I. Introduction
Polyimide is a class of aromatic heterocyclic polymers containing an imide ring in a repeating unit of a molecular chain. The polymers and their composites have been proved with excellent comprehensive performance, including heat resistance, radiation resistance, aging resistance, solvent resistance, dimensional stability, mechanical properties, insulation properties, dielectric properties, and etc [1]. Thus, polyimides are widely used in aerospace, engineering plastics, insulating coatings and especially in the field of microelectronics, such as interlayer dielectric material (ILD) [2] in the redistribution layers (RDLs).