# Interfacial Behavior During Reactions Between Sn and Electroplated Co–Zn Alloys

## TL;DR

This paper studies how varying zinc content in electroplated Co-Zn alloys affects the formation of intermetallic compounds when reacting with tin solder.

## Contribution

The study reveals that Zn doping significantly suppresses CoSn3 formation and improves interfacial stability during solid-state reactions.

## Key findings

- Higher Zn content (8.8 wt.%) reduces IMC formation by ~50% and creates a duplex structure.
- Zn doping suppresses CoSn3 growth by over 85% in solid-state reactions.
- Zn disrupts the crystallinity of Co, leading to partial amorphization.

## Abstract

This study investigates the electroplating characteristics of Co-Zn alloy coatings with varying Zn contents (0.55 wt.%~8.8 wt.%) and their influence on intermetallic compound (IMC) formation during reactions with Sn solder. Co-Zn alloy coatings were successfully fabricated by electroplating using cobalt plating solutions with different concentrations of zinc sulfate. The results reveal anomalous co-deposition behavior, where the less noble Zn preferentially deposits over Co. Surface morphologies and microstructures evolve significantly with increasing Zn content, transitioning from columnar to dendritic structures. Zn incorporation into the Co lattice disrupts its crystallinity, leading to decreased crystallinity and partial amorphization. Liquid-state and solid-state interfacial reactions with Sn solder demonstrate that Zn content considerably influences IMC formation. In liquid-state reactions at 250 °C, lower Zn contents (0.55–4.8 wt.%) slightly enhance CoSn3 growth. It exhibits a dense layered-structure without IMC spallation. In contrast, a higher Zn content (8.8 wt.%) significantly reduces IMC formation by approximately 50% and produces a duplex structure with two distinct layers. In solid-state reactions at 160 °C, the suppression effect becomes even more pronounced. The Co-0.55Zn deposit exhibits significant inhibition of CoSn3 growth, while the Co-8.8Zn sample forms only a thin IMC layer, achieving a suppression rate exceeding 85%. These findings demonstrate that Zn doping effectively limits CoSn3 formation during solid-state reactions and improves interfacial stability.