Research Projects of Daniel J. Lewis
Modeling Solder Joint Shape with Surface Evolver
Through the use of Surface Evolver software and our attendance at the annual Solder Interconnect Design Team workshops, it has been possible to model sessile drop wetting behaivor, solder manufacturing parameters such as solder volume, and gull-wing solder joint meniscus geometry. More recently we have modified the generic catenoid problem to solve for the solder joint height in a Ball Grid Array (BGA) or Flip Chip type package.Chip type package.
Material parameters such as the density of the solder, and the surface tension have been included in the following models. Other basic inputs for the models include solder volume, and solder wetting angle. Many thanks go to the Author of Surface Evolver, Ken Brakke, whose help has been greatly appreciated. A repository of solder joint modeling utilizing Surface Evolver software can be found at the Center for Theoretical and Computational Materials Science, at NIST, Gaithersburg, Maryland. The following problems have been addressed:
Phase Equilibria in the Ag-Sb-Sn System
Multiple calculated versions of the Ag-Sb-Sn phase diagram exist. This phase diagram is difficult to determine experimentally due to the Sb2Sn3 phase that is stable only at high temperatures. Corre only at high temperatures. Correlations between DSC results and theoretical phase diagrams have been attempted. In addition a study of the Sb2Sn3 phase's history has also been completed.
Phase Equilibria in the Au-Pb-Sn System
The tin-lead-gold system was first studied by A. Prince in 1966 and again by G. Humpston in 1984. Both have determined the ternary eutectic composition to be within 2-4 at% Au. Prince's value lies near 2 at% Au and Humpston's value lies near 4 at% Au. A series of seven alloys were made at compositions spread around the two proposed ternary eutectic points with at least one alloy in all three primary phase fields. Metallography and DSC were used to determine which of the two compositions were closest to the true ternary eutectic. This information will help microelectronic component assemblers determine the limit of gold solubility in tin-lead solder before large precipitates of AuSn4 embrittle the solder joints. A preprint of the article is available as well as an interactive phase diagram with mircostructures.
Solidification Modeling
A numerical model for ternary transitional alloy solidification will be developed using the Scheil equation as the basis. Incorporation of diffusion will be based on existing analytical models where limited solid state diffusion takes place. In addition to considering diffusion during primary phion during primary phase solidification, diffusion through the intervening transitional reaction product layers will also be modeled. A short proposal on this topic is available.
Research involving Phase-Field modeling of solidification microstructures has also been attempted. Here are two animated GIFs of dendritic solidification and Ostwald ripening.
A mass-balance and diffusion model is being developed. The first test-run of this model in the Pb-Sn system is available.
Wulffman
The Wulffman software, developed at NIST,
was used to verify previously completed surface energy measurements. This
work was submitted as a poster for the 1997 MRS meeting. I have included
some simple instructions for using Wulffman
to generate a Wulff shape. The crystals modeled were LiF and Pyrochlore:
