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I have a PCB that I'm sending off for assembly, which includes going through a reflow oven. The last batch had a problem because the solder mask on some PCBs does not completely insulate a via that is underneath a metal connector, resulting in a short. What can I safely use (that will withstand reflow temperatures) to paint or insulate the via to prevent the short?

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  • \$\begingroup\$ As JYelton said, you're looking for a workaround when in fact you should be solving the original problem which is with the solder mask. Change your manufacturer if needed \$\endgroup\$ – Gustavo Litovsky Sep 25 '13 at 17:29
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    \$\begingroup\$ The real solution here is to properly obey the keep-out drawing in the connector's data-sheet, which will have almost certainly instructed you to not place traces or vias under metal connector parts that contact the PCB. What if this goes into a high-vibration environment (e.g. a car)? You'll wear though the soldermask in a hurry. I really hope this isn't a product you're hoping to sell. \$\endgroup\$ – Connor Wolf Sep 26 '13 at 6:10
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You want to "tent" your vias. This means changing your DRC such that vias under a certain drill size will have solder mask extend over them.

If you are using Eagle, this is accomplished by changing the "Limit" value on the Masks tab of the DRC. The limit specifies what size drill hit that the solder mask should cover up to.

Check out the tutorial from Sparkfun on better PCB's.

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    \$\begingroup\$ Thanks. I understand that I can fix the original layout, but this was overlooked, so I have a stack of PCBs that I don't want to waste. I'm looking for something that I can just mask that one via with (sharpie? paint pen?) to make use of my existing boards. \$\endgroup\$ – MX21 Sep 25 '13 at 16:55
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    \$\begingroup\$ Ah...in that case, perhaps some Kapton tape over the via? \$\endgroup\$ – dext0rb Sep 25 '13 at 17:09
  • \$\begingroup\$ @MX21 Sorry I interpreted the question differently. If you have some existing boards with non-tented vias that you want to protect, then I'd agree with Dextorb and recommend Kapton tape or wave solder masking tape. \$\endgroup\$ – JYelton Sep 25 '13 at 17:17
  • \$\begingroup\$ I do have access to some Kapton tape, but given the very small spot and the quantity of boards, it's going to be tedious. Is there any sort of marker pen that would work that anyone knows of? \$\endgroup\$ – MX21 Sep 25 '13 at 20:00
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I went ahead and used Kapton tape for this batch.

I also found that the industrial Sharpie pro permanent marker is rated to 500°F, so I might experiment with that later.enter image description here

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If you already have PCBs made without the benefit of a tented via, as you've indicated in comments, the only option that comes to mind is a wave solder masking tape (such as this one made by 3M).

There's a solder mask repair pen made by CircuitWorks, but as pointed out in a technical bulletin:

...CW3300G is a green-tinted acrylic conformal coating. Acrylic conformal coating is applied to the finished circuit board to protect board components from moisture and provide electrical insulation between circuit traces. CW3300G is not an epoxy resist material and does not have the same properties of durability and permanence as a permanent solder resist. It will decompose at temperatures above 270 °F, and therefore should only be applied to the board after all soldering has been done.

Therefore, it should not be used to cover the via prior to wave soldering.

But this does provide a clue: epoxy. The solder mask is an epoxy or epoxy-acrylate coating, so it might be possible to find a similar coating available as an epoxy dispenser of some sort, though I have not seen (and was unable to find) something that I would use on PCBs.

Usually vias under a metal pad on an IC are there to provide additional grounding or heatsinking to the copper. If you've got a via that should not connect to the component above it, I would recommend relocating it (instead of just tenting) because the tenting method does not guarantee that all boards will be electrically isolated. You might get a small hole in the mask which still allows an electrical connection with the component above.

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  • \$\begingroup\$ "You might get a small hole in the mask which still allows an electrical connection with the component above." Therein lies the problem. 80% of the time, the vias are fine. But 20% fail because it manages to make contact. This is not an IC. It's a USB connector with metal shell. I normally don't allow vias under things like that, but missed this one. \$\endgroup\$ – MX21 Sep 25 '13 at 23:16
  • \$\begingroup\$ I hear you. I think I'd go with the Kapton tape. I'm not sure it would be much slower than applying an epoxy with a pen or dispenser. How many boards do you have to fix? \$\endgroup\$ – JYelton Sep 25 '13 at 23:31

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