The extraction of volatiles from printed boards produces some intereseting
stories. 

 I have experienced the return of most of the weight loss to samples within a
few hours after baking; however, it seemed to arrange itself differently.  In
a test where delamination was the prime consideration, the samples were baked
and then let set for 2 days after baking at 70%RH to recover.  The baked and
reabsorbed samples were considerably more resistant to delam. than samples
which not previously baked.  Several theories were proposed, such as; it was
not the same volatile, and there was not a large agglomeration of water drops
as might have been in the unbaked samples, and that the reabsorbed moisture
was more widely dispersed, or that it absorbed in the surface layers and came
out during preheat.

Another oddity with polyimide.  When boards were baked at 110 deg C for 6
hours they would have a certain weight loss.  If other samples from the same
lot were baked at 150 deg C the weight loss was much larger; however after
letting them set for a couple of days to readsorb water from the air, they
returned to nearly the same weight, but on rebaking at the two temperatures,
the weigh loss was again higher for the higher temperature samples.. Theory:
water is adsorbed in the pores and it takes a greater energy input to break
the water away from the pores than it does to merely spit water into vapor,
which occurs quickly at 100 deg C. and you do not get it all out at the BP of
water. The solution was to bake less time at a higher temperature which also
by theory caused lest intermetallic growth for the time/temp used.

I had asked surface energy people to explain some of the above  phenomena,
and they asked a few questions: (1) are the volatiles, absorbed, adsorbed or
occluded. (2) are they free floating or are they bonded the the material by
such a H bonding or are they actually a portion of the polymers. (3) What is
the pore density and what is the space between the polymer chains.  They
wanted to do WVT testing, I did not have the answers or the funds to test so
the problem was resolved by the "good old production solution" bake a shorter
time at a higher temperature, if it does not work, bake longer .  If any of
you have the answers to any of the above let me know.

Thanks 
Phil Hinton
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