Practical problems of fine PCB production

With the development of electronic industry, the integration 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. Reducing line width and line spacing is to make the best use of limited area, and increasing the number of layers is to make use of space. The mainstream of the circuit board in the future is 2-3mil or less.

It is generally believed that every time the production circuit board increases or rises a grade, it must be invested once, and the investment capital is large. In other words, high-grade circuit boards are produced by high-grade 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. For example, it seems to be a better method to make a test and trial production according to the existing situation of the enterprise, and then decide whether to invest according to the actual situation and market situation. This paper describes in detail the limit of thin line width that can be produced under the condition of common equipment, as well as the conditions and methods of thin line production.

The general production process can be divided into cover hole etching method and graphic electroplating method, both of which have their own advantages and disadvantages. The circuit obtained by acid etching method is very uniform, which is conducive to impedance control and less environmental pollution, but if a hole is broken, it will be scrapped; Alkali corrosion production control is easy, but the line is uneven and the environmental pollution is also large.

First of all, dry film is the most important part of line production. Different dry films have different resolutions, but generally can display the linewidth and line spacing of 2mil / 2mil after exposure. The resolution of ordinary exposure machine can reach 2mil. Generally, the linewidth and line spacing within this range will not cause problems. At 4mil / 4mil linewidth line spacing or above, the relationship between pressure and liquid medicine concentration is not great. Below 3mil / 3mil linewidth line spacing, the nozzle is the key to affect the resolution. Generally, fan-shaped nozzle is used, and the development can be carried out only when the pressure is about 3bar.

Although the exposure energy has a great impact on the line, most of the dry films used in the market generally have a wide exposure range. It can be distinguished at level 12-18 (level 25 exposure ruler) or level 7-9 (level 21 exposure ruler). Generally speaking, a low exposure energy is conducive to the resolution. However, when the energy is too low, the dust and various sundries in the air have a great impact on it, resulting in 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 effect of developing conditions on resolution is more obvious when the line is smaller. When the line is above 4.0mil/4.0mil, the developing conditions (speed, liquid medicine concentration, pressure, etc.) The influence 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 decrease significantly. At the same time, 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-shaped nozzle is large, and the impulse can still reach the bottom of the dry film when the line spacing is very small Development: the pressure of the conical nozzle is small, so it is difficult to develop the fine line. The direction of the other plate has a significant impact on the resolution and the side wall of the dry film.

Different exposure machines have different resolutions. At present, one exposure machine is air-cooled, area light source, the other is water-cooled and point light source. Its nominal resolution is 4mil. However, experiments show that it can achieve 3.0mil/3.0mil without special adjustment or operation; it can even achieve 0.2mil/0.2/mil; when the energy is reduced, it can also be distinguished by 1.5mil/1.5mil, but the operation should be careful In addition, there is no obvious difference between the resolution of Mylar surface and glass surface in the experiment.

For alkali corrosion, there is always mushroom effect after electroplating, which is generally only obvious and not obvious. For example, 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 great. The dry film forms a mushroom shape due to the overflow of lead and tin during electroplating, and the dry film is clamped inside, which makes it very difficult to remove the film. 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, and the thicker dry film is 50-55 microns, which is more expensive. This dry film is subject to acid etching 3. Low current electroplating. But these methods are not complete. In fact, it is difficult to have a very complete method.

Because of the mushroom effect, the stripping of thin lines is very troublesome. Because the corrosion of sodium hydroxide to lead and tin will be 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.

In alkaline etching, the line width and speed are different for different line shapes and different speeds. If the circuit board has no special requirements on the thickness of the produced line, the circuit board with the thickness of 0.25oz copper foil shall be used to make it, or part of the base copper of 0.5oz shall be etched, the plated copper shall be thinner, the lead tin shall be thickened, etc. all play a role in making fine lines with alkaline etching, and the nozzle shall be fan-shaped. Conical nozzle is generally used Only 4.0mil/4.0mil can be achieved.

During acid etching, the same as alkali etching is that the line width and line shape speed are different, but generally, during acid etching, the dry film is easy to break or scratch the mask film and surface film in the transmission and previous processes. Therefore, care should be taken during production. The line effect of acid etching is better than alkali etching, there is no mushroom effect, side erosion is less than alkali etching, and the effect of fan-shaped nozzle is obviously better than conical nozzle The impedance of the line changes less after acid etching.

In the production process, the speed and temperature of film coating, the cleanliness of plate surface and the cleanliness of diazo film have a great impact on the qualification rate, which is particularly important for the parameters of acid etching film coating and the flatness of plate 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 spacing) boards without special adjustment; however, the qualification rate is affected by the proficiency and operation level of environment and personnel. Alkali corrosion is suitable for the production of circuit boards below 3.0mil/3.0mil. Except that the non-base copper is small to a certain extent, the effect of fan-shaped nozzle is obviously better than that of conical nozzle.