Mechanical Characterization and Aging Induced Evolution of Cyclic Properties and Microstructure of Lead-Free Solder Materials
Type of DegreePhD Dissertation
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Solder joints in electronic assemblies are often subjected to cyclic (positive/negative) mechanical strains and stresses. Such exposures can occur in variable temperature application environments or during accelerated life thermal cycling tests used for qualification. Lead free solder materials are widely used in the electronic packaging industry due to environmental concerns. However, experimental testing and microstructural characterization have revealed that Sn-Ag-Cu (SAC) lead free solders exhibit evolving properties that change significantly with isothermal aging and cyclic loading. This dissertation addresses those changes in the microstructure and properties of lead free solders by conducting four different projects. In the first project, three new Bi-doped SAC solder materials (SAC_R, SAC_Q, and Innolot) have been chemically analyzed and then mechanically tested at high temperature in order to determine the nine Anand parameters. The nine Anand parameters were determined for each unique solder alloy from a set of uni-axial tensile tests performed at several strain rates and temperatures. Later, cyclic stress-strain behavior of these three doped alloys have been investigated and compared with the standard lead-free solder alloy. In the second project, the effects of isothermal mechanical cycling on damage accumulation in lead free SAC305 and SAC_Q solder joints have been explored. Uniaxial samples of SAC305 and SAC_Q have been prepared and subjected to various durations of prior mechanical cycling. The cycled samples were then subsequently subjected to stress-strain or creep testing. The cycling induced degradation of several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) have been characterized and quantified with the amount of prior cycling. Moreover, these evolutions observed in the two lead free alloys have been correlated with the observed changes in their microstructures that occurred during the cycling. In the third part of this dissertation, the aging induced evolution of the cyclic stress-strain behavior of three different doped SAC solder materials has been investigated. The specimens were aged at 125 °C for different durations (0-6 months) and the evolution of the hysteresis loop area, peak load, and plastic strain range have been characterized. Later, the effects of prior aging on damage accumulation occurring in SAC305 and SAC_Q have been investigated while subjected to mechanical cycling (fatigue testing). Regions of interest near the center of the sample were marked using a nanoindentation system. Samples were then subjected to aging at 125 °C. 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), the fixed were examined using scanning electron microscopy (SEM) Using the recorded images, the microstructural evolution in the fixed regions were observed, and the effects of the initial aging on the results were determined. In the last project, the mechanical cyclic induced evolution in mechanical properties of SAC 305 individual solder joint have been investigated. The test assemblies in this study were (3X3) BGA packages composed of total nine 0.75 mm diameter lead free solder balls which were formulated by reflowing solder spheres soldered onto 0.55 mm diameter Cu pads on FR4 coupons. The solder balls were cycled using an Instron Micro mechanical tester along with a newly designed fixture which facilitates the tester to cycle a solder ball individually. Nanoindentation tests were performed on the specimens to study the evolution in mechanical properties (e.g. elastic modulus, hardness and creep strain rate) of the solder balls as a function of duration of cycling.