Issues on Viscoplastic Characterization of Lead-Free Solder for Drop Test Simulations

Abstract

Reliable drop test simulations of electronic packages require reliable material characterization of solder joints. Mechanical properties of lead-free solder were here experimentally investigated for both monotonous and cyclic loading at different strain rates. With regards to the observed complex material behavior, the non-linear mixed hardening Armstrong and Fredrick model combined with the Perzyna viscoplastic law was chosen to fit the experimental data. This model was subsequently implemented into a commercial finite element code and used to simulate drop tests. Actual drop test experiments were conducted in parallel and experimental results were compared to simulations. Prediction discrepancies were analyzed and explanations suggested. IntroductionA few years ago, two major issues arose within electronics: the global ban of leaded solder and the increasing demand for handheld products to be able to sustain accidental drops. Prior to these two new challenges, extensive work had focused on thoroughly characterizing the thermo-mechanical properties of leaded solder in order to assess thermo-cycling reliability. When subjected to temperature variations, thermal expansion mismatch causes the polymeric printed circuit boards (PCB) to expand and contract more than the ceramic components that are soldered on. The resulting combination of tensile and shear stress in connection with the high homologous temperature (Eutectic Pb-Sn (61.9-38.1)wt% melts at 183°C and thermocycling experiments are typically conducted between -40°C and 125°C) lead to large plastic strains eventually causing mechanical and electronic failure. When focus shifted from leaded to lead-free solder, it seemed reasonable to keep on using the previously developed constitutive models but with new sets of material parameters, see [1] for a gathering of material parameters both for leaded and lead-free solders. Among various candidates, SnAgCu solders early emerged as suitable alternatives partly because of their relatively low cost and relatively low melting temperatures (Sn-Ag-Cu (96.1-3.4-0.5)wt%, for example, melts at 217°C). All over the world a vast amount of experiments were conducted on different weight compositions, at different temperatures and at different strain rates. This approach was successful and Finite Element Simulations with lead-free solder joints were soon to give reliable results. Publications dealing with thermo-cycling reliability are numerous and can be divided into two groups depending on whether time independent and time dependent plasticity are modeled separately or not. In the first approach, strain hardening plasticity and creep are assumed to be completely uncoupled; an advantage is that their contribution to plastic strain over thermocycles can be compared and analyzed [2]. More advanced models feature two creep regimes