PCB Troubleshooting Techniques

Troubleshooting a multi-layer PCB is often quite a challenge

Mistakes and component failure are a fact of life. Circuit boards are sometimes made with mistakes in them, components can be soldered in backward or in the wrong position, and components go bad. All of this makes a circuit work poorly or not at all. PCB troubleshooting can be a monumental task that taxes both the will and the mind. Luckily, a few tricks and techniques can greatly speed up the search for the troublesome "feature."

PCB Troubleshooting

Printed circuit boards, or PCBs, are a mass of insulators and copper traces that connect densely packed components together to create a modern circuit. Troubleshooting them is often quite a challenge, with factors such as size, number of layers, signal analysis, and types of components playing a large role. Some more complicated boards require specialized equipment to properly troubleshoot, but most troubleshooting can be done with basic electronic equipment to follow traces, currents, and signals through the circuit.

Human hand repairing printed circuit board, close up
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Have the Right Tools

Most basic PCB troubleshooting can be done with just a few tools. The most versatile tool is a multimeter, but depending on the complexity of the PCB and the problem, an LCR meter, oscilloscope, power supply, and logic analyzer may also be needed to dig deep into the operational behavior of the circuit.

Do a Visual Inspection

Visually inspecting your PCBs can find several potential issues. Overlapped traces, burnt out components, signs of overheating, and missing components can be found easily through a thorough visual inspection. Some burnt components, damaged through excessive current, cannot be seen easily, but a magnified visual inspection or the smell can indicate the presence of a damaged component. Bulging components is another good indicator of a problem, especially for electrolytic capacitors.

Do a Physical Inspection

One step beyond a visual inspection is a physical inspection with power applied to the circuit. By touching the surface of the PCB and the components on the board, you can detect hot spots without the use of an expensive thermographic camera. When a hot component is detected, cool it with compressed canned air to test the circuit operation with the component at lower temperatures.

This technique is potentially dangerous and should only be used on low voltage circuits with the proper safety precautions.

When physically touching a powered circuit, you should take several precautions. Make sure only one hand makes contact with the circuit at any time. This prevents a potentially fatal electrical shock from traveling across the heart. Keeping one hand in your pocket is a good technique when working on live circuits to prevent such shocks. Ensure all potential current paths to ground, such as your feet or a non-resistive grounding strap, are disconnected to reduce the danger of shocks.

Touching various parts of the circuit also changes the impedance of the circuit, which can change the behavior of the system and can be used to identify locations in the circuit that need additional capacitance to work correctly.

Conduct Discrete Component Testing

Testing each individual component is often the most effective technique for PCB troubleshooting. Testing each resistor, capacitor, diode, transistor, inductor, MOSFET, LED, and discrete active components can be done with a multimeter or LCR meter. If the components have less than or equal to the stated component value, they're typically good. But, if the component value is higher, it's an indication that either the component is bad or the solder joint is bad. Diodes and transistors can be checked using the diode testing mode on a multimeter. The base-emitter (BE) and base-collector (BC) junctions of a transistor should behave like discrete diodes and conduct in one direction only with the same voltage drop. Nodal analysis is another option that allows unpowered testing of components by applying power just to a single component and measuring its voltage vs current (V/I) response.

ICs Testing

The most challenging components to check are ICs. Most can be easily identified by their markings and many can be operationally tested using oscilloscopes and logic analyzers, but the number of specialty ICs in various configurations and PCB designs can make testing them very challenging. Comparing the behavior of a circuit to a known good circuit is often a useful technique and should help anomalous behavior to stand out.