Micro-Particle Induced X-ray Emission Study of Lead-Free and Lead-Based Solders and Interactions with Copper Wires

Abstract

Pb-free electrical solders, such as Cu–Sn alloys, work well for reflow soldering under tightly controlled conditions. Hand soldering, however, often results in poor quality joints compared to conventional Pb–Sn solders. To investigate this under realistic workshop conditions, micro-particle induced X-ray emission (micro-PIXE) with 2 MeV protons has been employed. Commercial flux-cored Cu–Sn and Pb–Sn solder wires are studied. Solder blobs under two cooling conditions as well as tinning Restriction of Hazardous Substances (RoHS)-compliant and legacy component wires are investigated. The results show that the long heating and slow cooling of Cu–Sn solder blobs lead to formation of an acicular precipitate that can be ascribed to Cu6 Sn5. Pb–Sn solder under the same conditions shows phase separation with regions of high Sn and regions with high Pb. In the case of rapidly cooled blobs where a shiny surface is produced, no phase separation in either solder is observed. Tinning of RoHS-compliant and legacy Cu component wires with the two solders produce significantly different interfacial depth profiles with varying degrees of grading, indicative of intermetallic phase formation.