Damage Mechanics Model for Interface Fracture Process in Solder Interconnects

In this study, cohesive damage zone model is evaluated and employed to model solder/intermetallics (IMC) interface crack initiation and propagation in solder interconnects. Interface materials damage is quantified in terms of stress-to-strength ratios of orthogonal components in a quadratic failure criterion along with a mixed-mode displacement formulation for crack initiation event. Subsequent crack propagation is predicted based on fracture energy considerations. The mechanics of solder/IMC interface decohesion is examined through finite element modeling of a typical solder ball shear test. The 3D model consists of Sn40Pb solder, Ni₃Sn₄ intermetallics and Ni layers, copper substrate and a rigid shear tool. Unified inelastic strain theory describes the strain rate- and temperature-dependent response of the solder. The strength and work of fracture of the bi-material interface are derived from load-displacement data of solder ball pull tests and ball shear tests. The quasi-static solder ball shear test of reflowed solder sample is simulated at 30°C with a prescribed displacement rate of 0.01 mm/sec. Results show that complex stresses developed on the interface plane due to applied shear and induced bending effects by the shear tool clearance. A nonlinear damage evolution is predicted at each interface material point during the test. Stresses in the "fractured" material points diminish as the crack front progresses. The progression of damage indicates a straight crack front for the brittle solder/IMC interface fracture, as observed experimentally. The corresponding fractographic analysis on the sheared interface indicates that the crack initiated and propagated along the bi-material solder/IMC interface.