Mechanical Behavior and Microstructural Evolution of Lead Free Solder Alloys in Harsh Environment Applications

Abstract

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. Lead free solders are common as interconnects in electronic packaging due to their relatively high melting point, attractive mechanical properties, thermal cycling reliability, and environment friendly chemical properties. However, 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 to create a database of extreme high-temperature mechanical properties and the associated microstructural changes of several lead free solder alloys. In the first project, several SAC and SAC+X lead free solder alloys, recommended for high reliability applications have been chemically analyzed and then mechanically tested in order to determine the temperature dependent mechanical properties of these alloys. The alloys include SAC305, Ecolloy (SAC_R), Cyclomax (SAC_Q), and Innolot. The mechanical behavior of these alloys have been explored at several extreme high temperatures from 125 to 200 °C. For each of 4 elevated temperatures (T = 125, 150, 175, and 200 °C), tensile stress-strain tests were performed at three strain rates (SR = 0.001, 0.0001, and 0.00001 sec-1). For each alloy and testing temperature, the stress-strain curve shape and high temperature tensile properties (initial modulus, yield stress, and ultimate tensile strength) were measured and compared. In the second project, temperature dependent stress-strain behavior of SAC305, SAC_Q, and Innolot solders subjected to high temperature aging at 125 and 200 °C have been reported. Before testing, the solder uniaxial specimens were aged (preconditioned) at the extreme high temperature of either T = 125 °C or T = 200 °C. At each of these aging temperatures, several durations of aging were considered including 0, 1, 5, and 20 days. Stress-strain and creep tests were then performed on the aged specimens. Using the measured data, the evolutions of the stress-strain and creep behaviors were determined as a function of aging temperature and aging time. In third project, SAC305 and doped SAC solder alloys (SAC_Q and Innolot) recommended for high reliability applications have been chemically analyzed and then mechanically tested in order to determine the nine Anand parameters. Anand parameters were determined for SAC305 with both water quenched and reflowed microstructures. For SAC_Q and Innolot, only reflowed microstructures were explored to determine the Anand parameters. 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 125, 150, 175, and 200 °C. In the fourth 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. The changes in mechanical behavior of solder alloys that occur during isothermal aging are a result of the evolution of microstructure. In the final project, the microstructural evolution of solder alloys were investigated for different aging conditions. In particular, aging was performed at T = 125, 150, and 175 °C for up to 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. These images were used to predict the microstructural evolution in SAC305 solder joints exposed to high temperature aging. 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.