Experimental Investigation of Temperature and Mean Stress Effects on High Cycle Fatigue Behavior of SnAgCu-Solder Alloy

Solder joints in automotive electronic assemblies are exposed to thermomechanical and vibrational loads. Usually, passive thermal cycling results in thermomechanical loads in the low strain rate plastic and creep regime of the solder alloy. In the case of vibrational loads, high strain rate plastic deformations in solder interconnections are expected. In order to investigate the deformation and failure behavior of the solder material in the high strain rate regime, we performed several high cycle fatigue (HCF) experiments on standardized specimens of a SnAgCu alloy under varying mean stresses and ambient temperatures. As a first step, high cycle fatigue tests with a frequency of 40 Hz at room temperature have been performed. From a statistical evaluation of the number of cycles to failure at different stress amplitudes and zero mean stress, we obtained a high cycle fatigue Woehler curve. Subsequently, the mean stress and temperature levels were changed, and the load frequency has been kept constant. The aim of this high cycle fatigue Woehler experiments was to explore the temperature and mean stress effects on the fatigue performance of the solder alloy. In order to assess the reliability of a solder ball grid array (BGA) under vibrational loading by means of finite elements (FE) simulations, a viscoplastic material model is calibrated based on the experimentally observed stress-strain material behavior in the HCF measurement. FE simulations using a fatigue material model were used to address the lifetime of the BGA solder joints under vibration during electrodynamic shaker testing on board level. The FE-based lifetime prognosis is discussed and compared to experimental statistical failure data of real solder joints obtained from electrodynamic shaker testing.