A solder technique used in electronics manufacturing is proving useful in keeping optical device lenses clear and in place when they are used inside a vacuum.

When cameras and microscopes are placed in a vacuum, their lenses and prisms, usually fixed with adhesives, release gases. The gas molecules settle on the lenses and alter their optical properties. Also, at high temperatures, or when used with lasers in the ultraviolet range, the optics' adhesives can become soft or brittle, and the optical components can slip by several micrometers.

Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering (Fraunhofer IOF) in Jena have now developed an alternative in a joint project with engineers from Pac Tech GmbH in Nauen.

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A dosing head heats small solder balls with a laser beam and accurately shoots the melted droplets onto the lenses to fix them perfectly in place. (Photo copyright ©Fraunhofer IOF)

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“We solder the optical components instead of gluing them,” said IOF group manager Erik Beckert, PhD. “This has a definite advantage: The solder material is resistant to extreme temperatures and radiation, and also conducts heat and electricity.”

To enable them to apply the solder to the lenses just as flexibly as an adhesive, the researchers adapted the “solder bumping” technique normally used in electronics manufacturing. Small balls of solder contained in a dosing head slip one by one into a capillary, where a laser beam heats them until they become liquid. The liquid solder droplets are then shot by a nitrogen pressure pulse to the spot where they are needed to fix the lens. Once in place, the solder cools in just a few milliseconds and solidifies.

“This process can be automated and is very flexible. We can apply the solder downwards or from various other angles and in places that are difficult to access,” said Beckert.

Solder bumping is much faster than gluing. While it takes 10 to 30 seconds to apply an adhesive and let it harden, soldering takes less than one second. To ensure that the solder does not come off the glass lenses, they have to be metallized in advance, for instance in a sputtering process which can be carried out on a large scale. A special feature of the solder bumping process is that it does not need a fluxing agent.

“Normally, flux is used to ensure that the solder fully coats the metal. However, in a vacuum, the flux residues would outgas in a similar way to adhesives. The lens system would have to be thoroughly cleaned before being used. That’s not necessary with the flux-free version,” Beckert said.

The researchers will present prototype optical components fixed in place by solder bumping at Optatec in Frankfurt on June 17-20 (Hall 3, Stand D53). Beckert said he hopes that the system will be ready for production in a year or two.

For more information, visit: www.iof.fraunhofer.de/index_e.html