I have three through-hole PCBs that are missing front and back solder mask openings for two of the components. The solder mask (usually green) is covering the front and back solder pads. All other pads for all other parts are exposed.
Is there a good way to remove the solder mask so that I can solder the components to the board?
You can use a burnishing pen, scratch brush,or fiberglass pen. It looks like an eraser pen but has fiberglass brush bristles come out. It's much more precise, gentle, and safe than a knife, dremel, or sandpaper. Just pretend it's an eraser. Normally use to scrape away oxidiation and rust.
In the past I've had success of scraping off solder mask with a scapel or exacto knife. Another thing that can be used is a precision flat blade screwdriver with a sharp edge. Get a microscope and see if you can unbury the pads.
(I've even dug through a full layer of FR4 with a precision screwdriver to reach an inner layer)
A sturdy (eg. Olfa) utility knife can work, but it's a bit of a blunt instrument and if it slips bad things can happen. Eye protection is a good idea in case the tip snaps off. I think X-Acto knives are a bit too flexible for the horizontal forces but okay for very small precise areas.
You could probably use a rotary tool (eg. Dremel or the more professional Foredom type) with a sanding bit but I can't specifically recall trying it.
A small 4-flute end mill in a CNC engraver will work, but it's a bit fussy getting the Z set.
A mini grinder pen might come in very handy for a job like this and many others. I think it is a fantastic tool also for repairing missing tracks in PCBs.
(From here)
