Microstructure and Damage Evolution During Thermal Cycling of Sn-Ag-Cu Solders Containing Antimony

S A Belyakov,B Arfaei,J W Xian,C M Gourlay,R J Coyle

doi:10.1007/s11664-020-08507-x

Abstract

Antimony is attracting interest as an addition to Pb-free solders to improve thermal cycling performance in harsher conditions. Here, we investigate microstructure evolution and failure in harsh accelerated thermal cycling (ATC) of a Sn-3.8Ag-0.9Cu solder with 5.5 wt.% antimony as the major addition in two ball grid array (BGA) packages. SbSn particles are shown to precipitate on both Cu6Sn5 and as cuboids in β-Sn, with reproducible orientation relationships and a good lattice match. Similar to Sn-Ag-Cu solders, the microstructure and damage evolution were generally localised in the β-Sn near the component side where localised β-Sn misorientations and subgrains, accelerated SbSn and Ag3Sn particle coarsening, and β-Sn recrystallisation occurred. Cracks grew along the network of recrystallised grain boundaries to failure. The improved ATC performance is mostly attributed to SbSn solid-state precipitation within β-Sn dendrites, which supplements the Ag3Sn that formed in a eutectic reaction between β-Sn dendrites, providing populations of strengthening particles in both the dendritic and eutectic β-Sn.

Highlights

Accelerated thermal cycling (ATC) test programs have shown that Sn-Ag-Cu solders can outperform the Sn-37Pb solder they were designed to replace.[1]
It has been found that the beneficial effects of Ag on accelerated thermal cycling (ATC) reliability diminishes as the severity of the thermal cycle increases,[1] making Sn-Ag-Cu solders unsuitable for some emerging applications in, for example, the automotive, aerospace and military sectors
Five phases were detected in the bulk solder of these joints by combining EDS with EBSD: b-Sn, Cu6Sn5, Ag3Sn, SbSn and InSb, InSb was only present as traces

Summary

Introduction

Accelerated thermal cycling (ATC) test programs have shown that Sn-Ag-Cu solders can outperform the Sn-37Pb solder they were designed to replace.[1]. It has been found that the beneficial effects of Ag on ATC reliability diminishes as the severity of the thermal cycle increases,[1] making Sn-Ag-Cu solders unsuitable for some emerging applications in, for example, the automotive, aerospace and military sectors. Free solders is under development where the alloy design approach has been to provide additional strengthening mechanisms, including solid solution strengthening and improved precipitation strengthening.[2,3,4] Many of the third-generation Pb-free solders are complex multicomponent alloys based on near-eutectic Sn-Ag-Cu compositions with significant additions of Bi, Sb and/or In, often with a combined Bi + Sb + In content of 3.5–6.5 wt.%.2. We focus on the influence of a 5.5 wt.%Sb addition to a near-eutectic Sn-Ag-Cu solder to build the understanding of its effect on microstructure evolution, precipitation strengthening, and failure mechanisms in thermal cycling Free solders is under development where the alloy design approach has been to provide additional strengthening mechanisms, including solid solution strengthening and improved precipitation strengthening.[2,3,4] Many of the third-generation Pb-free solders are complex multicomponent alloys based on near-eutectic Sn-Ag-Cu compositions with significant additions of Bi, Sb and/or In, often with a combined Bi + Sb + In content of 3.5–6.5 wt.%.2 In this paper, we focus on the influence of a 5.5 wt.%Sb addition to a near-eutectic Sn-Ag-Cu solder to build the understanding of its effect on microstructure evolution, precipitation strengthening, and failure mechanisms in thermal cycling

Methods

Results

Conclusion