Is prepreg hygroscopic or hydrophobic? - Page 1
Forum section: electronics / projects

author: nexus
author_rank: Regular Contributor
timestamp: January 21, 2021, 07:39:46 pm
content: Hi all,This is a question that popped into my mind since I was reworking a board. I accidently scratched off a little soldermask while prepping a through hole. The board appears to be a generic FR4 1.6mm 4 layer stack, and there is no copper on the top layer where the scratch is. It looks like a little bit of the first prepreg layer is now exposed.Since soldermask also protects against oxidation and moisture, I was wondering if typical prepregs like 2116 absorb or resist moisture? The bigger question is whether I should put a little touch up of acrylic conformal coat on the scratch or if that offers no benefits?Picture of the scratch attached (300x magnification).

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2021, 07:51:25 pm
content: Don't worry about it, it's all just a mixture of glass fibre and epoxy. The edges of the board aren't coated in solder mask and, the board being a laminar structure, would be a much larger area for moisture ingress - if it was a problem in the first place.

author: nexus
author_rank: Regular Contributor
timestamp: January 21, 2021, 08:37:39 pm
content: Good point; it didn't occur to me that the same material at the edges of the board would be covered if moisture was a problem.I'm probably too OCD for my own good.

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2021, 08:52:02 pm
content: It's an occupational hazard!Prepreg is short for pre-impregnated, so it already has resin in it when the board is layered.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 22, 2021, 08:50:28 pm
content: It's not dangerous at all, but it's ugly. I would spend a drop of green mask to paint over.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2021, 06:29:49 pm
content: Incidentally, itisa matter of concern -- only at certain points in production. No worry for normal operation or handling.The laminate does absorb moisture: a small amount, like a percent or something. When heated quickly, the release can cause delamination and blistering, particularly in boards with large (solid) copper areas, particularly multilayer boards.So, storage of unpopulated boards can prefer the use of desiccant, or a bake-out step before use.And no, the soldermask isn't much of a barrier -- it's usually a similar material (epoxy), so is just as permeable to gasses as the rest of the stackup. (Don't worry about the above defect)Worth keeping in mind for repair as well, but in repair you're usually not heating the whole board, or all that fast, so it's not usually an issue. Best practices would dictate a nice gradual heating curve; multilayer boards, you're probably using a hot air machine with a preheater anyway.I'm not sure that conformal coatings usually offer any substantial barrier to gasses. Their purpose is to occupy physical space, so that condensation cannot occur between the coating and the board (or solder or pin) surface and cause corrosion. It actually doesn't matter very much whether they're permeable or not!Now, there are plastics with different permeabilities to different gasses -- it could be there's one coating that does better, if you need it (perhaps for high vacuum service). This is very important for food products, though: shelf stability depends on the choice of plastic, for resistance to O2 and H2O. Metallized film and plastic-coated foil are also excellent choices, and indeed find the same use in electronics -- high MSL components are shipped in thermal-sealed foil packs.Oh yeah, not to mention components themselves -- they can absorb moisture, and can popcorn(!) on assembly or repair.Tim