Laser processing of high-density flexible circuit boards
Flexible circuit boards have many advantages such as space saving, weight reduction, and flexibility. Global demand for flexible circuit boards is increasing year by year. In view of the special properties of flexible circuit board materials, this article introduces some issues that need to be considered when laser processing high-density flexible circuit boards and drilling microvias. Figure 1: Vector-scanning technology uses computer-controlled mirrors and CAD/CAM software equipped with galvanometers to create tailoring and drilling patterns, and uses a telecentric lens system to ensure that the laser is directed perpendicular to the surface of the workpiece. â—†Molding â—† Slice â—†Drilling Figure 2: Hole drilled in Kapton using UV laser The CO2 laser can also use the conformal mask technology to drill microvias. Using this technique, using a copper surface as a mask, holes are first etched by ordinary printing etching, and then a CO2 laser beam is irradiated on the holes of the copper foil to remove the exposed dielectric material. Laser selection CO2 Laser (Automation Alternatives) UV-DPSS laser This article concludes
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The high-density flexible circuit board is a part of the entire flexible circuit board, generally defined as a flexible circuit board having a line pitch of less than 200 μm or a micro-via of less than 250 μm. High-density flexible circuit boards have a wide range of applications, such as telecommunications, computers, integrated circuits, and medical devices.
The unique characteristics of flexible circuit boards make it an alternative to rigid circuit boards and traditional wiring schemes in many applications. At the same time, it also promotes the development of many new areas. The fastest growing portion of the flexible circuit board is the computer hard disk drive (HDD) internal connection. The head of the hard disk should be moved back and forth on the rotating disk. Flexible wires can be used instead of wires to realize the connection between the moving head and the control circuit board. Hard disk manufacturers increase output and reduce assembly costs through a technology called "floating flexible board" (FOS). In addition, wireless suspension technology provides better shock resistance and improves product reliability. Another high density flexible circuit board used in hard drives is the interposer flex used between the suspension and the controller.
The second fastest growing field of flexible circuit boards is the new integrated circuit package. Chip-scale packages (CSPs), multi-chip modules (MCMs), and flexible circuit board-on-board (COFs) are used for flexible circuits. The market for CSP interconnects is particularly large because it can be used in semiconductor devices and flash. Speed ​​memory is widely used in PCMCIA cards, disk drives, personal digital assistants (PDAs), mobile phones, pagers, digital video cameras, and digital cameras. In addition, liquid crystal displays (LCDs), mylar switches, and inkjet printer cartridges are another three high-growth applications for high-density flexible circuit boards.
The market potential of flexible circuit technology in portable devices (such as mobile phones) is very large, which is very natural, because these devices require small size and light weight to meet the needs of consumers; in addition, the latest application of flexible technology is also Including flat panel displays and medical devices, designers can use it to reduce the size and weight of products such as hearing aids and human implant devices.
The huge growth in the above areas has also led to an increase in the production of flexible circuit boards worldwide. For example, the annual sales volume of HDDs is expected to reach 345 million units in 2004, almost twice that in 1999, and the conservative sales volume of mobile phones in 2005 is also estimated at 600 million units. These increases will lead to high-density flexible PCB production expected to be annual. It will increase by 35% to reach 3.5 million square meters in 2002. This high output demand requires efficient and low-cost processing. Laser processing is one of them.
Lasers have three main functions in the manufacture of flexible circuit boards: forming (cutting and cutting), slicing and drilling. As a non-contact processing tool, laser can apply high-intensity light energy (650mW/mm2) at a very small focal point (100-500μm). Such high energy can be used to cut, drill, and make materials. Marking, welding, scribing, and various other processes, processing speed and quality are related to the nature of the material being processed and the laser characteristics used such as wavelength, energy density, peak power, pulse width, and frequency. Flexible circuit boards use ultra-violet (UV) and far-infrared (FIR) lasers. The former typically use an excimer or UV diode-pumped solid-state (UV-DPSS) laser, while the latter typically uses a sealed CO2 laser. 
