Low temperature fine pitch Flex-on-Flex (FOF) assembly using nanofiber Sn58Bi solder anisotropic conductive films (ACFs) and ultrasonic bonding method

For few decades, anisotropic conductive films (ACFs) consist of conducting particles and adhesive polymer resin have been widely used for manufacturing electronic devices due to its fine pitch capability, light weight and thin structure. However, as the electronic devices became more advanced, the electrode pitch size became extremely smaller. This miniaturization resulted in more complicated interconnection problems such as short circuit and open circuit. And in most ACF bonding process, high bonding temperature is necessary for full ACF resin curing and stable electrical joint formation. Therefore, lowering the bonding temperature is more desirable for reducing damages, increasing productivity and most importantly lowering the process cost. As a result, low temperature conductive particle incorporated nanofiber Sn58Bi solder ACFs were introduced in order to increase the reliability and productivity while preventing any interconnection problems in fine pitch assembly. In this study, nanofiber Sn58Bi solder ACFs using low melting temperature Sn58Bi solder particles were investigated with respect to electrical/mechanical properties and reliability. Nanofiber solder ACFs are composed of polymer adhesive resin and conductive particle incorporated nanofibers. In order to lower the bonding temperature below 200 °C, we use Sn58Bi (42 % Sn and 58 % Bi) solder particles with a low melting temperature of 138 °C. The conductive particles were incorporated with variety of polymers such as PAN, and PVDF nanofibers by using a technique called “electro-spinning”. The nanofiber solder layer with a thickness below 30 μm was then laminated between two non-conductive films (NCFs) through roll and vacuum lamination to produce nanofiber Sn58Bi solder ACFs. As for FOF bonding, ultrasonic bonding method was used to remove solder oxides while reducing the bonding times. As a result, by incorporating low melting temperature Sn58Bi solder particle, 100μm fine pitch FOF bonding was successfully performed below 200°C with stable contact resistance and joint formation. And incorporated solder particles with nanofiber showed excellent insulation effects which eventually prevents short circuit between neighboring electrodes in FPCs.