PCBs are showing faults and it looks like an ESD problem

[treez]

Hello,
I am working in a place where PCBs with small package microcontrollers with many pins and lots of little ICs are being worked on in an office with a carpeted floor. There is an ESD mat on the table, and a wrist strap, but that is it.
The boards are suddenly stopping working or giving intermittent faults. Some boards work and some which are the same don’t work.
I have worked in two other places with carpeted floors and we were similarly chasing ghosts.
Can we agree that we must put a ESD mat on the floor, with snap-on connection to earth ground via a yellow grounding plug. Do we also agree that as well as wrist straps, we need footstraps?
Also, preferably ESD jackets.

 

[ads-ee]

A carpeted floor, can easily generate far in excess of 30,000 Volts of static charge. Just going near the bench and hooking into the wrist strap might have already damage something sitting on the bench (on the grounded mat). There is a reason why any competent shop won't even allow people within 3 ft of a bench without the proper ESD precautions (i.e. ESD coat, wrist strap and preferably a wrist strap and heel strap with an ESD conductive floor).

Seems like that place your working at doesn't know anything about ESD.

 

[dick_freebird]

I can't agree with your "must" assertions - you don't
know, for example, that the carpet isn't a sort that can
suppress ESD, that you need footstraps and so on. These
are all reasonable things, in themselves. But to me this is
backwards somewhat, and it's your certainty I object to.

You could do a proper ESD survey and -find- the sources
of static overvoltage, and address the big ones first. All
the mats, footstraps and so on will help little or none if
(say) it's the charged up stockroom cart that's blowing
the boards up after they leave the workstation.

A chain of many links, like. Be smart about it or you'll
just be seen throwing money at it without result, and
that means more junk product and maybe junked career
meanwhile.

If your company has not got religion, then maybe you
do not want to be the prophet (often this role comes
with serious downsides, per the literature). Might be
better to convince them to bring in an expert who can
school them properly, do the survey, write up best
practices and so on without you "getting any of it on you".

I'd expect your customers would eventually do a vendor
audit and lackadaisical ESD procedures would be a big
black eye. And a batch of field returns has a way of
tanking small companies that are barely or not quite
profitable. I think you could convince your management
that they need help, and need to get right, better than
you could convince them to buy and install stuff on your
say-so. Right or wrong doesn't enter into it.

 

[pjmelect]

Most components and integrated circuits are relatively immune to static electricity even though they are supposed to be static sensitive, however there are exceptions that are very static sensitive and require every static precaution, even if fitted in circuit. These include CMOS monostables, gas sensors and FET,s If your boards include any of these devices then full precautions should be taken.

 

[treez]

you don't
know, for example, that the carpet isn't a sort that can
suppress ESD

Thansk, its just a nomal office carpet, so i doubt its got ESD protection features.
I suppose that i literally have to go swiping my feet on the carpet then touching microcontrollers that work and see if they dont after swiping feet.

Three RMS-to-DC chips suddenly met their grief on one day.
Lots of other boards are not working. Some are, some aren't

 

[ads-ee]

Most components and integrated circuits are relatively immune to static electricity even though they are supposed to be static sensitive, however there are exceptions that are very static sensitive and require every static precaution, even if fitted in circuit. These include CMOS monostables, gas sensors and FET,s If your boards include any of these devices then full precautions should be taken.

It's those 1K to 2K V zaps that you can't even feel, that can damage the I/O of a part which results in latent failures that don't happen now or a week from now but happen sometime in the future when the customer has the product and it doesn't work quite right. Now you didn't just damage the device you damaged the companies reputation, because you believe that "ICs are relatively immune to static electricty".

Things like FETs, MOSFETs, etc are so sensitive they immediately die, that is NOT a definitive test for static electricity testing. Use an ESD field meter and measure it.

Thansk, its just a nomal office carpet, so i doubt its got ESD protection features.
I suppose that i literally have to go swiping my feet on the carpet then touching microcontrollers that work and see if they dont after swiping feet.

And if it still works, I guess you'll think everything is fine and dandy with the carpet. "Parts don't fail after the carpet test, so we must be ESD certified!" Yeah right, remind me, where do you work, I want to make sure we never buy anything from your company.

 

[FvM]

I agree with every word said about the importance of ESD protection. But hearing the report about the massive aggregation of failure events, I don't believe that they are ESD related.

 

[RobertFalbo]

What kind of carpet?
Some are worse then others.

