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

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 Cu-6 Sn-5. 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.