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

•Highly faceted Cu6Sn5 IMC are observed during solid-state interfacial reactions.•The “time-to-impingement” of intermetallics is established for the first time under solid-state reactions.•The addition of Ag slows down the growth kinetics of intermetallics. 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 10μm. 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.