[time-nuts] Double ovened 10811-60158 on ebay

Pete peterawson at earthlink.net
Mon Jul 21 15:49:47 UTC 2008


PCA coatings thin enough & chemically "friendly"
enough to permit component replacement simply
delay the effects of increasing ambient humidity
on the circuits exposed. If the PCA fabrication
processes leave the finished surface VERY clean,
then several brands of conformal coatings will
permit the surface to remain clean as humidity
rises and falls. This is not to say the circuit will
be unaffected by these changes, just that they
become predictable.

For most designs, particularly with fiber reinforced
substrates, moisture intrusion from untreated edges
is often overlooked. PCA cutting processes tend
to tear many of the reinforcement fibers, making
extended voids along the fiber length. Subsequent 
chemical processes fill these voids with varying 
amounts of electrolytes, which cannot be effectively
removed. This situation results in difficult to predict
response to high humidity. Here's one place where
a careful (expensive) application of epoxy would help.

Short of complete encapsulation, PCAs should first
be designed to work properly at the highest humidity
the customer demands: second, be absolutely clean; 
finally, coated only to stay clean.

Pete Rawson





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