With the development of the electronic 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 PCB will become smaller and smaller, and the number of layers will increase. 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. The mainstream of future circuit boards will be 2-3 mil, or less.

It is generally believed that each time the production circuit board is increased or upgraded 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 to do experiments to collect process data and trial production after 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 conditions. This article describes in detail the limits 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 capping acid etching method and pattern plating method, both of which have their own advantages and disadvantages. The lines obtained by the acid etching method are very uniform, which is conducive to impedance control, and has less environmental pollution, but if a hole is broken, it will lead to scrap; the production control of alkali etching is relatively easy, but the lines are not uniform and the environmental pollution is also large.

First of all, the first thing to do 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 ordinary exposure machines can reach 2mil, generally here Line width and line spacing within the range will not cause problems. 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-shaped nozzles are used, and the pressure is 3BAR or so to develop.

Although the exposure energy has a great influence on the circuit, the exposure range of most dry films currently on the market is 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. Therefore, the actual production should be combined with the cleanliness of the darkroom, so that the circuit boards that can be produced can be selected according to the actual situation. Minimum line width and line spacing.

The influence of developing conditions on the resolution is more obvious when the line is smaller. When the line is above 4.0mil/4.0mil, the influence of developing conditions (speed, concentration of liquid medicine, pressure, etc.) is not obvious; when the line is 2.0mil/2.0/mil, the shape and pressure of the nozzle play a key role in whether the line can be developed normally. At this time, the development speed may be significantly reduced, 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. When the line spacing is small, the impulse can still reach the bottom of the dry film. Therefore, 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 the resolution and the sidewall of the dry film.

Different exposure machines have different resolutions. One of the exposure machines currently used is an air-cooled, surface light source, and the other is a water-cooled, point light source. Its nominal resolution is 4mil. However, experiments have shown 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 requires Be careful, and the influence of dust and debris is very large. In addition, there is no obvious difference between the resolutions of the Mylar and glass surfaces in the experiments.

For alkaline etching, there is always a mushroom effect after electroplating, which is generally only an obvious and indistinct 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, and the dry film is sandwiched inside. 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, the cost is higher, this dry film is in acid etching 3. Electroplating with low current. But these methods are incomplete. In fact, it is difficult to have a very complete method.

Because of the mushroom effect, defilming 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.

Different line widths have different speeds and different line shapes during alkaline etching. If the circuit board has no special requirements for the thickness of the lines to be produced, use a circuit board with a thickness of 0.25oz copper foil or a 0.5oz base A part of the copper is etched away, the copper electroplating is thinner, and the lead and tin are thickened, all of which have an effect on the use of alkali etching to make thin lines, 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 is easy to break or scratch the masking film and the surface film in the transmission and previous processes. Therefore, care should be taken during production. The effect of acid etching is better than that of alkaline etching. There is no mushroom effect and side etching is less than 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 board surface, and the cleanliness of the diazo sheet have a great influence on the pass rate, and it is particularly important for the parameters of the acid etching film and the flatness of the board surface; Degree is important.

Therefore, it is believed that ordinary equipment can produce 3.0mil/3.0mil (referring to the film line width and spacing) without special adjustment; however, the pass rate is affected by the environment and the proficiency and operation level of personnel. Alkaline etching is suitable for For the production of circuit boards below 3.0mil/3.0mil, unless the base copper is small to a certain extent, the effect of the fan nozzle is obviously better than that of the conical nozzle.