Effects of silver addition on Cu-Sn microjoints for chip-stacking applications

The solid-state reactions under confined space between Cu substrate and Sn and Sn-Ag solders are systematically investigated, with the objective of providing fundamental data that are critical to the design of robust microjoints for chip-stacking applications. Sandwich structures of Cu/Sn/Cu and Cu/Sn3.5Ag/Cu are prepared by thermal compression bonding. The thickness of Sn and Sn3.5Ag layer is controlled at 10 μm. High temperature storage tests are conducted at 120 °C, 150 °C, 180 °C, and 200 °C for different time periods. Unlike the well-known void formation in the Ni/Sn/Ni system, there is no such void at all in the Cu/Sn/Cu system. The addition of Ag consistently slows down the growth kinetics of intermetallics. The time-to-impingement of intermetallics is established for the first time for both systems under solid-state reactions.