Microstructure evolution of Cu/Sn/Cu bonding stacks impacts on their mechanical properties

We examined the effects of the bonding temperature on the bonding morphology, electrical resistance and mechanical properties of a Cu/Sn/Cu bonding structure. By carefully characterizing the bonding morphology using several analytical methods (SEM, TEM, and EDS), we successfully unravelled that bonding the samples under various bonding temperatures produced greatly different bonding morphologies — and different prevailing phases in the bonded layer. Mechanically testing the samples bonded under different conditions generates the joint strength data of the samples, which confirm a sensitive dependence of the joint strength on the bonding conditions. In general, the samples bonded at the higher temperatures show a higher bonding strength and the strength escalates with the bonding temperature increasing. Connecting the mechanical data with the SEM analysis results reveals an intimate process-microstructure-property interrelationship that bonding samples under various process conditions produce different microstructures, which, in turn, lead to much different joint strengths.