Thermal-Mechanical Characterization and Microstructural Evolution of Lead Free Solder Alloys in Harsh Environment Applications
Type of DegreePhD Dissertation
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With the emergence of the modern electronic packaging technology over the last few decades, lead free solder alloys have been the primary interconnects material used in electronic packaging industry due to their relatively high melting point, attractive mechanical properties, thermal cycling reliability, and environment friendly chemical properties. As solder joints provide mechanical support, electrical and thermal interconnection between packaging levels in microelectronics assembly systems, proper functioning of these interconnections and the reliability of the electronic packages depend largely on the mechanical properties of the solder joints. However, the mechanical properties as well as the reliability of a solder, which is the primary interconnect of electronic assemblies are strongly influenced by its microstructure, which is controlled by its thermal history including its solidification rate and thermal exposures after solidification. Environmental conditions, such as, operating temperature, aging temperature, and aging time significantly affect these properties due to the microstructural evolution of the solder that occurs during aging. Moreover, electronic devices, sometimes experience harsh environment applications including well drilling, geothermal energy, automotive power electronics, and aerospace engines, where solders are exposed to very high temperatures from T = 125-200 °C. Mechanical properties of lead free solders at elevated temperatures are limited. This research involves several projects combining finite element and experimental approaches to investigate new reliability parameters and create a database of mechanical properties for different temperature exposure and the associated microstructural changes of several conventional and new lead free solder alloys. In the first project, a combined numerical and experimental study have been carried out to establish Poisson’s ratio as a new reliability parameter for lead free solder joints. The effects of Poisson’s ratio on the stress/strain behavior in the BGA components subjected to thermal cycling has been investigated by a three dimensional finite element analysis (FEA). The test vehicle from which various models were built using finite element analysis included various sizes of BGA components including 19 x 19 mm, 15 x 15 mm, 10 x 10 mm and 5 x 5 mm packages with SAC305 and SAC105 solder joints. Temperature dependent stress/strain behaviors were included for the packaging materials, with a thermal cycling between T = -40 to 125 oC. Experimental investigations have been carried out to determine the exact Poisson’s ratio of several SAC and SAC+X lead free solders. Uniaxial tensile stress-strain tests have been performed with two strain rates (0.0001, and 0.00001 (sec-1)) and four testing temperatures (T = 25, 50, 75, 100 oC). A set of additional experiments have been performed to determine the effects of aging. In particular the samples were aged (preconditioned) at a temperature of T = 125 °C for several durations including 0, 1, 2, 3, 4, 5, 6, 12, and 24 hours at a particular strain rate of 0.00001 (sec-1). In the second project, the mechanical behavior, microstructural evolution, and reliability of several different SAC+Bi alloys with various levels of Bismuth (1.0%, 2.0%, and 3.0%) have been investigated. To examine the base mechanical behavior, stress-strain tests were performed for each SAC+Bi alloy with three strain rates (0.001, 0.0001, and 0.00001 (sec-1)), and five different testing temperatures (T = 25, 50, 75, 100, and 125 oC). The Anand parameters were calculated for each alloy from the stress-strain data. In addition, the temperature dependent mechanical properties of the various SAC+Bi solders were measured and compared including effective modulus, yield stress, and ultimate tensile strength. The effects of aging has been studied for the various SAC+Bi alloys using both mechanical testing and microstructure observations. For the solder mechanical response, the fabricated uniaxial specimens were aged (preconditioned) at T = 125 °C for several durations of aging including 0, 1, 5, and 20 days. Stress-strain tests on the aged specimens were then performed at a single strain rate of 0.001 (sec-1), and temperatures of 25, 50, 75, 100, and 125 °C. Microstructural evolutions of the new solder alloys (1%, and 2% Bi) were also observed for aging at T = 125 °C for the same durations of 0, 1, 5, and 20 days. In particular, aging induced coarsening of the IMCs was studied for each alloy using Scanning Electron Microscopy (SEM), and correlated to corresponding material property evolution findings. In third project, the effect of extreme high temperature exposure has been studied using SAC305 lead free solder alloy by determining mechanical properties and Anand model parameters. In particular, uniaxial tensile stress-strain tests were carried out on SAC305 specimens using a micro tension/torsion testing machine with three strain rates (0.001, 0.0001 and 0.00001 (1/sec)), four extreme high testing temperatures (T = 125, 150, 175, and 200 oC), and four different pre-aging conditions 0, 1, 5, and 20 days at an isothermal aging temperature of T = 125 oC. The nine Anand parameters were determined using the uniaxial tensile tests results at several strain rates and temperatures mentioned above. In the fourth project, the microstructural evolution of SAC305 (96.5Sn-3.0Ag-0.5Cu) BGA joints have been investigated for different aging conditions utilizing Scanning Electron Microscopy (SEM). In particular, aging induced microstructural changes occurring within fixed regions have been monitored in selected lead free solder joints to create time-lapse imagery of the microstructure evolution. Aging was performed at T = 125, and 150 °C for several durations including 0, 1, 5, 10, and 20 days, and the topography of the microstructure of a fixed region was captured using the SEM system. This process generated several images of the microstructure as the aging progressed containing the visual and quantitative information of diffusion of copper in the β-Sn matrix, growth of the Cu6Sn5 IMC layer as a function of aging time and temperature at the solder joint to PCB copper pad interface, and the quantitative analysis of the evolution of Ag3Sn IMC particles during aging. In the final project, nanoindentation methods were utilized to explore the creep behavior, and aging effects of SAC305 solder joints at several extreme high testing temperatures from 125 to 200 °C. 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 four different testing temperatures (T = 125, 150, 175, and 200 °C). 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. Finally, correlation has been shown between the nanoindentation test results and the microstructural evolution of SAC305 BGA joints.