The laser processing precision is highly versatile and is an ideal tool for forming a flexible circuit board. Whether it is a CO2 laser or a DPSS laser, the material can be processed into any shape after focusing. It installs a mirror on the galvanometer to focus the focused laser beam anywhere on the workpiece surface (Figure 1), and then uses a vector scanning technique to perform computer numerical control (CNC) on the galvanometer table and uses CAD/CAM software to make it. Cutting graphics. This "soft tool" allows instant control of the laser during design changes. With the adjustment of the amount of light scaling and various different cutting tools, laser machining can accurately reproduce the design pattern, which is another significant advantage of it.
The vector scan can cut a substrate such as a polyimide film, cut out the entire circuit, or remove an area on the circuit board such as a slot or a square. During the forming process, the laser beam is always open when the mirror scans the entire machined surface. This is in contrast to the drilling process where the laser only opens after the mirror is fixed in each drilling position.
“Slicing†is, in jargon, the process of using a laser to remove another layer of material from a layer of material. This process is perfect for lasers. The same vector scanning technology as before can be used to remove the dielectric and expose the underlying conductive pads. At this time, the high precision of laser processing shows great benefits once again. Since the FIR laser beam is reflected by the copper foil, a CO2 laser is usually used here.
Although microholes are still formed in some places by mechanical drilling, punching, or plasma etching, laser drilling is still the most widely used method for forming microvias in flexible circuit boards, mainly because of its productivity. High, flexible, and uptime.
High-precision drills and dies are used for mechanical drilling and punching. Holes with a diameter of approximately 250 μm can be made on the flexible circuit board. However, these high-precision equipments are very expensive and relatively short-lived. Because the required aperture for high-density flexible circuit boards is smaller than 250 μm, mechanical drilling is not favored.
The use of plasma etching can make microvias smaller than 100μm in size on 50μm thick polyimide film substrates, but the equipment investment and process costs are quite high, and the plasma etching process is also expensive to maintain, especially for some chemical wastes. Processing and consumables and other related costs, in addition plasma etching in the establishment of a new process takes a considerable time to make consistent and reliable micro-vias. The advantage of this process is its high reliability. It is reported that the pass rate of microvias made by it reaches 98%. Therefore, in medical and avionics equipment, plasma etching processing still has a certain market.
In contrast, using lasers to make micro vias is a simple, low-cost process. Laser equipment investment is very low, and the laser is a non-contact tool, unlike the mechanical drilling will have an expensive tool replacement costs. In addition, modern sealed CO2 and UV-DPSS lasers are maintenance-free, minimizing downtime and greatly increasing productivity.
The method of producing micro vias on a flexible circuit board is the same as on a rigid PCB, but due to the difference in substrate and thickness, some important parameters of the laser need to be changed. Sealed CO2 and UV-DPSS lasers can all be drilled directly on the flexible circuit board using the same vector scanning technology as the molding process. The only difference is that the drilling application software scans the scanning mirror from one micro via to another. The laser is turned off during the via hole and the laser beam is turned on only when it reaches another drilling position. In order to make the hole perpendicular to the substrate surface of the flexible circuit board, the laser beam must be directed perpendicular to the circuit board substrate, which can be achieved by using a telecentric lens system between the scanning mirror and the substrate (Figure 2). 
The use of excimer lasers can also make micro vias by means of a projection mask. This technique requires that an image of a micro via or the entire micro via array be mapped onto a substrate, and then an excimer laser beam illuminates the mask to mask the micro vias. The film map is mapped to the surface of the substrate to drill holes. The quality of excimer laser drilling is very good. Its disadvantages are low speed and high cost.
Although the type of laser used to process flexible circuit boards is the same as that of rigid PCBs, the difference in material and thickness will greatly affect the processing parameters and speed. Sometimes excimer lasers and lateral excitation gas (TEA) CO2 lasers can be used, but these two methods are slow and costly to maintain, limiting the increase in productivity. In comparison, since CO2 and UV-DPSS lasers are widely used, fast, and cost-effective, the use of these two types of lasers is mainly used in the manufacture and processing of micro-vias for flexible circuit boards.
