A Component Level Test Method for Evaluating the Resistance of Pb-free BGA Solder Joints to Brittle Fracture under Shock Impact

Abstract

A high-speed shear tester has been used to evaluate the interface strength of Pb-free solder balls with varying Ag and Cu content to a BGA laminate interposer with different finishes. The shear rate used is 0.45 m/s. Components with SAC405 and SnPb solder balls have been used as a reference. Results are compared with SAC305, SAC305 with NiGe addition, Castin 258 (2.5Ag-0.8Cu-0.5Sb), SAC105 (1.0Ag-0.5Ag), SAC101 (1.0Ag-0.1Cu-0.02Ni-xIn), SACX (0.3Ag-0.7Cu-0.1Bi), LF35 (1.2Ag-0.5Cu) and Sn3.5Ag. Electroplated NiAu finishes on the BGA laminate interposer studied come from different suppliers. Three failure modes are observed in the high speed shear test: brittle fracture in the intermetallic layer at the ball/interposer interface, ductile fracture in the solder bump and peel off of the solder pad from the interposer. Analysis of the fracture interface shows that almost all Pb-free solder alloys fail either in the intermetallic layer or by pad peel off. Only SAC101 and Sn3.5Ag show ductile fracture in the bump or pad peel off, but no fracture in the intermetallic layer. This is comparable with results for eutectic SnPb under these test conditions. A JEDEC board level drop test on same packaging with SAC101, SAC305 and SnPbAg shows that the number of drops to failure for packages with SAC 101 is also higher than those with SAC 405 and even slightly higher than with eutectic SnPb solder balls. Failure analysis is done to understand the difference in shock resistance with different solder bumps. From the observations in both of the tests, it is evidenced that the high speed shear test, when combined with proper analysis of the failure mode, is a simple but very powerful tool to evaluate the resistance of Pb-free BGA solder joints to brittle fracture under shock impact.