Laminar pattern induced by cycling thermomechanical stress in two-phase materials

This article reports on the highly interesting microstructural evolution from a fine eutectic to a coarse laminar pattern caused by the cycling thermomechanical stress in eutectic PbSn flip-chip solder joints. The temperature was cycled between −55 and 125 °C for up to 2000 cycles. The shear strain on the corners joints, which experienced the maximum thermomechanical stress among all the solder joints in a flip-chip package, was estimated to be 0.13 for each cycle. A coarse laminar pattern, made up of the Pb-rich phase and the Sn-rich phase, gradually developed during the temperature cycling with phase boundaries that were approximately parallel to the direction of the shear stress. It is argued that the laminar pattern had developed in such a manner that the material with the lowest yield stress (the Pb-rich phase) could align itself with the direction of the shear stress. Because continuous soft layers were arranged in a direction that was approximately parallel to the direction of the shear stress, the stored deformation energy could be dissipated more efficiently.