Novel low-temperature processing of polymer dielectrics on organic substrates by variable frequency microwave processing

The demand for faster electronic devices with increased functionality has necessitated the use of new high performance dielectric materials with lower dielectric constant and higher thermal stability. Polyimides are most suited for these applications because of their excellent electrical, chemical, and thermo-mechanical properties. However, polyimides require long cure cycles at elevated temperatures, which are above the decomposition temperature of organic boards such as FR4 epoxy-fiberglass. Thus, it has not been possible to use high performance polyimides in conjunction with low-cost epoxy-based boards for applications such as microvia build-up layers. In this study, variable frequency microwave (VFM) processing was used to rapidly cure polyimides on organic boards. For the first time, rapid curing of a high performance polyimide on epoxy-based low-temperature organic substrates without degradation of the epoxy has been reported. Two different polyimide resins, BDPA-PDA and BTDA-ODA, were used for the cure studies. Substrate VFM heating rate characterization and thermal stability analysis were performed to identify the process window for VFM curing. FTIR studies showed no difference in the chemical structure of polyimide films cured on these substrates as compared to thermally cured films. For example, 98% imidization of polyimide BDPA-PDA was achieved after curing for 5 min at 200°C. The electrical and optical properties of the cured films were characterized and compared to thermally cured films. The results show that nearly complete imidization can be achieved by reacting the polyimide films for much shorter cure times and at lower temperatures as compared to films cured in a conventional thermal furnace