Effects of the Thermocompression Bonding on the Microstructure and Contact Resistance for the Ultrafine Pitch Chip-on-Glass Packaging With Nonconductive Film

Nonconductive film (NCF) is a challenging potential material to substitute the application of anisotropic conductive film in the ultrafine pitch chip-on-glass (COG) packaging. The NCF interconnection requires a high bonding temperature and pressure to form joints, and this causes new reliability concerns. This study investigated effects of the thermocompression bonding parameters on the microstructure and geometric size in the joints to a COG module packaged with NCF. The results revealed that the high temperature and pressure compressed the joints to become wider and shorter. A dual layer of intermetallic compounds consisting of $AuSn2$ (⁠$ε$ phase) and $AuSn4$ (⁠$η$ phase) was found in each joint. They were the two kinds of interphases with different melting points (⁠$AuSn2:309°C$ and $AuSn4:257°C$⁠) during the interfacial reaction between Au and Sn. At the low temperature (below the melting point), the high pressure induced the residual inner stress to generate the cracks in the joints, and this also increased the contact resistance of the joints. The contact resistance increased with the pressure elevating at the same temperature and with the temperature degrading at the same pressure. In the COG packaging with NCF, a proper elevating of the bonding temperature could produce a stable direct connection with the low contact resistance.