With the development of electronic industry, the integration of electronic components is getting higher and higher, but the volume is getting smaller and smaller, and BGA type packaging is widely used. Therefore, PCB lines will be smaller and smaller, with more and more layers. Reducing the line width and line spacing is to make the best use of the limited area, while increasing the number of layers is to make use of the space. In the future, the mainstream of the circuit board will be 2-3 mil, or less.
Generally speaking, every time the production of circuit boards increases or rises to a higher level, it is necessary to invest once, and the investment funds are large. In other words, high-grade circuit boards are produced by high-grade equipment. However, large-scale investment is not affordable for every enterprise, and it takes a lot of time and money for trial production to do experiments to collect process data after investment. For example, it seems to be a better method to conduct experiments and trial production according to the existing situation of the enterprise, and then decide whether to invest according to the actual situation and market conditions. In this paper, the limit of the width of thin wire that can be produced under the usual equipment conditions, and the conditions and methods of thin wire production are described in detail.
The general circuit board production process can be divided into cover hole acid etching method and pattern electroplating method, both of which have their own advantages and disadvantages. The circuit obtained by acid etching method is very uniform, which is beneficial to impedance control and has less environmental pollution. However, a hole is broken, which leads to scrapping. It is easy to control the alkali corrosion production, but the lines are uneven and the environment is polluted.
First of all, the dry film is the first thing in circuit manufacturing. Different dry films have different resolutions, but they can generally show a line width and line spacing of 2mil after exposure. The resolution of ordinary exposure machines can reach 2 mil, and generally, the line width and line spacing within this range will not cause problems. At the nozzle of the developing machine with a line width of 4mil/4mil or above, the relationship between the pressure and the concentration of the liquid medicine is not great. When the line width of 3mil/3mil is below the line distance, the nozzle is the key to the resolution. Generally, the fan-shaped nozzle is used, and the development can only be carried out with a pressure of about 3BAR.
Although the exposure energy has a great influence on the circuit, the exposure range of most dry films currently used in the market is quite wide. It can be distinguished at 12-18 levels (25-level exposure ruler) or 7-9 levels (21-level exposure ruler). Generally speaking, a lower exposure energy is beneficial to the resolution. However, when the energy is too low, dust and various sundries in the air will have a great influence on it, which will cause open circuit (acid etching) or short circuit (alkali etching) in the later process. Therefore, the actual production should be combined with the cleanness of the darkroom, so that the minimum line width and line spacing of the producible circuit board can be selected according to the actual situation.
The influence of developing conditions on resolution is more obvious when the line is smaller. When the line is above 4.0mil/4.0mil, the influence of developing conditions (speed, drug concentration, pressure, etc.) is not obvious; When the line is 2.0 mil/2.0 mil, the shape and pressure of the nozzle play a key role in the normal development of the line. At this time, the development speed may obviously decrease, 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, and the impact force can still reach the bottom of the dry film when the line spacing is small, so it can be developed. The pressure of cone nozzle is small, so it is difficult to develop fine lines. The direction of the other plate has obvious influence on the resolution and dry film sidewall.
Different exposure machines have different resolutions. At present, one of the exposure machines used is air-cooled surface light source, and the other is water-cooled point light source. Its nominal resolution is 4mil. However, the experiment shows that 3.0 mil/3.0 mil can be achieved without special adjustment or operation. Even 0.2 mil/0.2 mil; When the energy is reduced, 1.5mil/1.5mil can also be distinguished, but the operation should be careful at this time, and dust and sundries have great influence. In addition, there is no obvious difference between the resolution of Mylar surface and glass surface in the experiment.
As for alkali etching, there is always mushroom effect after electroplating, which is generally only obvious and unobvious. If the line is larger than 4.0mil/4.0mil, the mushroom effect is small.
However, when the line is 2.0mil/2.0mil, the influence is very great. The dry film is mushroom-shaped due to lead and tin overflow during electroplating, and the dry film is caught in it, which leads to rejection and calibrationPulse electroplating is used to make the coating uniform; 2. Use a thick dry film, the general dry film is 35-38 microns, the thick dry film is 50-55 microns, the cost is higher, and this kind of dry film has a good effect in acid etching; 3. Electroplating with low current. But these methods are not thorough. In fact, it is difficult to have a very complete method.
Because of the mushroom effect, it is very troublesome to remove the thin line film. Because the corrosion of sodium hydroxide to lead and tin will be 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 width speeds are different, and different line shapes and speeds are also different. If there is no special requirement for the thickness of the produced line, it is necessary to make a circuit board with a thickness of 0.25oz copper foil or etch a part of 0.5oz base copper, make the copper plating thinner, thicken the lead and tin, etc., all of which have an effect on making fine lines by alkaline etching, and the other nozzle needs a sector. Generally, the conical nozzle can only achieve 4.0mil/4.0mil.
In acid etching, the same thing as in alkali etching is that the line width and line shape speed are different. However, in general, when acid etching is used, the dry film is easy to break or scratch the mask film and the surface film in the process of transmission and front. Therefore, care should be taken in production. The line effect of acid etching is better than that of alkali etching, the side etching without mushroom effect is less than that of alkali etching, and the effect of fan nozzle is obviously better than that of cone nozzle. After acid etching, the impedance change of the wire is smaller.
In the production process, the speed and temperature of film pasting, the cleanliness of board surface and the cleanliness of diazo sheet have great influence on the qualified rate, especially on the parameters of acid etching film pasting and the flatness of board surface. For alkali etching, the cleanliness of exposure is very important.
Therefore, it is considered that ordinary equipment can produce 3.0mil/3.0mil (referring to film line width and pitch) plates without special adjustment; However, the qualification rate is affected by the environment, the proficiency and operation level of personnel. Alkaline etching is suitable for producing circuit boards below 3.0mil/3.0mil. Unless the base copper is small to a certain extent, the effect of fan nozzle is obviously better than that of cone nozzle.
