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TESTING 1, 2

Part 2: Standards: Prepping Your Prototype
by George Novacek

Start • Drainage • Temperature • Humidity • Shocks and Vibrations • Explosions • The Nasties • Sources

HUMIDITY

Section 6 addresses humidity’s effect on equipment performance. This is probably the last bastion of the COTS opponents, who hang on to the superiority of the ceramic packaging. This argument, however, becomes purely academic if the ceramic package is not available.

Nonetheless, we still need to decide how to address the effects of humidity on our circuits. The primary concern: long-term storage of components in places like missile silos or depots, where moisture seeping into the components and circuit boards causes their eventual failure. Moisture also causes PCB surface leakage, especially in high-impedance circuits. The solution lies in the manufacturing process.

Therefore, during the storage of parts and the PCB assembly, care must be taken not to allow moisture to be trapped anywhere. Plastic encapsulated ICs (PEMs) can crack (pop) during soldering when moisture turns into steam. A dry, assembled, and soldered PCB needs to be conformally coated, sealing it from moisture penetration. In the simplest case, PCBs are sprayed or dipped in a specially formulated varnish. The most critical equipment must be vacuum coated with a uniform, 10-mm–thick plastic coat.

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