# Unravelling Cu6Sn5 precipitate coarsening mechanisms in SAC solders under thermomechanical cycling

## TL;DR

This study explores how Cu6Sn5 precipitates in SAC solder grow under thermal and mechanical stress, finding that mechanical strain significantly influences their coarsening.

## Contribution

The paper identifies strain-enhanced coarsening as a dominant mechanism over Ostwald ripening in SAC solder under thermomechanical cycling.

## Key findings

- Strain-induced coarsening is more significant in high-strain regions than in lower-strain shear bands.
- Bismuth delays dynamic recrystallisation and reduces both coarsening mechanisms.
- Cu6Sn5 precipitates are 1.5–3 times larger in recrystallised regions due to strain-enhanced coarsening.

## Abstract

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.

The online version contains supplementary material available at 10.1038/s41598-025-21633-y.