Interfacial Microstructure Evolution for Cu/Cu3Sn/Cu Solder Joints during Ultrasonic-Assisted TLP Soldering Process

In this study, the interfacial microstructure evolution and mechanism of ultrasonic action during ultrasonic-assisted TLP soldering process (260°C, 600W, 20KHz) were investigated. The bonding time forming full Cu3Sn solder joints of traditional TLP and ultrasonic-assisted TLP soldering was 600min and 50s respectively. Before forming full IMCs solder joints, the Cu6Sn5 at Cu/Sn interface grew in a non-scallop-like shape during ultrasonic-assisted TLP soldering process, meanwhile, the non-interfacial Cu6Sn5 distributed within the Sn layer, the Cu3Sn at Cu/Cu6Sn5 interface grew in a non-wave-like shape or non-planar-like shape, the non-interfacial Cu3Sn in the Cu6Sn5 contacted with the Cu3Sn layers at Cu/Cu6Sn5 interface with the increasing of ultrasonic bonding time, which was different from the formation of scallop-like Cu6Sn5 layers, wave-like and planar-like Cu3Sn layers by traditional TLP soldering. The mechanism of ultrasonic action was regarded as that the solder joints experience generation of micro-cracks in the Cu6Sn5, separation from Cu6Sn5 layers at Cu/Sn interface, being smashed to smaller size of separate Cu6Sn5 and moving into the liquid Sn of smaller Cu6Sn5 in turn, while the formation of non-wave-like or non-planar-like Cu3Sn layers was considered to be the precipitation at Cu3Sn/Cu6Sn5 interface of Cu atoms, the formation of non-interfacial CU3Sn was attributed to the traversing Cu3Sn layers at Cu/Cu6Sn5 interface into Cu6Sn5 of Cu atoms. In addition, the ultrasonic wave accelerated the diffusion of Cu atoms and Sn atoms to form IMCs.