High strain rate behaviour of lead-free solders depending on alloy composition and thermal aging

Abstract

This work focuses on the mechanical behaviour of lead free solder alloys under high strain rate and thermal aging conditions. Tensile experiments have been accomplished using miniature specimens with a diameter of 1 mm to stay close to the dimensions of real solder joints. The specimens have been manufactured by a casting process. A controlled cool down process was applied having a cooling rate of 50 K/min. This way the solder specimens solidified in a comparable manner to reflowed solder joints. Two lead-free solder alloys SnAg1.3Cu0.5 (SAC) and SnCu0.7 (SC) were used for specimen manufacturing. The specimen preparation also contained a anneal step and strain gauge assembly. The experiments were conducted using a high strain rate tensile tester providing a constant deformation speed up to 5 m/s. The stress and strain within the specimen was recorded with high resolution using strain gauge and laser triangulation sensors respectively. Stress data were used to evaluate the yield stress dependency on strain rate and aging conditions. Furthermore, fracture site appearance and ultimate strain were analysed for a better understanding of the deformation behaviour. Cross sections were used to enable an insight into the deformation mechanism taking place depending on the applied strain rate and aging condition. Experiments were done at room temperature for specimens in the as cast state and after a considerable isothermal storage at 150 °C for 1000 h. The applied strain rates covered a range of 25 s−1 to 870 s−1. The strain rate depending material behaviour considering alloy composition and aging condition was incorporated in a finite element model and used to analyse the solder joint stress during a standard drop test experiment. SAC revealed a high sensitivity on the applied strain rate. Its yield stress increased with the strain rate. The yield stress of SC showed a lower sensitivity and a decrease towards higher strain rates. The ultimate total strain decreases with strain rate but shows quite high values and therefore pointing at an overall ductile behaviour for both solders. However, fracture site features of both solder alloys indicate a change of the deformation and fracture behaviour within the tested strain rate interval. Cross sectional analysis of SAC specimens also verify a change in the deformation behaviour. If the SC specimens are exposed to a prior isothermal storage the yield stress level decreases. The measured yield stress behaviour was fitted using the Cowper-Symonds-Equation for both solders and ageing conditions. Utilizing the as cast SAC strain rate sensitive material description for modelling a standard JEDEC drop test experiment the stress distribution shows clearly potential solder joint failure sites.