Thermomechanical Behavior and Reliability of Redistribution Layers with Ni-plated Cu Traces for High Power Applications

The re-distribution layer (RDL) has been widely used in various high-performance applications because it greatly increases package density by rearranging the I/O terminals of the chip to an area with available space. However, the increase in current density and frequency in the Cu trace layer generates a large amount of Joule heat, especially in high power scenarios, which causes internal stresses in the RDL due to the mismatch of coefficients of thermal expansion (CTE) of different materials in the RDL, leading to reliability issues such as interface delamination between Cu trace and polyimide (i.e., Cu/PI interface), wafer warpage and so on. Aiming at providing guidance for the structural optimization of the Cu RDL in high-power applications, this study performs 3D finite element simulations to characterize the thermomechanical behavior of conventional Cu RDLs in fan-out wafer-level packages at different currents and Cu trace thicknesses, so as to predict potential failure sites and modes. A novel Ni-plated RDL is also used to comparatively investigate the effects of the ratio of Cu to Ni and the total thickness of the metal trace layer on the thermomechanical behavior of the Cu-Ni RDL. The results show that both Cu and Cu-Ni RDLs exhibit current crowding and stress concentration at straight trace sections and pad corners, with the maximum stresses appearing at pad edges. The interfaces between the metal trace, under bump metallization (UBM), and solder balls are critical for RDL reliability. For Cu RDL, increasing the thickness of the Cu trace can reduce current density and temperature, but result in an increase in maximum stress. So, a suitable Cu thickness needs to be selected for high-power applications. Besides, the raise of Ni content leads to the increase of the maximum stress and maximum current density in Cu traces, while the maximum temperature depends on its thickness. However, due to the superior adhesion of Ni to PI, its effect on interfacial reliability needs to be further investigated.