Critical interlayer thickness for transient liquid phase bonding in the Cu–Sn system Original Research Article

The study focuses on the critical interlayer thickness for averting pore formation during transient liquid phase (TLP) bonding in the Cu–Sn system. Such pores are a consequence of the growth and subsequent contact of Cu6Sn5 intermetallic grains on the two surfaces to be bonded, prior to the formation of the transient liquid phase. A criterion for the critical interlayer thickness is developed, based on the heights of the largest intermetallic grains. Experiments are performed to ascertain the growth kinetics and the morphology of the intermetallic as a function of heating rate. On this basis, TLP bonding experiments are designed and implemented to demonstrate the transition from pore-containing to pore-free bonds as the interlayer thickness exceeds the predicted critical value. The benefits of high heating rates on both the critical interlayer thickness and the bonding time needed to achieve a targeted terminal microstructure are illustrated.