Richard J. Braun,
Geoffrey B. McFadden and Bruce T. Murray, ACMD
William J. Boettinger, Materials Science and Engineering Laboratory
Soldering has been a subject of ongoing research in EEEL and MSEL, and is of great practical interest to a wide variety of industries. The understanding of solder joint formation can lead to more cost-effective and safe joining processes. Environmental legislation banning the use of lead in plumbing applications has aroused the interest of many groups (e.g., the electronic and automotive industries) in lead-free solder systems. We have undertaken the development of some fundamental theory for standardized solder tests in order to better understand the complex physical phenomena observed in soldering processes. The development and understanding of such standard tests is one of NIST's fundamental missions.
The formation of solder joints is strongly dependent on the how well the solder can wet the substrate. Various standard experiments are carried out in an attempt characterize the wettability of solder on various substrates. Two such tests are standard area of spread tests and standard wetting balance tests. In the area of spread test, a droplet of solder with fixed volume is place on a hot, flat substrate, and the molten solder then spreads various amounts depending on the composition of the solder, the substrate and flux used, as well as other variables. In the wetting balance test, a wire (or rod) is submerged into a bath of molten solder, and the mechanical force required to support the wire yields a measure of how well the solder wets the rod. These experiments have been and are currently being carried out within Metallurgy Division, and workers there are finding that there is a real paucity of theory describing the complex transport phenomena during these experiments, and also in the spreading of solder in joining processes.
The investigators have begun analytical and numerical modeling of the fluid dynamics and heat and mass transport aspects of the standard tests mentioned above. Two models have been developed for the area of spread test. The first is a combination of analytical and numerical techniques; the solder drops become very thin, and so asymptotic methods may be used to derive simplified nonlinear partial differential equations for the evolution of the drop surface and the alloying element distributions in the drop. In order to solve these evolution equations, we use a variation of the method of lines developed by the investigators in CAML. The model has not been able to reach the extreme parameter regimes required by solder materials, but has yielded insight into the spreading and transport processes. The results have been published as an internal report which has been submitted to a refereed journal. The second model has been built within a sophisticated commercial finite-element package procured by one of the investigators. Some results have been obtained for fixed contact line position (no spreading) with realistic parameters values. Results from each model are used to understand and improve the other, and the results provide insight into the experiments carried out in Metallurgy Division. Modeling work on the wetting balance test is still in very early stages.
This work will provide insight into the relative importance of the spreading and transport processes, and to have predictive capability in other, related situations involving solder spreading.