Ultrasonic bonding of Cu/Ni and its thermal reliability

In this study, we ultrasonically bonded Cu and Ni sheets and evaluated the thermal reliability of the joints at 473 K by high-temperature testing. Furthermore, we observed the interfacial microstructures of the joint and evaluated their effects on the bondability of the joint. Cu and Ni sheets were metallurgically bonded under the optimum condition of the ultrasonic bonding. Cu base metal strongly deformed compared with deformation of Ni base metal during the ultrasonic bonding. The ultrasonically bonded interface of Cu and Ni sheets was composed of bonded region and unbonded region. The bonded region was composed of two types of micro structures. Cu and Ni were locally stirred and approximately 200-nm-thick of solid soluted region of Cu and Ni atoms was formed in the bonded region. The Cu grains were fined, moreover, the Ni grains became fined from the bonded interface to Ni side 1-2.m. These fine grains can be formed by deformation of bonded base metals during the ultrasonic bonding process, suggesting that plastic deformation was formed from the soft Cu base metal to the hard Ni base metal. The ultrasonically bonded joints showed good thermal reliability at 473 K through the holding time up to 1000 h. Some of the Cu grains became coarse in the Cu base metal. Phase separation of solid soluted region of Cu and Ni atoms may occur during the high-temperature test performed at 473 K.