Effect of Cyclic Fatigue Damage Accumulation on the Elastic-Plastic Properties of SAC305 Solders

摘要

This study examines the effect of cyclic damage on the constitutive response of Sn3.0Ag0.5Cu (SAC305) solder material. Cyclic damage is induced through isothermal, mechanical cycling tests at high strain rate and room temperature, using a modified lap-shear microscale specimen (180 μm wide solder joint). The elastic, plastic, and yield response of the cycled solder joints are measured as a function of increasing cyclic damage, across various strain amplitudes. Significant piece-to-piece variability is observed in the measurements. This variability is consistent with previously reported viscoplastic measurements. Previous studies on the microstructure of solder in the initial state [1] and subsequent microstructural evolution due to cyclic damage [2] are summarized here to provide insights into the measured properties. The scatter in the measurements arises since as-reflowed SAC solder joints at length scales of 200 μm consist of only a few highly anisotropic grains of Sn [1-2]. This mechanically inhomogeneous microstructure results in anisotropic mechanical response that is dependent on the grain orientation in each specimen. The initial variability in the mechanical properties is seen to decrease with increasing cyclic damage, possibly due to the increasing homogenization caused by microstructural recrystallization [2]. These observations are indicative of the damage evolution we can expect in solder joints under life-cycle loading [3-9]. The elastic-plastic response and yield strength of SAC305 solder degrades with the accumulation of cyclic damage. The rate of degradation of the properties is proportional to the severity of the load cycle. The yield stress is observed to be equal in both the loading and unloading paths of each hysteresis loop, suggesting that SAC305 obeys an independent hardening rule, rather than the isotropic or kinematic hardening [10] which is reported. A continuum damage mechanics model based on Kachanov's damage description [11] is proposed, to describe the degradation in the constitutive properties as a function of cyclic load drop. The proposed damage model does a reasonably good job of capturing the degradation in plastic and yield properties. However, for elastic properties, the proposed damage model is effective only upto 25% of the life of most specimens tested. Possible reasons for the discrepancy are discussed in the paper.