This study investigated the interface microstructure that developed between 50In-50Pb (wt%) solder and copper (Cu) base material as a function of solid-state aging. The aging temperatures and times were in the range of 55°C – 170°C and 1 – 350 days, respectively. The analysis examined the intermetallic compound (IMC) layer compositions; the rate kinetics of IMC layer growth; and the role of the IMC layer on solder joint shear strength. The IMC layer transitioned from pseudo-equilibrium compositions towards an equilibrium composition of Cu11In9 (φ phase) with an increased degree of aging, illustrating the non-equilibrium nature of the interface. The rate kinetics for solid-state IMC formation exhibited a time exponent, n, of 0.47±0.09, which indicated a diffusion-controlled reaction. The relatively low, apparent activation energy, ΔH, of 23±4 kJ/mol implied an anomalously-fast diffusion mechanism. The shear stresses were 22±2 MPa and 19±1 MPa for the 0.190 mm and 0.380 mm joint clearances, respectively, representing the as-fabricated condition; the difference reflected the plane strain effect. The crack path remained in the In-Pb solder so that the In-Pb microstructure, not the thickness, composition, or morphology of the IMC layer, controlled shear strength for either joint clearance. The shear strength trends differed between joint clearances due to competing processes in the In-Pb solder. Precipitation and re-solutionization of Cu dissolved in the In-Pb solder controlled the effects of aging on the shear strength of the 0.190 mm joint clearance while traditional recovery and recrystallization mechanisms determined the aging response of the 0.380 mm solder joints.
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