relationship between the power difference and the temperature (or time) between the input material and the reference material under the program temperature control. It is an analytical method to study the relationship between heat and temperature. According to this relationship, the physical, chemical and thermodynamic properties of materials can be studied and analyzed. DSC is widely used, but in PCB analysis, it is mainly used to measure the curing degree and glass transition temperature of various polymer materials used on PCB. These two parameters determine the reliability of PCB in subsequent processes.

Thermo-Mechanical Analyzer (TMA)

Thermal Mechanical Analysis is used to measure the deformation properties of solids, liquids and gels under the action of heat or mechanical force under programmed temperature. It is a method to study the relationship between thermal and mechanical properties. According to the relationship between deformation and temperature (or time), the physical, chemical and thermodynamic properties of materials can be studied and analyzed. TMA has a wide range of applications, and is mainly used for the two most critical parameters of PCB in PCB analysis: measuring its linear expansion coefficient and glass transition temperature. PCBs with substrates with excessive expansion coefficients often lead to fracture failure of metallized holes after soldering and assembly.

Thermogravimetric Analyzer (TGA)

Thermogravimetry (Thermogravimetry Analysis) is a method of measuring the relationship between the mass of a substance and the temperature (or time) under program temperature control. TGA can monitor the subtle mass changes of substances in the process of program-controlled temperature change through a precise electronic balance. According to the relationship between the mass of the substance and the temperature (or time), the physicochemical and thermodynamic properties of the material can be studied and analyzed. In terms of PCB analysis, it is mainly used to measure the thermal stability or thermal decomposition temperature of PCB materials. If the thermal decomposition temperature of the substrate is too low, the PCB will explode or delaminate at high temperatures during the soldering process.