Electronic – PCB solder mask inconsistency

manufacturingpcbsolder-mask

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.

Best Answer

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.