PCB Tech - Practical problems of PCB multi-layer circuit board thin line production

Practical problems of PCB multi-layer circuit board thin line production

With the development of the electronics industry, the integration of electronic components is getting higher and higher, and the size is getting smaller and smaller, and BGA-type packages are widely used. Therefore, the circuits of the multilayer PCB will become smaller and smaller, and the number of circuit board layers will increase. To reduce the line width and line spacing is to use a limited area as much as possible, and to increase the number of layers is to use space. The main circuit of the future PCB multilayer circuit board will be 2-3mil or less.

It is generally believed that every time the consumer circuit board increases or rises by a level, it is necessary to invest once, and the capital invested is relatively large. In other words,high-end circuit boards are consumed by high-end equipment. However, not every enterprise can afford large-scale investment, and after the investment, it will do experiments to collect process materials, and trial production will cost a lot of time and money. For example, it is a better way to conduct experiments and trial production based on the current situation of the company, and then decide whether to contribute capital based on the actual situation and market conditions. This article describes in detail the limit of the width of thin lines that can be consumed under normal equipment conditions, as well as the conditions and methods for the consumption of thin lines.

The general production process can be divided into cap hole acid etching method and pattern electroplating method, both of which have advantages and disadvantages. The circuit obtained by the acid etching method is very uniform, which is conducive to impedance control, and has less environmental pollution, but if a hole is broken, it constitutes a waste; the alkali etching consumption control is relatively simple, but the circuit is uneven, and the environmental pollution is also large.

First of all, dry film is the main thing for circuit production. Different dry films have different resolutions but generally can show a line width of 2mil/2mil after exposure. The resolution of general exposure machines can reach 2mil, which is generally here. There will be no problems with the line width and line spacing within the scale. For the nozzle of a developing machine with a line width of 4mil/4mil or more, the pressure and concentration of the drug solution are not very relevant. Below 3mil/3mil line width and line spacing, the nozzle is the key to the resolution. Generally, fan-shaped nozzles are used, and the pressure is About 3BAR can develop.

Of course, the exposure energy has a very large impact on the circuit, but in general, most of the dry film exposure scales currently used in the market are appropriately wide. It can be distinguished at levels 12-18 (25 levels of exposure ruler) or 7-9 (levels of 21 exposure ruler). Generally speaking, a lower exposure energy is good for resolution, but when the energy is too low, the dust and various impurities in the air can be distinguished. Substances have a great influence on it, and the subsequent process will form an open circuit (acid corrosion) or short circuit (alkali corrosion). Therefore, the cleanliness of the darkroom should be separated during practical consumption. In this way, choose the PCB multi-layer that can be consumed according to the actual situation. The minimum line width and line spacing of the circuit board circuit.

The influence of developing conditions on the resolution becomes more pronounced when the line is smaller. When the circuit is above 4.0mil/4.0mil, the development conditions (speed, syrup concentration, pressure, etc.) have no significant effect; when the circuit is 2.0mil/2.0/mil, the shape and pressure of the nozzle are related to whether the circuit can be developed normally. The key role is that the developing speed may be significantly reduced at this time, and the concentration of the syrup may affect the appearance of the circuit. The possible reason is that the pressure of the fan-shaped nozzle is large. In the case of a small distance between the lines, the impulse can still reach the dry film. Bottom, so it can be developed; the cone-shaped nozzle pressure is small, so it is difficult to develop thin lines. The direction of the other board has a significant effect on the resolution and the sidewall of the dry film.

Different exposure machines have different resolutions. One type of exposure machine currently in use is air-cooled, surface light source, and the other is water-cooled, point light source. Its nominal resolution is 4mil. But the test shows that without special adjustment or operation, it can achieve 3.0mil/3.0mil; so that it can achieve 0.2mil/0.2/mil; 1.5mil/1.5mil can be distinguished when the energy drops, but the operation needs to Seriously, and the impact of dust and debris is great. In addition, there is no significant difference between the resolution of the Mylar surface and the glass surface in the experiment.

With regard to alkaline etching, there is always a mushroom effect after electroplating, and it is generally only a distinction between significant and insignificant. If the line is larger than 4.0mil/4.0mil, the mushroom effect is smaller.

When the line is 2.0mil/2.0mil, the impact is very large. The dry film is formed into a mushroom shape due to the overflow of lead and tin during electroplating, and the dry film is caught in it and it is very difficult to remove the film.

The processing methods are:

1. 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, the cost is higher, this kind of dry film is better for use in acid etching;

3. Use low current electroplating. But these methods are not complete. In practice, it is difficult to have a very sound method. Because of the mushroom effect, the film removal of thin lines is very troublesome. Because the corrosion of lead-tin by sodium hydroxide is very significant at 2.0mil/2.0mil, it can be treated by thickening lead-tin and lowering the concentration of sodium hydroxide during electroplating.

During alkaline etching, the speed of different line widths is different, and the speed of the line shape is different. Assuming that the circuit board has no special requirements in terms of the thickness of the produced line, choose a PCB multilayer circuit board with a copper foil thickness of 0.25oz or 0.5 Part of the base copper of the oz is etched away, the electroplated copper is thinner, and the lead-tin thickening, etc., all have an effect on the use of alkaline etching to make fine lines, and the nozzle needs to be fan-shaped. Conical nozzles can generally only achieve 4.0mil/4.0mil.

During acid etching, the same thing as alkaline etching is that the different line widths and line shape speeds are different, but in general, the dry film is simple to use in acid etching. The masked film and the surface film are broken or scratched during the transfer and the previous process. Therefore, you need to be careful when you consume it. The line effect of acid etching is better than that of alkaline etching. There is no mushroom effect and the side etching is less than alkaline etching. In addition, the effect of fan-shaped nozzles is significantly better than that of cone-shaped nozzles. The impedance of the wire changes slightly after acid etching.

In the production process, the speed and temperature of the film, the cleanliness of the circuit board surface, and the cleanliness of the diazo film have a greater impact on the pass rate. It is particularly important for the parameters of the acid etching film and the flatness of the board; for alkali etching, exposure Cleanliness is very important.

Therefore, it is believed that the general equipment can be completed with 3.0mil/3.0mil (referring to the film line width and distance) without special adjustments; but the pass rate is affected by the proficiency and operation level of the environment and the operation of the personnel, and the alkali corrosion is appropriate Consumption 3.0mil/3.0mil