Environmental Effects on Copper Thick Film Microcircuits

Performance and reliability of copper thick film microcircuits have been investigated under ambient and accelerated aging environments. Results of the multilayer circuits tested at ambient conditions have demonstrated their quality performance in both unprotected as well as hermetically packaged systems. Performance of these microcircuits match and in some aspects surpass the functional behavior of conventional gold thick film hybrids. Since copper is inherently more reactive than noble materials, close attention has been given to oxidation, corrosion, and migration under elevated temperature/humidity conditions both with and without electrical biasing. Observations of unprotected copper multilayer microcircuits showed that exposure to high humidity promotes oxidation, lowers insulation resistance, reduces breakdown voltage, increases the dissipation factors, and under specific conditions develops electrical shorts. Comparable tests using conventional gold thick film hybrids showed similar electrical behavior but to a lesser degree. Moisture sensitivity in multilayer structui'es has been directly correlated with the dielectric porosity and is dependent on both material composition and processing relationships. Electrical insulation properties of thick film dielectrics are basically controlled by the glass type, refractory materials, and the formulation ratio. Processing in neutral atmosphere furnaces introduces some constraints on the choice of paste vehicles in that total pyrolysis is necessary to achieve nonporous microstructures. Because of the demonstrated moisture sensitivity and potential activity of copper thick film materials, hermetic packaging is advised for high reliability applications. The use of conformal coatings and/or thick film encapsulants will reduce oxidation, modify corrosion behavior, and prevent electrical shorts caused by condensation, but will not entirely eliminate changes in electrical properties due to moisture permeation of the thick film dielectrics. Research and engineering efforts are continuing tO develop low porosity dielectrics, to identify encapsulant materials for protection from moisture, and to improve long-term reliability of copper thick film microcircuits.