Recent legislation in Europe and Japan banning the use of lead in electronic circuit boards (used, for example, in computers and mobile phones) is forcing the industry to develop competitive replacement solder alloys. Pb-free solders based on near-eutectic Sn-0.7Cu-xNi alloys provide excellent solderability during wave soldering with cost advantages compared to Ag-containing alternatives. However, there is only limited knowledge about the solidification mechanisms in this alloy system and, furthermore, the ternary Sn-Cu-Ni phase diagram is not yet fully established. The influence of Ni additions on the solidification mechanisms in the Sn-0.7Cu alloy has been studied by microstructural analysis, fluidity measurements, directional solidification experiments and X-ray fluorescence imaging. The addition of 500-600 ppm Ni to Sn-0.7Cu creates a eutectic microstructure, changes the solidification mechanism and improves the fluidity during solidification. The microstructural observations allow for comparison with previous Bridgman studies on binary Sn-0.9Cu. The influence of Ni addition on the solidification mechanisms is then assessed by comparing the microstructures of the ternary and binary alloys. Finally, the ternary alloy microstructures are used to discuss the contrasting Sn-Cu-Ni phase diagrams reported in the literature. The results demonstrate the complex phase relations in the Sn-Cu alloy system and the important role of a few hundred ppm of various solute elements.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering