Ultraviolet laser and FPC industry

- Dec 26, 2019-

Ultraviolet laser and FPC industry

Flexible Printed Circuits (Flexible Printed Circuits, FPC) can achieve a variety of designs that cannot be achieved with traditional rigid circuit boards. For example, making circuits on flexible materials can create new applications that challenge the limits, including a variety of multilayer functions and solutions for the space, telecommunications, and medical industries.


At present, the trend of the FPC industry is miniaturization, because designers try to reduce the size of the circuit, while eliminating factors that limit the density of the installation or the distance between the circuits on the circuit board. Meeting these requirements usually requires arbitrary shaping, but the basic square circuit is too inflexible to meet the requirements of many modern applications.


These design requirements are challenges, including the problem of segmentation or the process of removing circuits from the board. How can you accurately cut smaller arbitrary circuits with high mounting density without damaging the components or the circuit itself? Flexible circuit materials are unique and can cause damage even with the smallest stress on the circuit during cutting.

 In order to avoid this damage, the diversity of design is limited. The buffer space around each cut must be considered in the design, which means that the width of the cut 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 complicated as required. Without a viable solution to this type of problem, these limitations will overwhelm innovation, as unsatisfactory sharding will be a major design consideration.


Automatic circuit board cutting (routing) and traditional mechanical board separation methods (such as die punching) will result in a large cutting width and excessive stress for complex flexible circuits. Even the CO2 laser cutting method is also unsatisfactory in this regard, because this method will produce a larger heat affected area.


However, when it comes to FPC panel cutting, a technology that has met the challenge has emerged: UV laser cutting. This technology can eliminate the physical stress of the mechanical process and greatly reduce the thermal stress of the CO2 UV laser, which can meet the design trends described above. Exploring various factors can reveal why UV laser cutting has become an option when it comes to flexible circuit cutting.


Circuit stress and cutting width


All flexible cutting methods will generate a certain 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 board separation method, there may be two types of stress on the flexible printed circuit board: mechanical stress or thermal stress.


When mechanical separation methods such as die punching or routing are used, mechanical stress will occur. The effects of mechanical stress on flexible circuits include: glitches, 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, which requires considerable cutting buffer space. In die punching and routing, the typical FPC cutting width is 1 mm, but this width is too large for many complex, random flexible circuits. Such wide cuts can result in reduced mounting density or reduced circuit mounting on each board. In an era where flexible printed circuits are becoming smaller and tighter, this has risen to technical and cost issues.


Because the mechanical cutting method cannot meet the flexible design standards, users switch to laser cutting, but it will produce a different type of impact on the circuit: thermal stress. The effects of thermal stress and mechanical stress are very different, and 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 burnt and incision width inconsistencies. However, these effects are more common when using pulsed CO2 laser systems. These systems are equipped with high-energy-density power supplies and the wavelength of the laser is in the warmer, more absorptive infrared spectrum. UV laser systems are equipped with cold UV lasers that operate at lower energy levels to minimize the effects of thermal stress.


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


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


Figure 3. A 125 μm thick Kapton polyimide material cut using an ultraviolet laser system.

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


Technical application

Holmium UV laser systems are capable of cutting almost any circuit material, whether they are flexible or not. Common flexible applications include polyimide (such as Kapton), PET materials (such as Akaflex), and composite materials (such as 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 UV laser beam is tapered, meaning the deeper the material penetrates, the wider the cut will be. Typical cut widths range from 25 to 50 μm. The repeat accuracy of the top-level UV laser system is ± 4μm, which can ensure the maximum accuracy of the design cut. UV laser cutting speed depends on the material being processed. In the Kapton application shown in Figure 3, the cutting speed is 95 mm / sec, which is about 2 to 3 times faster than the Routing method, while eliminating the harmful stress generated by other flexible cutting methods. Considering the other functions of the UV laser cutting system, such as cover cutting, drilling, drilling, and surface etching, it will never be surprised that the market demand for UV laser systems has grown rapidly in recent years.


Meet the needs of the trend


    Flexible circuit designers benefit from UV laser technology to discover the most sophisticated arbitrary 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 edges of the circuit. In addition, UV laser cutting can ensure maximum mounting density and reduced bridge space between circuits, which has greater potential for developing circuit boards. With the advent of UV laser cutting, cutting of flexible circuits has become easier. In addition to diversified applications, the stress on the board is reduced, the width of the cut is narrow, and the machining is precise, so UV laser cutting is the right choice for flexible cutting solutions.

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