Influence of material property of photosensitive polyimide on typical structural stress in redistribution layer of fan-out package

Abstract

Photosensitive polyimide (PSPI) has been widely used in advanced microelectronic packaging as a buffer coating, passivation layer, alpha particle barrier and interlayer insulator due to its excellent photosensitivity, heat and chemical resistance and low dielectric constant. In recent years, with the rapid development of 5G, artificial intelligence, IoT and autonomous driving, PSPI has become one of the dominant photosensitive dielectrics in fan-out package redistribution layer (RDL) fabrication due to its greatly simplified lithography process. RDLs are mainly composed of PSPI and Cu, interconnected by Cu wires and different forms of hole structures, and protected by PSPI. However, due to the different coefficients of thermal expansion (CTE) between the PSPI and Cu, a series of reliability problems such as fracture failure of the RDL structure, delamination and warpage at the interface between the PI and Cu layers may occur in practical applications. In this paper, the effect of CTE and modulus of PSPI material on the stress of RDL structure is studied by finite element analysis (FEA). Meanwhile, the effects of temperature-dependent modulus and elastoplastic mechanical behavior of PSPI materials under different loading rates are further studied. The results show that as the maximum Mises stress decreases linearly with the decrease of CTE and modulus. And, in the temperature-dependent elastic model, the change in modulus during the simulation process affects the maximum Mises stress at each step, but has little effect on the final simulation results. Besides, the input of the data from plastic section of the strain-stress curve of PSPI has no effect on the simulation results because the maximum Mises stress of PI layer value is far below the stress of its plastic deformation. At last, the tensile curves at 1-16 N/min exhibit similar results, indicating that there is no significant difference in the maximum Mises stress.