Solder joint reliability evaluation of Sn–Zn/Au/Ni/Cu ball-grid-array package during aging

This study investigated the interfacial reaction between eutectic Sn–9 wt.% Zn solder and Au/Ni electroplated Cu BGA substrate during aging at temperatures between 70 and 180 °C, as well as the shear strength of the resulting joints. After reflowing, the intermetallic compound (IMC) formed at the interface was AuZn3. The formed AuZn3 layer became detached from the interface during reflowing. In the case of aging at 70 °C, only an AuZn3 IMC layer was observed, whereas after aging for 2400 h at 100 and 120 °C, the Ni5Zn21 IMC was also observed on the Ni substrate. In the cases of aging at 150 and 180 °C, AuZn3 and Ni5Zn21 IMCs were observed at all the interfaces. The thickness of the detached AuZn3 layer did not change with increasing aging temperature and time, whereas that of the Ni5Zn21 IMC layer formed on the Ni layer increased with increasing aging temperature and time. After aging at 150 and 180 °C, the detached AuZn3 IMC layers were separated into AuZn3 and (Au, Ni)Zn3. After aging at 180 °C for 2400 h, the Ni5Zn21 IMC grew more extensively near both edges of the substrate and the entire Ni layer was completely consumed. In addition, the Cu5Zn8 phase was formed between the Ni5Zn21 IMC and Cu substrate. The shear strength of the aged Sn–9Zn/Au/Ni/Cu joint was significantly related to the detachment of the AuZn3 IMC layer and all fracturing occurred on the detached AuZn3 IMC layer. Therefore, this detachment of the AuZn3 layer must be prevented to ensure the reliability of the Sn–Zn/Au/Ni/Cu joint.