Thermal stress analysis of die stacks with fine-pitch IMC interconnections for 3D integration

The thermo-mechanical reliability of stacked die structures is a critical issue in 3D packaging. The assessment of the stress and the warpage of silicon dies in 3D stacked structures become important in achieving low-stress and low-warpage 3D packaging. However the parametric analyses of thermal stress and die-warpage by rigorous finite element analysis can be time consuming for 3D systems, since it involves many layers of materials such as silicon dies and organic layers. In this paper, we used the finite element method (FEM) with a simple 2D model to analyze the stress under thermal cycling condition on the die stack system and applied the 1D multilayered beam theory to perform parametric analyses of the die-warpage for the thermal stress condition. We used a 3D slice model to analyze the stress in the intermetallic compound (IMC) joints. The die-warpage values and the high stress sites in stacked structures obtained by these analyses were consistent with the measured data and experimental observations from the thermal cycle tests on full-area-array 40 μm bump pitch stacked die test vehicles with intermetallic compound joints.