Etching process is one of the basic steps in PCB production. Simply speaking, it is the process that the base copper is covered by the resist layer, and the copper that is not protected by the resist layer reacts with the etchant, so that it is bitten off, and finally the design circuit pattern and pad are formed. Of course, the principle of etching can be easily described in a few words, but in fact, the realization of etching technology is still quite challenging, especially in the production of micro-circuits, which requires a small line width tolerance and does not allow any errors in the etching process. Therefore, the etching results should be just right, not widened or over-etched.
Detailed analysis, principle and advantages of PCB vacuum etching technology
Further explaining the etching process, PCB manufacturers prefer to use horizontal etching lines for production, so as to achieve maximum production automation and reduce production costs. However, horizontal etching is not perfect, and the “pool effect” that cannot be eliminated makes the upper and lower surfaces of the board produce different etching effects. The etching rate at the edge of the board is faster than that at the center of the board. Sometimes, this phenomenon will make the etching results on the board surface quite different.
That is to say, the “pool effect” will cause the over-etching of the lines on the board edge to be larger than the over-etching of the lines in the center of the board, and even careful line correction (appropriately widening the line width on the board edge) to compensate for different etching rates will fail, because to obtain ultra-fine lines, it is necessary to carefully control the etching tolerance.
The change of etching rate caused by this situation is very significant. In the part above the circuit board, near the board edge, the etching solution is easier to flow out of the board, and the new and old etching solutions are easier to exchange, thus maintaining a good etching rate. However, in the center of the plate, it is easy to form a “pool” situation, so the flow of etchant is restricted, and it is relatively difficult for the solution rich in copper ions to flow out of the plate. As a result, compared with the edge of the plate or the bottom of the plate, the etching efficiency decreases and the etching effect becomes worse. Actually, it is impossible to avoid the “pool effect” in practice, because the chain-type horizontal drive rollers will prevent the etching solution from being discharged, resulting in the accumulation of etching solution between the rollers. This phenomenon is more obvious when producing large-area boards or ultra-fine lines, even if special production process control and compensation methods are adopted, such as spray system which can be independently adjusted horizontally in the conveying direction, adding oscillating spray pipes and adding corrective re-etching sections, etc. If there is no huge technical investment, this problem can’t be solved well, so the goal of avoiding “pool effect” has to go back to the starting point and start again.
At the end of last year, PILL e.K released a new process technology, which can improve the fluidity of the etching solution in the upward part of the board only by sucking the used etching solution with a water pump, thus preventing the puddle effect. This method is called vacuum etching.
The first vacuum etching line was demonstrated to the public at Productronica in November, 2001. At the same time, the tests conducted by PCB manufacturers also confirmed that the vacuum etching process can achieve excellent results only with less energy to control the engineering conditions.
After vacuum etching, the etching effect is very uniform on both sides of the board.
The principle of vacuum etching technology is very simple. In the etching section, not only nozzles are installed, but also suction units are installed at the positions between nozzles which are relatively close to the surface of the circuit board. These pumping units suck up the used etching solution and then return it to the liquid tank of the module through a closed circuit.
Here, vacuum refers to the negative pressure in the operating area of the system and the low suction force just enough to prevent the etching solution from producing puddle effect. Even the thinnest inner plate can’t be sucked up by the suction unit, and the production accuracy needs to be guaranteed. By connecting the rail of the air pump with the upper fixed roller in the conveying system, the designer ensures that the distance between the air pumping process and the plate surface is the best value, and whether the thin plate or the thick plate is produced can be processed. This means that no matter what kind of PCB board, the uniform extraction rate of etching solution can be obtained. On the surface of the whole 24 “x 24” board, only 1 micron copper thickness fluctuation was found on the upward side of the circuit board. By comparison, the etching effect of the upward part of the plate is basically the same as that of the downward part.
The circuit quality of the board produced by vacuum etching technology is also very good. Detailed tests with different PCB manufacturers show that the new vacuum etching technology can produce straighter conductor sections, so that the produced boards can more accurately approach the requirements of wiring.
In the vacuum etching process, the shrinkage rate of the etching medium under the resist film and the etching factor used to describe the etching depth and lateral etching amount of the wire are very high.
Of course, there are a series of other factors that are basically unaffected by the manufacturer, which will affect the actual etching effect. For example, the thickness of resist, the quality of exposure and development processes, and the copper thickness of etched substrate all have great influences. Generally speaking, it is estimated that the etching process or the update frequency of etching solution only accounts for half of the etching effect. But Oliver Briel, project manager of PILL, stressed that “it turns out that we let this 50% be completely controlled”.
Vacuum etching technology also shows a series of advantages in other aspects:
The productivity of etching process can be fully utilized. As the etching speed increases, the production time is shortened, so the yield of the etching process increases.
Because the first etching can achieve satisfactory results, there is no need to rework and re-etch.
It can reduce the related factory control projects and reduce the corresponding costs.
The vacuum etching system can produce ultra-fine wires with relatively simple technology, and it is no longer necessary to install a swinging spray manifold.
The nozzle structure with intermittent adjustable injection pressure can no longer be used. This design is mainly used to ensure that the puddle effect is reduced. Now, simply using the suction system can accomplish this function. Vacuum etching technology allows the process module to be shorter and more compact, and can simultaneously complete the functions of suction and etching in the same module.
An additional advantage of the vacuum etching technology system is that the spray manifold can be arranged transversely in the direction of travel. In the traditional jet manifold used to produce fine wire guide plates, the nozzles are usually arranged longitudinally along the traveling direction, so that different jet pressures can exist at the edge of the plate and in the plate. The angle between the nozzle and the direction of travel is appropriate, which is convenient for maintenance and requires less time for replacement. Moreover, this arrangement method can also carry out simple flow electrical monitoring for each injection manifold. If an irregular situation occurs, the user can immediately identify which injection manifold has a problem, and then can directly adjust it without delay.
Vacuum etching technology has great potential in the future, because this process is especially suitable for the production of thin wires and ultra-fine wire structural plates. The preliminary test of conductive patterns below 50 microns can get the promised results. At present, the ability of vacuum etching technology to produce thick copper plate lines is further evaluated, and all the data at present show that the results are good. Notably, not only the traditional copper chloride was used as the etching medium, but also ferric chloride (Ⅲ), which is widely used in Asia at present, was used as the etching medium. Although it takes a long time to use this etching medium, the effect is better when the conductor section is steep, and it undoubtedly provides an alternative for the process that has been accepted as a standard at present, especially for the production of ultra-fine lines.
If required, the vacuum etching line can be equipped with an environmentally friendly regeneration system: according to the technology developed by HUMLEITEC, hydrogen peroxide is no longer used, and oxygen in the air is used, that is, pure oxygen is not needed to be added, and copper chloride solution is re-oxidized. In the long-term practical application, it has been proved that the system adopting this technology is successful, and the investment cost can be recovered soon.
