Maskless screen printing technology for 20μm-pitch, 52InSn solder interconnections in display applications

Traditionally, ACF (Anisotropic Conductive Adhesive) technology has been used for CoG (Chip-on-Glass) and FoG (Flex-on-Glass) interconnections in display packaging area. The electrical contacts of ACF technology are based on the mechanical contacts between the electrodes on substrates and conductive particles in ACF. As pitches of these interconnections tend to get finer than 30 μm and bonding temperature needs to be decreased because of warpage concerns during the bonding process, a novel interconnection technology for the advanced display systems is necessary. In this paper, a maskless screen printing technology is proposed to form and bond 20 μm-pitch, 52InSn solder interconnections for advanced display systems. InSn solder is selected to decrease the bonding temperature because its melting point is 118°C. A novel material, called as solder bump maker (SBM) is developed to have InSn solder powder in SBM used for InSn bumping process. The polymer matrix and deoxidizing agent in SBM are carefully designed to make InSn solder powder in SBM wet on Cu or Au electrodes on a substrate during the bumping process. Since InSn solder powder resides only on electrodes on a substrate with temperature variations because of the surface tensions between the solder powder and metal electrodes, a maskless screen printing process can be adopted for the InSn, fine-pitch bumping process. Using a maskless screen printing process with SBM, 20 μm-pitch, InSn solder interconnections on a glass substrate are successfully formed. We, also, developed a no-flow underfill material, name as fluxing underfill, for a bonding material of InSn interconnections. It plays roles of flux and underfill at the same time during the bonding process. The bonding process for 20 μm-pitch, InSn solder interconnections is successfully achieved using fluxing underfill. Its peak temperature of the bonding process is 130°C.