Solder cracking mechanism correlation to alloy composition under thermal cycling stress (part 1)

Interconnect reliability of Micro BGA in enterprise and portable electronics were studied. Two Micro BGA with similar dimension but different alloy compositions were used in the life testing to observe crack propagation mechanism under thermal cycling stress. Standard metallurgical x-section, dye penetration and SEM were performed at time zero as well as after thermal cycling on 1st pass Micro BGA-solder-PCB interconnect system as well as on reworked. Solder crack propagation path along substrate-solder and solder-PCB pad interface were observed on both alloy systems, SAC305 and SAC125Ni. The stochastic trend of failure interface was rationalized with CTE variation and phase recrystallization in microstructure. Observation of crack initiation at grain boundary or triple-grain junction and recrystallization induced grain growth and coarsening were referred to earlier study. The study indicate that the Interconnect life under thermal cycling stress and cracking propagation were highly correlated to alloy composition but less related to 1^st pass SMT reflow or low cycle reworked process. The crack initiation mechanism and second phase strengthening correlation to phase dispersion are in-depth discussed.