Fixing lenses in place

Lenses in optical devices are kept in place by adhesives but this can cause problems when the microscopes and cameras are used in a vacuum, as the adhesives may release gases that contaminate the lenses.

At high temperatures, too, or when using lasers in the UV range, the adhesives also cause problems: they become soft or brittle, and the optical components can slip by several micrometres.

One alternative is to solder them instead. And, working in conjection with engineers from Pac Tech, that's exactly what engineers at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena have developed a new system to do.

'We solder the optical components instead of gluing them,' said IOF group manager Dr-Ing. Erik Beckert. '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 manufacture. 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. But to ensure that the solder does not come off the glass lenses, they have to be metallised in advance in a sputtering process which can be carried out on a large scale.

The researchers will present prototypes of optical components fixed in place by solder bumping at Optatec, which will take place in Frankfurt in June. Beckert hopes that the system will be ready for use in production in a year or two’s time.