Mechanisms of Creep Deformation in Pure Sn Solder Joints

The work reported here concerns the creep of pure Sn solder joints with Cu metallization (Cu||Sn||Cu). Steady-state creep tests in shear are combined with electron backscatter diffraction (EBSD) analysis of the evolution of the microstructure during creep to clarify the deformation mechanism and the nature of the microstructural evolution. The creep behavior of the joint changes significantly with temperature. At low temperature (65°C), two distinct creep mechanisms are observed. Low-stress creep is apparently dominated by grain boundary sliding, as evidenced by the low stress exponent (n ≈ 4), low activation energy (Q ≈ 42 kJ/mole), and significant grain rotation during creep. High-stress creep is dominated by bulk deformation processes, evidenced by a high stress exponent (n ≈ 9), an activation energy like that for bulk diffusion (Q ≈ 70 kJ/mole), and a relatively fixed microstructure. At high temperature all aspects of its behavior are consistent with deformation by bulk creep mechanisms; the stress exponent and activation energy are high (n ≈ 5 to 7, Q ≈ 96 kJ/mole), and despite significant grain coarsening, the microstructure retains (and strengthens) a fixed 001] texture. The results suggest that a “segmented” constitutive equation of Dorn type is most suitable for the low-temperature behavior, while a “hyperbolic” constitutive equation may be preferable at high temperature.