Effect of Cu/Al substrate thickness on the deformation and fatigue Life of Bumps Array

Since 3D-TSV technology has been more and more popular aiming at the demand of lightweight and multi-functions of electronic devices in recent years, the μ-C4 bumps with smaller size which interconnect the stacked die will suffer more serious operating stress, and even one of the thousands bumps fails, the device will fail. Hence, it is meaningful to study the reliability of smaller size and higher density bumps. In this paper, influence of the Cu/Al substrate thickness on the bumps stress, strain and fatigue life during thermal cycle is studied by finite element analysis (FEA), since Cu/Al have large coefficient of thermal expansion (CTE) and can induce large stress and strain on bumps. In this model, the bumps diameter and height are 15 μm, interval spacing is 100 μm, and selected substrate thicknesses are 1 mm, 2 mm and 4 mm, respectively. After 10 thermal cycles, it is observed that compared with the Cu substrate with different thickness, there is an approximate linear relationship between the sum displacement and substrate thickness, while the stress and strain have only little change. And this is same to Al substrate with different thickness. Compared Cu with Al substrate effect on bumps with thickness of 1mm, 2 mm and 4 mm, respectively, the ratio of the sum displacement and Δα of Al and Cu have approximate value, 1.35 and 1.43, respectively. Furthermore, the maximum effective stress nodes of bumps on two different substrates locate at different places: on the Al substrate, the max stress points locate on the edge of the bump-Al substrate and the value is 66.3 MPa; on the Cu substrate, the points locate on the edge of the bump-Si with the average value of 64.1 MPa.