IMCs Microstructure Evolution Dependence of Mechanical Properties for Ni/Sn/Ni Micro Solder-Joints.

The current miniaturization trend of microelectronic devices drives the size of solder joints to continually scale down. The miniaturized joints considerably increase intermetallic compounds (IMCs) volume fraction to trigger mechanical reliability issues. This study investigated precise relationships between varying IMC volumes and mechanical properties of Ni/Sn(20μm)/Ni micro-joints. A designed method that followed the IMC volume as the only variable was used to prepare micro-joint samples with different IMC volumes. The continuously thickened NiSn IMCs exhibited a noticeable morphology evolution from rod-like to chunky shape. The subsequent tensile tests showed unexpected tensile strength responses as increasing NiSn volume, which was strongly associated with the NiSn morphological evolutions. Fractographic analysis displayed that the ductile fracture dominates the 20%-40% IMC micro-joints, whereas the brittle fracture governs the 40%-80% IMC micro-joints. For the ductile fracture-dominated joints, an abnormal reduction in strength occurred as increasing IMCs volume from 20% to 40%. This is primarily due to severe stress concentrations caused by the transformed long rod-typed morphology of the NiSn. For the brittle fracture-dominated joints, the strength appeared a monotonous increase as the NiSn volume increased. This may be attributed to the increased crack resistance resulting from continuous coarsening of the chunky NiSn without any voids. Moreover, the finite element analysis was provided to further understand the joint failure mechanisms.