# Effects of bismuth on the microstructure and crack propagation in Sn–Ag–Cu-Bi solder joints during thermal cycling

## TL;DR

This study examines how adding bismuth to solder joints affects their durability under extreme temperature changes.

## Contribution

The paper reveals how bismuth influences solder joint microstructure and thermal fatigue resistance in BGA packages.

## Key findings

- High bismuth content improves thermal fatigue resistance in Sn–Ag–Cu-Bi solders.
- Excessive bismuth increases stress transfer to the intermetallic layer, reducing reliability.
- Microstructural changes like precipitation and recrystallization correlate with solder joint failure.

## Abstract

Bismuth is added to Sn–Ag–Cu-based solders to increase strength and solder joint reliability but accelerated thermal cycling (ATC) tests have shown variable thermal fatigue performance. To better understand their thermal fatigue mechanisms, we investigate three Sn–Ag–Cu-Bi solder alloys containing between 3 and 6 wt.% bismuth in two types of ball grid array (BGA) packages tested using a harsh ATC profile of -55/125 °C. We study how the bismuth additions affect the microstructure after soldering, after ~ 4 years storage at room temperature and after thermal cycling. The Weibull reliability statistics are then correlated with the microstructural evolution including solutionising, homogenisation, bismuth precipitation, recrystallisation and crack paths. The results show that high strength Sn–Ag–Cu-Bi solders can improve the resistance to thermal fatigue, but high solder strength needs to be balanced against excessive stress transfer to the intermetallic attachment layer.

The online version contains supplementary material available at 10.1007/s10854-025-15979-2.