Using DMM on static sensitive parts?

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Ian MacGregor

Well-known member
Joined
Jun 3, 2004
Messages
280
Location
Echo Park, Los Angeles, CA, USA
Ok,
So as a side job to support myself while attending college, I consult for a company that designs/reverse engineers PCB's (I know, pretty sweet job for a DIY'er like me). Anyway, when we reverse engineer PCB's I take the pcb, figure out the schematic and enter it into our software (PowerLogic, btw) to get it ready for PCB layout.

Here's my problem, I'm reverse engineering a little SMD board that has a little CMOS chip that is most likely ESD sensitive. The board is double layer and the chip is covering a good deal of traces. I can't take any of the parts off because the customer needs the board operable in the very short term.

Here's my question: Will using my DMM (Fluke 83) on the resistance setting have any chance of damaging the chip? I would guess that it would because it uses some amount of voltage/current to determine resistance.


Any ideas??

Thanks
Ian
 
i think you will be alright as long as the chip is in the circuit. the components around the chip, ie caps and resistors, will usually provide protection.

i think the biggest worry is humans touching the chips outside the circuit, as people can carry a lot of charge. most chips nowdays have some sort of internal protection. is the DMM 9 volt or AA batts? If you want to be extra safe, use a DMM with AA batts.
 
Umm...I have some expereince with this so I'll lend my 2 cents....

Concerns with ESD involve discharges of several hundred to several thousand volts from the human body or some other static causing device that can build charges on a body, usually. A DMM will not cause or store a large static charge (unless its surrounded by styrofoam or some such...silly under normal circumstances).

It is generally accepted that CMOS and other static sensitive devices are protected as long as they are in circuit. IMO, that would depend on whether or not the circuit designer has put the necessary components in place to protect the inputs of the chip...this might or might not be the case with the PCB you are working with, Ian. And until you finish the schemo, no one can tell you this, either! But even so, you will be making contact with the pins of the chips and so they may have no effective protection from ESD while you are working on them!

The main precaution you should be taking is to wear a grounding strap that is grounded to a "good" ground while you are working on this PCB. The next step would be to work at an "ESD protected workstation". This IMO, involves a grounded, conductive worksurface (can be as simple as a static mat), no carpet on the floor (preferably static dissapative flooring), and an ESD chair (anti-stat upholstery and a drag-chain making contact with the aforementioned flooring).

I'm sorry to say that I don't think that using a 3V vs a 9V battery wil make any diff. If you don't have a grounding strap on, you could still trash out components. Also note that static damage can be cumulative and/or not be immediately catastrophic. You can damage a chip and it just work under spec. or work for a while and then suddenly fail. ESD protection is like this: Insurance...you have to buy all you need before the accident, because afterward its too late. And since you can't see or assess the damage you should buy all you think you need and then a little bit more if you want to be safe.

Hope that this is helpful!
Charlie
 
Doesn't the spec for 4000 series logic warn against applying power to pins without the chip itself being powered? I thought in some instances by using the ohms range on a DMM you can pull things to a reverse polarity state if the chip is unpowered.
 
It's no problem to probe like this. Wear static protection yourself (wrist strap, don't wear a wool sweater), and it'll be fine. The issue with applying a signal to the input or outputs of CMOS circuits only happens when you power the chip up with an input applied. The ohms range on most meters (say, Fluke DMM's) are voltage and current limited so they don't hurt the circuit, even if the power supply is 9 volts. Don't be scared.

The scariest circuits to trace are ones that hold their program in RAM and a single slip of a probe causes the device to quit working. Like satellite decoders and things like that.
 

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