Vacuum etching technology

Vacuum Etching Technology: An Improved Etching Technique for Ultra-Fine Structures.

The etching process is a basic production step of PCB manufacturing. First, the resist is used to cover the wires and component pins that need to be retained on the copper-clad board, and the copper that is not covered by the resist and does not need to be retained is removed by etching. Conductive pattern. Etching technology is more challenging, especially in the production of fine wires, the tolerance requirements are very strict and no errors are allowed. Therefore, the effect after etching needs to be as consistent as possible.

Due to the high degree of automation of horizontal wet process lines, PCB manufacturers use more in production, which further complicates the etching process. As the cost of low cost provided by automated equipment, we cannot avoid the puddle effect, because the etching effect of the upper and lower plate surfaces of the horizontal line is different. Specifically, the etching rate of the edge of the board is faster than the etching rate of the middle part of the board. In some cases, it can cause severe disproportion (see Figure 1).

Figure 1: Puddle effect

The puddle effect will cause the side corrosion of the wires on the edge of the board to be much larger than that of the wires on the center of the board. Even with careful wiring corrections to compensate for differences in etch rates (slightly widening the line width of the wires along the edge of the board), failing to avoid the failure due to the strict tolerance requirements of ultra-fine lines.

The cause of the difference in etch rate is very obvious. In the horizontal transmission line, the etching solution flowing away from the board edge of the circuit board is relatively easy to flow away, so it can often be replenished with fresh etching solution. On the contrary, a puddle effect is generated in the central part of the board, and because the etching solution is difficult to get updated, the etching rate in this area is slower than the side of the board or the side of the board where the etchant is easy to drain / flow away. In actual operation, the horizontal transfer of the plutonium obstructs the discharge of the etching solution and causes it to accumulate in the plutonium, so the occurrence of a puddle effect cannot be avoided.

The puddle effect becomes particularly noticeable when producing large or ultra-fine wire boards. Unless a large-scale technical investment is made, even if special process engineering measures are carried out, such as: adjusting the individually adjustable nozzle to be parallel to the conveying direction, swinging the injection manifold or correcting the re-etching system, the problem is all Cannot handle properly. So avoid puddles during etching from the beginning.

At the end of last year, PILL e.K. released a new process technology, which can improve the fluidity of the etching liquid on the upward side of the board by only using a pump to suck the used etching liquid, thereby preventing the occurrence of puddle effect. This method is called vacuum etching (see Figure 2-3).

The first vacuum etching line was demonstrated to the public in Productronica in November 2001. At the same time, tests conducted by circuit board manufacturers have confirmed that the vacuum etching process can achieve excellent results with less effort to control engineering conditions.

After vacuum etching, the etching effect is very uniform on the entire surface of both sides of the board (see Figure 4-5).

The principle of vacuum etching technology is very simple. In the etching section (see Figure 6), not only the nozzle is installed, but also a suction unit is installed at a relatively close distance between the nozzle and the circuit board surface. These extraction units suck the used etchant away and return to the module’s tank through a closed loop.

Vacuum here refers to the negative pressure in the operating area of ​​the system and the lower suction just enough to prevent the puddle effect of the etchant. Even the thinnest inner layer board cannot be sucked up by the suction unit, and production accuracy needs to be guaranteed. By connecting the track of the air extractor with the upper fixed 辘 in the conveying system, the designer has ensured that the distance between the air extraction process and the plate surface is the optimal value, regardless of whether the thin plate or the thick plate is processed. This means that no matter what type of PCB is used, a uniform etching solution extraction rate can be obtained. On the entire surface of the 24 “X24” board, the circuit board is facing up, and only 1 micron copper thickness fluctuation is found. By comparison, the etching effect of the upward facing part and the downward facing part of the plate are basically the same.

The circuit quality of the board produced using vacuum etching technology is also very good. Detailed testing with different PCB manufacturers shows that the new vacuum etching technology can produce a straighter conductor profile, so that the board can be produced more closely to the requirements of the wiring.

In the vacuum etching process, the shrinkage rate of the attack amount of the etching medium on the side of the wire under the resist film and the value of the etching factor used to describe the wire etching depth and the side etching amount are very high. (Figure 7-8).

Figure 7: Etching effect of the board facing down Figure 8: Etching effect of the board facing up

Of course, there are a number of other factors that are largely unaffected by the manufacturer that can affect the actual etching results. For example, the thickness of the resist, the quality of the exposure and development processes, and the copper thickness of the etched substrate each have a large impact. In general, it is estimated that the effect of the etching process or the frequency of the update of the etching solution on the etching effect accounts for only half . But Oliver Briel, PILL project manager, emphasized that “it turns out we have 50% of this under full control.”

The vacuum etching process also has a series of other advantages:

Can make full use of the productivity of the etching process. As the etching speed increases, the production time is shortened, so the yield of the etching process increases.
Since satisfactory results can be achieved for the first time, no rework is required for re-etching.
Can reduce related plant control engineering and reduce corresponding costs.
The vacuum etching system uses relatively simple technology to produce ultra-fine wires without the need to install a swingable spray manifold.
Nozzle construction with intermittently adjustable injection pressure can be eliminated. This design is mainly used to ensure that the effect of puddles is reduced, which can now be accomplished simply by using a suction system.
The vacuum etching technology allows the process module to be shorter and tighter, and the functions of suction and etching can be completed simultaneously in the same module.
An additional advantage of the vacuum etch technology system is that the spray manifold can be positioned laterally in the direction of travel. Conventional spray manifolds for the production of fine wire plates usually have nozzles that need to be positioned longitudinally along the direction of travel so that different spray pressures can exist at the edges and in the plate. The proper angle between the nozzle and the direction of travel is convenient for maintenance and requires less time for replacement, and this arrangement method can also perform simple electrical flow monitoring for each injection manifold individually. If irregularities occur, users can immediately identify which jet manifold is faulty, and then make adjustments directly without delay.

Vacuum etching technology has great potential in the future, because this process is particularly suitable for the production of thin wires and ultra-fine wire structure boards. Preliminary testing of conductive patterns below 50 microns yields promising results. Further evaluation of the ability to produce thick copper plate lines using vacuum etching technology is underway, and all current data indicate good results. It is particularly noteworthy that not only traditional copper chloride was used as the etching medium, but also iron (III) chloride, which is currently commonly used in Asia, was used as the etching medium. Although it takes a long time to use this etching medium, the effect is better when the conductor profile is steep and straight, and it undoubtedly provides an alternative to the process that has been accepted as a standard, especially for special Production of fine lines.

If required, the vacuum etching line can be installed with 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, the copper chloride solution is reoxidized without adding additional pure oxygen. In long-term practical applications, systems using this technology have proven successful and the investment costs can be quickly recovered.

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