The design of PCB line width and spacing is getting smaller and smaller, which poses certain challenges for PCB manufacturers, and the equipment investment is large. This article introduces the limit of the thin line width that can be produced under normal pcb equipment conditions and the production conditions and methods.

With the development of electronic industry, the integration of electronic components is higher and higher, but 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 full use of limited area, and increasing the number of layers is to make use of space. The main line of the future PCB is 2-3mil or less.

It is generally believed that every time the PCB is increased or upgraded, 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 the large-scale investment, and it takes a lot of time and money to do experiments and collect process data after investment.

It seems to be a better way to do test and trial production according to the current 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 the width of the fine line that can be produced under the normal equipment, and the conditions and methods for the production of the fine line.

The general production process can be divided into cover hole acid etching and pattern plating method(Alkaline etching method), both have advantages and disadvantages.

Acid etching -The circuit obtained by acid etching method is very uniform, which is conducive to impedance control and has less environmental pollution.  However, if there is a hole broken, it will be scrapped;

Alkali etching- Alkali etching production control is easier, but the line is uneven, and the environmental pollution is also large.

First of all, dry film is the most important part of circuit making.  Different resolution of dry film is different, but it can display the line width and line spacing of 2mil / 2mil 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. When the line spacing of 4mil / 4mil or above, the relationship between pressure and concentration is not very big.  When the line spacing is less than 3mil / 3mil, the nozzle is the key to affect the resolution. Generally, fan-shaped nozzles are used, and the pressure is about 3BAR to develop.

Although the exposure energy has a great impact on the circuit, most of the dry film exposure range currently used in the market is quite wide. It can be distinguished at 12-18 (25 exposure scale) or 7-9 (21 exposure scale).  Generally speaking, lower exposure energy is beneficial to resolution.  However, when the energy is too low, dust and various impurities in the air 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 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 that the smaller the line, the more obvious the effect. When the line is above 4. 0mil/4. 0mil, the influence of developing conditions (speed, concentration, pressure, etc. ) is not obvious; the line is 2. 0mil/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 be significantly reduced.  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 force can still reach the bottom of the dry film when the distance between the lines is very small, so it can develop; the pressure of the conical nozzle is small, so it can developIt is difficult to thin the line. In addition, the direction of the PCB has obvious influence on the resolution and the dry film side wall.

Different exposure machines have different resolutions. At present, one kind of exposure machine is air-cooled, which is surface light source, and the other is water cooling, which is point light source. The nominal resolution is 4mil. But experiments show that, without special adjustment or operation, it can achieve 3. 0mil/3. 0mil; even can achieve 2. 0mil/2. 0mil; when the energy is reduced, 1. 5mil/1. 5mil can also be distinguished, but at this time the operation should be careful, and dust and debris have a great impact. In addition, there is no significant difference between the resolution of Mylar and glass.

For alkali 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 small.

However, when the circuit is 2. 0mil/2. 0mil, the influence is very big.  The dry film is difficult to remove because of the mushroom shape caused by the overflow of lead and tin during electroplating. The solutions are as follows:

1.  Using pulse electroplating to make the coating uniform;

2.  Using a thicker dry film, the general dry film is 35-38 μ m, and the thicker dry film is 50-55 μ m, which has higher cost.  This dry film has better effect in acid etching;

3.  Low current electroplating.

But these methods are not thorough. In fact, it's hard to have a very complete approach.

Because of the mushroom effect, it is very troublesome to strip the fine circuit. Since the etching of sodium hydroxide to lead and tin is obvious at 2. 0mil/2. 0mil, it can be solved by thickening lead and tin and reducing the concentration of sodium hydroxide.

In alkaline etching, different line width and speed are different, and different circuit shapes and speeds are also different.  If the circuit board has no special requirements on the thickness of the line, using 0. 25oz copper foil thickness circuit board or etching a part of 0. 5oz base copper, plating copper thin, thickening lead and tin, etc. , all have effects on making fine circuit by alkali etching, and the nozzle needs fan-shaped. Generally, the conical nozzle can only achieve 4. 0mil/4. 0mil.

During acid etching, the same as alkaline etching is the different line width and line shape speed. but in general, the dry film is easy to break or scratch the masking film and the surface film during the conveying and previous processes when using acid etching, so be careful during production.  The circuit effect of acid etching is better than that of alkali etching, there is no mushroom effect, side etching is less than alkali etching, and fan nozzle effect is obviously better than conical nozzle. The impedance change of the line after etching is smaller.

In the production process, the speed and temperature of the film, the cleanliness of the plate surface and the cleanliness of the diazo film have a great influence on the pass rate, which is particularly important for the parameters of the acid etching film and the flatness of the plate surface; it is very important for the cleanliness of the alkali etching and exposure.

Therefore, it is considered that: ordinary equipment can produce 3. 0mil/3. 0mil  PCB without special adjustment; however, the qualification rate is affected by the environment and personnel’s operation proficiency and operation level, and alkali etching is suitable for producing PCB below 3. 0mil/3. 0mil.  Unless the base copper is small enough, the effect of fan nozzle is obviously better than that of cone nozzle.

Of course, the use of high-end equipment such as LDI exposure equipment and pulse vertical plating line can easily realize the processing of fine line/space PCB less than 3 /3mil.