SHINHOP laser

I. Technical Background
Laser welding technology can be said to be one of the most widely used processing techniques in current laser applications, with rapid technological development and a relatively good welding level in material welding. However, glass, as a transparent and fragile brittle material, cannot be easily absorbed by traditional laser sources, and the glass that absorbs heat tends to crack during welding due to its large thermal expansion coefficient, making it unsuitable for processing with traditional laser welding methods.
You may have heard of plastic laser welding, but you may not be very familiar with glass welding. In fact, there are mainly two methods for laser welding glass compared to welding plastic.
1. One method is to coat an opaque pigment at the welding interface or add an intermediate layer to increase the laser absorption rate. The material near the interface absorbs the laser, causing the temperature to rise, and then after the material melts, it solidifies to achieve the connection of transparent materials.
2. The second method uses special welding light sources for welding. By using high power density lasers, nonlinear absorption occurs between transparent materials, forming effective weld points. More and more researchers and engineers are turning their attention to the application of laser welding processing with special light sources.
II. Research Status
In recent years, various weldings between glass, glass, and single crystal silicon have been achieved using special light sources.
The American company PolaOnyx has utilized specialized laser single-line/multi-line scanning to achieve glass welding and sealing. Hélie et al. employed lasers to micro-weld 100μm thick glass end caps onto micro-structured optical fibers, successfully welding end caps for both standard optical fibers and micro-structured optical fibers. Tamaki et al. conducted research using a laser with a wavelength of 1558nm, successfully achieving welding between dissimilar glasses and between glass and silicon wafers, resulting in welding strengths of 9.87MPa and 3.74MPa, respectively.
However, the results of most scholars studying laser welding of glass show that the welded fusion area presents a droplet shape, mainly consisting of three parts: a circular cavity at the top, a molten area in the middle, and a linear structure composed of a small cavity at the bottom. The cavities at the top and bottom are prone to stress concentration, and if the parameters are not well controlled, cracks are likely to occur. Additionally, due to its droplet shape, poor control of the line spacing may lead to discontinuous and unconnected weld seams.

Figure 1: Laser Welding Glass End Face Slice
III. Experimental Materials and Methods
The materials used in the experiment are optical glass, with sample dimensions of 25×25×1mm. The welding test steps are as follows:
1. Clean the glass surface. Soak the glass sheet in an alcohol solution for 5-10 minutes, then rinse the glass surface 3-5 times with distilled water, and finally use a hot air blower to dry the water stains on the glass surface.
2. Press and weld the glass sheets. Place the stacked glass sheets in the positioning groove of the fixture, adjusting the welding fixture mechanism to press down the surrounding area of the glass sheets to ensure they fit tightly. Previous research has shown that for glass welding, the required glass fitting gap should be less than 100nm (some opinions suggest less than a quarter of the laser wavelength).
3. Adjust the focus to the junction of the two glass sheets. When the laser passes from air into the glass, refraction occurs, causing the focus to shift. Therefore, the focus is found by using the fitting glass sheets, adjusting the height of the vertical mirror lens at equal intervals, and scanning the glass sheets with the same energy box. After removing the glass sheets, observe the position of the laser absorption interface on the glass, which indicates the focus position.
4. Glass laser welding. Repeat steps (2) and (3) to press and weld the glass sheets and adjust the focus to the glass junction. Adjust suitable laser welding process parameters (power, speed, scanning pattern, etc.) to weld the glass. When the high-energy laser at the glass junction exceeds a certain threshold, it induces multiphoton ionization of the glass material. The ionized free electrons accelerate and collide with other atoms, causing avalanche ionization, raising the material temperature, melting the glass, and achieving welding after cooling and solidifying during the light-off time.
IV. Special Laser Glass Welding Process
Using special lasers for glass welding tests, the entire welding surface area (4×4mm) is uniformly formed, with minimal material deformation after welding, and the flatness of the material has not changed significantly. The welding fusion zone is located at the junction of the two materials with a small thickness, and no thermal damage is observed on both sides of the glass in the fusion area.

Figure 2: Welded Bonding Layer Morphology
The post-weld glass end face slice shows that when using a new type of laser welding process, the welding fusion area does not present a droplet shape, and there are no defects such as a circular cavity at the top and a small cavity at the bottom that are sources of welding cracks. The molten area in the middle also does not show discontinuous unformed linear crack defects. After conducting welding strength tests, the material broke in the base material area, and the weld point did not fall off, indicating good welding strength.

V. Summary and Outlook
Using special laser light sources and adjusting suitable welding process parameters, the glass material nonlinearly absorbs the laser, melts, and solidifies, forming a strong welding zone between two pieces of transparent glass without adding filler. Direct welding between optical glasses has been successfully achieved. The fusion area of optical glass is integrated into one body, with no significant macro or micro cracks, and does not present a droplet shape. Therefore, there are no top droplet-shaped circular cavities or bottom linear damage areas, and no linear non-fusion occurs at the junction, effectively avoiding the generation of crack sources. After strength testing, the weld point remains on the sample surface, demonstrating high connection strength.
In recent years, with the rapid development of 5G, wireless charging, optical communication, and chip technology, glass materials have gradually become the mainstream materials for 3C electronic structural components (such as Mobile glass cases), semiconductor devices (chips and panels), optical components, and camera modules due to their outstanding advantages of low electromagnetic signal shielding, high hardness, light weight, low cost, and suitability for mass production. Glass laser welding processing will usher in broad application prospects.