Mistakes and component failure are a fact of life. Circuit boards will be made with mistakes in them, components will be soldered in backwards or in the wrong position, and components go bad all of which will make a circuit work poorly or not at all. PCB troubleshooting can be a monumental task that taxes both the will and the mind. Luckily there are a few tricks and techniques that can greatly speed up the search for the troublesome 'feature.'
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 a multi-layer PCB is often quite a challenge, with factors such as size, number of layers, signal analysis, and types of components playing a large role in the ease of troubleshooting. Some more complicated boards will 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.
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 PCBs and the problem, an LCR meter, oscilloscope, power supply and logic analyzer may also be needed to dig deep in to the operational behavior of the circuit.
A visual inspection of PCBs can find several potential issues. Overlapped traces, burnt out components, signs of over heating, and missing components can be found easily through a thurough 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 source of a problem, especially for electrolytic capacitors.
One step beyond a visual inspection is a powered physical inspect with power applied to the circuit. By touching the surface of the PCB and the components on the board, hot spots can be detected without the use of an expensive thermographic camera. When a hot component is detected, it can be cooled 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, several precautions should be taken. Make sure that only one hand makes contact with the circuit at any time. This prevents an electrical shock from traveling across the heart, a potentially fatal shock. Keeping one hand in your pocket is a good technique when working on live circuits to prevent such shocks. Ensuring all potential current paths to ground, such as your feet or a non-resistive grounding strap, are disconnected is also essential to reduce the danger of shocks.
Touching various parts of the circuit will also change 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.
Discrete Component Testing
Often the most effective techniques for PCB troubleshooting is to test each individual component. Testing each resistor, capacitor, diode, transistor, inductor, MOSFET, LED, and discrete active components components can be done with a multimeter or LCR meter. Components that have less than or equal to the stated component value, the component is typically good, but if the component value is higher it is an indication that either the component is bad or that 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.
The most challenging components to check are ICs. Most ICs 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 ICs very challenging. Often a useful technique is to compare the behavior of a circut to a known good circuit, which should help anomalous behavior to stand out.