Application of laser processing technology in flexible circuit board

High density flexible circuit board is a part of the whole flexible circuit board, which is generally defined as the line spacing less than 200 μ M or micro via less than 250 μ M flexible circuit board. High density flexible circuit board has a wide range of applications, such as telecommunications, computers, integrated circuits and medical equipment. Aiming at the special properties of flexible circuit board materials, this paper introduces some key problems to be considered in laser processing of high-density flexible circuit board and micro via drilling p>

The unique characteristics of flexible circuit board make it an alternative to rigid circuit board and traditional wiring scheme in many occasions. At the same time, it also promotes the development of many new fields. The fastest growing part of FPC is the internal connection line of computer hard disk drive (HDD). The magnetic head of the hard disk shall move back and forth on the rotating disk for scanning, and the flexible circuit can be used to replace the wire to realize the connection between the mobile magnetic head and the control circuit board. Hard disk manufacturers increase production and reduce assembly costs through a technology called “suspended flexible plate” (FOS). In addition, wireless suspension technology has better seismic resistance and can improve product reliability. Another high-density flexible circuit board used in hard disk is interposer flex, which is used between suspension and controller.

The second growing field of FPC is new integrated circuit packaging. Flexible circuits are used in chip level packaging (CSP), multi chip module (MCM) and chip on flexible circuit board (COF). Among them, CSP internal circuit has a huge market, because it can be used in semiconductor devices and flash memory, and is widely used in PCMCIA cards, disk drives, personal digital assistants (PDAs), mobile phones, pagers Digital camera and digital camera. In addition, liquid crystal display (LCD), polyester film switch and ink-jet printer cartridge are other three high growth application fields of high-density flexible circuit board\

The market potential of flexible line technology in portable devices (such as mobile phones) is very large, which is very natural, because these devices require small volume and light weight to meet the needs of consumers; In addition, the latest applications of flexible technology include flat panel displays and medical devices, which can be used by designers to reduce the volume and weight of products such as hearing aids and human implants.

The huge growth in the above fields has led to an increase in the global output of flexible circuit boards. For example, the annual sales volume of hard disks is expected to reach 345 million units in 2004, almost twice that of 1999, and the sales volume of mobile phones in 2005 is conservatively estimated to be 600 million units. These increases lead to an annual increase of 35% in the output of high-density flexible circuit boards, reaching 3.5 million square meters by 2002. Such high output demand requires efficient and low-cost processing technology, and laser processing technology is one of them.

Laser has three main functions in the manufacturing process of flexible circuit board: processing and forming (cutting and cutting), slicing and drilling. As a non-contact machining tool, laser can be used in a very small focus (100 ~ 500) μ m) High intensity light energy (650MW / mm2) is applied to the material. Such high energy can be used for cutting, drilling, marking, welding, marking and other processing. The processing speed and quality are related to the properties of the processed material and the laser characteristics used, such as wavelength, energy density, peak power, pulse width and frequency. The processing of flexible circuit board uses ultraviolet (UV) and far infrared (FIR) lasers. The former usually uses excimer or UV diode pumped solid-state (uv-dpss) lasers, while the latter generally uses sealed CO2 lasers div>

Vector scanning technology uses computer to control the mirror equipped with flow meter and CAD / CAM software to generate cutting and drilling graphics, and uses telecentric lens system to ensure that the laser shines vertically on the workpiece surface < / div >

Laser Drilling processing has high precision and wide application. It is an ideal tool for forming flexible circuit board. Whether CO2 laser or DPSS laser, the material can be processed into any shape after focusing. It shoots the focused laser beam anywhere on the workpiece surface by installing a mirror on the galvanometer, then carries out computer numerical control (CNC) on the galvanometer by using vector scanning technology, and makes cutting graphics with the help of CAD / CAM software. This “soft tool” can easily control the laser in real time when the design is changed. By adjusting the light shrinkage and various cutting tools, laser processing can accurately reproduce the design graphics, which is another significant advantage.

Vector scanning can cut substrates such as polyimide film, cut out the whole circuit or remove an area on the circuit board, such as a slot or a block. In the process of processing and forming, the laser beam is always turned on when the mirror scans the whole processing surface, which is opposite to the drilling process. During drilling, the laser is turned on only after the mirror is fixed at each drilling position div>

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“Slicing” in jargon is the process of removing one layer of material from another with a laser. This process is more suitable for laser. The same vector scanning technology can be used to remove the dielectric and expose the conductive pad below. At this time, the high precision of laser processing once again reflects great benefits. Since FIR laser rays will be reflected by copper foil, CO2 laser is usually used here.

drill hole

Although some places still use mechanical drilling, stamping or plasma etching to form micro through holes, laser drilling is still the most widely used micro through hole forming method of flexible circuit board, mainly because of its high productivity, strong flexibility and long normal operation time.

Mechanical drilling and stamping adopt high-precision drill bits and dies, which can be made on the flexible circuit board with a diameter of nearly 250 μ M, but these high-precision devices are very expensive and have a relatively short service life. Due to the high-density flexible circuit board, the required aperture ratio is 250 μ M is small, so mechanical drilling is not favored.

Plasma etching can be used at 50 μ M thick polyimide film substrate with a size less than 100 μ M, but the equipment investment and process cost are quite high, and the maintenance cost of plasma etching process is also very high, especially the costs related to some chemical waste treatment and consumables. In addition, it takes quite a long time for plasma etching to make consistent and reliable micro vias when establishing a new process. The advantage of this process is high reliability. It is reported that the qualified rate of micro via is 98%. Therefore, plasma etching still has a certain market in medical and avionics equipment div>

In contrast, the fabrication of micro vias by laser is a simple and low-cost process. The investment of laser equipment is very low, and laser is a non-contact tool. Unlike mechanical drilling, there will be an expensive tool replacement cost. In addition, modern sealed CO2 and uv-dpss lasers are maintenance free, which can minimize downtime and greatly improve productivity.

