Studies on various 2-metal chip-on-flex (COF) packaging methods

Various chip-on-2-metal flex (2-metal COF) packaging methods using such as ACF and NCF adhesives, and AuSn metallurgical bonding methods, were investigated in terms of electrical characteristics, flip chip joint quality, and reliability performances. 2-metal flex substrate and test chip were designed to include different pitches, 35 um, 25 um, and 20 um pitch. Thermal cycling test (TC test, −40 °C ~ +125 °C, 1000 cycles), and high temperature storage test (HTS test, 125 °C, 1000 hrs) were conducted to verify reliability of the 2-metal COF packages by various bonding methods. All the COF packages showed good TC and HTS reliability, whereas electrically shorted joints were observed during reliability tests only at the 20 um pitch of ACF joints. Therefore, for less than 20 um pitch of 2-metal COF packages, NCF adhesives bonding and AuSn metallurgical bonding methods are recommended, while all the ACF and NCF adhesives bonding and AuSn metallurgical bonding methods can be applied for larger than 25 um pitch applications.