Characterization of Material Damage and Microstructural Evolution Occurring in Lead Free Solders Subjected to Cyclic Loading

Abstract

In this investigation, the effects of isothermal mechanical cycling on damage accumulation in lead free SAC305 and SAC_Q (SAC+Bi) solder joints have been explored. In particular, we have examined both the evolution of the solder microstructure and the degradations occurring in the solder constitutive behavior (stress-strain and creep) as cycling progresses during fatigue testing. Such knowledge is necessary to develop accurate fatigue criteria for lead free solder alloys. In addition to quantifying damage accumulation during cyclic testing, we have also measured the evolution of the cyclic stress-strain behavior (hysteresis loop area, plastic strain range, and peak stress) during single fatigue tests run to failure. To study the evolution of the cyclic stress-strain behavior during a single fatigue test, the uniaxial specimens have been subjected to cyclic stress/strain loading at constant temperature until fatigue failure. From the recorded cyclic stress-strain curves, the evolution of the solder hysteresis loops with number of cycles has been characterized. Also, the dependencies of the hysteresis loop area (plastic strain energy density), plastic strain range, and peak stress on the duration of cycling have been studied. Near fatigue failure, the decreases in loop area (56%, 54%) and peak stress (48%, 54%) were similar for the two materials. The plastic strain range remained essentially constant during the cycling for both materials. To study the effects of cycling on damage accumulation and degradation of the constitutive behavior of SAC305 and SAC_Q, uniaxial samples have been prepared and subjected to various durations of prior isothermal mechanical cycling (e.g. 0, 50, 100, 300, 600, 900, 1200 cycles) that were below the fatigue life of the materials. The cycled samples were then subsequently subjected to stress-strain or creep testing. Using the data from the various tests, we have been able to characterize and quantify the cycling induced damage through the observed degradations of several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) with the amount of prior cycling. The degradations in creep strain rate were especially significant for both materials due to exacerbated grain boundary sliding from intergranular fatigue cracking. The cycling induced material property evolutions observed in the two lead free alloys have been correlated with the observed changes in their microstructures that occurred during the cycling. Special polished test specimens were prepared so that the microstructural evolution of a small fixed region in the solder specimen could be followed as the cycling progressed. It was observed that the cycling induced damage consisted primarily of small intergranular cracks forming along the subgrain boundaries within dendrites. These cracks continued to grow as the cycling continued, resulting in a weakening of the dendrite structure, and eventually to the formation of large transgranular cracks. The distribution and size of the intermetallic particles in the inter-dendritic regions were observed to remain essentially unchanged.