Au-Rich/Sn-Bi Interconnection in Chip-on-Module Package

A new package structure for CMOS image sensor (CIS) is proposed as COM (Chip on Module), based on a novel soldering wire bonding technique whose second bond point is achieved by soldering with the help of eutectic Sn-57Bi solder paste, forming the Au-rich/Sn-Bi interconnection which is the focus of this paper. The reliability of the interconnection has been confirmed by functional tests, pull tests and interfacial observation after high temperature and humidity storage (THS) reliability tests at 85°C/85%RH for 120h, 240h, 480h and 1000h. All COM packaged samples have passed the camera functional tests after aging. The tensile strengths of the gold wires, which have finished soldering wire bonding, always maintain above 9 grams during aging, which is higher than the standard strength in conventional wire bonding (8 grams). Furthermore, as for the interfacial structures in Au/Sn-Bi couple, AuSn and AuSn
₂
are found at the interface, whose thickness can be described as a parabolic relationship with the annealing time, suggesting a bulk diffusion controlling mechanism. Besides, the effects of 0.3%~0.35% Ag addition, in Sn-57Bi solder, on the reliability of the interconnection have also been investigated. Owing to the barrier effects of Ag and Ag
₃
Sn on the diffusion of Au atoms into Sn-contained solder, a more reliable joint has been obtained during aging, with a higher tensile strength (above 19 grams), thinner IMC thickness (20%~30% thinner) and unobvious Kirkendall voids at the interface.