The method of generating micro vias on flexible circuit board is the same as that on rigid pcb, but some important parameters of laser need to be changed due to the difference of substrate and thickness. Sealed CO2 and uv-dpss lasers can use the same vector scanning technology as molding to drill directly on the flexible circuit board. The only difference is that the drilling application software will turn off the laser during the scanning mirror scanning from one micro via to another. The laser beam will not be turned on until it reaches another drilling position. In order to make the hole perpendicular to the surface of the flexible circuit board substrate, the laser beam must shine vertically on the circuit board substrate, which can be achieved by using a telecentric lens system between the scanning mirror and the substrate (Fig. 2) div>

Holes drilled on Kapton using UV laser

CO2 laser can also use conformal mask technology to drill micro vias. When using this technology, the copper surface is used as a mask, the holes are etched on it by ordinary printing etching method, and then the CO2 laser beam is irradiated on the holes of the copper foil to remove the exposed dielectric materials.

Micro vias can also be made by using excimer laser through the method of projection mask. This technology needs to map the image of a micro via or the whole micro via array to the substrate, and then the excimer laser beam irradiates the mask to map the mask image to the substrate surface, so as to drill the hole. The quality of excimer laser drilling is very good. Its disadvantages are low speed and high cost.

Laser selection although the laser type for processing flexible circuit board is the same as that for processing rigid pcb, the difference in material and thickness will greatly affect the processing parameters and speed. Sometimes excimer laser and transverse excited gas (tea) CO2 laser can be used, but these two methods have slow speed and high maintenance cost, which limit the improvement of productivity. In comparison, CO2 and uv-dpss lasers are widely used, fast and low cost, so they are mainly used in the fabrication and processing of micro vias of flexible circuit boards.

Different from gas flow CO2 laser, sealed CO2 laser（ http://www.auto-alt.cn ）The block release technology is adopted to limit the laser gas mixture to the laser cavity specified by two rectangular electrode plates. The laser cavity is sealed during the whole service life (usually about 2 ~ 3 years). The sealed laser cavity has compact structure and does not need air exchange. The laser head can work continuously for more than 25000 hours without maintenance. The biggest advantage of the sealing design is that it can generate fast pulses. For example, the block release laser can emit high-frequency (100kHz) pulses with a power peak of 1.5KW. With high frequency and high peak power, rapid machining can be carried out without any thermal degradation div>

Uv-dpss laser is a solid-state device that continuously sucks neodymium vanadate (Nd: YVO4) crystal rod with laser diode array. It generates pulse output by an acousto-optic Q-switch, and uses the third harmonic crystal generator to change the output of Nd: YVO4 laser from 1064nm & nbsp; The IR basic wavelength is reduced to 355 nm UV wavelength. Generally 355nm < / div >

The average output power of uv-dpss laser at 20kHz nominal pulse repetition rate is more than 3W div>

Uv-dpss laser

Both dielectric and copper can easily absorb uv-dpss laser with output wavelength of 355nm. Uv-dpss laser has smaller light spot and lower output power than CO2 laser. In the process of dielectric processing, uv-dpss laser is usually used for small size (less than 50%) μ m) Therefore, the diameter less than 50 should be processed on the substrate of high-density flexible circuit board μ M micro via, using UV laser is very ideal. Now there is a high-power uv-dpss laser, which can increase the processing and drilling speed of uv-dpss laser div>

The advantage of uv-dpss laser is that when its high-energy UV photons shine on most non-metallic surface layers, they can directly break the link of molecules, smooth the cutting edge with “cold” lithography process, and minimize the degree of thermal damage and scorching. Therefore, UV micro cutting is suitable for high demand occasions where post-treatment is impossible or unnecessary div>

CO2 laser (Automation alternatives)

Sealed CO2 laser can emit a wavelength of 10.6 μ M or 9.4 μ M FIR laser, although both wavelengths are easy to be absorbed by dielectrics such as polyimide film substrate, the research shows that 9.4 μ The effect of M wavelength processing this kind of material is much better. Dielectric 9.4 μ The absorption coefficient of M wavelength is higher, which is better than 10.6 for drilling or cutting materials μ M wavelength fast. nine point four μ M laser not only has obvious advantages in drilling and cutting, but also has outstanding slicing effect. Therefore, the use of shorter wavelength laser can improve productivity and quality.

Generally speaking, fir wavelength is easily absorbed by dielectrics, but it will be reflected back by copper. Therefore, most CO2 lasers are used for dielectric processing, molding, slicing and delamination of dielectric substrate and laminate. Because the output power of CO2 laser is higher than that of DPSS laser, CO2 laser is used to process dielectric in most cases. CO2 laser and uv-dpss laser are often used together. For example, when drilling micro vias, first remove the copper layer with DPSS laser, and then quickly drill holes in the dielectric layer with CO2 laser until the next copper clad layer appears, and then repeat the process.

Because the wavelength of UV laser itself is very short, the light spot emitted by UV laser is finer than that of CO2 laser, but in some applications, the large-diameter light spot produced by CO2 laser is more useful than uv-dpss laser. For example, cut large area materials such as grooves and blocks or drill large holes (diameter greater than 50) μ m) It takes less time to process with CO2 laser. Generally speaking, the aperture ratio is 50 μ When m is large, CO2 laser processing is more appropriate, and the aperture is less than 50 μ M, the effect of uv-dpss laser is better.