A recent blog by Ronald C. Lasky, a senior technologist at Indium Corp., on the Pb-Free.com website notes that SAC305 has failed to become the defacto standard for lead-free assembly. SAC305 is a combination of 3 percent silver, .05 percent copper and the balance tin. The problem, it appears, is that SAC305 does not hold up sufficiently to shock. “In drop tests where a device is dropped on a cement floor from a certain height, SAC305 performs worse than tin-lead solder and worse than SAC105,” says Lasky. SAC105 contains 1 percent silver and .05 percent copper with a balance of tin. He notes that some mobile device manufacturers have already switched to SAC105. “The current very high cost of solver has helped ‘grease the skids’ for this transition,” says Lasky.
He explains the superior strength of SAC105: “It is accepted that the silver in SAC305 causes silver tin intermetallic platelets. These platelets cause mechanical stress raisers that lower shock resistance. Since SAC105 has less silver, it forms few platelets and as a result performs better in drop tests.”
Problem is, SAC105 melts at about 227C, about 10C higher than SAC305’s 217C. Lasky notes that SAC105 also performs poorly in thermal cycle testing. So it’s back to the drawing board for a reliable standard. “Folks are investigating adding small amounts of other alloying elements to further improve shock resistance, obtain better thermal cycle fatigue and also lower the melting temperature closer to 217C,” says Lasky.