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.

2) You are seeing an higher current draw, and the current is increasing.

Turn off power immediately. Check polarity of all capacitors while wearing eye protection. An electrolytic or a tantalum may have been installed incorrectly.

3) The output of a circuit is clamped to a lower voltage than nominal. Or, voltage is not reaching a proper level.

Check output TVSes for a correct rating.