Mechanical Properties of Lead-Free Solders

Mechanical testing of solder joint arrays was used to characterize lead-free solder alloys under both shear and tensile loading. The alloys studied included Sn4.0Ag0.5Cu, Sn3.5Ag, Sn0.7Cu, Sn3.0Ag0.5Cu, and Sn1.2Ag0.5Cu0.05Ni. All of the lead-free alloys exhibited time and temperature dependent creep behavior over the entire temperature range from -55 C to 125 C. A SINH creep law was effective in describing the steady state creep behavior over the entire range of conditions. Lead-free alloy strength increases with increasing Ag content. The relative strength of tin-lead versus lead-free alloys depends on the strain rate and temperature regime. At high strain rates and low temperatures, eutectic tin-lead is the strongest alloy. At low strain rates and high temperatures, eutectic tin-lead is the lowest strength alloy. Simulated stress-strain hysteresis loops for both accelerated test and field use conditions showed dramatic differences between the various alloys. Higher Ag content results in a larger stress range but a smaller strain range during temperature cycling.