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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
