The Anand parameters for SAC solders after extreme aging

Abstract

The mechanical behavior of lead free solder materials is often represented using the Anand viscoplastic constitutive model. This nine parameter model is built into popular commercial finite element codes, and is widely used in the electronic packaging industry. Reliability prediction results are often highly sensitive to the specified Anand parameters, and there are great variations in the available literature values for common solder alloys. In this work, we have explored the range of Anand parameters possible for four common SAC (Sn-Ag-Cu) alloys by testing samples with a wide range of microstructures. The lead free solder materials tested include 98.5Sn1.0Ag0.5Cu (SAC105), 97.5Sn2.0Ag0.5Cu (SAC205), 96.5Sn3.0Ag0.5Cu (SAC305), 95.5Sn4.0Ag0.5Cu (SAC405). These SACN05 solders have various Ag contents from N = 1.0 to 4.0%, and all contain 0.5% Cu. For each lead free solder alloy, four different cooling profiles and resultant microstructures have been investigated that yielded vastly different mechanical behaviors. These included water quenched (WQ), reflowed (RF), reflowed + 6 months of aging at 100°C, and reflowed + 12 months of aging at 100°C. The nine Anand parameters were determined for each unique solder alloy and microstructure from a set of stress strain tests performed at three different strain rates and five different temperatures (15 sets of conditions). After deriving the Anand parameters for each alloy and microstructure, the stress-strain curves have been calculated for various temperatures and strain rates, and excellent agreement was found between the predicted results and experimental stress-strain curves. The large range of microstructures examined has allowed us to explore the extreme values of the material properties and Anand parameters possible for a given SACN05 alloy. The WQ microstructures are extremely fine, and yield high mechanical properties at the upper limits possible for the solder alloys. The RF + 6 months of aging and RF + 12 months of aging microstructures are highly coarsened, and yield similar and highly degraded mechanical properties. After such a long durations of aging, any further changes in the microstructure, mechanical response, and mechanical properties will be rather small. Thus, the results for these “extreme aging” cases can be regarded as approaching the highest level of mechanical behavior degradation possible for a lead free solder material. Such limiting values found for a severely aged SAC alloy can be used by designers as a conservative set of constitutive parameters in finite element simulations.