We have a small PCB that has been manufactured and assembled several thousand times. However, the last lot showed inconsistencies with the solder mask:

Solder mask inconsistencies

It looks like the solder mask is very thin, almost exposing the copper. The expected color would be what you see next to the SOT23 pads.

We contacted the PCB manufacturer about this, who claims that it is most likely an issue from too high soldering temperature or too long solder duration.

So we contacted the assembly company, but they say that the soldering process did not change from the previous runs and it must be a problem with the PCB manufacturing process.

  1. Did anyone experience this kind of issue and what is the likeliest cause?
  2. How critical is this? I can't quantify if I am looking at slightly less solder mask or nearly no solder mask at all.
  • 3
    \$\begingroup\$ Looks like the usual cheap crappy china PCB solder mask. \$\endgroup\$
    – PlasmaHH
    Jun 14, 2016 at 8:09
  • \$\begingroup\$ @PlasmaHH: The thing is that both companies were picked because they tend to be pretty reliable and are ISO 9001 certified, since this PCB is for automotive use. \$\endgroup\$
    – Rev
    Jun 14, 2016 at 8:19
  • 6
    \$\begingroup\$ even chinese fish markets are ISO9001 certified. Having worked in the aviation industry, I can tell you that this alone means nothing. It only means that you hold up to some self created standards, however low they might be. It doesn't mean that never fuck up products and try to sell it anyways. Or that you fucked up scheduling and outsource to someone else. To check who is right, you should just have a look at some unpopulated boards. \$\endgroup\$
    – PlasmaHH
    Jun 14, 2016 at 8:22
  • 1
    \$\begingroup\$ @DmitryGrigoryev: I do not think the designer had heat sinking in mind but low impedance to the plane. I am not familiar with the circuit. \$\endgroup\$
    – Rev
    Jun 14, 2016 at 8:38
  • 3
    \$\begingroup\$ @PlasmaHH Indeed, ISO9001 doesn't prevent a manufacturer to sell crap. But the awesome is that, once certified, they can prove it's crap! Another thing that has always amused me: Once certified, the manufacturers pays ISO for the certification and each renewal. If not certified, there are no renewals to pay, of course... So, guess what ISO does? \$\endgroup\$
    – dim
    Jun 14, 2016 at 8:41

1 Answer 1


We had a detailed discussion with the PCB manufacturer. Suffice to say, that they guarantee (according to their "Process and Capability Manual") a solder mask of at least 5um.

According to them, everything nearing the 5um thickness would already result in the "copperish" color shining through the solder resist.

The solder resists viscosity may vary slightly and cause differing results, especially at the edges, where some resist may flow off the copper.

Nevertheless we get better and more consistent results from other manufacturers, so it still stands that their processing quality may not be optimal.

  • \$\begingroup\$ The ultimate question is: Does it work? \$\endgroup\$
    – pipe
    Jun 14, 2016 at 16:53
  • \$\begingroup\$ I just saw a very similar issue to this with one of my designs at work. I ordered them from Advanced Circuits and they sent back boards with what looked to be bare copper in certain areas. It is fully-coated, it's just thicker in some places than in others. Despite the look, it still functions correctly. \$\endgroup\$
    – DerStrom8
    Jun 14, 2016 at 17:36
  • \$\begingroup\$ @pipe: The boards tested so far are all working, but we did not expect any problems there. \$\endgroup\$
    – Rev
    Jun 14, 2016 at 18:46

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