The effect of Cu and Ag on the yielding behaviour of lead-free solders at high strain rates

Electronics for automotive or mobile applications are exposed to vibration and shock loads causing PCB and interposer vibration. This leads to high strain rates within the component solder joints. It is known that solder joints show strain rate dependent yield behaviour [1-4] as well as that the strain rate dependency is dependent on the solder alloy composition. The solder joint alloy composition is determined by the solder paste and/or ball composition and the metallisation of the component and the substrate. There have been numerous publications on the strain rate depending yield behaviour of several alloy compositions used in industry. It has been shown that solder alloys behave more stiff and brittle if there is a higher silver content. Still, the direct dependency of yield behaviour and strain rate sensitivity on the silver and copper content has not been investigated and published yet. In this work the base material Sn99.9 and six lead-free solder alloys namely SnAg1.3 (wt.%), SnAg3.5, SnCu0.5, SnCu0.7, SnCu0.9 and SnAg1.3Cu0.5 have been studied for their yielding behaviour, strain rate sensitivity, deformation and fracture behaviour. Selected alloys have been tested in the as cast and isothermally aged state. The ageing was done at 150°C for 1000 h. Specimens were manufactured by casting applying fast cooling with 50 K/min. The specimen geometry as shown in figure 1 is a miniature dogbone shape to achieve a specimen micro- and grain structure comparable to solder joints. A high deformation speed tester introduced in earlier work [5] was utilised to conduct high strain rate experiments at rates from 20 to 800 s-1. High resolution online stress measurement revealed the strain rate dependent yielding behaviour. Fracture site inspection giving information on the damage behaviour was done by electron microscopy. The EBSD (electron backscatter diffraction) option was used to analyse the deformation behaviour at the grain structure level. Local fracture strain measurement revealed a very ductile behaviour of all specimens.