Screening tests of conformally-coated printed wiring boards

Identical printed wiring boards (PWB) were fabricated using glass-triazene as substrates and were conformally coated with either conothane or parylene. The boards were then placed in sealed canisters along with TATB and other organic materials found in recent weapon systems. While subjected to electrical potentials of 120V and 250V, these PWBs were either aged at 600, 700, or 800 or temperature cycled for as long as 16 months. The resistivities of the boards were precisely measured before and after aging to determine the effect of time, temperature, and environment on the electrical characteristics of board materials. In all cases board resistivity increased with temperature and duration of test. This increase was believed to be related to either a reduction in water content in the board substrate or a completion of cure of the conformal coating with time elevated temperature. Both mechanisms would result in a reduction in conductive species. In spite of high humidity levels, no evidence of current activated filament formation between adjacent active circuit paths on aged PWBs was seen.