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Solid UV laser source and FPC industry

Jun 28 , 2022

Solid UV laser source and FPC industry

 

Flexible Printed Circuits (FPC) can realize diverse designs that cannot be realized by traditional rigid circuit boards. For example, fabricating circuits on flexible materials enables new and challenging applications, including a variety of multilayer functions and solutions for the space, telecommunications, and medical industries.

 

The current trend in the FPC industry is toward miniaturization, as designers find ways to reduce circuit size while eliminating factors that limit mounting density or the distance between circuits on a circuit board. Meeting these requirements often requires arbitrary shaping, but the basic square circuit is too elastic to meet the requirements of many modern applications.

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These design requirements are challenges, including issues of partitioning or the process of removing circuits from the board. How can smaller arbitrary circuits with high mounting density be precisely cut without damaging the components or the circuit itself? Flexible circuit materials are unique in that even the smallest stress on the circuit during cutting can cause damage.

 

In order to avoid this damage, the variety of designs is limited. The buffer space around each cutout must be considered in the design, which means the cutout width will be wider than needed, the components cannot be placed close to the edge of the board or close to each other, and the forming cannot be as complex as needed. Without viable solutions to these types of problems, these constraints can drown innovation, as unsatisfactory splitting becomes a major design consideration.

 

Automated circuit board cutting (Routing), as well as traditional mechanical depaneling methods such as die punching, result in large cut widths and excessive stress for complex flex circuits. Even the CO2 laser cutting method is unsatisfactory in this respect as it creates a larger heat affected area.

 

However, when it comes to FPC depaneling, a technology has emerged to rise to the challenge: UV laser cutting. This technique eliminates the physical stress of the mechanical process and greatly reduces the thermal stress of the CO2 UV laser, meeting the design trends described above. Exploration of various factors will reveal why UV laser cutting has emerged as an option when it comes to flexible circuit cutting.

 

Circuit Stress and Cut Width

 

All flex cutting methods create some amount of stress on the circuit board, but there are differences in the type of stress introduced and the degree to which the stress affects the circuit. When considering the above-mentioned splitting method, there may be two types of stress on the flexible printed circuit board: mechanical stress or thermal stress.

 

Mechanical stress occurs when mechanical splitting methods such as die punching or routing are used. The effects of mechanical stress on flexible circuits include: burrs, deformation, and damage to circuit components. These effects are very serious for flexible materials. For example, die punching is a high-impact process that vibrates circuits and damages components, and requires considerable cutting buffer space. In die punching and routing, the typical FPC cut width is 1mm, but this width is too large for many complex, random flex circuits. Such wide cutouts can result in: reduced mounting density, or reduced circuit mounting per board. At a time when flexible printed circuits are becoming smaller and more compact, this raises the question of technology and cost.

 

Users turned to laser cutting because mechanical cutting methods could not meet flex design criteria, but it produced a different type of effect on the circuit: thermal stress. The effect of thermal stress is very different from the effect of mechanical stress. The laser beam has no physical contact with the circuit. For this reason, laser cutting can be more accurately described as laser ablation. The most common effects of thermal stress are scorch and inconsistent kerf width. However, these effects are more common with pulsed CO2 laser systems. These systems feature high-energy-density power supplies and lasers with wavelengths in the warmer, more absorptive infrared spectrum. In contrast, UV laser systems feature cold UV lasers operating at lower energy levels to minimize thermal stress effects.

 

Figures 2 and 3 show the cutting of a 125 μm thick Kapton polyimide board using a CO2 laser and a UV laser, respectively. The beam size of both laser sources was 20 μm. In this case, the higher-energy CO2 laser produces extremely hot cuts, and the stress applied to the material causes severe scorching and deformation. As a consequence of the stress, the effective kerf width is extended to 120 μm. Although this figure is much narrower than the 1mm kerf width of the mechanical cutting method, the kerf is uneven and of poor quality.

 

Figure 2. 125μm thick Kapton polyimide board cut with CO2 laser system.

 

Figure 3. 125μm thick Kapton polyimide material cut by UV laser system.

 

When cutting the same material with a UV laser system, the thermal energy is reduced, resulting in a "cold" cut (also known as cold ablation), resulting in a nearly stress-free cut with a 30µm cut width and smooth vertical cutting edges. Reducing the stress applied to circuits is critical for cutting polyimide and other flexible materials. Due to the low power, UV laser cutting can ensure the integrity of the FPC cutting as much as possible, keeping it clean and straight.

 

Technology application

 

UV laser systems are capable of cutting virtually every circuit material, whether they are flexible or not. Common flexible applications include polyimide (eg Kapton), PET materials (eg Akaflex), and composite materials (eg Pyralux). UV laser systems can also process almost any rigid material in rigid-flex applications. Common applications include FR4 and other epoxy interlayers, Rogers materials, ceramics, PTFE, aluminum, and copper. The tapered shape of the UV laser beam means that the deeper you go into the material, the wider the incision will be. Typical kerf widths range from 25 to 50 μm. The top-of-the-line UV laser system has a repeatability of ±4μm, ensuring maximum precision in designing cuts. UV laser cutting speed depends on the material being processed. The Kapton application shown in Figure 3, with a cutting speed of 95 mm/sec, is approximately 2 to 3 times faster than routing, while eliminating the detrimental stresses associated with other flexible cutting methods. Considering the other functions of UV laser cutting systems, such as cap cutting, drilling, drilling, surface etching, it is no surprise that the market demand for UV laser systems has grown rapidly in recent years.

 

meet trends

 

Flexible circuit designers benefit from UV laser technology to discover the most sophisticated random designs. Because innovation is no longer limited by technology, it can break through the shape and size of traditional circuits.

 

Due to the narrow and clean cuts processed by the UV laser system, circuit components can be placed closer to each other and closer to the edge of the circuit. In addition, UV laser cutting can ensure maximum mounting density and reduced bridge space between circuits, with greater potential to develop circuit boards. With the advent of UV laser cutting, flexible circuit cutting has become easier. In addition to the variety of applications, reduced stress on the board, narrow kerf width, and precision machining make UV laser cutting the right choice for a flexible cutting solution.

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