Deformation characteristics and microstructural evolution of SnAgCu solder joints

Three SnAgCu solder alloys (Sn2AgO.5Cu, Sn3.4AgO.8Cu, Sn4AgO.5Cu) have been tested to determine their deformation behavior in the temperature range 23-110°C, strain-rates varying 10^-7-10^-1 1/s. It is shown by optical micro-graphs of CSP solder joints that microstructure of SnAgCu may undergo through significant changes due to various loading conditions, which can occur during usage of microelectronic devices, such as thermal cycling, mechanical bending, and drop impact. The solidification microstructure consists typically of very large Sn-matrix colonies, with eutectic structure and intermetallic particles distributed within the colonies in cellular form. Typical observed temperature- and deformation-induced microstructural evolution includes recrystallization and twinning. The deformation mechanisms of the alloys have been predicted based on the values of measured activation energies and stress exponents. The constant stress and constant strain-rate tests have been performed in the shear configuration, which enables a stress-state of nearly pure shear in the solder joint. In the intermediate stress regime, the deformation appears to occur by the slip mechanism, and the rate is likely to be controlled by the dislocation climb process. The measured shear stress-strain data are utilized to determine the constants for the visco-plastic Anand´s constitutive model.