Volume shrinkage induced by interfacial reactions in micro joints

In 3D-IC, solder joints of a few microns in size are used. Theoretically, there is 11.4% shrinkage in volume when Ni reacted with Sn to form Ni
₃
Sn
₄
. Surface profilometer and SEM analysis were carried out to measure the actual volume shrinkage for micro Ni/Sn/Ni sandwich structures during an isothermal annealing at 180°C for 0-7 days. The results showed there was about 7.3% shrinkage in volume and the remaining 3.2% shrinkage was released by forming voids. The internal stress and the forming of voids caused by solid state reaction might induce potential reliability issues.