Adhesive failure of rigid-flex printed circuit boards (RF-PCBs) during use has degraded the performance of military avionics systems. Adhesive failure is often caused by differences in the coefficient of thermal expansion (CTE) between the materials used in RF-PCBs and by moisture absorption by the adhesives and polyimide (PI) films. High-performance thermoplastics were investigated to replace the epoxies, PIs, and adhesives currently used in RF-PCBs. Because thermoplastic materials are remeltable, adhesive bonding may be replaced by fusion bonding to join RF-PCBs. Fusion bonding would eliminate problems with material compatibility and differences in the CTE encountered with adhesive bonding. Industries that would benefit from this research are manufacturers of aerospace instrumentation, medical equipment, automotive systems, computers, telecommunications equipment, industrial instrumentation and controls, and consumer products, such as stereo systems and calculators.
An extensive survey of high-performance-engineering thermoplastic materials was performed, and samples of the more promising materials (both films and chopped fiber reinforced) were obtained for preliminary screening tests. The tests performed were chemical resistance, water absorption, tensile strength, flexibility, and solder resistance of bare dielectric. From these tests, a glass-filled liquid crystal polymer film made by Hoescht Celanese Performance Films best met the performance criteria compared with the thermoplastics tested.