New Technology Brings Laser Glass Welding into Focus

Ultrashort pulse lasers plus sophisticated autofocus technology deliver the quality and process reliability needed to make laser glass welding a practical reality for volume production

February 22, 2022 by Coherent

Ultrashort Pulse Lasers

The unique and desirable properties of glass have led to its increased use in a wide range of high technology products in areas as diverse as biomedicine, aerospace, and microelectronics. We’ve described before how this creates challenges for manufacturers, particularly in terms of high volume, precision glass cutting. It also presents difficulties in terms of bonding – this means welding individual glass components together, as well as joining glass to other materials like metals and semiconductors.

Getting it together

The problem is that all of the traditional methods for welding glass struggle to deliver the combination of precision, bond quality, and production speed that are necessary for cost-effective volume manufacturing. For example, adhesive bonding is an inexpensive method, but can leave residual glue on the parts and even cause outgassing.

Frit welding involves putting powdered material at the joint, and then melting it to create a bond. Whether this melting is done in an oven or with a laser, a lot of heat gets pumped into the part. That’s a problem for microelectronics and many medical devices.

Ionic bonding is an elegant method that delivers an extremely strong bond. Two pristine and extremely flat glass surfaces are pressed into contact and become literally fused together by molecular bonding. But, performing this on a production basis isn’t really practical.

Laser glass welding

What about laser welding? Well, some of the very same properties that make glass so useful – like it’s extremely high melting point, transparency, brittleness, and mechanical stiffness – also make it difficult to weld with lasers. As a result, the typical industrial lasers and methods used for welding metals and other materials don’t apply well for glass.

Just like with precision glass cutting, the secret is to use an infrared wavelength ultrashort pulse (USP) laser. Glass is transparent in the infrared, so the focused laser beam simply passes right through it. That is, up to the point where the focused beam narrows down and becomes so concentrated that it triggers “non-linear absorption.” This only happens because of the high peak power of the ultrashort pulses, which is why you can’t use other laser types to accomplish the same thing.

So, the glass absorbs the laser light and rapidly melts in a very small region (usually less than tens of micrometers in diameter) around the point of focus. The focused beam is scanned along the desired weld path to create the bond, just like in other forms of laser welding.

USP laser glass welding method delivers three key benefits. First, it produces a strong bond, since both parts are partially melted and then resolidify together to form the weld joint. And, the technique works equally well joining glass to glass, glass to metal, and glass to semiconductors.

Second, the process puts very little heat into the part, and only does that over an area that is less than a couple hundred microns wide, at most. This allows the weld seam to be placed very close to electronic circuitry or other heat-sensitive components. This provides both designers and manufacturers with greater freedom, and supports greater product miniaturization.

Finally, if USP laser glass welding is implemented just right, then it doesn’t create micro-cracks in the area surrounding the weld. Micro-cracks mechanically weaken the glass. Plus, they can be a source of eventual failure in devices that subsequently experience temperature cycling (which happens to just about everything).

Coherent makes USP laser glass welding practical

The advantages of USP laser glass welding derive from the fact that the glass is only heated over a very small volume. But that also creates a challenge when actually implementing it. It means the laser focus position has to be very precisely maintained right at the interface between the two components being welded, even as the parts move. And, this is hard to do because real-world parts aren’t perfectly flat. Also, they may not be positioned exactly level when they’re placed in the welding system.

One solution is to use an axially elongated focus. This “stretches” the laser beam focus out to overcome the problem with position sensitivity. However, the negative of this approach is that this elongated beam focus creates a melt pool in the glass that is no longer round in cross-section. A less than round melt pool is much more likely to form micro-cracks as the glass solidifies in the melt zone.

Optogenetics

Coherent has taken a different approach that delivers micro-crack free welds while still accommodating substantial changes in interface distance during the process. The secret is to utilize high numerical aperture (NA) optics to produce a small focused spot, in combination with highly dynamic focusing technology.

So, the Coherent system achieves a highly spherical melt pool, which avoids micro-cracking. And, it senses the interface distance and continuously adjusts the optics so that focus is always perfectly maintained. The result is high-quality welds on pretty much any shape part, along with a process that is immune to part tolerances and position.

Learn more about Coherent glass welders, ready for integration into customer tools or production lines, as well as complete, stand-alone glass welding systems.

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