Unravelling Cu6Sn5 precipitate coarsening mechanisms in SAC solders under thermomechanical cycling

Thermo-mechanical cycling of microelectronic devices creates complex stress-states in Sn-Ag-Cu (SAC) solder balls, leading to Cu₆Sn₅-precipitate coarsening. Two key mechanisms - strain-induced coarsening and Ostwald ripening - are examined separately. Strain-induced coarsening, studied via plastic shear deformation, is more significant in dynamically recrystallised high-strain regions than in lower-strain shear band regions. Ostwald ripening is investigated via in-situ FESEM, and its interplay with strain-enhanced coarsening is analysed in thermo-mechanically cycled solders with varying Bi-contents. Results show that Bi, solved in the β-Sn matrix, delays dynamic recrystallisation and reduces both strain-enhanced coarsening and Ostwald ripening of Cu₆Sn₅. Nonetheless, Cu₆Sn₅-precipitates are 1.5-3 times larger in recrystallised high-strain regions than in single-crystalline lower-strain regions regardless of Bi-content, due to strain-enhanced coarsening during thermo-mechanical cycling. The findings indicate that mechanical strain plays a dominant role in precipitate growth, suggesting that strain-enhanced Cu₆Sn₅ coarsening, and thusly decreased precipitate strengthening effects, correlate with increased thermo-mechanical fatigue.