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

Abstract

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 Cu 3 Sn solder joints of traditional TLP and ultrasonic-assisted TLP soldering was 600min and 50s respectively. Before forming full IMCs solder joints, the Cu 6 Sn 5 at Cu/Sn interface grew in a non-scallop-like shape during ultrasonic-assisted TLP soldering process, meanwhile, the non-interfacial Cu 6 Sn 5 distributed within the Sn layer, the Cu 3 Sn at Cu/Cu 6 Sn 5 interface grew in a non-wave-like shape or non-planar-like shape, the non-interfacial Cu 3 Sn in the Cu 6 Sn 5 contacted with the Cu 3 Sn layers at Cu/Cu 6 Sn 5 interface with the increasing of ultrasonic bonding time, which was different from the formation of scallop-like Cu 6 Sn 5 layers, wave-like and planar-like Cu 3 Sn layers by traditional TLP soldering. The mechanism of ultrasonic action was regarded as that the solder joints experience generation of micro-cracks in the Cu 6 Sn 5 , separation from Cu 6 Sn 5 layers at Cu/Sn interface, being smashed to smaller size of separate Cu 6 Sn 5 and moving into the liquid Sn of smaller Cu 6 Sn 5 in turn, while the formation of non-wave-like or non-planar-like Cu 3 Sn layers was considered to be the precipitation at Cu 3 Sn/Cu 6 Sn 5 interface of Cu atoms, the formation of non-interfacial CU3Sn was attributed to the traversing Cu 3 Sn layers at Cu/Cu 6 Sn 5 interface into Cu 6 Sn 5 of Cu atoms. In addition, the ultrasonic wave accelerated the diffusion of Cu atoms and Sn atoms to form IMCs.