Is soldermask required for fine pitch IC soldering?

Intrigued about a discussion on the forum about whether soldermask is needed between fine pitch pads, Brian decided to test it.

In the video of the test he placed solder paste on both the FR4 board material, and the soldermask, as well as some on a tined pad. Once done reflowing his results were in. Both the FR4 and the soldermask acted the same, no solder got stuck to either of them, while the tined pad got soldered as expected. The first video shows and explains the setup, while the one below has the results.

Via the forum

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  1. Now, I have a question about this. The two portions that are under test both appear to me to have soldermask. Isn’t the green colored portion the soldermask? The difference in the light and dark green appear to be the presence of a ground/power plane pour. The soldermask is present over both. The use of a fidicual would be a better test for this hypothosis since a fidicual will have a bare copper/tinned dot surrounded by a keepout that contains no solder mask and then the usual solder mask on the regular/other portions of the board. The video appears to have tested a tinned pad versus an area with a ground plane pour covered with solder mask, and an area with no copper underneath covered with solder mask. If I am wrong, please correct me.

    1. I will double check tonight. You could be right. I speculated in the forum thread that it was a lamination under the copper layer. I don’t have any better board to use to test than the one I did. I also did further tests without flux in the forum thread though no video of them.

      1. So I check again and will comment in the forum and my blog. I think you are correct. However I just tried it out with the paste with flux in it and it doesn’t really change the wetting results. I don’t have a great large flat area to try it on so I was forced to use the edge of several boards stacked… Not great for video I am afraid.

  2. It probably a little bit better to have solder mask because it levels the surface and makes bridging between two close copper areas less probable. Otherwise there is a dip between such areas where some stray solder can get.

  3. I agree with John. Both non-soldering pad areas look like they are covered in green solder mask. FR4 usually is more light green or yellow when not covered with solder mask. Many board manufacturers are also removing the white silk screen printing from areas that are not covered with solder mask. The upper pad contour of the battery holder is placed on the claimed non solder mask area but is still outlined with white silk screen.

  4. If you really want to test this, then just draw something up in Eagle with the solder mask removed from a test area. You can even create the exact same patterns, with and without mask, to see how the paste performs differently. Of course, it’s going to cost you the price of a prototype board to test this accurately, but maybe it’s worth it to you.

  5. I recently had some prototype PCBs (for the ToorCon 13 badge) made, some with soldermask and some without (because I could get them made overnight without soldermask and I was in a hurry). I was able to get the soldermask-free boards working, but it was much more work, and they were much less reliable. I had several traces running under an LQFP, and it was hard to avoid shorts underneath the part. This was annoying, but I was able to fix it. I had worse problems with the QFN part (CC2400) on the board. I must have re-soldered, re-heated, or otherwise touched up that part 50 times, and I still had problems where it would stop working after a few hours of testing. Honestly I would have been better off just waiting for the boards with soldermask to arrive.
    At the charity auction at the end of ToorCon, we auctioned off a bag full of bad badges that had been returned for replacement, and I added my original two soldermask-free prototype boards to the bag, partly because they might be cool for someone to have and partly because I never want to see those boards again!
    I didn’t have any trouble with 0603s or 0402s. If you must do SMT without soldermask, avoid no-lead parts and be careful of traces under ICs (avoid having them in the layout if possible).

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