PCB Troubleshooting
Printed circuit boards, or PCBs, are a mass of insulators and copper traces that connect densely packed components to create a modern circuit. Troubleshooting PCBs is often 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. However, most troubleshooting can be done with basic electronic equipment to follow traces, currents, and signals through the circuit.
Have the Right Tools
Most basic PCB troubleshooting requires only a few tools. The most versatile tool is a multimeter. However, 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.
Perform a Visual Inspection
Visually inspecting a PCBs uncovers more obvious problems including overlapped traces, burnt out components, signs of overheating, and missing components. Some burnt components, damaged through excessive current, cannot be seen as 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.
Perform 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. When you touch a powered circuit, take several precautions. Make sure only one hand makes contact with the circuit at any time to prevent a potentially fatal electrical shock from traveling across your heart. If possible, 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 thus 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. Test each resistor, capacitor, diode, transistor, inductor, MOSFET, LED, and discrete active components with a multimeter or LCR meter. If the components register less than or equal to the stated component value, the components are typically good. If the component value is higher, it’s an indication that either the component is bad or the solder joint is bad. Check diodes and transistors using the diode testing mode on a multimeter. The base-emitter and base-collector 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 to a single component and measuring its voltage-versus-current (V/I) response.
ICs Testing
The most challenging components to check are ICs. Most can be easily identified by the markings, and many can be operationally tested using oscilloscopes and logic analyzers. However, the number of specialty ICs in various configurations and PCB designs can make testing challenging. Comparing the behavior of a circuit to a known good circuit is often a useful technique and should help anomalous behavior stand out.
