Failure Mechanisms Associated with Die-to-Header Bonds of Planar Transistors

An investigation of failure mechanisms associated with die-to-header bonds in planar transistors was centered on five thermally induced physical processes that occur in these bonds. The five processes include solid-state diffusion of intermediate materials into the bonding layer and outgassing of materials on the header, both resulting in void formation; nickel-silicon intermetallic formation leading to cracking; thermal reordering of gold-silicon eutectic alloys accompanied by electrical resistance changes; and a phase transformation in copper-gold braze alloys, also resulting in changes in electrical resistance. The kinetics of these processes were examined with relation to actual device materials and geometries to determine their effects on transistor electrical characteristics. The results indicate that these processes are probably not major life-controlling mechanisms. However, any one or a combination of these processes may result in parameter drift and incipient failure during extended device operation. Cerain immediate corrective actions are available to prevent these deleterious effects. Thermal stabilization or outgassing steps in the device fabrication and close control of materials are suggested. Other corrective actions will require an investigation of more efficient barrier layers on the header, substitution of materials to improve wettability, or perhaps a complete redesign of certain devices.