Study of Interactions Between RDL Polyimides and Underfills on Reliability of Flip-Chip Interconnects in Thermal Cycling

Redistribution layers (RDL) have been widely used in advanced microelectronic packaging to improve routing and enable better integration. Polyimide is used in RDL as a buffer coating, passivation layer, alpha particle barrier and interlayer insulator due to its heat and chemical resistance and low dielectric constant. With increasing requirements for speed, advanced flip-chip packages are often designed with Cu-pillar bumps on RDL [1] and polyimide passivation layers. RDL allows a designer to create bumps away from original pad locations without making significant changes to the original device. However, this approach could pose significant reliability challenges on copper pillar bumps in flip-chip packages due to the higher stiffness of copper pillar relative to standard solder bump. During temperature cycling tests and high temperature storage, greater stress could induce cracking in polyimide of RDL and Cu trace due to a thermal mismatch between the chip and substrate [2]. In this paper, the effect of Coefficient of Thermal Expansion (CTE) and modulus of polyimide of RDL structures on stress inside flip chip packages with different underfill materials were studied using Finite Element Analysis (FEA). A simple design of experiments (DOE) study to compare a commercial high temperature polyimide and a low temperature PFAS (Per- and polyfluoroalkyl substances) free polyimide with several underfill materials was conducted. Major factors impacting bump failure including underfill material properties, polyimide material properties, die thickness, bump height, etc. are also studied. Results of the study demonstrate a good correlation between simulation results and experiments. The study reveals that PFAS free low temperature polyimide material performed significantly better than high temperature polyimide. The use of the low temperature polyimide material showed improved resistance to common failure modes, such as polyimide crack and delamination.