Most engineers and technicians troubleshoot electronic boards with typical tools such as multi-meters, oscilloscopes and other standard test equipment. While this works well for most troubleshooting, sometimes a problem comes along that is very difficult to troubleshoot. Power rail shorts are a perfect example. The reason? Power supplies typically source several integrated circuits, each of which usually has one or more decoupling capacitors. If any one of these components are shorted, the entire circuit will appear shorted. To clarify, a true short would be measured as a zero-ohm reading, but for the purpose of this discussion, shorts also include non zero-ohm measurements, but much less than expected – typically less than 10 ohms.
Once a power rail short is discovered on a board assembly, the traditional troubleshooting technique is as follows:
- Inspect the board to try to identify any obvious solder bridges that can be removed.
- If there are only a few components, remove them one at a time, measuring after each removal to see if the short goes away; when a component is removed and the short goes away, replace that component, and install all removed components.
- If there are significant components to remove, try to identify a component that can be removed that will isolate portions of the circuit; this will likely be a resistor, ferrite bead or inductor. The short will likely only be on either side of the circuit and will reduce the component count to remove. Continue isolating portions of the circuit as much as possible to reduce the number of components that need to be removed, then remove one at a time, measuring for shorts.
- Some boards do not have components in place that can break down the circuit, so the only way to determine where a shorted component is located would be to make a best guess at which component is shorted, then remove, measure and repeat until the short is found.
- Some boards have hundreds of such components and the most cost-effective solution may be to scrap the board.
This can be a very long, and not very cost-effective, method of troubleshooting shorts. An alternate way of troubleshooting boards with power rail shorts is to use a thermal camera.
What you will need:
- The board schematic and possibly assembly drawing
- A thermal camera (Flir C2 used for this discussion)
- A power supply with current-limiting capabilities
- Wire, preferably single core red and black for convenience; this wire must be able to handle the current you will use; choose the smallest wire you can for convenience
- Something metal that you can point with, such as tweezers, a small screwdriver, wire or anything similar will work
- Determine a good location to solder power and GND wires to. This will usually be a large bypass capacitor on that power rail. It doesn’t matter where it is in the circuit. The most important part is to have easy access to solder it to. Assuming a capacitor was found, solder a red wire to the positive side and a black wire to the negative side. Length should be sufficient to connect the other ends to the power supply and be able to flip the board to the other side.
- Set up the power supply. Start with the current limit at 0.5A and the voltage set to the desired voltage on the board. If the power supply cannot supply the proper voltage, that is fine – just set it to the maximum (when shorted, the voltage will be much less anyway). Also at this point, evaluate the schematic to determine the highest amperage that the components can handle.
- Remaining setup:
- Turn on thermal camera and let it boot up
- Make sure circuit board is not connected to anything else
- Connect the wires to the power supply
- Turn the power supply ON
- View both sides of the board with the thermal camera. Look for hot spots. Note that many cameras use the colors to show relative temperatures, not an absolute temperature, so what looks like a hot spot may be just warmer than the rest of the board. The Flir C2 and others have a bullseye that allows you to measure the actual temperature. Some notes:
- At this point, ignore any temperature less than 100°F
- Any metal will show up as a hot spot and is usually safe to ignore them
- 0.5A may not be enough current to start showing a problem
- If a problem component was found, replace the component. If not, increase the power supply current limit 0.5A at a time and repeat until a problem component is found. DO NOT go over the current that the weakest component can handle.
If any hot spots are found, heat up the metal pointing device between your fingers and point to the hot spot to confirm the component that is visually hot. The overlay image may not be aligned perfectly. Also note the measured temperature in the bullseye on the upper left corner of the screen.
Hot capacitor found (seen in image): This is likely a damaged capacitor that when power was applied developed a short. Remove it to confirm the short is gone. If not, repeat the process as more may have been found.
Warm inductor or ferrite bead found: This may be normal, or it may indicate current flowing through that component. That inductor or ferrite bead is likely okay but may point to a part of the circuit to focus on. Remove it and repeat the thermal camera test looking for additional clues.
A warm area of the board is observed: This is likely due to a solder bridge. The bridge can handle sufficient current so that it is not getting very hot but is warming up. This will usually only be seen at higher tested currents. Inspect the board looking for solder bridges in that area that the board warmed up.
No observable warm or hot spots: This is probably due to a large solder short or a PCB short that can handle all the current and more that you applied. The thermal camera test is not very effective in this situation and standard troubleshooting techniques should be applied.
Additionally, a thermal camera can be used to detect lack of heat where expected, indicating that a portion of the circuit is not powered up or working. As the trusted electronics manufacturer you’ve been looking for, ACDi is committed to delivering the highest-quality printed circuit boards and box-build electronics assemblies. Troubleshooting is part of the product engineering support services that we offer to our clients. Contact us to learn more.