Evolution of the hardness and Young’s moduli of interlayers in Sn99Cu1/Cu solder joints subjected to isothermal ageing

The interlayers at solder/pad interface are critical to the reliability of solder joints; hence, their mechanical properties is of vital importance. However, the correlation between service duration and evolution of mechanical characteristics of these interlayers has seldom been reported. In this work, hardness and Young’s moduli of Cu6Sn5, Cu3Sn and Cu were evaluated by nanoindenation after ageing for every 100 h up to 500 h. It was found that hardness and Young’s moduli of Cu6Sn5 and Cu3Sn dropped with aging and reached the bottom at 200 h and 300 h, respectively, followed by a gradual increase. This U-shape curve was generally opposite to the evolution of corresponding parameters in Cu. Evolution of mechanical properties of IMCs can be attributed to constrained volume shrinkage induced by solidstate reactions that producing IMCs. The resultant stress ultimately affected load–displacement curves recorded by nanoindentation tests. The observed reverse evolution trend of examined parameters of Cu and adjacent IMC layers was a result of mutual constraint posed by Cu3Sn/Cu interface.