Journal of SMT Article


Authors: Mohammad M Hossain1, Tommi Reinikainen2, Viswam Puligandla
Company: 1 Intel corporation, Chandler, AZ; 2 Nokia Institute
Date Published: 10/1/2007   Volume: 20-4

Abstract: A solder is considered lead free if lead (Pb) content in it is less than 0.2 wt % (USA) and 0.1 wt % (EU). Sn-Ag-Cu (Tin, Silver, and Copper) solder is among the selected alloys as one of the options to replace tin-lead solders in electronic assemblies. In this paper is reported the mechanical and thermo-mechanical aspect of Sn-Ag-Cu solder alloys that have been tested for their deformation behavior. Commercially available Sn3.8Ag0.7Cu was considered as base alloy in this study. Constant stress and strain-rate tests were performed in tensile and shear configuration. Deformations were measured using bulk solder tensile specimens and grooved lap shear specimens which enabled a stress-state of nearly pure shear in the solder joint. The strain rate range was between 0.1/sec and 10-6/sec, and test temperatures were 25°C, 75°C and 125°C. Both freshly prepared and thermally aged samples were tested. The influence of Au addition in SAC solder alloy on it effect on the strength in tensile loading is also studied. The measured tensile stress-strain data were utilized to derive the constants for the visco-plastic Anand’s constitutive model. Coefficient of thermal expansion (CTE) for those Sn-Ag-Cu lead (Pb) - free alloys were measured in the temperature range of -40 to 160°C using a thermo-mechanical analyzer (TMA). The tensile and shear properties of 95.5Sn3.8Ag0.7Cu solder and solder joint specimens were found to be dependent on test temperature and strain rate. The mechanical strength of SAC bulk solder and solder joint decreases with increase in temperature and increases with increase in strain rate. CTE for the SAC leadfree alloys were found to be relatively lower compared with tin-lead solder. The steady-state creep test data for 95.53.8Ag0.7Cu solder was curve-fitted to a hyperbolic-sine creep model. Despite change in solder specimen methodology and a change in lead free failure model methodology the material constitutive parameters developed are in line with similar studies.

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