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Laser Soldering of a Vacuum Measuring Cell

A solder ring has to seal a metallised ceramic cell inside a vacuum chamber, through a viewport – and leave the bore to the reference chamber open while doing it.

Photonics & Laser

Laser Soldering of a Vacuum Measuring Cell

Photonics & Laser

01Starting point

A solder ring has to seal a metallised ceramic cell inside a vacuum chamber, through a viewport – and leave the bore to the reference chamber open while doing it. An earlier trial had managed this for the first time, but with around 15 minutes of continuous laser time. That proves it can be done and, at the same time, that it is not yet a process. The real question was: why does the solder wet the metallisation so reluctantly, and how do you get the energy in faster without destroying something?

02Approach

With a 150 W source, coupled through a 600/720 µm high-power fibre, and a two-lens optic that images the fibre cross-section at roughly 1:10 onto a 6 mm beam diameter – deliberately not focused but spread out, so the ceramic substrate heats evenly. Plus a loss budget written before the first shot: 7 % per lens, 7 % reflection and 2.5 % transmission loss at the viewport, giving 23.5 % total loss and roughly 114.75 W actually arriving. And a thermal simulation that came out pleasingly linear: about 1.75 °C per watt of laser power and about 1 °C per degree of substrate temperature.

03Result

Fifteen minutes became ten seconds – a factor of 90. At 150 W for 10 s, that is 1.5 kJ, good wetting was achieved between solder and substrate. More important still: the process is now described in numbers rather than in instrument settings. Anyone who changes the optics knows what has to be recalculated.

What's inside

Build-up of the laser source with fibre and optics, the optical loss budget, a thermal simulation over laser power and substrate temperature, trials on several cell types including sectioned images of the joint – and a test series across 20 cells in which the pump-down time before soldering was deliberately varied between roughly 3 and roughly 13 hours, with the pressure logged between 3.5 · 10⁻⁷ and 8.6 · 10⁻⁷ mbar.

And what happened next

The most instructive result was a failure. Double the energy – 150 W for 20 s, 3 kJ – produced not a better joint but no joint at all: the PdAg metallisation was in all probability vaporised and deposited itself on the substrate and on the viewport. Above the working point the process does not get worse, it changes mechanism – and in doing so it damages the very window it works through. And one cell that looked fine from the outside showed, in section, a dark, poorly wetted zone right at the substrate. Which is why we went on cutting rather than looking.

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