Practical issues in the production of fine wires for circuit boards
Abstract: With the development of the electronics industry, the integration degree of electronic components is getting higher and higher, and the size is getting smaller and smaller, and BGA-type packages are generally used. Therefore, PCB lines will become smaller and smaller, and the number of layers will increase. Decreasing the line width and line spacing is to use the limited area as much as possible, and increasing the number of layers is to use the space. The mainstream of future circuit boards will be 2-3mil, or less.
Usually, every time the production circuit board is increased or increased, it must be invested once, and the investment capital is relatively large. In other words, high-end circuit boards are produced by high-end equipment. However, not every enterprise can afford large-scale investment, and investing in experiments and collecting process data later, trial production takes a lot of time and money. It seems that a better method is to conduct trials and trial production according to the current situation of the enterprise, and then decide whether to invest based on actual conditions and market conditions. This article details the limits of the width of thin wires that can be produced under common equipment conditions, as well as the conditions and methods of producing thin wires.
The general production process can be divided into capped acid etching method and pattern plating method, both of which have advantages and disadvantages. The line obtained by the acid etching method is very uniform, which is favorable for impedance control and less environmental pollution, but a hole is broken to cause scrapping. Alkaline etching production control is easier, but the line is uneven and environmental pollution is also large.
First of all, dry film is the first line of production. Different dry films have different resolutions but can generally display line width and line spacing of 2mil / 2mil after exposure. The resolution of ordinary exposure machines can reach 2mil, generally here. Line widths and spacing within the range will not cause problems. At 4mil / 4mil line width line pitch or above, the relationship between pressure and solution concentration is not very big. Below 3mil / 3mil line width line pitch, the nozzle is the key to affect the resolution. Generally, fan-shaped nozzles are used. 3BAR can develop.
Although the exposure energy has a very large impact on the circuit, most of the dry film currently used in the market generally has a wide exposure range. It can be distinguished between 12-18 steps (25-level exposure ruler) or 7-9 steps (21-level exposure ruler). Generally speaking, a lower exposure energy is good for resolution, but when the energy is too low, dust and various impurities in the air It has a great influence on it, and it will cause open circuit (acid corrosion) or short circuit (alkali corrosion) in the later processes. Therefore, the actual production should be combined with the cleanliness of the darkroom, so that the selectable circuit board can be selected according to the actual situation. Minimum line width and line spacing.
The influence of developing conditions on the resolution becomes more obvious when the line is smaller. When the line is more than 4.0mil / 4.0mil, the development conditions (speed, concentration of liquid, pressure, etc.) have no significant effect. When the line is 2.0mil / 2.0 / mil, the shape and pressure of the nozzle play a key role in developing the line. The development speed at this time may be significantly reduced, and at the same time, the concentration of the drug solution has an effect on the appearance of the line. The possible reason is that the pressure of the fan nozzle is large, and the impulse can still reach the bottom of the dry film when the line spacing is small. Therefore, it is possible to develop; the pressure of the conical nozzle is small, so it is difficult to develop a fine line. The orientation of the other board has a significant impact on the resolution and the side wall of the dry film.
Different exposure machines have different resolutions. One of the currently used exposure machines is air-cooled, surface light source, and the other is water-cooled, point light source. Its nominal resolution is 4mil. However, experiments show that 3.0mil / 3.0mil can be achieved without special adjustment or operation; even 0.2mil / 0.2 / mil can be achieved; 1.5mil / 1.5mil can also be distinguished when the energy is reduced, but the operation needs to be performed at this time. Be careful, and the impact of dust and debris is large. In addition, the resolution of Mylar and glass surfaces was not significantly different in the experiment.
For alkali etching, there is always a mushroom effect after electroplating, and it is generally only a clear and inconspicuous distinction. If the line is larger than 4.0mil / 4.0mil, the mushroom effect is small.
When the line is 2.0mil / 2.0mil, the effect is very large. The dry film is formed into a mushroom shape due to the lead and tin overflowing during plating. . Use pulse electroplating to make the coating uniform; 2. Use a thicker dry film, the general dry film is 35-38 microns, the thicker dry film is 50-55 microns, and the cost is higher. This dry film is acid etched Good effect in medium use; 3. Plating with low current. But these methods are not thorough. In fact, it is difficult to have a very complete method.
Due to the mushroom effect, the peeling of thin lines is very troublesome. Because the corrosion of lead and tin by sodium hydroxide is very obvious at 2.0mil / 2.0mil, it can be solved by thickening lead and tin and reducing the concentration of sodium hydroxide during electroplating.
In alkaline etching, different line widths and speeds are different, and different circuit shapes are also different in speed. If the circuit board has no special requirements in terms of the thickness of the wire, use a 0.25oz copper foil circuit board Copper is partially removed, electroplated copper is thinner, lead and tin are thickened, etc. all have an effect on making fine lines with alkali etching, and the nozzle needs to be fan-shaped. The conical nozzle can only achieve 4.0mil / 4.0mil.
In acid etching, the same as alkaline etching is different in line width and line shape speed, but generally dry film is easy to break or scratch the masking film and surface film during transportation and previous processes when acid etching is used. Therefore, care must be taken during production. The effect of acid etching is better than that of alkaline etching. There is no mushroom effect. Side etching is less than that of alkaline etching. In addition, the effect of fan nozzles is significantly better than that of conical nozzles. The resistance change of the wire is smaller after acid etching.
In the production process, the speed and temperature of the film, the cleanliness of the board surface, and the cleanliness of the diazo sheet have a greater impact on the pass rate, which is particularly important for the parameters of the acid-etched film and the flatness of the board surface; for the cleaning of alkali etching and exposure Degree is important.