White Silicon on PCB help - Page 1
Forum section: electronics / repair

author: moffy
author_rank: Super Contributor
timestamp: September 03, 2025, 06:39:37 am
content: I have a board to repair with both through hole and surface mount but the manufacturer has used liberal amounts of what looks like white slicone which has ended up over some of the surface mount components. I have to excavate slowly but already one of the surface mount components has been dislodged. Any advice as to how to do it safely would be appreciated.

author: moffy
author_rank: Super Contributor
timestamp: September 03, 2025, 07:33:40 am
content: Using some hot air judiciously at 300C definitely helps it to flake away more easily. I guess short burst to try not to overheat surrounding components or melt plastics.

author: elektryk
author_rank: Regular Contributor
timestamp: September 03, 2025, 08:16:43 am
content: I'm also looking for sth that will melt it or at least soften it.I've tried DCM on transparent silicone potting but it evaporates too quickly.

author: Swake
author_rank: Frequent Contributor
timestamp: September 03, 2025, 08:39:51 am
content: Would love to hear about a definitive solution for that...Depending on what that white glue really is you might want to try:- Freezing. It might become brittle and break more easily.- A 50/50 mix of IPA and naphtha (lighter fluid) weakens and softens silicone. It takes some time to do its thing, I've had good results in the bathroom....I would also consider acetone, but that might have a negative impact on other plastics.

author: moffy
author_rank: Super Contributor
timestamp: September 03, 2025, 08:53:00 am
content: Heating and removing layers does seem to work in small areas at a time, it turns slightly yellow and loses its adhesion. I am wary of using anything other than IPA because it can adversely effect the components unless it's approved for the use. Had a bad experience with acetone recently where it disolved a piece of plastic I was trying to clean. Does naptha have a negative impact on plastic?

author: elektryk
author_rank: Regular Contributor
timestamp: September 03, 2025, 09:04:51 am
content: It shouldn't hurt most of the plastics but may affect some rubbers.

author: moffy
author_rank: Super Contributor
timestamp: September 03, 2025, 10:45:12 am
content: Quote from: Swake on September 03, 2025, 08:39:51 amQuote from: elektryk on September 03, 2025, 09:04:51 amOk, I'll see if I can get some and give it a try, would naptha flakes dissolved in IPA be a suitable substitute?P.S. It looks like I can get something called 'Shellite' which is a lighter fluid and 'solvent Naphtha'

author: IconicPCB
author_rank: Super Contributor
timestamp: September 03, 2025, 12:37:18 pm
content: Don't confuse moth balls and lighter fluid.

author: moffy
author_rank: Super Contributor
timestamp: September 03, 2025, 01:37:12 pm
content: Quote from: IconicPCB on September 03, 2025, 12:37:18 pmThey are both composed of napthalene but I guess the mothballs must have some solidifying agent?

author: Swake
author_rank: Frequent Contributor
timestamp: September 03, 2025, 01:59:33 pm
content: naphtha is also known as White Spirit, petroleum spirits and UN 1300

author: moffy
author_rank: Super Contributor
timestamp: September 03, 2025, 09:35:39 pm
content: Quote from: Swake on September 03, 2025, 01:59:33 pmThanks for the heads up, 'White Spirits' at Bunnings is listed as a 'silicone solvent' also as a 'dry cleaning agent' so I guess it is a replacement for the once ubiquitous carbontetrachloride.

author: wraper
author_rank: Unknown
timestamp: September 03, 2025, 09:47:28 pm
content: Use plastic spudger or something similar to remove it, it greatly reduces the chance of damaging something. Do not use metal tools such as screwdriver. Also I'd avoid removing it at all unless strictly necessary. Most of repairs require removing it only locally, and there is no need removing it for checking TH components such as capacitors and MOSFET which fail most often.

author: moffy
author_rank: Super Contributor
timestamp: September 03, 2025, 11:04:43 pm
content: Quote from: wraper on September 03, 2025, 09:47:28 pmI have found two failed components so far both surface mount and both buried under the silicone, hence the excavation, one was a 500mA fuse the other a 1A diode, which might have failed from heat stress being buried under a mound of silicone. Using plastic is good advice as well as only excavating where needed.

author: wraper
author_rank: Unknown
timestamp: September 03, 2025, 11:18:43 pm
content: What diode? In mains voltage rectifier? If that's the case, then IME you already have found all that has failed. Those generally fail due to stress caused by inrush current.

author: moffy
author_rank: Super Contributor
timestamp: September 04, 2025, 01:06:13 am
content: Quote from: wraper on September 03, 2025, 11:18:43 pmOutput diode on the +/-18V flyback supply, that and the output fuse might be all, but I will need to check at least the flyback controller, one of those single chip devices.

author: wraper
author_rank: Unknown
timestamp: September 04, 2025, 09:07:41 am
content: Quote from: moffy on September 04, 2025, 01:06:13 amPrimary circuit failing due to secondary side fault almost never happens. If mains input fuse is fine, checking that is counterproductive.

author: moffy
author_rank: Super Contributor
timestamp: September 04, 2025, 10:36:58 am
content: Quote from: wraper on September 04, 2025, 09:07:41 amChecking fuses is always a starting point, I guess your experience is different to mine.