Troubleshooting complex boards

When troubleshooting complex boards, your troubleshooting toolkit must contain many tools.

I define complex as those boards being SMD, multilayer, analog combined with digital, and high-speed designs.

Here are some scenarios I have run across.

1) If you have a dead short (rail to common), a few methods are available:

• Lowest resistance IC by IC check. A four-wire or a milliohm meter is preferrable over a DMM. Some high-precision bench DMMs in high-count and manual range mode, zeroed (or nulled) can be used if solid (good quality) probes are used.

• You can power up the board if it was powered up before, and quickly check the board over with a liquid crystal sheet or thermal imaging to see which chip heats up. NOTE: the chip that heats up may be the chip which is actually attempting to drive (output to) a shorted chip.

• You can apply low-voltage current to the board and check for heating with a crystal sheet or thermal imaging. This has the benefit of being able to also diagnose physical (copper) shorts. The risks include a)overheating a component, b) burning out a component, and c)burning out the copper short without knowing where it happened.

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