Green shmutz on trimpot screwheads

[aeccles]

My company has recently received PCBAs from our CM that have some sort of green discoloration on the trimpot screwheads.
I do not think that this is oxidation, it seems sort of like glyptol or something. I know that this isn't a lot to go on but I would appreciate hearing from anyone who has seen something like this.
These are brand-name trimpots and a very reputable CM. Obviously we'll be contacting the CM and, if necessary, the trimpot manufacturer.
Thanks!

[coppercone2]

does it smear? they must be lubricated with a damping compound of some sort to allow for smooth adjustment, possibly with a electrical grease in the wiper area and then a purely mechanical damping grease in the shaft area. they have alot of grades of the stuff. orange green and red are common colors of this grease used for ID purposes

[floobydust]

It could be "service cement", sometimes applied to trimpot adjustment screws after calibration to mark that it's done and hold the screw.
I might guess your CM is used to that step, of applying the stuff after PCBA test, and it was done missing the part where you'll apparently do the final test/cal.

[thm_w]

Need photos.

[Smokey]

HA!  I've heard people talk about pot "glip" for years without knowing it's actually "glyp", short for "glyptal".

https://en.wikipedia.org/wiki/Alkyd

[aeccles]

Here's a picture of the trimpots. The middle board shows the green color. The board above and the board below show the normal color.

[schmitt trigger]

Is either a locking lacquer or a “witness mark”, which allows quality inspectors to easily determine whether a particular critical operation has been accomplished. Or not.

[Smokey]

Is either a locking lacquer or a “witness mark”, which allows quality inspectors to easily determine whether a particular critical operation has been accomplished. Or not.

If the boards are just coming back from the assembly CM, and assuming they are just doing assembly and not testing/calibration, there shouldn't be a reason for any adjustment locking.
Does your CM have any business adding locking to those pots?  Are they doing testing too?

[aeccles]

The CM does not adjust the trimpots and they do not seal them. Thanks.

[Dr. Frank]

That looks like oxidized copper to me.
The trimming axles might be made of brass (which would contain copper), judging from the yellow-ish color of the other pots.
I can't imagine, why only the middle PCB is affected.
Also take notice, that the leftmost trim pot in the middle, marked "BW" is as well discolored / darkened, compared to the lower one.
Are these trim pots from BOURNS?
Frank

[aeccles]

we have seen the problem with Bourns and BI Tech. Which leads me to tend to exonerate the trimpot manufacturer.

[reboots]

The jumper pin to the immediate right of the "LIN" pot looks tarnished, although this could easily be a reflection in the photo. I would perform an extremely careful and thorough inspection of the entire PCB. Any other discoloration might indicate exposure to a corrosive environment during or after assembly.

[BrokenYugo]

Looks like corrosion to me, the locking/tamper seal goop has a certain look to it and this is not that.

[coppercone2]

you need to take a close picture with a magnifying glass and to test if its tarnish scrape some off and put it in hot vinegar (microwave styrofoam cup is fine) or lemon juice. it will dissolve if its tarnish.

the only thing I can think of that might be causing film tarnish like that is a spray flux process which has a acidic enough flux to get brass to corrode but it might not effect common tin plated parts. all fluxes have a acidic nature (when hot) because that is the only thing that will get rid of metal oxide. The idea for electronics flux is that at room temperature it is not acidic or nearly non acidic so it becomes inert after soldering.

I see there is SMT parts there too. Manufacturers do alot of creative things with flux to get boards that solder together right.




the part with the label PS looks very much like oxidized metal

[aeccles]

Here's a closeup. FWIW, I'm coming around to corrosion sort of thing. Like a patina.

[coppercone2]

lol what the hell the one on the left looks like corrosion but the one on the right looks like green laquer

it clearly looks like laquer when you start to chip it off and then you get the white color from the bond line breaking and air gap between the laquer and substrate

can they be trying to protect the parts from the solder process then forget to remove the protection? ingress seal of some kind

[reboots]

Can your CM provide samples of trimpots from the same batch used for assembly?

If the color in the extreme close-up is a varnish, it might have been applied before the pot was assembled. Or maybe it's a clear coating that sealed in corrosive contaminants.

[coppercone2]

oh yeah ultra thin conformal coats are becoming common now they might do this

[ajb]

Were the boards washed?  Did the CM possibly coat the pots with a sealant/masking compound for washing and missed removing it from a few units?

[floobydust]

It's an interesting puzzle. I suspect the PCB soldering/wash, something that attacks brass. The LED holder has crusties of some residue inside it, the gold jumper pins look damaged as well. I think the board got hit with some chemical that de-zincified the brass.
You can check the trimpot date/lot codes, but a blue one got it too as well as the grey ones.