PCB Technical - PCB circuit board thin line production problem

With the development of the electronics industry, PCB manufacturers are increasingly integrating electronic components, and their volume is getting smaller and smaller, and BGA-type packages are generally used. Therefore, PCB circuits will become smaller and smaller, and the number of layers will increase. To reduce the line width and line spacing is to use the limited area as much as possible, and to increase the number of layers is to use space. The main line of the future circuit board will be 2-3mil, or smaller.

It is generally believed that every time the production circuit board is increased or increased by a grade, 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 it takes a lot of time and money for trial production to do experiments to collect process data after the investment. It seems a better way to do experiments and trial production based on the current situation of the company, and then decide whether to invest based on the actual situation and market conditions. This article describes in detail the limit of the width of thin lines that can be produced under normal equipment conditions, as well as the conditions and methods of thin line production.

The general production process can be divided into cover 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 one hole breaks will cause scrap; the alkali corrosion production control is easier, but the circuit is uneven, and the environmental pollution is also large.

First of all, dry film is the main thing for PCB circuit board production. Different dry films have different resolutions but generally can display a line width of 2mil/2mil after exposure. The resolution of ordinary exposure machines can reach 2mil. The line width and line spacing within this range will not cause problems. 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, the nozzle is the key to the resolution. Generally, fan-shaped nozzles are used, and the pressure is It can be developed around 3BAR.

Although the exposure energy has a great influence on the circuit, most of the dry film currently used on the market has a wide exposure range. 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. It has a great influence on it, and it will cause open circuit (acid corrosion) or short circuit (alkali corrosion) in the subsequent process. Therefore, the actual production should be combined with the cleanliness of the darkroom, so that the circuit board that can be produced is 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 circuit is above 4.0mil/4.0mil, the development conditions (speed, syrup concentration, pressure, etc.) have no obvious influence; when the circuit is 2.0mil/2.0/mil, the nozzle shape and pressure play a key role in whether the circuit can be developed normally At this time, the developing speed may be significantly reduced, and the concentration of the medicine has an effect on 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 momentum can still reach the bottom of the dry film. Therefore, it can be developed; the cone-shaped nozzle pressure is small, so it is difficult to develop thin lines. The direction of the additional board has a significant impact 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 experiments show that you can achieve 3.0mil/3.0mil without special adjustment or operation; even 0.2mil/0.2/mil; 1.5mil/1.5mil can be distinguished when the energy is reduced, but the operation must Be careful, and the influence of dust and debris is great. In addition, there is no obvious difference between the resolution of the Mylar surface and the glass surface in the experiment.

For alkaline etching, there is always a mushroom effect after electroplating, and it is generally just a distinction between obvious and inconspicuous. If the line is larger than 4.0mil/4.0mil, the mushroom effect is smaller.

When the circuit 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 it is very difficult to remove the film due to the dry film being clamped inside. The solutions 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, and the cost is higher. This kind of dry film has better effect in acid etching; 3. Use low current electroplating. But these methods are not thorough. In fact, it is difficult to have a very complete method.

Because of the mushroom effect, the film removal of thin lines is very troublesome. Since 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.

During alkali etching, the speed of different line widths is different, and the speed is different for different line shapes. If the circuit board has no special requirements on the thickness of the line to be produced, use a circuit board with a thickness of 0.25oz copper foil or make a 0.5oz base Part of the copper 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 thin 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 easy to break or scratch the masked film and the surface film during the transfer and the previous process. Therefore, you need to be careful during production. 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 using fan-shaped nozzles is obviously better than that of cone-shaped nozzles. The impedance of the wire changes slightly after acid etching.

In the PCB layout and design and production process, the speed and temperature of the film, the cleanliness of the 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, The cleanliness of exposure is very important.

Therefore, it is believed that ordinary equipment can produce 3.0mil/3.0mil (referring to film line width, spacing) boards without special adjustments; but the pass rate is affected by the environment and the proficiency and level of operation of personnel, and alkaline etching is suitable For the production of circuit boards below 3.0mil/3.0mil, unless the base copper is small to a certain extent, the effect of fan-shaped nozzles is significantly better than that of cone-shaped nozzles.