High-Speed Imaging Probes Mysteries of Spreading Metal

Hank Hogan

When Eduardo Saiz, a staff scientist at Lawrence Berkeley National Laboratory in Berkeley, Calif., points out the importance of liquid metals, he's not just talking about the mercury found in thermometers. Heated enough, metals liquefy and spread, a behavior that plays an important role in brazing, soldering, thin films, microelectronics, optoelectronics and other areas.

There is much empirical knowledge about liquid metal spreading, but little in the way of predictive theory. As a result, the development of new solders or brazes and of new processes often requires lengthy trial-and-error efforts. Saiz and other researchers at the laboratory are working to correct that deficiency.

Using high-speed imaging, researchers observed the spreading dynamics of liquid metal. Analysis of the films was performed using optical and atomic force microscopy. Images by Robert Couto. Courtesy of Lawrence Berkeley National Laboratory.

Out of necessity, they've turned to high-speed imaging. "Due to the speed at which the liquid moves, it would be impossible to analyze spreading without high-speed photography," said Saiz, lead author of a December Nature Materials paper on the work with Antoni P. Tomsia.

For the experiments, they developed a system to dispense the metal drops. They used two high-speed cameras from Photron USA Inc. of San Diego to look into a furnace where the molten metal -- mixtures of silver and molybdenum, silver and tungsten, copper and nickel, gold and nickel, and germanium and silicon -- spread. One, the FastCam-X 512 PCI, is capable of 2000 fps at a resolution of 512 × 512 pixels, or of up to 32,000 fps at reduced resolution. The second, the FastCam Super 10K/2K1, runs at a maximum of 2000 partial frames per second. To capture the images without melting the equipment, they employed a telescope setup designed by ramé-hart inc. of Mountain Lakes, N.J.

Although extremely high frame rates are possible, Saiz noted that lighting limitations act as a brake on imaging speed. At the highest possible speeds, the captured images and the movies derived from them would be too dark to be useful. The need for adequate illumination restricts the imaging speed to the 2000- to 8000-fps range, meaning that a frame lasts 0.5 ms or less.
Previously reported times for the spreading of small metal droplets ranged from milliseconds to minutes or longer. The high-speed images, in contrast, indicate that the drops spread in 20 to 30 ms, comparable to organic liquids of similar viscosities.

In a new discovery, the researchers also observed that liquid metals form precursor -- or Marangoni -- films when spreading. These types of thin layers, which stretch ahead of the main liquid, are common in the spreading of lower-temperature films.

They also found that the spreading speed of liquid metals is not controlled by viscosity and surface tension, as is the case for organics. Instead, the speed seems to be dictated by friction at the triple line -- the contact line at the foot of the drop. "The controlling step seems to be similar to the one controlling surface diffusion," Saiz said.

The scientists hope that this information will lead to the development of successful theories that can predict metal spreading. This will perhaps provide solutions to technological problems.

Some possibilities, Saiz suggested, include solders to bond ceramics at low temperatures, lead-free solders with improved characteristics and processes to control liquid metal flow at smaller dimensions. Rapid progress might be possible if technologists were armed with a good theory, he added.