Sn-Ag-Cu Solder Joints Interconnection Reliability of BGA Package during Thermal Aging and Cycling

This study illustrates test results and comparative literature data on the influence of isothermal aging and thermal cycling associated with Sn-1.0Ag-0.5Cu (SAC105) and Sn-3.0Ag-0.5Cu (SAC305) ball grid array (BGA) solder joints finished with ImAg, ENIG and ENEPIG on board side. The resulting degradation data suggests that ENIG is the best surface finish for applications involving long-term isothermal aging. ENEPIG ranks second, followed by ImAg. SAC305, with a higher relative fraction of Ag3Sn IMC within the solder, performs better than SAC105. SEM and polarized light microscope analysis show most cracks happened at package side, propagated from corner to center or even to solder bulk, which eventually cause fatigue failures. Three factors are discussed: IMC, Grain Structure and Ag3Sn particle. The continuous growth of Cu-Sn intermetallic compounds (IMC) and grains increase the risk of failure, while Ag3Sn particle seems helpful to block the crack propagation.