Understanding the Effect of Reflow Profile on the Metallurgical Properties of Tin–Bismuth Solders

Karel Dušek,Julia Del Re,Martin Plaček,Anna Pražanová,Petr Veselý,David Bušek

doi:10.3390/met12010121

Abstract

Sn–Bi alloys are desirable candidates for soldering components on printed circuit boards (PCBs) because of their low melting point and reduced cost. While certain tin–bismuth solders are well characterized many new alloys in this family have been developed which need proper characterization. The following study looks at the behavior of four different Sn–Bi alloys—traditional 42Sn58Bi and 42Sn57Bi1Ag and two new tin–bismuth alloys—in solder paste during the reflow soldering process. Each alloy was processed using different reflow profiles that had varying times above liquidus (TALs) and peak temperatures. The PCBs were then analyzed to see how the processing variables influenced wetting, voiding, microstructure, intermetallic layer composition, and thickness. After analysis, the PCBs were then subjected to thermal cycling experiments to see how reflow profile impacted microstructure evolution. The results demonstrated that reflow profile affects properties such as metal wetting and voiding. It does not however, greatly impact key metallurgical properties such as intermetallic layer thickness.

Highlights

A range of peak temperatures and times above liquidus (TALs), which are typical of those used in the surface mount technology (SMT) industry, were selected to understand how these two variables influence the qualities of the reflowed solder joint
Short TALs and low peak temperatures are known to minimize the formation of the intermetallic compound (IMC) layer between the solder and the board, this may lead to poor soldering
The current study has demonstrated how four different low temperature alloys beplotted against the square root of time

Summary

Introduction

Solder alloys play a pivotal role in surface mount technology (SMT). When used in materials such as solder paste, they are vital for the assembly of components on printed circuit boards (PCBs) through processes such as reflow soldering [1]. Two key variables in reflow soldering are peak reflow temperature and the time above liquidus (TAL) [2]. Solder alloys are exposed to peak temperatures that extend. 25–50 ◦ C above their melting point to ensure that components are properly fused to the PCB [3]. The exact temperature at which they are processed as well as how long they are kept at this temperature, TAL, are known to affect the reflowed solder joint [4]

Methods

Results

Conclusion