Hybrid Experimental-Computational Approach for Solder/IMC Interface Shear Strength Determination in Solder Joints

Damage-based models for solder/intermetallics (IMC) interface often require the interface properties such as tensile and shear strengths. The minute size of the solder joint renders direct experimental determination of these properties impractical. This paper presents a hybrid experimental-computational approach to determine the shear strength of solder/IMC interface. Displacement-controlled ball shear tests are performed on as-reflowed and thermally-aged solder specimens. The observed sudden load drop in the load-displacement curve corresponds to the crack initiation event and the load is indicative of the shear strength of the solder/IMC interface. Finite element simulation of the ball shear test is then employed to establish the complex stress states at the interface corresponding to the onset of fracture. The finite element model consists of Sn40Pb solder, Ni₃Sn₄ intermetallic and Ni layers, copper pad and a rigid shear tool. Unified inelastic strain theory describes the strain rate-dependent response of the solder while other materials are assumed to behave elastically. Quasi-static ball shear test is simulated at 30°C with a prescribed displacement rate of 0.5mm/min. Results show that steep stress gradients develop in the shear tool-solder contact and solder/IMC interface regions indicating effective load transfer to the interface. The bending (normal) stress is found to be of the same order of magnitude as the maximum shear stress. Higher stress values are predicted near the leading edge of the solder/IMC interface. The equivalent shear stress condition to the triaxial stress state at the interface, represented by the absolute maximum shear stress, τ_max,abs should have reached the shear strength of the interface at fracture. The resulting shear strength of Sn40Pb/Ni₃Sn₄ interface is determined to be 87.5 MPa.