Recovering boards with too much solder mask: soldering after lasering - Page 1
Forum section: electronics / general

author: bikeNomad
author_rank: Contributor
timestamp: September 19, 2020, 03:25:44 pm
content: I just sent in a JLC PCB order for 10 boards and then realized that I'd not checked the soldermask layer. When I did check, I found out that the manufacturer-supplied footprint lacked paste and mask on the pads of a 64-lead QFP package.My intention is to reflow solder this part myself (JLC didn't have it) using a stencil cut from the full-size stencil.I'll be using leaded solder paste and either a modified toaster oven or a hot air pencil.I was thinking about using my 40W laser cutter to blast off the soldermask over the pads.Has anyone done this?If so, how did you do it?I can experiment with the engraving power, and I can try to use the proper soldermask pattern to avoid damaging the board. But it might be hard to perfectly align. Easier would be to make four stripes over the pad areas:I was wondering whether a power level adequate to vaporize the soldermask would damage the PCB material.Also, is the result clean enough to easily solder? I figured I could use Scotchbrite pads to remove the remaining junk.And does anyone know what this solder mask is made of? I have to avoid lasering PVC or any materials that generate corrosive gases like HCl or HBr.

author: MR
author_rank: Regular Contributor
timestamp: September 19, 2020, 03:55:06 pm
content: ...10 boards...2 options out of many others:- some fine small rasp (maybe some rough dremel tool, and scratch it off by hand?)- fiberglass scratch pen

author: IconicPCB
author_rank: Super Contributor
timestamp: September 20, 2020, 04:15:59 am
content: A CO2 laser will not remove the soldermask. It will only burn the board.A pulsed laser on the other hand has a chance to ablate the epoxy layer.It may take a number of passes to remove the offending material.

author: ar__systems
author_rank: Frequent Contributor
timestamp: September 20, 2020, 11:48:54 am
content: Interesting, whenever I had similar small mistakes on my boards like unexposed ground pad, PCB suppliers always caught those.

author: chris_leyson
author_rank: Super Contributor
timestamp: September 20, 2020, 12:18:20 pm
content: Pulsed YAG might ablate it, CO2 won't. I would would go for the easy option, fiberglass pen, just remember not to breath in the glass dust.QuoteYou should see what laser wire stripping does to PVF insulation, the by-products ate through stainless steel wire guides in no time at all, nasty.

author: GerardG
author_rank: Regular Contributor
timestamp: September 21, 2020, 06:51:09 am
content: Hello bikeNomad,I have removed soldermask by CO2 laser before. (80W)40W CO2 will burnoff the soldermask and will not do any damage to the copper.You have 1 "big" problem. If the FR4 between the pads is burned it will carbonize and you have a short.You could use the cut stencil as a mask, CO2 will not burn into the stainless steel.For 10 boards I would take a sharp hobby knife and scrape the soldermask carefully from the pads.

author: bikeNomad
author_rank: Contributor
timestamp: September 24, 2020, 01:50:11 pm
content: gerardg using the stencil as a mask is a great idea. I think I can get the alignment good enough to make this work.

author: tooki
author_rank: Super Contributor
timestamp: October 10, 2020, 03:30:45 pm
content: Why not use the stencil as a mask for a fiberglass pen?

author: GerardG
author_rank: Regular Contributor
timestamp: October 12, 2020, 07:03:50 am
content: Fiberglass pen will not work on a fully cured soldermask.

author: tooki
author_rank: Super Contributor
timestamp: October 12, 2020, 08:30:39 am
content: Quote from: GerardG on October 12, 2020, 07:03:50 amLOL sure it does. I've used it plenty of times to remove solder mask. I have no idea where you'd get the idea that it doesn't work!

author: bikeNomad
author_rank: Contributor
timestamp: October 18, 2020, 09:20:12 pm
content: The laser removes *most* of the soldermask. But the fiberglass pen doesn't touch the remaining mask. I've been using a dental scaler to scrape off the remaining mask for the two boards I wanted to test.My ultimate solution: pay another $45 for 10 of these boards, including shipping. These will have solderable pads for the processors. Plus I switched the 71 LEDs from blue to white and got 10x the original brightness.

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2020, 05:04:17 am
content: I mean... are you guys not applying any pressure with the fiberglass pen? Remember, it’s basically a sandpaper pencil, it won’t work without some force. And you have to hold it perpendicular to the PCB, not at an angle.

author: bikeNomad
author_rank: Contributor
timestamp: October 24, 2020, 09:33:44 pm
content: Yes, I tried pressing very hard, and it barely touched the mask.

author: tooki
author_rank: Super Contributor
timestamp: October 25, 2020, 02:54:50 am
content: I dunno what to tell you. Fiberglass pens are commonly used to remove solder mask, and I've done it many times.Mind you, it may not look like much is happening as you wear down the mask, but suddenly it'll pop through.