Effects of Aging on the Thermal Cycling Reliability of Ball Grid Array Assemblies

Lead-free free solders are highly susceptible to aging effects, where their mechanical behavior and failure properties degrade with time when exposed to isothermal or variable temperature environments. Such changes are caused by the unstable microstructures present in SAC (Sn-Ag-Cu) solders at very low temperatures, and they can lead to a significant reduction in the reliability of electronic products with time. In this work, we report on the effects of isothermal aging (preconditioning) on the board level reliability of plastic Ball Grid Array (BGA) assemblies subjected to subsequent thermal cycling accelerated life testing. We first summarize the prior work in this area found in the literature, and identify common findings as well as needs for further work. We then report on the initial results found in our ongoing studies on aging effects and qualification of lead-free electronics for military-grade applications. These investigations include several new features related to aging effects in BGAs. In particular, they include consideration of additional solder alloys (Sn-Pb, SAC305, Innolot = SAC + Bi + Ni +Sb); fine pitch small BGAs (0.4 mm and 0.8 mm joint pitch); multiple PCB surface finishes (ENIG and OSP); use of several aging conditions for preconditioning (1 and 6 months at 125°C, 6 months at 75°C); and subsequent thermal cycling at the ‒55 to 125°C range used for qualifying military rated electronics.