With the development of electronic industry, the integration degree of electronic components is higher and higher, and the volume is smaller and smaller, and BGA type packaging is widely used. Therefore, the circuit of PCB will be smaller and smaller, and the number of layers will be more and more. 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 the space. In the future, the main line of the circuit board will be 2-3mil or less.
It is generally believed that every time the production circuit board increases or rises a level, it must be invested once, and the investment fund is large. In other words, high-end circuit boards are produced by high-end equipment. However, large-scale investment is not affordable for every enterprise, and it will take a lot of time and money to do experiments to collect process data after investment. For example, according to the current situation of the enterprise, it seems to be a better way to make a test and trial production first, and then decide whether to invest or not according to the actual situation and market situation. This paper describes in detail the limit of the width of the thin line that can be produced under normal equipment conditions, as well as the conditions and methods for the production of the thin line.
The general production process can be divided into two methods: cover hole etching and pattern electroplating, each of which has its own advantages and disadvantages. The lines obtained by the acid etching method are very uniform, which is conducive to impedance control, with less environmental pollution, but one hole is broken, resulting in scrap; the alkali etching production control is relatively easy, but the lines are uneven, and the environmental pollution is also large.
First of all, dry film is the first thing to make the circuit. Different dry film resolutions are different, but generally, the line width and line spacing of 2mil / 2mil can be displayed after exposure. The resolution of ordinary exposure machine can reach 2mil. Generally, the line width and line spacing within this range will not cause problems. The relationship between the pressure and the concentration of liquid medicine is not very big when the line width of 4mil / 4mil or above is the nozzle of the developer. When the line width of 3mil / 3mil is below, the nozzle is the key to affect the resolution. Generally, the fan-shaped nozzle is used, and the development can only be carried out when the pressure is around 3bar.
Although the exposure energy has a great influence on the circuit, most of the dry film used in the current market has a wide exposure range. It can be distinguished at level 12-18 (level 25 exposure scale) or level 7-9 (level 21 exposure scale). Generally speaking, the lower the exposure energy is beneficial to the resolution, but when the energy is too low, the dust and various sundries in the air will have a great impact on it, which will cause open circuit (acid corrosion) or short circuit (alkali corrosion) in the later process. Therefore, the actual production should be combined with the cleanliness of the darkroom, so as to select the minimum line width and line distance of the circuit board that can be produced according to the actual situation.
The influence of development conditions on resolution is more obvious when the circuit is smaller. When the line is above 4.0mil/4.0mil, the influence of development 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 nozzle play a key role in the normal development of the line. At this time, the development speed may be significantly reduced, and the concentration of liquid medicine has an impact on the line appearance. The possible reason is that the pressure of fan-shaped nozzle is large In the case of small line spacing, the impulse force can still reach the bottom of the dry film, so it can be developed; the pressure of the conical nozzle is small, so it is difficult to develop the fine line. In addition, the direction of the plate has an obvious effect on resolution and dry film sidewall.
Different resolution of exposure machine is different. At present, one kind of exposure machine is air-cooled, surface light source, the other is water-cooled, point light source. Its nominal resolution is 4mil. But the experiment shows that, without special adjustment or operation, it can achieve 3.0mil/3.0mil; even 0.2mil/0.2/mil; when the energy is reduced, 1.5mil/1.5mil can also be distinguished, but at this time, the operation should be careful, and the influence of dust and debris is great. In addition, there is no obvious difference between the resolution of Mylar surface and glass surface in the experiment.
For alkaline etching, there is always mushroom effect after electroplating, which is generally only obvious and not obvious. 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 overflows to form mushroom shape due to the lead tin overflow during electroplating, and the dry film is clamped inside, resulting in the withdrawal of the lead tin?. The coating is uniform by pulse plating; 2. A thick dry film is used, the general dry film is 35-38 μ m, the thick dry film is 50-55 μ m, the cost is higher, the dry film has better effect in acid etching; 3. Low current plating. But these methods are not thorough. In fact, it's hard to have a very complete method.
Because of the mushroom effect, it is very troublesome to remove the film of the thin line. Because the corrosion of sodium hydroxide to lead tin is very obvious at 2.0mil/2.0mil, it can be solved by thickening lead tin and reducing the concentration of sodium hydroxide.
In alkali etching, the line width and speed are different, and the line shape and speed are also different. If there is no special requirement for the thickness of the line, the circuit board with 0.25oz copper foil thickness or the base copper with 0.5oz is etched away, the copper plating is thinner, the lead tin is thickened, etc. all have effect on the thin line with alkali etching, and the nozzle needs to be fan-shaped. Generally, the conical nozzle can only achieve 4.0mil/4.0mil.
In acid etching, the same as alkali etching is different in line width and line shape and speed, but in general, when using acid etching, dry film is easy to break or scratch the mask and surface film in the transmission and the previous process, so it is necessary to be careful in production, the effect of acid etching on the line is better than alkali etching, there is no mushroom effect, side etching is less than alkali etching, and the effect of fan-shaped nozzle is obviously better than cone-shaped nozzle 。 The impedance changes a little after acid etching.
In the production process, the speed and temperature of the film, the cleanness of the plate surface and the cleanness of the diazo sheet have a great influence on the qualification rate, which is particularly important for the parameters of the acid etching film and the flatness of the plate surface; for the alkali etching, the cleanness of the exposure is very important.
Therefore, it is considered that ordinary equipment without special adjustment can produce 3.0mil/3.0mil board, but the qualification rate is affected by the environment and the operation proficiency of personnel, alkali etching is suitable for the production of circuit board below 3.0mil/3.0mil, except for the non base copper to a certain extent, the effect of fan-shaped nozzle is significantly better than that of cone-shaped nozzle.
Sep 05, 2019
