The ongoing project is being conducted under six major tasks. Each task is composed of integrated experimental -- at Iowa State University (ISU), and Advanced Photon Source (APS) -- and modeling at Iowa State University (ISU). In this work, using four microgravity solders from the In-Space Soldering Investigation (ISSI) study, two ISSI terrestrial solders along with a third set of freshly made terrestrial 40wt%Pb-60wt%Sn solder and 4th set of 50wt%Pb-50wt%Sn solders, significant progress has been made in Task A to Task E. Under these tasks, we report results from the advanced microstructural characterization and resultant micro-to-nano mechanical response of solders in terrestrial vs. microgravity environments, 1g vs. ~1×10-5g under different temperature conditions.

In Task A (Year 1), we conducted 3-D Microstructural Characterization of Solder Samples at the Advanced Photon Source. We performed non-destructive tomography to characterize the 3-D distribution of Pb-rich dendrites and voids in the Pb-Sn solder. However, high-resolution tomography scans were limited to sample thicknesses of 700 µm or less so we had to cut the solder sample. We cut them down to 700 µm x 1 mm rectangular sections in order to perform the tomography experiment.

In Task B, C and D (Partially completed in Year 1), we polished the solder samples section by section for a detailed microstructural characterization and nanomechanical testing of microgravity and terrestrial solder samples. The detailed analysis of voids distribution in the microgravity and terrestrial samples demonstrated that the number of voids in the ISSI microgravity wire wrap solder is ~13 times higher than the ISSI terrestrial solder and ~5 times higher than the recently prepared terrestrial solder. Additionally, ISSI microgravity wire feed solder demonstrated ~3.5 times higher number of voids (larger in size) than the ISSI microgravity wire wrap solder. The higher number of voids in the wire feed solder demonstrated that the excess amount of flux was remained internal to the liquid as compared to the wire wrap solder. The ISSI microgravity wire wrap solder demonstrated a homogeneous distribution of Pb-rich phase. In contrast to the ISSI microgravity wire wrap solder, ISSI microgravity wire feed solder demonstrated the accumulation of voids and Pb-rich dendrites in one region. This difference in the solder microstructure is probably caused by the strong Marangoni flow in the wire feed solder. Further characterization of this sample revealed that this Marangoni effect was probably caused by the presence of a large void in the wire feed solder, which moved Pb-rich dendrites and voids from low temperature region to the high temperature region.

Task D: A further consideration, not mentioned in the earlier ISSI report is the substantial effect of aging on the solder microstructure and properties over the past 17 years. Previous investigations have shown that Pb-Sn soldering alloys (due to their low melting temperatures) exhibit grain coarsening and reduced mechanical properties with age, even under ambient conditions (room temperature). Microstructural changes stem from the diffusion of Sn in to the Pb rich regions, creating a finer dispersion of grains as well as voids near phase boundaries. After aging 30 days in ambient conditions, the microstructure and mechanical properties appeared to stabilize. In earlier NASA Physical Sciences Informatics (PSI) reports, we have discussed the aging properties of 40Pb-60Sn solder. In the report, we demonstrate how the microstructure and properties of solder joints change as a function of aging in 50%Pb-50%Sn. We noticed that at room temperature, nanomechanical testing showed a decrease in hardness of the top sample from 0.183GPa on the first day to 0.147GPa by day 90.

Additionally, solder joints in the space environment are subjected to large temperature variations, such as -120 ºC to +25 ºC for the Mars missions, and -157 ºC to +121 ºC outside the International Space Station (ISS). However, as deep space exploration missions become longer and more distant, the solder and other metal joints' current reliability is inadequate for the harsh conditions they encounter. Therefore, we performed in-situ scanning electron microscope (SEM) micro-pillar compression experiments at various cryogenic temperatures to understand the effect of ductile to brittle transition and beta Sn to alpha Sn transformation on the solder properties. Our experimental analysis showed that, as the temperature went down, solder demonstrated higher hardening and strength under the compressive loading conditions. The in-depth data analysis of these experiments is still under investigation to understand the effect of phase transformation on the solder properties.

This project has resulted in two undergraduate projects: 1) Investigate the effect of aging on microstructure and properties of 50Pb-50Sn solders under ambient conditions; 2) Non-destructive 3-D microstructural characterization of Pb-Sn and Sn-Ag-Cu solders. This project has provided two junior undergraduate students (UG) and two sophomore UG students with an opportunity to work on in-space soldering and to get hands-on experience on the lab instruments. In addition, this project has resulted in six presentations at various conferences (e.g., ISSRDC, ASGSR, TMS, NCUR, etc.) and six awards (e.g., Wayne G. Basler Scholarship, ASGSR travel award, FHMP Scholar Award, TMS Acta Materialia scholarship). [Ed. Note: See Bibliography.]