1
\$\begingroup\$

I just finished soldering a QFN package to a breakout board, and was about to go at it with my multimeter in continuity testing mode to check for bridges, when it occurred to me that some semiconductors can be damaged by negative voltages. I presume that continuity testing works by applying a small voltage, in line with a large resistor, and checking for a voltage drop... could that small voltage damage the IC? I only have the one multimeter so I can't exactly check what voltage it feeds in, but I assume it's large enough to get through your average diode's forward drop.

I'm probably just being paranoid, but it would be nice to get a categorical answer to put my mind at ease. Is the voltage applied by a run-of-the-mill DMM in continuity mode ever enough to risk damaging an unpowered digital IC?

\$\endgroup\$
3
  • \$\begingroup\$ If the circuit is very low voltage and current, maybe. Crappy DMM with high current and voltage is a more likely candidate, or ESD. \$\endgroup\$
    – winny
    Sep 22, 2016 at 18:17
  • \$\begingroup\$ This is a decently nice DMM so I think it's safe to assume it's following best practices, whatever those are. To clarify, I have no evidence that there's anything wrong with the circuit... it hasn't touched anything ungrounded yet. \$\endgroup\$
    – Sneftel
    Sep 22, 2016 at 18:22
  • 1
    \$\begingroup\$ Probably okay for ordinary chips and 99.9% of DVMs- ESD is more of a risk- but take care if dealing with UHF/microwave chips or transistors such as SiGe types with very small geometry and very low breakdown (eg. 1.2V absolute maximum). \$\endgroup\$
    – Spehro Pefhany
    Sep 22, 2016 at 21:09

1 Answer 1

Reset to default
3
\$\begingroup\$

DMM diode test or continuity currents are typically 1mA which is well under the 5mA guaranteed level for ESD protection diodes. Latchup only occurs if power is on and any input or output exceeds the supply rail by 0.7V

However if you failed to discharge your meter and yourself to ground before probing, you may have exceeded the 3kV ESD protection for a std 100 pF rating, for a standard finger.

No Risk, touch gnd pad first. or use strap.

\$\endgroup\$
6
  • \$\begingroup\$ Is it just current, though? My understanding was that some semiconductors could be damaged by sufficient reverse voltage, regardless of current. \$\endgroup\$
    – Sneftel
    Sep 22, 2016 at 18:20
  • \$\begingroup\$ Yes most LEDs are not ESR protected and like transistors have limited Vr = -5V but CMOS has two diodes (to each rail) with Series R rated for 5mAdc or from 3kV@100pF.. thus 4 diodes all together per input. \$\endgroup\$
    – Tony Stewart EE75
    Sep 22, 2016 at 18:21
  • \$\begingroup\$ As far as I know not every IC includes protection diodes. \$\endgroup\$
    – bitshift
    Sep 22, 2016 at 18:21
  • \$\begingroup\$ Almost all do contain protection diodes. \$\endgroup\$
    – winny
    Sep 22, 2016 at 18:38
  • 1
    \$\begingroup\$ AFAIK, every IC that did not include basic clamping diode protection on I/O ports already has died, and companies making them are out of business 25 years ago. \$\endgroup\$
    – Ale..chenski
    Sep 22, 2016 at 19:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.