Some problems and possible solutions for hybrid microcircuit reliability

Thick film hybrid microcircuits developed for various telecommunications applications and made by various technologies have been examined by overstress testing and by failure analyses, which have yielded a number of promising results and revealed a number of deficiencies. Examples are presented of the poor reliability obtainable not only from low-cost plastic-coated hybrids but also from supposedly superior hermetically-sealed hybrids—degradation occurring because of moisture permeation or desorption, the evolution of deleterious vapours, bond failure due to inter-metallic reactions, and so on. Reliability problems of this sort are often encountered when the technologies or manufacturing standards are not directed towards high reliability applications, despite conformity to accepted quality standards, so demonstrating that approval of manufacturing capability does not necessarily provide reliability assurance. Nevertheless, the reliability prospects for hybrids are quite bright, because problems can and have been overcome — for example, that some circuits even in low-cost encapsulations can approach the highest reliability standards, that the long term stability of thick-film resistors can be better than 1%, that bonding problems can be obviated by using the right interconnection materials and that less hazardous adhesives are available for use in hermetic encapsulations. The observations show that while it cannot be claimed that hybrid technologies are inherently reliable, neither does it appear that the opposite is true — potentially high reliabilities being achievable by exercising intelligent choice.