Ab initio-aided CALPHAD thermodynamic modeling of the Sn-Pb binary system under current stressing

Area of Science:

• Materials Science
• Physical Chemistry
• Solid-State Physics

Background:

• Soldering, an ancient joining technique, is vital in modern electronics.
• High electric currents passing through solder joints have revealed a new phenomenon: supersaturation.
• Observed effects include unexpected supersaturation of the solder matrix and formation of unusual ring-shaped grains.

Purpose of the Study:

• To provide a plausible explanation for the observed supersaturation and grain formation in solders under high electric currents.
• To investigate the influence of electric current stressing on the phase stability of lead-tin (Pb-Sn) solders.
• To elucidate the underlying physical mechanisms driving these phenomena.

Main Methods:

• Utilizing *ab initio*-aided CALPHAD (Computer Aided Phase Diagram) modeling.
• Translating the effects of electric current into excess Gibbs free energies of the phases.
• Comparing Pb-Sn phase diagrams with and without current stressing.

Main Results:

• Demonstrated that electric current stressing shifts the phase equilibrium in Pb-Sn solders.
• Quantified the changes in phase stabilities induced by high electric currents.
• Provided a theoretical basis for the observed supersaturation and ring-shaped grain formation.

Conclusions:

• The study successfully explains the novel physical phenomena observed in solders under high electric currents.
• Changes in phase stability, driven by electric current, are identified as the origin of supersaturation and unusual grain structures.
• This research offers critical insights into solder behavior under electrical load, relevant for electronic device reliability.