Wednesday, March 23, 2016

Detect defective components using handheld multimeter

Previous part covering methods on how to detect hotspot for potential failure during PCBA troubleshooting can be found here.

Apart from dry joints, defective or out of tolerance components are among common causes of failures on a circuit board. Component checking using a handheld digital multimeter is commonly performed when determining the root cause of failures.

Diode test

Typically, diodes and transistors are checked using the diode test function. The digital multimeter sends a current through the semiconductor junction and measures the voltage drop between the test points of the semiconductor device. For non-defective diodes, the digital multimeter normally shows a value of between 0.3 V to 0.8 V at the forward bias condition, and it shows an open circuit (with O.L. on the display) at the reverse bias condition. Defective diodes will appear as either an open circuit or a short circuit in both directions. Sometimes, diode test reading may vary due to the resistance of other pathways between the test probe tips. When this happens, it is recommended to isolate the component by removing it from the circuit board, and performing the test again.

Resistance and capacitance measurements

In many circumstances, electronic devices that failed in quality assurance testing during the production stage due to components such as resistors or capacitors being out of tolerance. The component accuracy range for resistors and capacitors are relatively large. The accuracy for resistors typically range from ±1% and above, whereas capacitors have an even bigger range normally starting from ±20%. These commonly found components on a circuit board are tested using a digital multimeter to verify if the values fall within the accepted range. These component tests allow the technician to quickly detect the problematic components at the work bench.

The typical capacitance measurement for 4 ½ digit handheld digital multimeter may starts at  10 nF range with 0.001 nF resolution. To test the capacitor value below this range, use a handheld LCR meter with a wider measurement range from 20 pF range with 0.001 pF resolution to 20 mF range.

The Keysight U1730C Series handheld LCR meters perform quick and basic LCR measurements of the components. Coupled with the U1732B SMD tweezer, SMD-type component measurement can be easily made.  By connecting to a power adapter, the LCR meter will operate like a benchtop eliminating the worry about the limited battery life.

Figure 4 Keysight U1732C handheld LCR meter is used at a PCBA debug station
Next posting will discuss about signal probing and other methods technicians use to accelerate troubleshooting tasks.

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