Characterization of Lead-Free Solders in Flip Chip Joints

The creep and crack propagation behavior of SnAg3.5, SnAg4Cu0.5, and SnPb37 (as reference) was investigated on flip chip solder joints V=1×10−12 m3. The test specimen consisted of two silicon chips (3.3×3.3 mm), bonded to each other by four flip chip joints (one on each corner). The steady-state creep rate was determined by reversible constant load shear tests. The stress exponents were n=11 for Sn96.5Ag3.5, n=18 for Sn95.5Ag4Cu0.5, and n=2 for Sn63Pb37. The apparent activation energies were Q=79.8 kJ/mol for Sn96.5Ag3.5, Q=83.1 kJ/mol for Sn95.5Ag4Cu0.5, and Q=44.9 kJ/mol for Sn63Pb37. Microstructural analyses indicated that small precipitates of Ag3Sn and η-Cu6Sn5 intermetallics are responsible for the high values for n and Q that were found for the Sn96.5Ag3.5 and Sn95.5Ag4Cu0.5. The crack growth rate was determined by isothermal fatigue experiments on Sn63Pb37 and Sn95.5Ag4Cu0.5 flip chip solder joints. The flip chip solder joints were loaded with strain amplitudes ranging from Δε=0.3–4% and test frequencies f=0.0001-100 Hz at a temperature of T=300 K.