Solder joints in electronic assemblies are subjected to mechanical and thermal cycling. These cyclic loadings lead to the fatigue failure of solder joints involving damage accumulation, crack initiation, crack propagation, and failure. Aging leads to significant changes on the microstructure and mechanical behavior of solder joints. While the effect of thermal aging on solder behavior has been examined, no prior studies have focused on the effect of long-term room temperature aging (25 °C) on the solder failure and fatigue behavior. In this paper, the effects of long-term room temperature aging on the fatigue behavior of five common lead-free solder alloys, i.e., SAC305, SAC105, SAC-Ni, SAC-X-Plus, and Innolot, have been investigated. Several individual lead-free solder joints on printed circuited boards with two aging conditions (no aging and 4 years of aging) have been prepared and subjected to shear cyclic stress–strain loadings until the complete failure. Fatigue life was recorded for each solder alloy. From the stress–strain hysteresis loop, inelastic work and plastic strain ranges were measured and empirically modeled with the fatigue life. The results indicated that 4 years of room temperature aging significantly decreases the fatigue life of the solder joints. Also, inelastic work per cycle and plastic strain range are increased after 4 years of room temperature aging. The fatigue life degradation for the solder alloys with doped elements (Ni, Bi, Sb) was relatively less compared to the traditional SAC105 and SAC305.
The most well-known and widely observed microstructural changes during aging are the coarsening of Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) present in the eutectic regions between β-Sn dendrites. In this investigation, Scanning Electron Microscopy (SEM) has been utilized to examine aging induced coarsening of IMCs occurring within lead free solders. Unlike many prior studies, fixed regions in the solder joint cross-sections were monitored throughout the aging process, rather than examining different samples and/or different regions after the various aging exposures. Sn-3.0Ag-0.5Cu (SAC305) lead free solder samples were formed with reflowed (RF) and water quenched (WQ) cooling profiles and resulting initial microstructures, and then polished microscopy cross-sections were prepared. Nanoindentation marks were added to the cross-sections at certain locations to facilitate locating the fixed regions of interest in subsequent microscopy observations. After preparation, the samples were then aged at T = 125 °C, and the microstructures were observed and recorded in the selected regions after various aging exposures using SEM. In addition, the coarsening of IMCs during aging has been quantitatively analyzed. Particularly, the aging induced changes in number of IMCs, total area of all IMCs, average particle area, and average particle diameter have been quantified for fixed regions in the samples.
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