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The fatigue fracture behaviors of a series of Cu/lead-free solder joints deformed under different directions of loadings were investigated in this study. Observation results showed that fatigue cracks generally initiate around the IMC/solder interface when the loading axis is vertical to the interface. For all the solder joints, the interfacial deformations are resulting from strain localization induced by the stain mismatch. Fracture surface observations reveal that the crack propagation path and fatigue resistance of the solder joints are affected by the yield strength and mechanical property of the solder. When the copper/solder interface is parallel to the loading axis, the interfacial IMC layer failed approximately perpendicular to the interface under the action of slip bands, and then the cracks propagated to the IMC/solder interface, leading to the fracture along the interface.

With the miniaturization of the solder joint, the dependence of the property on the microstructure orientation has become increasingly notable. In this work, a lead-free Sn3.8Ag0.7Cu solders with certain microstructure orientation were processed by the Bridgman method in a horizontal furnace. After tensile tests at different strain rates, it was found that the tensile strengths of the materials were strongly dependent on the growth direction. The strength of the sample with a gauge perpendicular to the growth direction was much lower than that parallel to the growth direction. While the radial Cu6Sn5 and Ag 3 Sn IMCs were distributed uniformly around the equiaxed tin grains perpendicular to the growth direction, the well-arranged needle shaped IMCs were oriented along the growth direction. At an applied tensile strain of 20%, the cross-slip lines were found throughout the polished surface and then impinged to the IMCs, resulting in the breaking of the IMCs. The variation of the tensile property with microstructural orientation may be attributed to the modification of the mobile slip system and the interaction of the slip lines with the IMC phase.