How to Check the Potential Quality Issues in the Laser Trimming Groove of Metal Film Resistors?

The metal film resistors (MELF and SMD) are commonly used in many different electrical fields (electric vehicle, EV charger, solar energy/power supply,feedback circuit,control circuit, etc.). The resistors are used for measuring and controlling the voltage or current. The resistance value change of the resistor will cause the circuit to appear the abnormal behaviors in the feedback, voltage reading, power conversion, etc., which will then cause the other components to be damaged or burned. For example, when the resistance value of a resistor of an EV battery charging/discharging circuit has changed, it may cause the wrong voltage reading of the battery, and then cause the battery to be overcharged and burned. Consequently, it causes a fire.

After the resistive layer has been sputtered onto a ceramic substrate, a laser will be used to burn/cut the resistive layer for trimming the resistance value. The laser trimming grooves should have a good insulation to maintain the resistance value, but if there is a metal particle in the laser trimming groove, there will be a potential risk of forming a short circuit in the laser trimming groove, which could cause the resistance value to change. This kind of potential defective resistor will not be able to sort out by the Ohmmeter (resistance meter), because the metal particle does not form the short circuit yet, and the test voltage is not high enough. After this potential defective resistor has been used for a while, the metal particle caused an internal short circuit to form inside the laser trimming groove, so its resistance value changed. For example, if there is an abnormal short circuit occurred in one of ten turns (cuts) of laser trimming groove(s) of a 100kΩ resistor, its resistance value may deteriorate from 100kΩ to 90kΩ (Figure 1).

▲Figure 1. The metal particle causes the internal short circuit in one of ten turns, so the resistance of a 100kΩ resistor changes from 100kΩ to 90kΩ.

The current production tests for metal film resistors do not include any very high voltage test item, so it is very hard to detect the metal particle in the laser trimming groove. Although the power quality test for metal film resistor requires to apply a 100~250Vac from AC source for around 0.5 sec, and then uses the Ohmmeter (resistance meter) to check whether the resistance value has changed or not, there are two major issues with using the AC source for the purpose of checking the metal particle(s) in the laser trimming groove(s):

1, The test voltage is not high enough for checking the insulation quality: When the 250Vac is applied to a metal film resistor that has 10 turns of laser trimming grooves, the voltage difference on each turn is only around 25Vac. In Figure 2, it shows that a less than 1um air gap requires over 350V test voltage for effectively checking the insufficient insulation distance (i.e., the discharge phenomenon is caused by electric breakdown of the air gap). Therefore, the metal film resistor needs to use at least a few thousand volts of test voltage in order to check whether the insulation distance of each turn of the laser trimming groove is enough.

2, The test power becomes too large when the test voltage is increased: When the test voltage of AC source is increased, the test time (duration) of the voltage output cannot be shortened, so the test power that is applied to the resistor becomes too large and then damages the resistor (Figure 3).

▲Figure 2. Breakdown voltage of air vs. Gap distance

▲Figure 3. CHROMA 19311 surge test vs. general AC source

Chroma 19311 can provide the surge test that has a high test voltage with a very short duration (<160uS) for checking the insulation distance of laser trimming grooves and exposing the poor insulation quality products. The high-voltage surge test checks whether the metal film resistor potentially has poor insulation quality or insufficient insulation distance for making sure its resistance value will not change or deteriorate under the normal operating conditions. This can reduce the chance of safety accidents and ensure the long-term reliability of electrical products.

▲Chroma 19311 Surge Tester