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

Abstract

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.