High thermo-mechanical fatigue and drop impact resistant Ni-Bi doped lead free solder

Abstract

Presently, Sn-Ag-Cu (SAC) solders are most commonly used as the interconnect materials in the semiconductor package. However, their thermal fatigue and drop impact resistant properties depends on the Ag content and therefore, most semiconductor package assemblers are forced to implement multiple SAC alloys depending on intended performance. Sn-xAg-Cu solders with high Ag content (x>3 mass%) give good temperature cycling (TC) reliability but poor drop impact reliability whereas Sn-xAg-Cu solders with low Ag content (x<;2 mass%) show poor temperature cycling reliability but good drop impact reliability. So, there is need to develop solders having both good TC and drop impact reliability. For the present study, we developed a new SAC solder by micro-alloying it with Ni and Bi. It was found that the thermal fatigue and drop impact resistant is improved dramatically simultaneously. The improvement in the drop impact resistance is attributed to the decrease in the bulk and joint IMC thickness and grain refining. The high TC reliability is due to the unique network like structure of Ag3Sn in the bulk solder microstructure. The newly developed solder show high and stable shear and pull strength as compared to SAC solders and the dominant fracture mode in the high speed shear test is ductile. This new solder has potential to become interconnect material for all types of semiconductor packages.