Richard Grugel, a materials scientist at the Marshall Space Flight Center, watched his video monitor in disbelief. A transmission from the International Space Station was playing. The scene: Astronaut Mike Fincke touches the tip of a soldering iron to a wire wrapped with rosin-core solder.
Here's what happened:
The solder, heated, became a molten blob with a droplet of rosin clinging tight to the outside. Solder melts: that's not too surprising. It's the behavior of the rosin that amazed. As the temperature increased, the droplet began to spin, round and round, faster and faster, like a miniature carnival ride.
"What a surprise," says Grugel. "I've never seen anything quite like it."
Grugel is the principal investigator of the In-Space Soldering Investigation, or "ISSI" for short, which Fincke was doing at the time of the discovery. ISSI's purpose is to find out how solder behaves in a weightless environment. This is important information for astronauts. If something breaks during a long trip to Mars, they'll likely reach for a soldering iron to repair it.
(Editor's Note: ISSI isn't the first experiment to investigate reduced-gravity soldering. See, e.g., Gravitational Effects on Solder Joints by R. D. Pettegrew et al..)
The solder Fincke used for ISSI is a mixture of lead, tin and rosin. The purpose of lead and tin is to form an electrically conducting connection. What does the rosin do?
Grugel explains: "When metals are exposed to air, they become coated with oxides." Iron, for example, rusts: iron oxide. "One purpose of rosin," he says, "is to wash away any oxides before the lead and tin solidify, clearing the way for a good strong connection."
Below: The action of rosin, from Integrated Publishing's Electrical Engineering Training series. [More]
Rosin has another purpose, too. On Earth and in space, surface tension tends to hold solder in awkward blobs. Rosin breaks the tension, an action called "wetting," allowing molten solder to flow.
But, as the video shows, weightless rosin doesn't always do what's expected. Is this a problem?
To find out, Grugel plans to slice the solder-blobs created by Fincke and examine what lies inside. He'll be able to see whether the solder made a clean connection to the wire. He'll also look for tiny bubbles of vaporized rosin inside the blobs; such bubbles, which also appear in solder joints on Earth, lessen the electrical and thermal conductivity of the connection.
The samples will be returned to Earth by astronauts in a Soyuz capsule or, perhaps, after the space shuttle returns to flight. The date isn't set.
Meanwhile, Grugel and his colleagues are brainstorming, trying to understand what causes the rosin to twirl. "We almost have it," Grugel says, but he's not ready to announce a solution yet. He does, however, have some advice for astronauts bent on soldering: wear your goggles and watch out for flying rosin.
Gravitational Effects on Solder Joints --
(Welding Journal) by R. D. Pettegrew, P. M. Struk, J. K. Watson, D. R. Haylett, and R. S. Downs
Rolling Boil -- (Science@NASA) Astronaut Don Pettit investigates the physics of boiling water using a soldering iron onboard the International Space Station.
Houston, we have a solution -- (Science@NASA) New research aboard the space station aims to adapt a tried-and-true repair tool to weightlessness.
In Space Soldering Investigation (ISSI) fact sheet -- from the Marshall Space Flight Center
More information about ISSI -- from NASA's Office of Biological and Physical Research