Small Trace PCB Manufacturing Points
With the development of electronic industry and electronic components integration, the electronic products size have become smaller and smaller,so small BGA package is widely used in PCB design,it’s necessary to reduce circuit trace width and line spacing to increase the number of layers as much as possible in limited area.
It is generally believed that the production circuit board must be invested once every increase or rise of a grade, and the investment is more capital. In other words, the high quality circuit boards are produced by high grade equipment. However, large-scale investment is not affordable for every enterprise, and after the investment, the experiment is conducted to collect the process data, and the trial production takes a lot of time and money. It is a better way to decide whether to invest or not, according to the actual situation and market situation, according to the actual situation and market situation of the enterprise. 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 hole acid etching method and figure plating method, both of which have their own advantages and disadvantages. The acid etching route is very uniform, which is advantageous to the impedance control, the environment pollution is few, but has a hole to cause the scrap; the alkali corrosion production control is easy, but the line is not uniform, the environment pollution is also big.
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 great 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 other plate has obvious influence on the resolution and the side wall of the dry film.
Different exposure machines have different resolutions. At present, the exposure machine is air-cooled, surface light source, and the other is water cooling, point light source. The 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 resolution of Mylar and glass surface.
For alkaline etching, there is always a mushroom effect after plating, which is usually obvious and not obvious. Such as the line larger than 4.0mil/4.0mil, the effect of small mushrooms.
When the line is 2.0mil/2.0mil when the impact is very big, because the dry film plating tin overflow forming mushroom, dry film is very difficult in the film clip lead back inside. The solution is 1.; with pulse plating the coating uniformity; a dry film 2. thick, dry film generally 35 to 38 microns thick dry film is 50 to 55 microns, the high cost, the dry film etching in use effect is good; 3. with low current electroplating. But these methods are not thorough. In fact, it’s hard to have a very complete approach.
Because the mushroom stripping effect, fine line is very troublesome. Due to the corrosion of sodium hydroxide on lead and tin in 2.0mil/2.0mil will be very obvious so that it can be thickened and reduce the electroplating tin concentration of sodium hydroxide solution.
The line width 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. The conical nozzle generally can only be 4.0mil/4.0mil.
In acid and alkali corrosion is the same line width and line shape of different speed different, but generally with the dry film etching process, easy in transmission and in front of the mask and the surface pore membrane break or scratch, so be careful with production, acid alkali corrosion is better effect than the line. There is no mushroom effect side etching alkali corrosion is less, the other with a fan-shaped nozzle is better than the conical nozzle. The impedance changes little after etching.
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.
So that ordinary equipment does not make special adjustments, can realize the production of 3.0mil/3.0mil (refers to the film width and spacing) board; but the qualified rate is influenced by the environment and personnel operation proficiency and operation level, suitable for the production of alkali corrosion under 3.0mil/3.0mil base copper circuit board, except the small to a certain extent, fan-shaped nozzle the effect is obviously better than the conical nozzle.