Retardation of the Cu5Zn8 Phase Fracture and Improvement of Shear Strength of the Sn-9 wt pct Zn/Cu Interface by Inserting an Electroless Palladium Layer

A uniform layer of the Cu5Zn8 phase is formed at the Sn-9 wt pct Zn (Sn-9Zn)/Cu interface after reflow at 503 K (230 °C) but fractures very severely during subsequent solid-state aging at 443 K (170 °C). Such a severe fracture of an interfacial intermetallic compound deteriorates the integrity of joint interface and leads to excessive atomic interdiffusion as well as intermetallic compound growth. In the current study, it was observed that by inserting an electroless palladium (Pd) layer at the Sn-9Zn/Cu interface, a Pd2Zn9 phase is formed between Cu5Zn8 and Sn-9Zn solders. This Pd2Zn9 phase exhibits higher microstructural stability at the interface during solid-state aging and effectively retards the fracture of the Cu5Zn8 phase. The integrity of the Sn-9Zn/Cu interface is improved remarkably. Mechanical tests reveal that the shear strength of the Sn-9Zn/Cu joint is enhanced by inserting an electroless Pd layer at the interface.