Development of low temperature Chip-on-Flex (COF) bonding process of 100°C

Recently, many researchers have introduced low temperature bonding technology using Anisotropic conductive film (ACF) or Nonconductive adhesive (NCA). In their studies, the bonding temperature is in the range between 150°C and 200°C. In this study, we developed a Chip-on-Flex (COF) bonding process of 100°C by using Sn-Ag bumps and nonconductive film (NCF). Sn-Ag bumps were formed by electroplating and reflowed to form dome shape. The COF bonding was performed between Sn-Ag bumps and Cu/Polyimide film substrates using a thermo-compression bonder at 100°C for 5 s. The low temperature curable NCF was applied during the bonding process. The Sn-Ag bumps were deformed and direct contact was made between Sn-Ag bumps and Cu/PI substrate during thermo-compression bonding. The initial contact resistance of all joints was less than 30 mΩ, and no COF joints failed electrically. To evaluate reliability of COF joints, Temperature & Humidity (T&H) test (85°C/85% RH) was performed for 1000 hr. The contact resistance was increased during reliability test. However, the failed joints were not observed after T&H test. The contact resistance change will be discussed in terms of microstructure change in the COF joints.