Role of Process and Microstructural Parameters on Mixed Mode Fracture of Sn-Ag-Cu Solder Joints Under Dynamic Loading Conditions

Electronic packages in mobile devices are often subjected to drops, leading to impact loading. Since solder joints, which serve as mechanical and electrical interconnects in a package, are particularly prone to failure during a drop, the fracture behavior of solders at high strain rates is a critical design parameter for building robust packages. Here we report on a methodology for measuring mixed-mode fracture toughness of Sn3.5Ag0.7Ag (SAC387) solder joints under dynamic loading conditions (at strain rates up to 100s⁻¹), and use this method to investigate the role of solder microstructure and interfacial intermetallic compound (IMC) layer thickness on the joint fracture toughness at different mode-mixities and strain rates. Modified compact mixed mode (CMM) samples with adhesive solder joints between Cu plates and a thin film interfacial starter crack were used for the measurements. The interfacial IMC layer thickness was adjusted by controlling the dwell time during reflow, while the solder microstructure was controlled via the post-reflow cooling rate and subsequent thermal aging. The critical strain energy release rate (G_c) was measured as a function of these microstructural and loading variables, and these data were correlated with the associated crack path, details of which were elicited through fractography as well as crack-profile observations. The crack profile studies were based on samples with double interfacial starter cracks, one of which propagated only partially. Associated with the alteration of the joint microstructure, transitions in the fracture behavior were noted. In all cases, the cracks remained confined to the interfacial region, although the details of the crack propagation path and its interaction with interfacial IMCs, the adjacent solder and the pad surface finish varied significantly. Fracture toughness decreased with an increase in the strain rate and decreased with increasing mode-mixity. A thicker/coarser interfacial IMC layer (due to high dwell times) decreased toughness, while coarser solder microstructures (due to slow cooling during reflow or post-reflow aging) increased toughness. Correlations between joint microstructure and the observed deformation and fracture mechanisms will be highlighted, and a qualitative model based explanation for the inter-play between solder and IMC, and the associated interfaces will be presented.