Thermal cure solder masks are certainly more difficult to strip that the
aqueous processible LPI solder masks. After final cure, some of the thermal
cure masks are nearly impossible to strip.

You could conclude that thermal cure masks are more chemically resistant that
the aqueous LPI masks and, I bleieve, you would be right!

The thermal cure masks are usually epoxy resin systems with thermal cross
linking agents. Once cured, they form a tight polymer network that is
extremely chemically resistant.

On the other hand, aqueous LPI solder masks are much more complicated
formulations and, in addition to the epoxy resins and thermal cross linking
agents, they contain photoinitiators and acrylate monomers (so they will
react to UV light during the exposure step) and they contain carboxylate acid
groups somewhere in the resin chain (so they will develop in mild alkaline
developers). Therefore, the aqueous LPI solder masks are very resistant to
acidic solutions, but are sensitive to  alkaline systems, especially if the
pH is very high (>13.0) and/or the temperature is high (>150 deg F). The
thermal cure masks have no such sensitivity to high temp, high pH alkaline
solutions (no carboxylate groups).

Hope this helps

Larry Fisher
Dexter Electronic Materials
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