Characterization of the microstructure of tin-silver lead free solder

Reliability and lifetime are the two most relevant design considerations in the production of safety critical assemblies. For example in a modern automobile dozens of electronic assemblies are integrated in which thousands of solder joints are mounting the electronic components to the printed circuit boards. There exists no standardised and universal observation method for characterising the fine micro structure of such solder joints. Previously we have developed a new method for the quantitative characteriz ation of lead - free solder alloys and in present study the validity of the proposed method is demonstrated. M icrostructure of Sn - 3.5Ag lead free solder alloy was investigated by electrochemical impedance spectroscopy. Solder samples were solidified with dif ferent cooling rates in order to induce differences in the microstructure. M icrostructure of the ingots was revealed by selective electrochemical etching. Electrochemical impedance spectr a (EIS) were measured before and after the selective etching process. The complex impedance spectra contain information about microstructure of the solder alloys. Comparison and mode l ling of two EIS spectra allowed obtaining a characteristic parameter of surface structure of the etched specimens. The EIS measurements were c omplemented with small angle neutron scattering measurements and scanning electron microscopy , in order to correlate the EIS parameter with the magnitude of the interface of the β - Sn and Ag 3 Sn phases