Critical Factors Affecting Structural Transformations in 3D IC Micro Joints

Nowadays, micro joint has been a widely used technology for the vertical integration of 3D IC. Even so, however, a common phenomenon after high temperature storage test or reflowing is that considerable voids are left in micro joints. Considering the strong relationship between reliability and the microstructures of micro joints, it is of necessity and interest to verify the factors which affect morphologies of micro joints. In this study, Ni/Sn/Ni sandwiches micro joints with two kinds of dimension were electroplated onto a silicon wafer, followed a solid-state aging at 180°C. The results showed, for the Ni/Sn/Ni sandwiches with the smaller aspect ratio (i.e. pillar-like sandwiches), solder outward leakage (i.e. surface diffusion) significantly affected surface morphologies of the sandwiches while the inner microstructures were mainly affected by volume shrinkage induced by interfacial reaction. Most interestingly for the Ni/Sn/Ni sandwiches with the larger aspect ratio (i.e. plate-like sandwiches), before Ni
₃
Sn
₄
growing in the opposite directions impinged to each other, voids were only formed near the Sn surface instead everywhere in the Sn layer. A reasonable explanation is that solder surface diffusion is a dominant factor for the sandwiches with the larger aspect ratio before the Ni
₃
Sn
₄
impingement. It is noteworthy that there seemed a size effect working on micro joints, if the aspect ratio reached a specific value, the whole solder layer would be within the range of solder surface diffusion. Nevertheless, further experimental results are necessary for this speculation.