The effect of Au plating thickness of BGA substrates on ball shear strength under reliability tests

Each metal layer plated on BGA substrates has its own specific functions. The Ni layer is used as a barrier to prevent Cu from diffusing into solder balls, as the Cu interacts with Sn to form Sn-Cu intermetallic compounds (IMC). The IMC will then affect the shear strength of solder balls. A thin Au layer is also plated on top of the Ni to prevent Ni layer oxidation. The Au layer also provides excellent wetting capability for Sn that is very beneficial for the surface mounting process. However, as Au diffuses into solder balls to form Au-Sn IMC at elevated temperatures, this could decrease the solder joint fatigue life. The effect of the Au plating thickness on the ball shear strength under reliability tests therefore becomes very interesting. This work probes the effects of Au plating on ball shear strength under various reliability tests and the solder ball failure mechanisms. The shear strength of solder balls on BGA substrates with three different Au plating thicknesses is studied. The specimens are first assembled and preconditioned and then undergo high temperature storage (HTS) tests, thermal cycle tests (TCT), thermal shock tests (TST) and pressure cooker tests (PCT). The solder ball shear strengths are measured after assembly, pre-conditioning, and each reliability test. SEM and EDX are used to analyse the failure mechanisms. The solder ball shear strength degradation after each reliability test is discussed. The effect of Au thickness on the shear strength and the solder ball failure mechanism in each case are also discussed.