Analysis of thermal stresses in redistribution layer of WLP with different arrangement of interconnections

Thick polyimide film and electroplated Cu lines are widely adopted in redistribution layer of wafer level packaging. One potential reliability problem is the stresses generated in the thermal process. In this paper, the evolution of thermal stresses and plastic strain in two-level interconnections in redistribution layer of WLP was analyzed by recourse to the finite element analysis and Taguchi method. Cu lines embedded in polyimide with different aspect ratio, aligned vertically or arranged in a staggered manner were considered. Attention was devoted to the thermal stresses and plastic strain evolution and their dependency on the geometry structure. The constitutive response of Cu was taken to be elastic-plastic, with the post-yield behavior following the kinematic hardening model. The stresses and plastic strain are found to be a little higher in the low-level lines, for both aligned and staggered arrangements. A larger aspect ratio is helpful to reduce the stress, but it will generate large plastic deformation. The aspect ratio also plays a key role in wafer curvature and large aspect ratio will decrease the curvature.