With the development of the electronics industry, the integration of electronic components is getting higher and higher, and the volume is getting smaller and smaller, and BGA type packages are generally used. Therefore, the lines of the multi-layer PCB will be smaller and smaller, and the number of layers of the circuit board will be more and more. Reducing 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. In the future, the main circuit of the PCB multi-layer circuit board will be 2-3 mil, or less.
It is generally believed that each time the consumer circuit board increases or rises one level, it must be invested once, and the capital invested 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 will cost a lot of time and money for trial production to conduct experiments to collect process materials after the investment. It seems to be a better method to do experiments and trial production according to the existing situation of the enterprise, and then decide whether to invest or not according to the actual situation and market situation. This article describes in detail the limit of the width of thin lines that can be consumed in the case of common equipment, and the conditions and methods of thin line consumption.
The general production process can be divided into cover hole acid etching method and pattern plating method, both of which have their own advantages and disadvantages. The line 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 will be invalid.
First of all, the first step in circuit production is dry film. Different dry films have different resolutions, but generally they can show 2mil/2mil line width and line spacing after exposure. The resolution of general exposure machines can reach 2mil. Generally, here Line width and line spacing within the scale will not be a problem. In the nozzle of the developer with a line width of 4 mil/4 mil or above, the relationship between the pressure and the concentration of the liquid medicine is not very large. Below the line width and line spacing of 3 mil/3 mil, the nozzle is the key to affecting the resolution. Generally, fan nozzles are used, and the pressure is between 3BAR or so to 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 on the market are quite wide. It can be distinguished at 12-18 levels (25-level exposure ruler) or 7-9 (21-level exposure ruler). Generally speaking, a lower exposure energy is beneficial to resolution, but when the energy is too low, dust and various impurities in the air can be detected. It has a great influence on it, and it will form an open circuit (acid etching) or a short circuit (alkaline etching) in the subsequent process. Therefore, the actual production should be separated from the cleanliness of the darkroom, so that the multi-layer PCB that can be produced should be selected according to the actual situation. The minimum line width and line spacing of the circuit board.
The influence of developing conditions on the resolution is more significant when the line is smaller. When the line is above 4.0mil/4.0mil, the influence of developing conditions (speed, liquid concentration, pressure, etc.) is not significant; when the line is 2.0mil/2.0/mil, the shape of the nozzle and the pressure play a role in whether the line can be developed normally. The key role is that the development speed may be significantly reduced at this time, and the concentration of the liquid medicine has an impact on the appearance of the line. The possible reason is that the pressure of the fan nozzle is large. In the case of a small line distance, the impulse can still reach the dry film. The bottom, so it can be developed; the pressure of the conical nozzle is small, so it is difficult to develop thin lines. The orientation of the additional plate has a significant effect on resolution and dry film sidewalls.
Different exposure machines have different resolutions. One of the exposure machines currently used is air-cooled, surface light source, and the other is water-cooled, point light source. Its nominal resolution is 4mil. However, 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 also be distinguished when the energy drops, but the operation needs to be done at this time. Seriously, and the effect of dust and debris is great. In addition, there was no significant difference in resolution between the Mylar and glass surfaces in the experiments.
With regard to alkaline etching, there is always a mushroom effect after electroplating, which is generally only a significant and insignificant 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 impact is very large. The dry film is formed into a mushroom shape due to the overflow of lead and tin during electroplating. The methods of processing are:
1. Use pulse electroplating to make the coating uniform;
2. Use a thicker dry film, the general dry film is 35-38 microns, and the thicker dry film is 50-55 microns, and the cost is higher. This type of dry film works better in acid etching;
3. Electroplating with low current. But these methods are incomplete. In practice it is difficult to have a very complete method. Because of the mushroom effect, defilming of thin lines is very troublesome. Because the corrosion of lead and tin by sodium hydroxide is very significant at 2.0mil/2.0mil, it can be processed by thickening lead and tin and reducing the concentration of sodium hydroxide during electroplating.
Different line widths and speeds are different during alkaline etching, and the speed of the line shape is also different. Assuming that the circuit board has no special requirements for the thickness of the lines to be produced, choose a PCB multilayer circuit board with a thickness of 0.25oz copper foil or 0.5 oz. The base copper of oz is etched away, the copper plating is thinner, and the lead and tin are thickened, all of which are useful for making fine lines with alkaline etching, and the nozzle needs to be fan-shaped. Conical nozzles can generally only achieve 4.0mil/4.0mil.
In acid etching, the same as alkali etching, the line width and line shape speed are different, but in general, when acid etching is used, the dry film simply breaks or scratches the masking film and the surface film in the conveying and previous processes. , so you need to be careful when using 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 that of alkaline etching. The impedance change of the wire after acid etching is smaller.
In the production process, the speed and temperature of the film, the cleanliness of the circuit board surface, and the cleanliness of the diazo sheet have a great influence on the pass rate. cleanliness is important.
Therefore, it is considered that: general equipment can complete the production of 3.0mil/3.0mil (referring to the film line width and distance) without special adjustment; but the pass rate is affected by the environment and the skill level and degree of operation of personnel, and alkaline corrosion is suitable. Consumption 3.0mil/3.0mil.
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