PCB Tech - Analysis of problems in laser processing high-density FPC

FPC has many advantages such as space saving, weight reduction and high flexibility. The global demand for FPC is increasing year by year. In view of the special properties of FPC materials, this article introduces some issues that need to be considered when processing high-density FPC by laser and drilling micro-vias.

High-density FPC is a part of the entire FPC, and is generally defined as an FPC with a line pitch less than 200μm or a microvia less than 250μm. High-density FPC has a wide range of applications, such as telecommunications, computers, integrated circuits, and medical equipment.

The unique characteristics of FPC make it an alternative to rigid circuit boards and traditional wiring schemes in many occasions, and it also promotes the development of many new areas. The fastest growing part of FPC is the internal connection line of the computer hard disk drive (HDD). The magnetic head of the hard disk must move back and forth on the rotating disk to scan, and flexible circuits can be used instead of wires to realize the connection between the moving magnetic head and the control circuit board. Hard disk manufacturers use a technology called "floating flexible board" (FOS) to increase production and reduce assembly costs. In addition, the wireless suspension technology has better shock resistance and can improve product reliability. Another high-density FPC used in hard drives is an interposer flex, which is used between the suspension and the controller.

FPC growth rate ranks second in the field of new integrated circuit packaging. Chip-scale packaging (CSP), multi-chip module (MCM), and chip-on-FPC (COF) all use flexible circuits. Among them, the market for CSP interconnect circuits is particularly huge because it can be used in semiconductor devices and flash memory The above is widely used in PCMCIA cards, disk drives, personal digital assistants (PDA), mobile phones, pagers, digital video cameras and digital cameras. In addition, liquid crystal displays (LCD), polyester membrane switches and inkjet printer cartridges are the other three high-growth application areas of high-density FPC.

The market potential of flexible circuit technology in portable devices (such as mobile phones) is very large. This is natural because these devices require small size and light weight to meet the needs of consumers; in addition, the latest applications of flexible technology are also Including flat panel displays and medical equipment, designers can use it to reduce the volume and weight of products (such as hearing aids and human implant devices).

Laser has three main functions in the FPC manufacturing process: forming (cutting and cutting), slicing and drilling. As a non-contact processing tool, laser can apply high-intensity light energy (650mW/mm2) on a small focal point (100～500μm). Such high energy can be used to cut, drill, and work on materials. For marking, welding, scribing and other various processing, the processing speed and quality are related to the properties of the processed material and the characteristics of the laser used, such as wavelength, energy density, peak power, pulse width and frequency. FPC processing 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.

Processing and molding

The laser processing has high precision and wide application. It is an ideal tool for FPC molding processing. 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 shoot the focused laser beam anywhere on the surface of the workpiece (Figure 1), and then uses vector scanning technology to perform computer numerical control (CNC) on the galvanometer, and use CAD/CAM software to make Cutting graphics. This "soft tool" can conveniently control the laser in real time when the design is changed. By adjusting the amount of light zoom and various cutting tools, laser processing can accurately reproduce the design graphics, which is another significant advantage of it.

Drilling

Although some places are still using mechanical drilling, stamping or plasma etching to form micro-vias, laser drilling is still the most widely used FPC micro-via forming method, mainly because of its high productivity, Strong flexibility and long uptime.

Mechanical drilling and punching use high-precision drills and dies, which can make holes with a diameter of approximately 250μm in the FPC, but these high-precision equipment is very expensive and relatively short-lived. Since the hole diameter required for high-density FPC is smaller than 250μm, mechanical drilling is not favored.

Plasma etching can make micro vias with a size of less than 100μm on a 50μm thick polyimide film substrate, but the equipment investment and process costs are quite high, and the maintenance cost of the plasma etching process is also high, especially some chemical waste Processing and consumables and other related costs, in addition, plasma etching requires a considerable amount of time when establishing a new process to make consistent and reliable micro vias. The advantage of this process is its high reliability. According to reports, the qualified rate of micro vias made by it has reached 98%. Therefore, plasma etching still has a certain market in medical and avionics equipment.

In contrast, making micro vias with lasers is a simple, low-cost process. Laser equipment investment 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 increase productivity.

The method of producing micro vias on FPC is the same as on rigid PCB, but due to the difference in substrate and thickness, some important parameters of the laser need to be changed. Both sealed CO2 and UV-DPSS lasers can use the same vector scanning technology as the molding process to directly drill holes on the FPC. The only difference is that the drilling application software will scan the scanning mirror from one microvia to another. The laser is turned off during the process, and the laser beam is turned on only when it reaches another drilling position. In order to make the hole perpendicular to the surface of the FPC substrate, the laser beam must be irradiated vertically on the circuit board substrate. This can be done by using a telecentric lens system between the scanning mirror and the substrate.

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

The method of using an excimer laser to pass through a projection mask can also be used to make micro vias. This technology requires mapping an image of a micro via or the entire micro via array onto the substrate, and then the excimer laser beam irradiates the mask to make the mask. The film map is mapped to the surface of the substrate to drill holes. The quality of excimer laser drilling is very good, but its disadvantages are low speed and high cost.

Laser selection

Although the type of laser used to process FPC is the same as that used to process 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 in speed and high in maintenance costs, which limit the increase in productivity. In comparison, because CO2 and UV-DPSS lasers are widely used, fast, and low in cost, these two types of lasers are mainly used for the production and processing of FPC micro-vias.

CO2 Laser (Automation Alternatives)

sealed CO2 lasers can emit FIR lasers with a wavelength of 10.6μm or 9.4μm. Although both wavelengths are easily absorbed by dielectrics such as polyimide film substrates, studies have shown that processing such materials with a wavelength of 9.4μm is much better. The 9.4μm wavelength of the dielectric has a higher absorption coefficient, and it is faster to use this wavelength to drill or cut materials than to use the 10.6μm wavelength. The 9.4μm laser not only has obvious advantages in drilling and cutting, but also has a very outstanding slicing effect. Therefore, the use of a shorter wavelength laser can improve the productivity and quality of FPC.

UV-DPSS laser

Both dielectric and copper can easily absorb UV-DPSS laser with output wavelength of 355nm. UV-DPSS laser has a smaller 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 50μm) process, so it is necessary to process the diameter of less than 50μm on high-density FPC substrate. For micro vias, UV lasers are ideal. Now there is a high-power UV-DPSS laser, which can increase the processing and drilling speed of the UV-DPSS laser.

Materials with higher UV etching thresholds like copper must be processed with high-energy low-repetition rate lasers; while low-threshold materials like polyimide films can only be processed with low-energy and high-repetition lasers. The energy and high repetition rate are to avoid damage to the copper pads and increase productivity. In order to increase production capacity, most large-diameter micro-vias are processed in two steps: first use UV-DPSS laser to drill the copper foil, and then use a CO2 laser to remove the exposed dielectric.