Practical problems in PCB production

- Nov 17, 2018-

With the development of electronics industry, the integration degree of electronic components is higher and higher, while the volume is smaller and smaller.As a result, PCB lines will be smaller and have more layers.Reducing line width and line spacing is to use the limited area as far as possible, increasing the number of layers is to use space.In the future, the main circuit of the circuit board will be 2-3mil, or smaller.

It is generally believed that every time the manufacturing circuit board is added or upgraded to a higher level, it must be invested once, and the investment is larger.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 takes a lot of time and money to invest in the future to collect technological information and test production.It seems to be a better way to make a trial and trial production according to the current situation of the enterprise, and then decide whether to invest according to the actual situation and the market situation.In this paper, the limit of fine line width and the condition and method of fine line production are described.

The general production process can be divided into cover hole etching and pattern plating, both of them have advantages and disadvantages.The circuit obtained by acid etching method is uniform, which is favorable for impedance control and less environmental pollution.The production of alkali corrosion is easy to control, but the line is not uniform and the environment is polluted.

First of all, the line is mainly made of dry film. The resolution of different dry film is different, but the line width and line spacing of 2mil/2mil can be generally displayed after exposure. The resolution of ordinary exposure machine can reach 2mil.The relationship between the nozzle, pressure and liquid medicine concentration of the developer is not very big at the line width of 4mil/4mil or above, but below the line width of 3mil/3mil, the nozzle is the key to affect the resolution. Generally, the fan nozzle is applied, and the pressure can only be developed at about 3BAR.

Although the exposure energy has a very large impact on the line, but generally the most used dry film exposure range is quite wide.It can be distinguished at levels 12-18 (level 25 exposure scale) or 7-9 (level 21 exposure scale). Generally speaking, low exposure energy is beneficial to the resolution, but when the energy is low, the dust and various sundries in the air have great influence on it. In the following process, it will cause open circuit (acid corrosion) or short circuit (alkali corrosion).Therefore, the actual production should be combined with the cleanliness of darkroom, so as to select the minimum line width and line spacing of the PCB line that can be produced according to the actual situation.

The influence of developing condition on the resolution is more obvious when the line is smaller.When the line is above 4.0mil/4.0mil, the development condition (speed, solution concentration, pressure, etc.) has no obvious influence;Line is 2.0 mil / 2.0 / mil, the shape of nozzle, pressure to normal development lines play a key role, then the development speed may be significantly lower, at the same time, the concentration of the potion to influence lines appearance, the possible reason was that the fan nozzle pressure, under the condition of the line spacing is very small, force can still be reached at the bottom of the dry film, thus enhancement;Conical nozzle pressure is low, so it is difficult to develop the thin line.The direction of the other placement plate has obvious influence on the resolution and the dry film lateral wall.

Different exposures have different resolution.At present, the use of the exposure machine one is air cooling, surface light source, another is water cooling, point light source.The nominal resolution is 4mil.However, experiments show that 3.0mil/3.0mil can be achieved without special adjustment or operation.You can even achieve 0.2mil/0.2/mil;The 1.5mil/1.5mil can also be resolved when energy is reduced, but the operation should be careful and the impact of dust and debris is significant.In addition, there was no significant difference in the resolution of Mylar surface and glass surface.

For caustic corrosion, there is always mushroom effect after electroplating, which is usually only distinguished between obvious and inapparent.If the line is larger than 4.0mil/4.0mil, the mushroom effect is smaller.

However, when the line is 2.0mil/2.0mil, the impact is very large. As the lead and tin spillage during electroplating forms mushroom, the dry film is clamped in, which makes it very difficult to remove the film.The solutions are:1. Make the plating layer uniform by pulse plating;2. Use a thick dry film. Generally, the thick dry film is 35-38 microns and the thick dry film is 50-55 microns, which is more expensive.Electroplate with low current.But these methods are not thorough.It's actually very difficult to have a very complete approach.

Because of the mushroom effect, the membrane of the thin line is very troublesome.Since the corrosion of sodium hydroxide to lead tin is very obvious when 2.0mil/2.0mil is applied, it can be solved by thickening lead tin and reducing the concentration of sodium hydroxide when electroplating.

In alkali corrosion different rate of line width, line shape different speed is different also, if the thickness of the circuit board in the production line has no special request, made by the 0.25 oz copper foil thickness PCB or will go to the part of the 0.5 oz base copper corrosion, electroplating copper thin some, such as lead tin upset all have role to do fine lines with alkali corrosion, another fan nozzle.Conical nozzles generally can only achieve 4.0mil/4.0mil.

With alkali corrosion in acid corrosion different line width, and the line shape is the same speed is different, but generally with the etched dry film easy in transfer and the process will be in front of the mask holes on the surface of the membrane and membrane broken or cut, so be careful during production, with the effect of acid etching the line better alkali corrosion, there is no lateral erosion mushroom effect relatively less alkali corrosion, with fan nozzle effect is much better than other conical nozzle.The impedance of the line changed a little after the acid corrosion.

In the production process, the speed and temperature of the coating, the cleanliness of the board, and the cleanliness of diazo sheet have a great influence on the qualification rate.Cleanliness of exposure is important for alkali erosion.

Therefore, it is believed that: ordinary equipment does not make special adjustments, and it can realize the production of 3.0mil/3.0mil (refers to film line width and spacing) plates;However, the pass rate is affected by the proficiency level of environment and human operation. Alkali corrosion is suitable for the production of circuit boards below 3.0mil/3.0mil.