Don't just jump on the ESD blame game.
Maybe other causes of sudden failure are to blame.

 

[dick_freebird]

Yes. Assignable cause, then corrective action. Actions
taken without target and proof may not be corrective.

 

[Mattylad]

Stop wearing that polyester underwear treez. :razz:

Can you temporarily move location? perhaps to somewhere that does not have such a carpet, then you can test the theory that it is the carpet by removing that from the equation.

 

[RobertFalbo]

ESD police can get out of hand.
How many times do I see zero ohm resistors in static bags?

Actually any resistor in a static bag.


If your worried about the carpet, spray it with salt water. That should drop the static.:-D

 

[treez]

If your worried about the carpet, spray it with salt water. That should drop the static

Thanks, seriously, is there stuff we can spray on the carpet to reduce static build up?

 

[dick_freebird]

I believe I have seen stuff like that, but was not interested
enough to remember it. Of course you'd like not only to
keep static from building up, but dissipate to someplace.
An ESD large area floor mat that underlies the whole
workstation, chair with a drag-chain, ionizers, and so
on could all help. But that's "could", not "will", because
the cause remains only theoretical.

You might "stalk" a board from its source, through the
workstation and to its test destination with an eye to
other sources.

You might take a known good board along the same route
except for the workstation activity, multiple times and
be sure that this is not test EOS, or damage picked up
at some other point before / after the workstation in
question. I've had parts blow up on first, second, third
test insertion due to flyback from force current, measure
voltage test steps and marginal contactors, for example.

Challenge your assumptions in an orderly way, or someone
will do it for you.

 

[RobertFalbo]

Is it the same component failing?
Some components are more sensitive. Also some are more likely to receive contact with a charged surface. IO chips connected to connectors that are exposed, most likely are the contact points. The path being from there, to another surface with a different potential, generally the person holding the board.

 

[treez]

We are not too sure, but it looks like the DALI input pin of the PIC18F26K20 micro is breaking. It just doesnt seem to detect the DALI signal. (on some it does, some it doesnt. The DALI signal is opto isolated from the DALI bus.
We have written simple test code to test if the input pin works but we cant get the test code to work....we dont know if thats because the input pinis broken or the code is wrong.

- - - Updated - - -

Here is our test code, but it doesnt work
https://www.edaboard.com/showthread.php?t=368575

...the code just puts out on RC2 what it receives on RB0. -which would check if RC2 is working as an input pin.

 

[c_mitra]

...its just a nomal office carpet,

There are carpets and there are carpets.

Cheapest solution is to get a cheap humidifier. If that solves your problem (check over min 2 months and keep proper statistics), get rid of the carpet (and happily live thereafter).

Expensive nylon carpets are the worst but why a carpet in an electronics work place?

Carpets are excellent in trapping dust and spills (during their early life) but they are a source of nuisance once they grow old.

- - - Updated - - -

If your worried about the carpet, spray it with salt water. That should drop the static.:-D

Till the water dries. Don't bother with the salt.

 

[RobertFalbo]

Salt holds on to the water longer

 

[betwixt]

I can comment on an ESD damage investigation I did for a company years ago. They were making satellite TV receiver LNBs so there were several ESD sensitive parts on the PCBs, including HEMT, and small mixer diodes. They had an unexpectedly high failure rate at first functional test stage (the nature of the product made ICT impossible) and I was called in to find the cause. I found:

1. Boards leaving soldering ovens the stacked in plastic crates. Owing to laxed workforce, these were placed directly in front of (touching) a CRT monitor screen. I could measure several KV from the screen at a distance equal to the board locations!

2. Functional test used a vacuum operated test rig where the board sat in a Neoprene gasket and had an acrylic lid placed over it to seal the air flow. The acrylic alone carried enough static to kill the devices but the rush of air over it and the board surface as it evacuated raised several KV as well!

3. Inspection stations where some bright spark (pun intended) had installed 'ozone generators' because they read somewhere that they remove harmful ions from the air and therefore thought it would reduce static risks. These things were a mains driven voltage multiplier with maybe 20 1N4007 diodes and capacitors in a ladder to produce high voltage which then went to a sharp metal pin where it emitted a corona and interesting sizzling noise. These probably did more harm than all the other deficiences added together!

If you do any of the above - consider that the object of the exercise is to remove voltage differences, not inadvertently increase them.

Brian.