Effects of Aging on Microstructure and Mechanical Properties of Lead Free Solder Materials

Abstract

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 environmental exposures such as isothermal aging and thermal cycling. 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 SAC_Bi lead free solder materials recommended for high reliability applications have been chemically analyzed and then mechanically tested in order to determine the nine Anand parameters. The alloys are referred to as Ecolloy (SAC_R), CYCLOMAX (SAC_Q), and Innolot by their vendors. For each alloy, three different microstructures were explored using different cooling profiles as well as subsequent isothermal aging. The nine Anand parameters were determined for each unique solder alloy from a set of uniaxial tensile tests performed at several strain rates and temperatures. Testing conditions included strain rates of 0.001, 0.0001, and 0.00001 (sec-1), and temperatures of 25, 50, 75, 100, and 125 oC. The mechanical properties and the values of Anand parameters for these new SAC-Bi alloys were compared with those for standard SAC105, SAC305, and SAC405 lead free alloys. In the second project, nanoindentation methods were utilized to explore the creep behavior, and aging effects of SAC305 solder joints at several elevated testing temperatures from 25 to 125 oC. A special high temperature stage and test protocol was used within the nanoindentation system to carefully control the testing temperature, and make the measurements insensitive to thermal drift problems. Solder joints were extracted from 14 x 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter). For all the experiments, only single grain solder joints were used to avoid introducing any unintentional variation from changes in the crystal orientation across the joint cross-section. After extraction, the single grain solder joints were subjected to various aging conditions. Nanoindentation testing was then performed on the aged specimens at five different testing temperatures (T = 25, 50, 75, 100, and 125 oC). In order to understand creep response of the solder joints at different temperatures, a constant force at max indentation was applied for 900 sec while the creep displacements were monitored. With this approach, creep strain rate was measured as a function of both temperature and prior aging conditions. Nanoindentation pile-up effects, although insignificant at room temperature, were observed during high-temperature testing and corrections were made to limit their influence on the test results. The changes in solder mechanical behavior that occur during isothermal aging are a result of the evolution of the SAC solder microstructure. In the third part of this dissertation, new procedures were developed to capture solder microstructure while the sample is being heated inside an oven (in-situ aging study). The heating stage and scanning probe microscopy (SPM) facility within a nanoindentation system were utilized to achieve the goal. The sample was kept within the nanoindentation system and exposed to a high temperature aging using the heating stage present in the instrument. In particular, aging was performed at T = 125 oC for up to 26 hours, and the topography of the microstructure of a fixed region (10 × 10 microns) was continuously scanned using the SPM system and recorded after one hour time intervals. Image analysis software was utilized to quantify microstructural changes (total area, number and average diameter of IMC particles, interparticle spacing etc.) with respect to aging time. In the last project, the board level thermal cycling reliability of Super Ball Grid Array (SBGA) packages has been investigated by simulation and experimental testing. Nanoindentation and strain gauge based CTE (coefficient of thermal expansion) measurement methods were utilized to extract mechanical and thermal properties of different layers of a 31 mm SBGA package such as substrates, mold, die, solder mask, Cu-pad, solder, adhesive material etc. Extensive microscopic study was performed to understand the construction of SBGA package, FR-4 and Megtron6 PCB laminates. Finite element modeling was used to predict the reliability if SBGA assemblies for different PCB laminate materials. The FEA results have been validated through correlation with thermal cycling accelerated life testing experimental data.