Unlike gas flow CO2 lasers, the sealed CO2 laser uses a block release technique that confines the laser gas mixture within the specified laser cavity of the two rectangular electrode plates. The laser cavity is used throughout its life (usually about two to three years). It is sealed. The sealed laser cavity is compact and requires no air change. The laser head can operate continuously for more than 25,000 hours without maintenance. The biggest advantage of the sealed design is the ability to generate fast pulses. For example, a block-release laser emits high-frequency (100 kHz) pulses with a peak power of 1.5 kW. Fast processing with high frequency and high peak power without any thermal degradation.
The UV-DPSS laser is a solid-state device that uses a laser diode array to continuously inject a neodymium vanadate (Nd:YVO4) crystal rod. It generates a pulsed output from an acousto-optic Q-switch and uses a third harmonic crystal generator to change the Nd:YVO4. Laser output, output from 1,064 nm
The IR base wavelength is reduced to 355 nm UV wavelength. Under normal circumstances 355nm
The UV-DPSS laser has an average output power of more than 3 W at a nominal pulse repetition rate of 20 kHz.
Sealed CO2 lasers can emit FIR lasers with a wavelength of 10.6 μm or 9.4 μm. Although both wavelengths are easily absorbed by dielectrics such as polyimide film substrates, studies have shown that processing such materials with a wavelength of 9.4 μm is much better. The absorption coefficient of the dielectric 9.4 μm wavelength is higher, and using this wavelength to drill or cut material is faster than using a wavelength of 10.6 μm. The 9.4μm laser not only has obvious advantages in drilling and cutting, but also has a prominent slice effect. Therefore, the use of shorter wavelength lasers can improve productivity and quality.
In general, the FIR wavelength is easily absorbed by the dielectric, but it is reflected back by the copper, so most of the CO2 laser is used for dielectric processing, slicing, and dielectric substrate and laminate layering. Since CO2 lasers have higher output power than DPSS lasers, CO2 lasers are used to process dielectrics in most cases. CO2 lasers and UV-DPSS lasers are often used in combination. For example, when drilling micro-vias, the copper layer is first removed with a DPSS laser, and then a CO2 laser is used to rapidly drill holes in the dielectric layer until the next copper layer is repeated. The process.
Since the UV laser itself has a very short wavelength, it emits a finer spot than the CO2 laser, but in some applications the large diameter spot produced by the CO2 laser is more useful than the UV-DPSS laser. For example, when cutting a large area material such as a groove or a block or drilling large holes (diameter greater than 50 μm), the time required for processing with a CO2 laser is shorter. In general, CO2 laser processing is appropriate when the aperture diameter is larger than 50 μm, and the UV-DPSS laser effect is better when the aperture diameter is less than 50 μm.
Both dielectric and copper can easily absorb UV-DPSS lasers with an output wavelength of 355 nm. The UV-DPSS laser has a smaller spot and lower output power than the CO2 laser. In the dielectric processing process, the UV-DPSS laser is usually used in a small-sized (less than 50 μm) process, so the diameter must be smaller than that of a high-density flexible circuit board substrate. 50μm microvias are ideal with UV lasers. High-power UV-DPSS lasers are now available, which can increase the processing and drilling speed of UV-DPSS lasers.
The advantage of the UV-DPSS laser is that its high-energy UV photons can directly break molecular links when shining on most non-metallic surface layers, using a "cold" lithography process to smooth the cut edges, while minimizing thermal damage and scorching Therefore, the UV micro-cutting process is suitable for demanding applications where no or no post-processing is required.
Materials with higher UV etch thresholds such as copper are processed with high-energy, low-repetition-rate lasers, while low-threshold materials such as polyimide films can only be processed with low-energy, high-repetition-rate lasers. The energy and high repetition rate is to avoid damage to the copper pad and increase production capacity. To increase production capacity, most large-diameter micro vias are processed in two steps: UV-DPSS laser is used to drill copper foil first, and CO2 laser is used to remove the exposed dielectric.