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Solder Bumping Improves Lens

Photonics.com
May 2008
JENA, Germany, May 7, 2008 -- 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.SolderedLenses.jpg
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)
“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


GLOSSARY
adhesive
An intermolecular substance that serves to hold materials together. Two types are used in the optical industry: one, which must be transparent and colorless, to cement lenses together; and a general-purpose adhesive for bonding prisms and other glass parts to their metallic supports.
camera
A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
electronics
That branch of science involved in the study and utilization of the motion, emissions and behaviors of currents of electrical energy flowing through gases, vacuums, semiconductors and conductors, not to be confused with electrics, which deals primarily with the conduction of large currents of electricity through metals.
flux
Time rate of flow of energy; the radiant or luminous power in a beam.
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
microscope
An instrument consisting essentially of a tube 160 mm long, with an objective lens at the distant end and an eyepiece at the near end. The objective forms a real aerial image of the object in the focal plane of the eyepiece where it is observed by the eye. The overall magnifying power is equal to the linear magnification of the objective multiplied by the magnifying power of the eyepiece. The eyepiece can be replaced by a film to photograph the primary image, or a positive or negative relay...
optical
Pertaining to optics and the phenomena of light.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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