Effect of Isothermal Aging on Ball Shear Strength in BGA Joints with Sn-3.5Ag-0.75Cu Solder

Abstract

The ball shear strength of BGA solder joints during isothermal aging was studied with Sn–3.5Ag–0.75Cu solder on three different pads (Cu, electroless Ni–P/Cu, immersion Au/Ni–P/Cu) at temperature between 70 and 170°C for times ranging from 1 to 100 days. The reliability of solder ball attachment was characterized by mechanical ball shear tests. As a whole, the shear strength of BGA joints decreased with increasing temperature and time. The shear strength for both the immersion Au/Ni–P/Cu and electroless Ni–P/Cu pads was consistently higher than that of the Cu pad for all isothermal aging conditions. The fracture surface showed various characteristics depending on aging temperature, time, and the types of BGA pad. The P-rich Ni layer formed at the interface between (Cu, Ni)6Sn5 and Ni–P deposits layer after aging, but fracture at this interface was not the dominant site for immersion Au/Ni–P/Cu and electroless Ni–P/Cu pad.