Process optimization to overcome void formation in nonconductive paste interconnections for fine-pitch applications

Nonconductive paste (NCP) is becoming an alternative material for chip-on-flexible (COF) substrate, where it is able to achieve a very fine-pitch interconnection for high resolution products. The application of NCP faces the problem that it is initially a high viscosity liquid, which easily causes voids in the interconnection. Voids in the interconnection reduce the mechanical strength in the joints and create moisture entrapment sites, which shorten the reliable performance of the interconnection. They were revealed using optical microscopy and nondestructive C-mode scanning acoustic microscopy (C-SAM). This study reveals the factors involved during process control in order to minimize void entrapment in the interconnection. These parameters include the following: (a) substrate pretreatment, (b) the speed at which the tool heater descends, (c) bonding force, (d) bonding temperature, and (e) holding time on the bonding stage. These parameters permitted a modification to the surface, and also brought chemical reaction and viscosity changes to the NCP during the bonding process, which leads to the minimization of void formation in the specimen.