Research On Failure Behavior Of BGA Solder Interconnections Under Fatique Loading

Lead–free solders are enforced to be adopted in micro–electronic mounting after the implementation of laws of Waste Electrical and Electronic Equipment in 2006. This paper focuses on the widely used BGA packaging in SMT (Surface Mounting Technology), studies the failure process of BGA solder interconnections under fatigue loading. These samples are made of three different solders: eutectic Sn–Pb solder (Sn63Pb37, wt %), mixed solder of Eutectic Sn–Pb solder paste and SAC305 (Sn–3Ag–0.5Cu, wt %) solder bump, and SAC305 lead–free solder. In my studies, the vertical sections of samples after different periods of cycling were observed under SEM and EBSD to study the changes of compositions and crystal structure inside the solder interconnections; and the hardness values of different positions on the vertical section were tested with micro–hardness gauge to find stress and strain concentration areas. After analysis, the failure principle of BGA solder interconnections under fatigue loading was obtained.When BGA solder interconnections are under thermal fatigue loading, stress and strain first concentrate in corners, near interfacial areas or near grain boundary areas. The stress concentration increases the density of defects and lattice distortion energy, and provides enough energy for recrystallization. Defects and high temperature condition accelerates the diffusition of Ag, Cu atoms to form big blocks of Ag3Sn and Cu6Sn5 on the grain boundaries that has been distributing dispersively inβ–Sn solders as small particles. At the same time, dislocations initiate and diffuse continuously until blocked by Ag3Sn and Cu6Sn5 compounds. After long time accumulation, subgrains are formed with small–angle grain boundaries or even large angle grain boundaries between Ag3Sn and Cu6Sn5 compounds. Subgrains rotate around c–axis and finally become separated grains. Recrystallization areas have weak mechanical property, and grains slip along grain boundaries, thus cracks intiate and extend along grain boundaries. At last, cracks propagate through BGA solder and caused its failure.Under mechanical fatigue loading in room temperature, diffusion and growth of Ag3Sn and Cu6Sn5 compounds are not as obvious as under thermal fatigue loading. Hence, dislocations were limited among dense particles of Ag3Sn and Cu6Sn5, which causes much smaller recrystallized grains in BGA solder interconnections under mechanical loading.