Influences of reflow time and strain rate on interfacial fracture behaviors of Sn-4Ag/Cu solder joints

Abstract

In this study, the influences of reflow time and strain rate on interfacial fracture behaviors of the Sn-4Ag/Cu solder joints were investigated through in-situ observation. The interfacial microstructures of the solder joints reflowed for different times were first observed by scanning electronic microscope and measuring laser confocal microscope. Then tensile tests of the solder joints were conducted using in-situ tensile stage at different strain rates, and the interfacial fracture processes were in-situ observed. The observation results reveal that the thickness and surface roughness of the interfacial Cu6Sn5 layers increase linearly with increasing square root of the reflow time. Due to the serious strain concentration, fractures of the solder joints occur around the solder/Cu6Sn5 interface. The solder joints reflowed for a short time usually fracture inside the solder close to the joint interface, while the long-time reflowed solder joints are more apt to fracture in the interfacial Cu6Sn5 layer, because the stress applied on the Cu6Sn5 grains increases with increasing reflow time and grain size of Cu6Sn5. The solder joints reflowed for moderate time have the highest fracture resistance. At higher strain rate, the solder can apply a higher stress on Cu6Sn5 layer prior to its fracture, making the latter more apt to fracture.