# Failure Behavior and Mechanism of Solder Joint Under Thermal Mechanical Coupling Loads

## TL;DR

This study explores how solder joints fail under repeated thermal stress, proposing a new method to predict their lifespan more accurately.

## Contribution

A novel hypothesis is proposed to improve thermal fatigue life evaluation by using viscoplastic strain in the Coffin–Manson model.

## Key findings

- Alternating shear strain concentrates at the joint–pad interface, initiating cracks.
- The predicted thermal fatigue life of the solder joint is 18,930 cycles.
- Higher viscoplastic strain energy density at the critical point indicates energy dissipation drives damage.

## Abstract

The periodic thermal loads to which electronic devices are exposed during operation induce alternating thermal stresses due to the mismatched coefficients of thermal expansion (CTE) between the solder joints and the surrounding materials. This leads to cyclic thermal strain, ultimately causing crack initiation, propagation, and failure of interconnect structures. This study investigates thermal fatigue failure of Sn3.5Ag solder joints induced by cyclic thermal stresses from CTE mismatch. Numerical simulations and experiments reveal that alternating shear strain concentrates at the joint–pad interface, serving as the crack initiation site. This study proposes a hypothesis: extracting the equivalent viscoplastic strain range from the steady-state hysteretic response after cyclic stabilization and applying it to the Coffin–Manson model can mitigate the strain overestimation inherent to methods based on the initial transient impact, thereby providing a more reasonable physical basis for thermal fatigue life evaluation. Based on this, the thermal fatigue life of the solder joint is predicted to be 18,930 cycles. Analysis confirms significantly higher viscoplastic strain energy density at this critical point, indicating energy dissipation drives damage. This study addresses the above hypothesis from three aspects: deformation mechanism, cyclic response, and energy dissipation, providing a key basis for developing a highly reliable method for assessing solder joint life.