Currently, lead-free solders are being widely used as an alternative to traditional Sn-Pb solders in micro-electronic packaging industry due to the environmental concern of lead. Fatigue failure of solder joints is one of the common failure modes in electronic packaging which might be attributed to the experiences of thermo-mechanical fatigue (e.g. Power switching) or mechanical fatigue (e.g. vibration) loading. To design these lead-free solders more strategically for specific applications, it is important to understand the failure mechanism of lead-free solders under fatigue loading. Moreover, the microstructure and constitutive properties of conventional lead free solder joints in electronic assemblies such as SAC305 changes when exposed to isothermal aging. These changes consequently reduce the reliability of lead free electronic assemblies significantly due to aging. In this study, we have examined the effects of prior aging on damage accumulation occurring in SAC305 and SAC_Q (SAC+Bi) solder materials subjected to mechanical cycling (fatigue testing). Uniaxial samples have been prepared and polished so that the microstructural changes could be tracked after the initial aging, and then subsequently with mechanical cycling. In particular, we have examined the microstructural changes that occurred in small fixed regions in the solder samples, rather than using several different regions. Regions of interest near the center of the sample were marked using small indents formed with a nanoindentation system. Samples were then subjected to aging at 125 °C for various durations to produce several different initial microstructures. Scanning electron microscopy (SEM) were used to investigate the aging induced microstructural changes in the regions of interest in the solder sample. After aging, the samples were then subjected to mechanical cycling. After various durations of cycling (e.g. 0, 10, 25, 50, 75, 100, 200, 300 cycles) that were below the fatigue life of the materials, the regions of interest were again examined using SEM. Using the recorded images, the microstructural evolutions in the fixed regions were observed, and the effects of the initial aging on the results were determined. In case of SAC305, It was found that the number of IMC particles decreased while the average diameter of the particles increases significantly due to the initial aging. The distribution and size of the intermetallic particles in the inter-dendritic regions were observed to remain essentially unchanged with the application of the mechanical cyclic load. Relative to the non-aged samples, there were significant differences observed in the rate and intensity of the micro crack growth occurring in the heavily aged samples that began with much coarser microstructures. Later, the cycling induced microstructure evolutions observed in the SAC_Q lead free alloy has been compared with the observed changes in the microstructure of SAC305 that occurred during the cyclic loading. Due to the presence of bismuth, significant difference in the microstructural evolution of the SAC_Q alloy during cycling were observed. Thus, the doped alloys have shown a high potential for use in thermal cycling conditions because of their improved resistance to aging-induced evolution.

Solder Joints are among the most vulnerable components within electronic packages, and solder joint fatigue is regarded to be one of the major methods of electronic package failure. The prediction of solder joint reliability is thus of great importance and most finite element packages utilize the Anand Viscoplastic Model to model the mechanical behavior of the solder joint material. In this work, 3 × 3 arrays of SAC305 solder joints of roughly 750 μm in diameter were reflowed in between two FR-4 printed circuit boards to create a sandwich structural sample. These samples were then subjected to creep testing in shear at various temperatures (T = 25, 50, 75, 100 °C) and stress levels (τ = 5, 10, and 15 MPa). A set of specially designed fixtures was used to grip the solder joint specimens. The nine Anand model constants were then extracted from the creep data. The Anand model predicted creep response curves were then compared with the experimental creep measurements to determine the accuracy of the model. The Anand model predictions were found to match the measured data very well over a wide range of temperatures and stress levels.

In temperature changing environments, solder joints often experience fatigue failure due to cyclic mechanical stresses and strains induced by mismatches in the coefficients of thermal expansion. These stresses and strains lead to damage accumulation and contribute to the crack initiation, crack propagation, and eventually to failure. In this study, we have investigated the cyclic stress-strain behavior of SAC305 and SAC_Q reflowed lead free solders that occur at various testing temperatures and with various prior aging conditions. Lead free solder uniaxial test specimens with circular cross-section have been prepared using vacuum suction method and then were aged for 0 to 20 days at 125 °C. The samples were then subjected to cyclic stress-strain loading using a Micro-mechanical tester at different testing temperatures from T = 25 C to T = 100 C. The evolution of hysteresis loops with duration of prior aging was characterized by measuring the strain energy density dissipated per cycle (loop area), peak stress, and plastic strain range. It was observed that aging degrades the mechanical fatigue properties due to microstructural coarsening. At elevated temperatures, a drop in the loop area and peak stress and an increase in the plastic strain range for both lead free reflowed solder materials were obtained. In addition, SAC_Q samples had a higher loop area and peak stress compared to SAC305.

Fatigue failure of solder joints is one of the most common methods by which electronic packages fail. Electronic assemblies usually must cope with a temperature varying environment. Due to the mismatches in coefficients of thermal expansion (CTEs) of the various assembly materials, the solder joints are subjected to cyclic thermal-mechanical loading during temperature cycling. The main focus of this work is to investigate the changes in microstructure that occur in SAC305 and SAC+Bi lead free solders subjected to mechanical cycling. In this paper, we report on results for the SAC+Bi solder commonly known as SAC_Q or CYCLOMAX. Uniaxial solder specimens were prepared in glass tubes, and the outside surfaces were polished. A nanoindenter was then used to mark fixed regions on the samples for subsequent microscopy evaluation. The samples were subjected to mechanical cycling, and the microstructures of the selected fixed regions were recorded after various durations of cycling using Scanning Electron Microscopy (SEM). Using the recorded images, 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.

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