Detecting hotspot for potential failure during circuit board troubleshooting

Previous part covers the introduction of the printed circuit board assembly troubleshooting can be found here.

Circuit board failures may be caused by overheated components such as power transistors and ICs. Traditionally, overheated components are detected by simply touching the surface component. This can be dangerous and has to be done with extreme caution to avoid getting burnt fingers and electrocuted. For circuit boards suspected of thermal failures, they can be detected for hotspots using an IR thermal imager without physically touching the components. Infrared thermal imaging is able to capture the temperature distribution of the whole circuit which makes it easy to see the hotspot at a glance. 

Figure 2 Detecting hotspot on a circuit board using Keysight U5850 series TrueIR Thermal Imagers with image logging capability

Typically, 320 x 240 pixels of in-camera fine resolution such as Keysight U5850 series TrueIR Thermal Imagers with the ability to focus on objects as close as 10 cm, are idea for measuring temperature of small components that are close to each other. Additionally, the thermal imager monitors temperature changes through image logging and temperature trending capabilities using the TrueIR Analysis and Reporting Tool software.

For more accurate temperature monitoring, the component temperature can be measured using a Keysight digital multimeter with a thermocouple probe attached to the surface of the component while the circuit board is powered on.

Figure 3 Monitoring the temperature of a circuit board with the  U1282A handheld digital multimeter

Next, we will talk about how to detect faulty components during PCBA troubleshooting using a handheld digital multimeter. 

Circuit board troubleshooting using handheld digital multimeters

Printed circuit board assembly troubleshooting

The printed circuit board assembly (PCBA) can be found in almost any electronic devices. These electronic devices range from consumer electronics such as scanners, exercise devices, charging stations, computer accessories and modems to commercial electronics in the industrial and medical fields with stringent requirements. Testing is crucial to ensure the electronic devices operate normally. In most cases of electronic device failures, the root cause is determined to be the PCBA itself. 

This blog describes some common techniques used in circuit board troubleshooting.  Before proceeding with the troubleshooting task, it is important to identify the failure symptoms. The failure symptoms may provide information about the possible cause of defect.

Checking the power supply voltage to the circuit

Voltage measurement of the circuit board’s power supply is an important testing process because it is critical that a proper level of power is supplied to most of the integrated circuits (ICs). It is essential to check if the voltage level output from the power supply is within the acceptable limit to prevent overheating or overloading of the circuit board. Handheld digital multimeters with 4 ½ digit display resolution deliver the precision, accuracy and repeatability which is compatible with the basic bench multimeters, would be a good alternative for the technicians to use in troubleshooting.

Visual inspection to look for burned or damage parts

Visual inspection is a straight forward and effective method for troubleshooting. Components or parts such as transformers, power output transistors, resistors and capacitors that carry the burn mark can be detected easily using this method. The burned parts normally appear as brownish in color and can be easily detected visually through the naked eye or with the aid of a magnifying glass. Sometimes, the overheated components will form a brownish mark on the circuit board and produce a burnt scent.

Figure 1 Example of dry joints with obvious crack line (figure extracted from internet)

Solder joint is another common item inspected during the visual inspection to look for any dry joints.  Dry joint is a common symptom of a defective board due to soldering defects. Dry joint cause poor contact at the solder joint and affects the current conduction in the circuit.  A good solder joint normally looks smooth, bright and shiny. Dull surface suggests a weak joint.  Dry joint can be checked using a handheld digital multimeter. Resistance test or continuity check is performed from one pin to another pin to locate any dry joints or open traces on the circuit board.

Next posting will discuss methods to detect hotspot for potential failure. 

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.