Ultrashort pulse laser combined with sophisticated self focusing technology provides the required quality and process reliability, making it possible to apply laser glass welding to mass production. The unique and excellent properties of glass make it widely used in various high-tech products in biomedicine, microelectronics and other fields. We have previously introduced the challenges it brings to manufacturers, especially in the field of high-volume and precision glass cutting. It also introduces bonding difficulties, including welding individual glass components together and welding glass to other materials such as metals and semiconductors.
inosculate as a whole
All traditional methods for welding glass are difficult to provide the required precision, bonding quality and production speed for cost-effective mass production. For example, adhesive bonding is an economical method, but it will leave adhesive material on the parts and even require degassing.
Medium welding is to place the powder material at the contact point, and then melt it to complete the bonding. Whether this melting is achieved by oven or laser, a large amount of heat will be pumped into the parts. This is a problem for microelectronic devices and many medical devices.
Ion bonding is an ingenious method, which can provide extremely high bonding strength. Two new and extremely flat glass surfaces are pressed together and truly fused by molecular bonds. However, it is not realistic to do this in a production environment.
Laser glass welding
What about laser welding? Glass has many useful properties, such as high melting point, transparency, brittleness and mechanical rigidity, but it also brings many difficulties to laser welding. Therefore, typical industrial lasers and methods 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. Glass is transparent in the infrared, so the focused laser beam can pass through it directly until the focused beam narrows and becomes concentrated, triggering "nonlinear absorption". This "nonlinear absorption" can only occur in ultrashort pulse lasers with high peak power, and other types of lasers cannot be used to accomplish the same thing.
Therefore, in a very small area (usually less than tens of microns in diameter) around the focus of the laser beam, the glass absorbs the laser and melts rapidly. The focused beam is scanned along the required welding path to complete the bonding, just like other forms of laser welding.
USP laser glass welding method has three main advantages.
First, it produces a strong bond because both materials are partially melted and then solidified together to form a weld. Moreover, the process is also applicable to bonding glass to glass, glass to metal and glass to semiconductor.
Secondly, in this process, only a small amount of heat enters the component, and this heat is generated in a region up to a few hundred microns wide. This allows welds to be placed very close to electronic circuits or other thermal components, which provides designers and manufacturers with greater freedom and supports better product miniaturization design.
Finally, if USP laser glass welding is carried out properly, microcracks will not occur around the weld. Microcracks reduce the mechanical strength of the glass. In addition, after periodic changes in temperature (which is inevitable for all things), microcracks may become the root cause of the final failure of the equipment.
Relevant companies put USP laser glass welding into practice
The advantage of USP laser glass welding is that the glass is heated only in a small volume. But it also brings challenges to the actual operation. This means that even if the part moves, the laser focus position must be kept very accurately at the interface between the two welding components. This is difficult to achieve because real-world components are not completely flat. In addition, the placement of components in the welding system may not fully fit.
One solution is to use an axially elongated focus. This will "stretch" the size of the laser beam focus to solve the position sensitivity problem. However, the disadvantage of this method is that the elongated beam focus will produce a molten pool with non-circular cross-section in the glass. When the glass solidifies in the melting zone, the non-circular molten pool is more likely to form microcracks.
Relevant companies adopt another method to achieve the welding effect without micro cracks, and can adapt to the significant changes in the interface distance in the process at the same time. The secret lies in the use of high numerical aperture (NA) optics to produce small focal spots in combination with high dynamic focusing technology.
Therefore, coherent's laser system has realized a high sphericity molten pool, thus avoiding microcracks. It also senses the interface distance and constantly adjusts the optics to maintain perfect focus at all times. As a result, high-quality welding can be guaranteed on almost any shape of components, and the process is not affected by component tolerances and positions.
Source: nuclear technology network
