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The through-holes of FPC flexible printed boards can also be drilled by numerical control like rigid printed boards, but they are not suitable for hole processing of double-sided metalized hole circuits in tapes and tapes. With the increase in the density of circuit patterns and the smaller diameter of metallized holes, coupled with the limitation of the diameter of numerically controlled drilling, many new drilling technologies have been put into practical use. These new drilling technologies include plasma etching, laser drilling, punching with small apertures, chemical etching, etc. These drilling technologies are easier to meet the hole forming requirements of the tape process than CNC drilling.
The through-holes of flexible printed boards can also be drilled by numerical control like rigid printed boards, but they are not suitable for hole processing of double-sided metalized hole circuits in tapes. With the increase in the density of circuit patterns and the smaller diameter of metallized holes, coupled with the limitation of the diameter of numerically controlled drilling, many new drilling technologies have been put into practical use. These new drilling technologies include plasma etching, laser drilling, punching with small apertures, chemical etching, etc. These drilling technologies are easier to meet the hole forming requirements of the tape process than CNC drilling.
1. CNC drilling
Most of the holes in the double-sided flexible printed board are still drilled with a CNC drilling machine.
The CNC drilling machine and the CNC drilling machine used in the rigid printed board are basically the same, but the drilling conditions are different. Because the flexible printed circuit board is very thin, it is possible to overlap multiple pieces of drilling. If the drilling conditions are good, 10 to 15 pieces can be overlapped for drilling. The backing plate and cover plate can use paper-based phenolic laminate or glass fiber cloth epoxy laminate, or aluminum plate with a thickness of 0.2 to 0.4 mm. Drills for flexible printed boards are available on the market, and drills for drilling rigid printed boards and milling cutters for milling shapes can also be used for flexible printed boards.
The processing conditions for drilling, milling, and the shape of the reinforced board are basically the same. However, since the adhesive used in the flexible printed board material is soft, it is easy to adhere to the drill bit, and it is necessary to frequently check the state of the drill bit. And it is necessary to appropriately increase the speed of the drill bit. For the multi-layer flexible printed board or multi-layer rigid flexible printed board drilling should be particularly careful.
2. Punching
Punching small apertures is not a new technology, and has been used as a mass production. Since the reeling process is continuous production, there are many examples of using punching to process the through-holes of the reel. But batch punching technology is limited to punching diameter O. Compared with the drilling of the CNC drilling machine, the 6～0.8mm hole has a longer processing cycle and requires manual operation. Because the size of the initial process is large, the punching mold is correspondingly large, so the mold price is very expensive. Although mass production is beneficial to reducing costs, the burden of equipment depreciation is large, and small batch production and flexibility cannot compete with CNC drilling, so it is still not popular.
However, in the last few years, great progress has been made in both the precision of punching technology and the numerical control drilling. The practical application of punching on flexible printed boards has been very feasible. Zui's new mold manufacturing technology can produce 75um holes in the adhesive-free copper clad laminate with a 25um base material thickness. The reliability of the punching is also quite high. If the punching conditions are suitable, it can even be punched in diameter. 50um hole. The punching device has also been numerically controlled, and the mold can also be miniaturized, so it can be well used for punching flexible printed boards, and neither CNC drilling nor punching can be used for blind hole processing.
3. Laser drilling
The smallest through holes can be drilled with a laser. The laser drilling machines used to drill through holes on flexible printed boards include excimer laser drills, impact carbon dioxide laser drills, YAG (yttrium aluminum garnet) laser drills, and argon gas. Laser drilling machine, etc.
The impact carbon dioxide laser drilling machine can only drill the insulating layer of the substrate, while the YAG laser drilling machine can drill the insulating layer and copper foil of the substrate. The speed of drilling the insulating layer is significantly faster than the speed of drilling the copper foil. Fast, it is impossible to use the same laser drilling machine for all drilling and production efficiency to be very high. Generally, the copper foil is etched first, the hole pattern is formed first, and then the insulating layer is removed to form the through hole, so that the laser can drill holes with extremely small apertures. However, at this time, the position accuracy of the upper and lower holes may restrict the hole diameter of the drilled hole. If it is to drill blind holes, as long as the copper foil on one side is etched away, there is no problem of up and down position accuracy. This process is similar to the plasma etching and chemical etching described below.
At present, the holes processed by excimer laser are the smallest. The excimer laser is ultraviolet light, which directly destroys the structure of the resin of the base layer, makes the resin molecules discrete, and generates very little heat. Therefore, the degree of heat damage to the periphery of the hole can be limited to the minimum range, and the hole wall is smooth and vertical. If the laser beam can be further reduced, holes with a diameter of 10-20um can be processed. Of course, the larger the thickness-to-aperture ratio, the more difficult it is to wet copper plating. The problem of excimer laser technology drilling is that the decomposition of polymer will cause carbon black to adhere to the hole wall, so some means must be taken to clean the surface before electroplating to remove the carbon black. However, when laser processing blind holes, the uniformity of the laser also has certain problems, which will produce bamboo-like residues.
The biggest difficulty of the excimer laser is that the drilling speed is slow and the processing cost is too high. Therefore, it is limited to the processing of high-precision and high-reliability micro-holes.
The impact carbon dioxide laser generally uses carbon dioxide gas as the laser source, and it radiates infrared rays. It is different from the excimer laser which burns and decomposes resin molecules due to thermal effects. It belongs to thermal decomposition, and the shape of the processed hole is worse than that of the excimer laser. The aperture that can be processed is basically 70-100um, but the processing speed is significantly faster than the excimer laser speed, and the cost of drilling is much lower. Even so, the processing cost is much higher than the plasma etching method and the chemical etching method described below, especially when the number of holes per unit area is large.
The impact carbon dioxide laser should pay attention to that when processing blind holes, the laser can only be emitted to the surface of the copper foil, and the organic matter on the surface does not need to be removed. In order to clean the copper surface stably, chemical etching or plasma etching should be used as a post-treatment. Considering the possibility of technology, the FPC laser drilling process is basically not difficult to use in the tape process, but considering the balance of the process and the proportion of equipment investment, it does not have an advantage, but the tape chip automation The welding process (TAB, TapeAutomatedBonding) has a narrow width, and the tape-reel process can increase the FPC drilling speed. There have been practical examples in this regard.
