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 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.