Practical problems of PCB thin line production

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 adopted. As a result, PCB's wiring will become smaller and smaller, and the number of layers will increase. Reducing the line width and line spacing is to make use of the limited area and increase the number of layers to make use of space. The main line circuit board in the future when 23mil, or less.


It is generally believed that if the production line increases or rises a grade, it must invest once, and the investment is larger. In other words, the high-end circuit boards are made out of high-end equipment. However, large-scale investment is not affordable to every enterprise. After the investment, the experiment is done, and the process data are collected, and the trial production takes a great deal of time and money. According to the existing situation of the enterprise, we do experiments and trial production first, then decide whether to invest according to the actual situation and market situation, it seems to be a better method. This paper describes the equipment under normal production lines, can limit the width of the condition and method and fine line production.


The general production process can be divided into cover, acid etching and graphic electroplating, both of which have their own advantages and disadvantages. Acid etching of the circuit is very uniform, is conducive to impedance control, less environmental pollution, but there is a hole is scrapped, caustic alkali production control is relatively easy, but the line is uneven, environmental pollution is also large.


First of all, the first is the dry film production line, different dry film of different resolution but can generally be shown after exposure line distance of 2mil/2mil, the resolution of the common exposure machine can reach 2mil, the general line in this range the distance will not cause problems. The nozzle, in line 4mil/4mil or above from developing machine pressure between liquid concentration is not large, the 3mil/3mil line width from below the nozzle is the key influencing the resolution, the general application of fan nozzle, pressure to about 3BAR in developing.


Although the exposure energy has a very big impact on the line, but most of the current market use of dry film exposure range is quite wide. In the 12 - 18 grade (grade 25 or 7 - 9 feet exposure level (21 level exposure ruler) can be discerned, generally with a low exposure energy is conducive to the resolution but low energy when the dust in the air and all kinds of debris on the impact, in the back of the process will cause the open circuit (acid) short circuit (or alkaline). Therefore, the actual production and the cleanliness of the darkroom combination, so choose according to the actual situation of the circuit board production line can be the minimum line width and line spacing.


The influence of developing conditions on resolution is more obvious when the line is longer. When the line above the 4.0mil/4.0mil developing conditions (speed, liquid concentration, pressure etc.) the influence is not obvious; line is 2.0mil/2.0/mil, the shape of the nozzle, the pressure for developing a normal line can play a key role, then the developing speed may decrease, while the concentration of liquid line appearance effect, its possible why is the fan nozzle pressure, the line spacing is small, momentum can still reach the dry film at the bottom, so it can be developed; the conical nozzle pressure is small, so have difficulty developing fine lines. The orientation of the plate has obvious influence on the resolution and the lateral wall of the dry film.


Different exposure machines have different resolutions. Currently used exposure machine, one is air-cooled, surface light source, and the other is water cooling, point light source. Its nominal resolution is 4mil. But experiments show that do not need adjustment or special operation, can do 3.0mil/3.0mil; even can do 0.2mil/0.2/mil; 1.5mil/1.5mil can also reduce energy resolution, but the operation carefully, and the effects of dust and debris in the great. In addition, there is no obvious difference between the Mylar and glass surfaces in the experiment.


The linewidth at different speeds of alkali corrosion of different line shape at different speeds are different, if the thickness of the circuit board in the production line has no special requirements, the circuit board made of copper foil thickness 0.25oz or 0.5oz base copper corrosion part, copper plating thin, lead tin thickening and so on with do fine line has the effect of alkali corrosion, the other with fan-shaped nozzle. Taper nozzles generally only do 4.0mil/4.0mil.


In the production process, the temperature of the surface of the film speed, cleanliness, cleanliness of the diazo film on the qualified rate of influence parameters and surface of etched foil flat is very important; in alkaline etching, exposure of cleanliness is very important.


 

(Source: internal information)

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