A number of years ago, testing was done on the "outgassing" of LPI solder
masks. It was found that the weight loss after final bake[1 hour at 300 deg
F](on a stainless steel substrate) was about 4%. If the LPI mask was given a
UV cure step before the final bake, the weight loss was reduced to about 2%.
Analysis of "gunk" residues in a curing oven showed the presence of monomers
and photoiniators. It was assumed that the UV cure step reduced the amount of
both free monomers and photoinitiators in the gunk, so the customer included
a UV cure to reduce his "gunk" buildup and the number of times he had to
clean the "gunk" from the oven.

Outgassing would also be possible from legend inks because you can never get
100% crosslinking of any polymer (there is probability going on here) and you
can have some free monomers/curing agents (like photoinitiators) that will
volatalize. To reduce these, you can increase the time/temperature of the
final bake or include a UV cure step (if there is free radical polymerization
involved in the legend ink being used)

Hope this helps.

Larry Fisher

Allen Woods & Associates