Chapter 2 the advantage of wet film on PCB process

brushing（ccl pre-treatment）---screen print---bake----expose—develop---plating— film strip — etch

Points of Operation for Wet Film Application

ü                      Brushing board

For the materials provided in the previous process (ie, production boards), it is required that the board surface is free from serious oxidation, oil stains, and wrinkles. We use pickling (5% sulfuric acid) spray to remove organic impurities and inorganic dirt, and then use 500 mesh nylon brush rollers to brush. After brushing the board, it must be achieved below result: the copper surface is free of oxidation, the copper surface is uniformly roughened, the copper surface has strict flatness, and there is no water trace on the copper surface.

This performance enhances the bonding force between the wet film and the surface of the copper foil to meet the requirements of the subsequent process. The surface state of the copper foil after brushing directly affects the yield of the PCB.

ü                      screen printing

In order to achieve the required thickness of the wet film, the screen should be selected before screen printing, and the thickness and mesh number of the screen (that is, the number of lines per unit length) should be paid attention to. The film thickness is related to the ink penetration of the screen. The theoretical ink penetration (Uth) of the ink is:

Uth=Dw2\(w+d)2*1000

(D- mesh thickness d- wire diameter w- opening width)

The actual amount of ink penetration is also related to the viscosity of the wet film, the pressure of the squeegee, and the moving speed of the squeegee. In order to achieve uniform coverage, the blade edge should be well ground. The thickness of the film after printing should be controlled between 15-25um. If the film is too thick, it is easy to cause insufficient exposure, poor development, and difficult to control pre-baking, which is easy to cause battlefield films and difficult to operate. If the film is too thin, it is easy to cause overexposure, good corrosion resistance, poor insulation during electroplating, and difficult to remove the film. To manufacture fine lines below 0.15um, the film should be less than 20um.

The viscosity of the wet film should be adjusted well before use, and it should be fully stirred and evenly stirred for 15 minutes. The environment of the screen printing room should be kept clean to prevent foreign debris from falling on the surface and affecting the pass rate of the board. The temperature should be controlled at about 20 °C. Relative humidity should be around 50%。

3. Pre-baking

The pre-baking parameters are used to bake the first side at 800-100°C for 7-10 minutes, and the second side is also baked at 80-100°C for 10-20 minutes. Pre-baking is mainly to evaporate the solvent in the ink. Pre-baking is related to the success of wet film application. Insufficient pre-baking makes film easy to stick to the panel during storage and move, also make it stick to the negative film during exposure, which will eventually cause line breakage or short circuit. Excessive pre-baking, It is easy to develop dirty, and the edges of the lines are jagged. Pre-baking directly matter the quality of the PCB. Therefore, in the usual operation, the thickness of the wet film should be tested frequently, and the parameters of the oven should be adjusted according to the change of the ambient temperature, and the blasting of the oven should be checked frequently. whether the circulation system is good or not, the board should be exposed as soon as possible after drying, and it is best not to leave it for more than 12 hours

4、Exposure

Exposure is a photopolymerization process in which monomer molecules in the wet film absorb light energy under the action of ultraviolet rays. Choose a high-power exposure machine to reduce exposure time and heat accumulation, ensure the stability of exposure graphics and reduce possibility of sticking to negative films. The exposure workshop should be kept clean in each shift to avoid trachoma, gaps and wire breakage caused by debris and dust attached to the panel surface. An exposure ruler should be used to adjust the exposure time to avoid excessive or insufficient exposure. The final exposure level should be controlled btw the 6th level and 8th level. During exposure process, the same panel should be exposed at the same position as much as possible, and try to ensure that the same type of board receives the same energy. The exposure is too large, the effect of anti-etching and anti-plating is good, but the film stripping effect is not ideal and the pattern line is reduced (using positive film) or enlarged (using negative film); insufficient exposure, resulting in poor development, poor anti-etching, lines edges are hairy, and the line spacing increases or decreases, which is easy to cause the line short and broken during etching

5. Development

Development is the process of removing the part of the wet film layer that has not been exposed, then obtain the wanted circuit pattern. Strictly control the developer concentration (10-12G/L) and temperature (30-34°C), if the developer concentration is too high or too low, it is easy to cause development not clean. Optimize the development speed to match the exposure, and clean the nozzles frequently to keep the pressure distribution of the nozzles same more or less. Tool long development time or too high temperature will cause deterioration of the wet film surface, serious penetration or side etching during electroplating or acid etching, which reduces the accuracy of pattern production

6. Etching and stripping

After Etching we gets the circuit pattern we need, and the etching solution can choose alkaline ferric chloride, acid copper chloride and ammonia water. When etching different copper foil thicknesses, we use different etch speed, and the etching speed should be aligned with the temperature and concentration of the etching solution. regular nozzle maintenance will help keep the pressure and spray liquid distribution uniform, otherwise it will eventually cause uneven etching performance and copper silk appeared by the line edge, then affects the quality of PCB.

To strip the film, we use 4%-7% sodium hydroxide solution at 50-60 °C, mainly because sodium hydroxide is the process of film expansion and subdivision. If Controlling the parameters, the wet film in the hole can also be removed. We have gotten good results by using wet film for graphic transfer on special double-sided panels

The issue solved 

Select the appropriate wet film, adjust a large number of parameters, and adjust the thickness of the wet film on the board surface according to different needs, so that the pass rate of semi-finished products reaches 99%. Basically solved the problem. We continued to track for three months without any major fluctuations. Sometimes due to a little change in ambient temperature and humidity, after we adjust the parameters, the problem is easily solved. Since wet film can solve the material cost, equipment cost, energy cost, and bring economic benefits to the company, we have been using wet film in the inner layer of multi-layer boards and single-sided boards

Besides, some extra discussion.

   Printed circuit board manufacturers have two methods of making panel pattern, one is dry film and the other is wet film, so what is the difference between dry film and wet film.

Dry film is easy to operate, especially with automatic film laminator, which can come true mass production. But the price of dry film is more expensive, but it can do buried hole and has high precision (not easy to stuck film)。

  The price of the wet film is much cheaper than the dry film, which is nearly 1:7, but it cannot be used in an automatic production line. In theory, the wet film is good for thin lines, but the wet film cannot finish the buried holes (cover the holes), and the oil film inlet control is not good. It is necessary to make stop points. In theory, the precision of wet film is higher than that of dry film. However, because the wet film is thinner, it is easy to clamp the film during electroplating as shown in the figure. It is difficult to make high-precision PCB, and secondary drilling is required.

Overall, dry film is simple and stable; wet film is cheap.