Analytical stress model for tin based solder material

In recent years the activity to predict lifetime focuses more and more on finite element analysis (FEA) using up growing computer power. The description of the material properties especially viscoplastic behavior have to be well known to bring FEA calculations in a good agreement with experimental results concerning solder joints [1, 2]. Typically, creeping of solder materials is specified by extensive mathematics modeling. New research activities concentrate on viscoplastic modeling according to Chaboche [3, 4]. This paper shows a new methodology for lifetime prediction based on stress measurements of tin solder specimens performed by Fraunhofer Society IKTS [5]. Characteristic hysteresis curves of the stress-strain behavior of tin based solder alloy were measured at different temperatures to enable the calculation of elastic and plastic elongation depending on time and temperature. This new approach gets by on complex mathematics equations and FEA. A stress based model combined with lifetime facts offers a rapid lifetime prediction of solder materials. Transferability to solder joints is shown by means of the component CR1206 and its reliability due to thermal cycling experiments. The adjusted analytic calculated results are evaluated by the experimental thermal cycling results and will be transformed to a calculation model for thermal cycle test procedure.