Mechanical test after temperature cycling on lead-free Sn-3Ag-0.5Cu solder joint

The solder balls on the plastic ball grid array (PBGA) packages provide both electrical input/output and mechanical supports. Mechanical properties and strengths of solder balls significantly influence the PBGA package reliability. Temperature cycling test has been widely used in flip-chip technology (FCT) in order to simulate the failure mode due to fatigue and aging under repeatedly heat treatment. In this study, ball shear test was employed for measuring mechanical properties in the joint of the Sn-3Ag-0.5Cu solder ball(phi =100mum) attached to Ti/Ni(V)/Cu UBM. The alloys studied included Sn-3Ag-0.5Cu and baseline eutectic Sn-37Pb. FE-EPMA was used to quantitatively analyze intermetallic compounds and to observe fracture surface. After repeatedly heat treatments, there are some differences on fracture surfaces between Sn-3Ag-0.5Cu and Sn-37Pb joint. The fracture surface of Sn-37Pb joint was ductile after TCT treatment. Only tin rich and lead rich phase were observed in the solder matrix of Sn-37Pb system, and no precipitated compound formed during the heat treatment. The force of shear test was almost the same after 1000 times TCT. In the Sn-3Ag-0.5Cu system, Ag would dissolve in Sn to form Ag₃Sn. There were (Cu, Ni)₆Sn₅ and (Cu, Ni)₃Sn IMC formation between Sn-3Ag-0.5Cu solder and Ti/Ni(V)/Cu UBM. After TCT treatment, the growth of (Cu, Ni)₆Sn₅ IMCs would affect the fracture surface. In this study, the compounds of Ag₃Sn and (Cu, Ni)6Sn5 in the Sn-3Ag-0.5Cu joint grew slowly. After shear test, the fracture surface appeared brittle. The shear strength of Sn-3Ag-0.5Cu was higher than Sn-37Pb. The relationships between microstructure and strength of the joints as functions of thermal cycling test (TCT) cycle were investigated and discussed.