Weaknesses in a strong film adhesive

The ease with which a preform of film adhesive can be used in hybrid circuits makes it appear a desirable method for bonding components to substrates. Another reason that this type of adhesion appears desirable is its high bond strengths. These two advantages of film adhesives over other standard epoxies may be overshadowed by their possible corrosiveness to hybrid metallizations and their catastrophic effects in the presence of humidity. The idea is not to eliminate film epoxies, but to properly evaluate their effects prior to use in hybrid circuits. Previously accepted tests for epoxies used in hybrids are the measurement of outgassing properties, weight losses, adhesive strength of bonds, and any effects caused by thermal shock. For a more complete evaluation of an epoxy, one must expose the cured epoxy to a humidity test and a standard corrosion test. The effects of humidity and the corrosive effects of a film epoxy were investigated with very unusual results. The type of film adhesive used was made from an epoxy impregnated fiberglass cloth. The epoxy resin and hardener used were previously tested for humidity effects and corrosive effects with satisfactory results. The addition of the fiberglass or another unknown drastically changed the results of the test. Exposing the film adhesive to humidity caused the film to lose its bond strength. Further investigation showed that the film became intensely corrosive in the presence of an electrical potential difference at elevated temperatures. These two results have shown that it can be potentially dangerous to use a film epoxy without proper testing.