Scraping PCB causes ESD damage to IC?

[treez]

Hi,

In the attached schematic, I had been soldering components on and off the board many times, in the vicinity of this circuit. On some occasions, I accidentally used a plastic handled metal pointy object to scrape away the burnt up solder resist and general gubbins off the PCB. The point was metal but I sometimes forgot and held it by the plastic handle. As such, I think I may have ESD damaged the ZR431.

The reason I think this is because the voltage at the opamp non-inverting input was at first 8V3. However after some time it went down to 8V…in spite of the 12V holding firmly at 12V.

The ZR431 was obviously drawing more current through its cathode...making the 8V3 fall to 8V. I believe this is leakage current due to ESD damage. Would you agree?

I believe the TL432AIDBZR would be a pin-for-pin alternative for ZR431 and would be less likely to suffer ESD damage…would you agree?
The TL432AIDBZR has a greater bias current than ZR431 and generally this means less susceptibility to ESD damage…would you agree?

ZR431 datasheet
https://www.diodes.com/assets/Datasheets/ZR431.pdf

TL432AIDBZR datasheet
https://www.ti.com/lit/ds/symlink/tl431.pdf

 

[wwfeldman]

12 V being 12 V doesn't matter
the shunt regulator regulates the voltage at the 8.3V location
R2, R3 and R4 divide the 8.3V to 2.5 V at the reference input to the shunt regulator

change the 12V to something else and/or change R1 to something else,
you still get 8.3 V assuming you meet the minimum cathode current spec

i assume Cambridge Massachusetts, where its wetter than usual and static is not an issue as it usually is in winter.

i don't think ESD is the issue
you may have overheated something repeatedly
the burnt up solder resist may be providing a parallel resistance to a resistor, thereby
changing the feedback, which changes the cathode voltage so the reference voltage is correct

get some alcohol wipes at the drug store and clean the board carefully
be sure the alcohol dries and there are no stray pieces of the wipe
before turning is on again

 

[betwixt]

I think he's in Cambridge UK, probably even wetter than Massachusetts!

In general in the UK, the humidity is high enough that static is rarely a problem, at least in factory/office/lab environments. I agree this sounds more likely to be a thermal issue. The presence of plastic in the scraper handle does not necessarily mean a static charge can build as there are so many leakage paths and plastic alone does not produce a charge, only friction against it in some circumstances. Gripping it firmly isn't likely to be one of them.

Brian.

 

[asdf44]

Heated buildings are often extremely dry in winter. Wearing a sweater around the carpeted office sitting in a rolling chair causes a lot of static.