It's like the Holy Grail! What is the magic solution for ensuring that your
solder mask has been cured? Methlyene chloride does give a clue, but it is
quite toxic and many people don't want to use it these days. So, what else?

1. There is an instrument that gives a quantitative measurement for solder
mask (plus other thermoset polymers) cure. It is call the Meseran and measures
the radioactive counts of carbon 14 (sounds nasty, but really isn't). The
method is non destructive, but the instrument is expensive.

2. Probably the best method is to measure things that you can do easily (oven
temperature and times) and pay close attention to these. If your solder mask
(just about any type - LPI, thermal cure, dry film, etc.) sees a one hour bake
at 300 deg F, then you can assume it is cured!

3. Most often, circuit boards see some harsh processes in the board shop after
the solder mask is cured (HASL, electroless Ni/Au, OSP, etc.). If the mask
survives these processes without any degradation, lifting, adhesion loss,
blistering, etc., then you can probably assume that mask has been cured
properly. Pencil hardness testing should help here too (should be 6H to 9H -
depending on the mask used).

4. Often times "solder mask cure" is blamed for problems at assembly when it
has nothing to do with it. If a mask has survived HASL and looks good, any
subsequent problems that occur are probably because of excessive stress on the
solder mask/PWB, not on solder mask cure.

Hope this helps.

Larry Fisher
Dexter Electronic Materials
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