Mysterious stickers and gunk on UPS PCB? - Page 1
Forum section: electronics / beginners

author: e-pirate
author_rank: Regular Contributor
timestamp: February 11, 2025, 09:39:18 pm
content: Hi folks.Recently I found that one of my 24x7 running small servers is very sensitive on mains power stability. I was sitting and watching stuff when the light blinked, then I notices that one of the servers (the old one) remained operational and seem not to notice that short power loss at all, while the other one rebooted. I noticed that second server having less uptime then the first one, but never figured out the reason. So, this time I was at home and looks like I found the reason for the second server to reboot twice a year.I had a 15 y.o. APC UPS on hand that was switched of for at least 5-7 years, so I decided to use it for the sensitive servers to check if it helps to prevent it from rebooting on every mains problem. I was not stupid enough to connect it to power prior inspection. First of all the 1207 battery was 0.2V dead, the second problem is that casing of the battery cracked all around and some acid stuff managed to get out and slightly damage the metal bottom of the UPS chassis. OK, the old battery should be utilized (this is a head ache actually) and replaced with a fresh one. But this is not what got me here.After getting the casing off, I found two gunky and messy spots. The sticky nasty gray-yellowish stuff almost decomposed even drip down the PCB. The right one (the black circular sticker) was exactly the same as the left one before clearing. Since I had nothing to lose, I first removed big parts of gunk with a metal tool and then cleaned what left with isopropyl alcohol. Under the sticky gunk there were a regular black circular stickers. The other sticker (the left one) didn't hold on and I removed it expecting something special under it. But no, it's just a PCB marked "PAD HERE" and nothing more.So, I got curious what that heck is this? Why they covered a regular PCB with black stickers and then covered them with soft rubber compound that later decomposed. I have never seen anything like this before. I've seen UV-erasable ROMs windows been covered with similar looking stickers - for obvious reasons. But there is nothing special under this two, at least one of them. On the other side of PCB there is also nothing interesting - just regular through hole parts.Whats going on there?

author: e-pirate
author_rank: Regular Contributor
timestamp: February 11, 2025, 09:48:11 pm
content: My guess would be this is just a spacer/separator to prevent metal casing from contacting and shorting the PCB. Because of the age, the stuff this parts were made from decayed so badly that turned into something like chewing gum. But why do they need stickers then? Besides, there are 4 rubber "legs" on the bottom, they turned slightly softie, but still hold pretty well.

author: GLouie
author_rank: Regular Contributor
timestamp: February 12, 2025, 04:11:42 pm
content: Those appear to be 3M Bumpons.https://www.digikey.com/en/products/filter/bumpers-feet-pads-grips/570?s=N4IgTCBcDaIEYFcC2AHA9gOxAXQL5AI think the yellow drip is extra glue they used, as the 3M adhesive is not very good. They are usually sold as "rubber feet" but are polyurethane. I find that some have decomposed after several decades, truning to goo, and the adhesive is not so good even on clean, dry, flat surfaces.I think you are right that they are just protection against the board touching any metal in the case.

author: e-pirate
author_rank: Regular Contributor
timestamp: February 12, 2025, 06:05:15 pm
content: Quote from: GLouie on February 12, 2025, 04:11:42 pmThank you very much on confirming my guess. It looks like there are really two substances: the top grayish layer is what used to be circular "feet", the yellowish trace beneath is an extra glue. Both completely decomposed.I wonder if this stuff could cause any short circuit or unwanted currents if I let it stay.

author: benj38
author_rank: Frequent Contributor
timestamp: February 13, 2025, 08:37:15 pm
content: My guess, by the amount of the drip and its consistency, is that this is not glue, but the deteriorated elastomer.I have seen many elastomers turn into goo and drip. This is much more common in climates with high temperatures and humidity.I suggest you clean this goo off of the PCB. Depending on the exact composition of this elastomer, it may contain chemicals (like halogen-based flame retardants) that given enough time can corrode the PCB traces.

author: CaptDon
author_rank: Super Contributor
timestamp: February 13, 2025, 11:04:06 pm
content: That glue looks like the old suspicious ply-o-bond rubberized elastomer glue. Be aware that as it ages it becomes more and more conductive. It appears you have already removed it. Good work!!

author: wraper
author_rank: Unknown
timestamp: February 13, 2025, 11:09:00 pm
content: Yes, looks like rubber bumps which had gone bad and leaked that junk.