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I just received my first PCB back from the manufacturer. After soldering on the SMT components I found that the board works absolutely fine. The only thing I'm puzzled about is that both the ground pour and the traces are a brown color and not the green I would expect it to be since I specified a green solder mask. This is what it looks like:

enter image description here

I've tested the resistance of the ground pour between two points and it is not conductive, so this does not appear to be pure copper. However, since this is my first PCB I am really puzzled about the brown color. Does anyone have an explanation?

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    \$\begingroup\$ Have you asked the PCB supplier? \$\endgroup\$
    – Leon Heller
    Apr 7, 2014 at 12:39
  • \$\begingroup\$ No, I didn't want to sound like an idiot if there's a really obvious explanation... \$\endgroup\$
    – David Högberg
    Apr 7, 2014 at 12:39
  • \$\begingroup\$ You should take it up with them, the solder mask is obviously the wrong colour. Who is the supplier? \$\endgroup\$
    – Leon Heller
    Apr 7, 2014 at 12:57
  • \$\begingroup\$ That looks like the solder mask isn't a full print, but almost something that is deposited only on the copper after a mask is placed on the copper portions not meant to be masked. I would also look and see if your mask layer gerber is like this or has full coverage where there isn't copper. \$\endgroup\$
    – Joe
    Apr 7, 2014 at 13:05
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    \$\begingroup\$ Overall, PCB manufacturing is pretty complex, so there's not really any stupid questions. Don't be afraid to call the PCB supplier, they will usually not hesitate to call you in a similar manner ("did you actually mean to have the silk screen in the middle layer or should we move it to the top?"). \$\endgroup\$
    – Lundin
    Apr 7, 2014 at 14:50

2 Answers 2

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This looks like they used clear solder mask. If there was no solder mask, the gold would have plated all over the copper.

Your photo is not clear enough to confirm that, but try looking closely at an oblique angle and see if you can see a small gap between a gold-plated pad and a clear shiny layer on the board.

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  • \$\begingroup\$ Ah, that would make more sense. There isn't missing solder mask where there is not copper, it just isn't easy to see in the photo. \$\endgroup\$
    – Joe
    Apr 7, 2014 at 13:29
  • \$\begingroup\$ I have looked at a lot of different angles under the light and I can't identify a clear shiny layer. I would have thought that if there was NO solder mask, though, that I would get zero resistance when I put my multimeter on two points of the ground pour. I'm a newbie though, so what do I know. \$\endgroup\$
    – David Högberg
    Apr 7, 2014 at 13:42
  • \$\begingroup\$ It looks like the copper is oxidized (which would also show high resistance). But I don't know how you could do that and have an ENIG finish... I only see options for green soldermask on the Wurth site. \$\endgroup\$
    – W5VO
    Apr 7, 2014 at 13:56
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Soldermask is nearly always somewhat translucent, so the color over the copper ends up a much darker shade than over bare areas of the board.

It looks to me like the soldermask they used on this board is a very feint/translucent shade of green (maybe someone accidentally mixed some clear soldermask with the green stuff) and/or is applied very thin. So on the copper areas the fient green of the soldermask combines with the pinkish color of the copper to produce a brown, while in the non-copper areas you see a very feint green.

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