Ultrashort pulse lasers combined with exquisite self-focusing technology provide the quality and process reliability required to make laser glass welding applications for volume production possible. The unique and excellent properties of glass make it widely used in various high-tech products in different fields such as biomedicine and microelectronics. But it poses challenges for manufacturers, particularly in the area of high-volume, precision glass cutting. It also presents difficulties with bonding, including welding individual glass components together and welding glass to other materials such as metals and semiconductors.
Blending as one
All conventional methods used for welding glass struggle to provide the precision, bonding quality and production speed required for economical and efficient series production. Adhesive bonding, for example, is an economical method, but leaves adhesive material on the part and even requires degassing.
Dielectric welding involves placing a powdered material at the point of contact and melting it to complete the bond. Whether this melting is achieved by oven or laser, there is a lot of heat being pumped into the part. This is a problem for microelectronic devices and many medical devices.
Ionic bonding is an ingenious method that provides extremely high bond strengths. Two new and extremely flat glass surfaces are pressed together and literally fused together by molecular bonding. However, it is not practical to perform this operation in a production environment.
Laser glass welding
What about laser welding? Glass has many very useful properties, such as a very high melting point, transparency, brittleness and mechanical rigidity, but at the same time it poses many difficulties for laser welding. Therefore, typical industrial lasers and methods used for welding metals and other materials are not applicable to glass.
Just like precision glass cutting, the secret lies in the use of infrared wavelength ultra-short pulse (USP) lasers. The glass is transparent in the infrared, so the focused laser beam can pass right through it until the focused beam narrows and becomes so concentrated that it triggers "non-linear absorption". This "non-linear absorption" can only occur with ultra-short pulsed lasers with high peak power, and the same cannot be done with other types of laser.
So, in a very small area (usually less than a few tens of microns in diameter) around the focus of the laser beam, the glass absorbs the laser and melts rapidly. This focused beam is scanned along the desired welding path to complete the bonding, just like any other form of laser welding.
The USP laser glass welding method offers three main advantages.
Firstly, it creates a strong bond as both materials being welded are partially melted and then solidified together to form the weld. Furthermore, the process is equally suitable for bonding glass to glass, glass to metal and glass to semiconductor.
Secondly, in this process, only a very small amount of heat enters the part, which is generated in an area at most a few hundred microns wide.This allows the placement of solder joints very close to electronic circuits or other thermally sensitive components, which provides greater freedom for designers and manufacturers and supports better product miniaturisation designs.
Finally, if USP laser glass welding is carried out properly, no micro-cracks will be created around the weld.And Microcracks reduce the mechanical strength of the glass.In addition, after temperature cycling (which is inevitable for everything), micro-cracks can be the source of eventual equipment failure.
Putting USP laser glass welding to work
The advantages of USP laser glass welding stem from the fact that the glass is heated in only a small volume. However, this also poses a challenge in practice.This means that the laser focus position must remain very precisely at the interface between the two welded components, even if the part moves. This is difficult to achieve because real-world components are not completely flat. In addition, the position in which the parts are placed in the welding system may not fit perfectly.
One solution is to use an axially elongated focal point. This "stretches" the size of the laser beam focal point to solve the position sensitivity problem.The disadvantage of this method, however, is that the elongated beam focus creates a melt pool in the glass with a non-circular cross-section. When the glass solidifies in the melting zone, the non-circular pool is more likely to form microcracks.
Another method has been adopted to achieve microcrack-free welding results and to accommodate significant changes in interface distances in the process at the same time.The secret lies in the combination of highly dynamic focusing technology, using high numerical aperture (NA) optics to produce a small focal spot.
As a result, the laser system achieves a high sphericity of the melt pool and thus avoids microcracking. It also senses the interface distance and constantly adjusts the optics so that perfect focus is always maintained.
The result is a high quality weld on almost any shape of part, and the process is independent of the tolerances and position of the part.
