Quantitative analyses of Ag3Sn intermetallic compound formation in SnAgCu solder alloys

Abstract

In this paper, a new explicit state variable was introduced which describes the Ag3Sn intermetallic morphology inside of lead-free solder joints at different cooling rates. In the experiment, samples were prepared by melting standard Ag-content (4 % in wt%) and low Ag-content (0.3 % in wt%) solder alloys, and solidifying them at five different cooling rates between 0.8 and 3 K/s. Images were taken about the cross-section of solder joints by brightfield light-microscope to calculate the explicit state variables, which describe the morphology of precipitating Ag3Sn intermetallic compounds. The captured images were processed then with an in-house developed automatic image processing algorithm. It was found that the area of Ag3Sn intermetallic compounds followed exponential distribution, so a new state variable could be introduced, such as the amplitude of a certain exponential curve fitted to the area histogram of the Ag3Sn IMCs. It was proven that our new variable described the different Ag3Sn IMC morphologies obtained at different cooling rates better; it was more sensitive at both aforementioned solder alloys (4 and 0.3 % Ag content) than the widely applied parameters, like the mean-intercept length or the average area of particles.