Characterisation of lead-free solders at high strain rates considering microstructural conditions

This work focuses on the mechanical behaviour of lead free solder alloys under high strain rate conditions. The knowledge about this specific material behaviour and the ability of consideration in finite element modelling are especially important for reliability investigations on mobile communication and entertainment devices. Since the mechanical behaviour of solders is strongly dependent on their micro- and grain structure two solder alloys SnAg1.3Cu0.5 (SAC) and SnAg3.5 (SA) have been considered in this work. After solder joint reflow changes of the micro- and grain structure occur due to thermal loading of the joints. Therefore isothermal ageing has been conducted to enable the comparison of as cast and aged solder condition. The strain rate dependent mechanical behaviour of the named alloys has been investigated by the usage of tensile experiments on miniature dog bone specimens. The effective specimen part with a diameter of 1 mm allowed to stay close to the dimensions of real solder joints and hence to their micro- and grain structure. Experiments have been accomplished at room temperature. The isothermal ageing at 150°C lasted for 1000 h to reach considerable structural changes. The applied strain rates covered a range of 25 s^-1 to 870 s^-1. The strain rate depending material behaviour of both solders was incorporated in a finite element model and used to analyse the solder joint stress during a standard drop test experiment.