We have previously discussed the challenges it presents to manufacturers, particularly in the field of high-volume, precision glass cutting. It also introduces difficulties in bonding, including welding individual glass components together and welding glass to other materials such as metals and semiconductors.
Integrate into one
All traditional methods used for welding glass struggle to provide the required precision, bond quality, and production speed for cost-effective mass production. For example, adhesive bonding is an economical method, but it leaves adhesive residue on the parts and may even require degassing.
Dielectric bonding involves placing powdered material at the contact points and then melting it to complete the bonding. Whether this melting is achieved through an oven or a laser, a significant amount of heat is pumped into the part. This poses a problem for microelectronic devices and many medical devices.
Ion bonding is an ingenious method that provides extremely high bond strength. Two brand-new, extremely flat glass surfaces are pressed together and truly fused together through molecular bonds. However, performing this in a production environment is not practical.
Laser glass welding
So, what about laser welding? Glass has many very useful properties, such as an extremely high melting point, transparency, brittleness, and mechanical rigidity, but these also present many challenges for laser welding. Therefore, typical industrial lasers and methods used for welding metals and other materials are not suitable for glass.
Just like precision glass cutting, the secret lies in using an infrared-wavelength ultrashort pulse (USP) laser. Glass is transparent in the infrared, so a focused laser beam can pass directly through it until the focused beam narrows and becomes concentrated, triggering "nonlinear absorption." This "nonlinear absorption" only occurs in ultrashort pulse lasers with high peak power, and cannot be accomplished using other types of lasers.
Therefore, within a very small area (typically less than tens of micrometers in diameter) around the focal point of the laser beam, the glass absorbs the laser and melts rapidly. This focused beam then scans along the desired welding path to complete the bonding, just like other forms of laser welding.
The USP laser glass welding method has three main advantages.
First, it creates a strong bond because both materials being welded partially melt and then solidify together to form a weld. Moreover, this process is equally suitable for bonding glass to glass, glass to metal, and glass to semiconductors.
Secondly, in this process, very little heat enters the component, generating heat in an area at most a few hundred micrometers wide. This allows the weld seam to be placed very close to electronic circuits or other heat-sensitive components, providing designers and manufacturers with greater freedom and supporting better product miniaturization designs.
Finally, if USP laser glass welding is performed correctly, microcracks will not form around the weld. Microcracks reduce the mechanical strength of the glass. Furthermore, after temperature cycles (which are unavoidable in all things), microcracks can become the root cause of eventual equipment failure.
Coherent has put USP laser glass welding into practical use.
The advantage of USP laser glass welding lies in the fact that the glass is heated within a very small volume. However, this also presents challenges in practice. This means that even if the parts move, the laser focus must be maintained very precisely at the interface between the two welded components. This is difficult to achieve because real-world parts are not perfectly flat. Furthermore, the positions of the parts in the welding system may not be perfectly aligned.
One solution is to use an axially elongated focal spot. This "extends" the size of the laser beam focus to address the position sensitivity issue. However, a drawback of this approach is that the elongated beam focus creates a molten pool with a non-circular cross-section within the glass. This non-circular pool is more prone to microcrack formation as the glass solidifies in the molten zone.
Coherent employs an alternative approach to achieve microcrack-free welding while simultaneously adapting to significant variations in interface distances throughout the process. Their secret lies in combining high dynamic range focusing technology with high numerical aperture (NA) optics to generate a small focal spot.
Therefore, Coherent's laser system achieves a highly spherical weld pool, thus avoiding microcracks. It also senses the interface distance and continuously adjusts the optics to maintain perfect focus at all times. As a result, high-quality welds are guaranteed on virtually any shape of part, and the process is unaffected by part tolerances or position.
