Interfacial reactions and joint reliability of Sn-9Zn solder on Cu or electrolytic Au/Ni/Cu BGA substrate

The interfacial reactions and shear strength of joints between Sn-9Zn solder and two different kinds of BGA (ball-grid-array) substrates (Cu and Au/Ni-P/Cu) were investigated for reflow conditions of 230 °C up to 60 min. For the Cu substrate, only Cu5Zn8 intermetallic compound was observed and other Cu-Sn IMCs were not observed. For the Au/Ni/Cu substrate, an AuZn3 IMC layer formed at the interface due to the fast reaction between Au and Zn. In addition, some of the AuZn3 IMC layer was detached from the interface after reflow. The thickness of the AuZn3 layer did not change with reflow time because the Au was consumed during the initial reflow process. After being reflowed for 60 min, a Ni5Zn21 IMC formed between AuZn3 IMC and Ni substrate. Also, many voids existed inside the upper AuZn3 IMC layer. Ball shear tests for the Cu substrate showed the shear strength significantly decreased after reflow for 60 min. A Cu5Zn8 IMC was also observed on the fracture surface. For the Au/Ni/Cu substrate, the shear strength value did not change much as a function of reflow time and fractures occurred within the solder.