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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?

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    \$\begingroup\$ I use a scalpel for this, with a slightly curved blade. \$\endgroup\$
    – jonathanjo
    Sep 28, 2022 at 16:25
  • \$\begingroup\$ Thanks. I briefly tried using a razor (straight edge), but it didn't seem like the right tool. I need to find a nice scalpel. \$\endgroup\$
    – njs
    Sep 28, 2022 at 16:28
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    \$\begingroup\$ "grinding pen" youtube.com/watch?v=pSuaS11QeWY \$\endgroup\$ Sep 29, 2022 at 10:16

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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.

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    \$\begingroup\$ @OP I've never used one smaller than the usual 4 mm tip size, but fibreglass pens are also available with a 2 mm tip if there isn't much space to work with. Be wary of the tiny glass fragments that break off with normal use (e.g., clean up with a damp small disposable cloth afterwards), and don't touch the tip. \$\endgroup\$ Sep 28, 2022 at 18:45
  • \$\begingroup\$ First time I used a fiberglass pen (love them!) I noticed sharp pains in my fingers long after, and washing the hands didn't help. Turns out tiny bristles got stuck in the skin. Fortunately it went away after a day or so. Probably migrated into the body and up to my brain, but at least the pain is gone! Next time I was more careful, and used gloves. \$\endgroup\$
    – pipe
    Sep 29, 2022 at 11:47
  • \$\begingroup\$ Thanks, @DKguyen. I have three on order. \$\endgroup\$
    – njs
    Sep 29, 2022 at 13:30
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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)

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  • \$\begingroup\$ Why would you use a precision screwdriver and not just twist a small drill back and forth between your fingers? Or a drill in a pin vise? \$\endgroup\$
    – DKNguyen
    Sep 28, 2022 at 18:20
  • \$\begingroup\$ I've done that before also, but I've found a precision screwdriver really allows you to not disturb the copper underneath because it's not as sharp as a scapel (because of the bluntness of the edge) it allows you to scrape soldermask or FR4 off of copper \$\endgroup\$
    – Voltage Spike
    Sep 28, 2022 at 18:28
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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.

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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.

Another model (From here)

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