PCB ink Selection Knowledge (2)

2. Coating

Coating refers to uniformly coating the copper surface with a layer of liquid photoresist. There are many methods, such as centrifugal coating, dip coating, screen printing, curtain coating, roller coating, etc.

Screen printing is a commonly used coating method, with low equipment requirements, simple and easy operation and low cost. However, it is not easy to coat both sides at the same time, the production efficiency is low, and the uniformity of the film cannot be completely guaranteed. In general screen printing, 100-300 mesh screen is used for full-page printing. This method is welcomed by most small and medium manufacturers.

Roller coating can realize simultaneous coating on both sides, high automatic production efficiency, and can control the thickness of the coating. It is suitable for large-scale production of boards of various specifications but requires equipment investment.

Curtain coating is also suitable for large-scale production and can evenly control the thickness of the coating layer, but the equipment requirements are high, and only one side can be coated before coating the other side, which affects the production efficiency.

If the photocoated film is too thick, it is easy to produce insufficient exposure, insufficient development, high pressure sensitivity, and easy to stick to the negative film. If the film is too thin, it is easy to produce overexposure, poor electroplating resistance, and easy to produce the phenomenon of electroplating metal coating, and the film removal speed is slow.

Working Conditions: Operate under yellow light in a clean room. Room temperature 23-25°C, and relative humidity 55±5%. The workplace should be kept clean and avoid direct sunlight and fluorescent lamps.

The following aspects should be paid attention to when coating operation:

1) If there are pinholes in the coating layer, there may be unknown objects in the photoresist. It should be cleaned with acetone and replaced with a new resist. It may also be that there are particles in the air falling on the board surface or other reasons that cause the board surface to be unclean. It should be carefully checked and cleaned before coating.

2) If the photocoated film is too thick during screen printing, it is because the screen mesh is too small. If the film is too thin, it may be because the screen mesh is too large. If the thickness of the coating layer is not uniform, diluent should be added to adjust the viscosity of the resist or adjust the coating speed.

3) Try to prevent the ink from entering the holes when coating the film.

4) No matter what method is used, the photoimageable covercoating should have a uniform thickness, no pinholes, bubbles, inclusions, etc., and the thickness of the film should reach 8-15um after drying.

5) Because the liquid photoresist contains solvent, the workplace must be well ventilated.

6) Wash hands with soap after work.

3. Pre-Baking

Pre-baking refers to drying the liquid photoresist film surface by heating and drying, so as to facilitate the exposure and development of the negative film to produce graphics. This process is mostly performed in the same room as the coating process. The two most commonly used pre-baking methods are drying tunnel and oven.

Generally, oven drying is used. The pre-baking temperature of the first side of both sides is 80±5°C for 10 to 15 minutes; the pre-baking temperature of the second side is 80±5°C for 15 to 20 minutes. This kind of pre-baking one after the other makes the pre-curing degree of the wet film on both sides different, and it is difficult to ensure that the development effect is completely consistent. The ideal is to coat both sides at the same time, and pre-bake at the same time, the temperature is 80±5℃, and the time is about 20-30 minutes. In this way, both sides are pre-cured at the same time, and the double-side development effect can be guaranteed to be consistent, and man-hours can be saved.

It is very important to control the temperature and time of the pre-baking. If the temperature is too high or the time is too long, it will be difficult to develop and it is not easy to remove the film; if the temperature is too low or the time is too short, the drying will not be complete, the film will be pressure sensitive, and it will be easy to stick to the negative film, resulting in poor exposure and easy damage to the negative film. Therefore, the pre-baking is appropriate, the development and film removal are faster, and the graphics quality is good.

Attention should be paid to this process:

(1) After pre-baking, the board should be air-cooled or naturally cooled before exposure to negative film alignment.

(2) Do not use natural drying, and the drying must be complete, otherwise it is easy to stick to the negative film and cause poor exposure. The hardness of the photosensitive film after pre-baking should be HB~1H.

(3) If an oven is used, it must be equipped with blast air and constant temperature control to make the pre-baking temperature uniform. And the oven should be clean and free of impurities, so as not to fall on the board and damage the film surface.

(4) After pre-baking, the time from coating to developing should not exceed 48hr at most. When the humidity is high, try to expose and develop within 12hr.

(5) The requirements for different types of liquid photoresists are also different. The instructions should be read carefully, and the process parameters, such as thickness, temperature, time, etc., should be adjusted according to production practices.

4. Fixed Postion

As high-density interconnect technology (HDI) applications continue to expand, resolution and positioning have become major challenges for PCB manufacturers. The higher the circuit density, the more precise the positioning is required. The positioning methods include visual positioning, movable pin positioning, fixed pin positioning and other methods.

Visual positioning is to use a Diazo film to coincide with the printed board hole through the pattern, and then paste the adhesive tape to expose. The diazo sheet is brown or orange-red translucent, which can ensure better coincidence and alignment accuracy. Silver Film can also use this method, but a transparent positioning disc must be made on the negative for positioning.

The movable pin positioning system includes a photographic film puncher and a double-hole de-pin locator. The method is as follows: firstly align the two negative films on the front and the back, and use the film puncher to punch two positions at random outside the effective graphics. If you take any one of the holes to program the drilling program, you can use the drilling machine to drill at one time. After the metallized holes and pre-copper plating of the printed board, you can use the double-hole knock-out locator to locate and expose.

The positioning of the fixed pins is divided into two systems, one for fixing the photographic plate and the other for fixing the PCB. By adjusting the positions of the two pins, the coincidence and alignment of the photographic plate and the PCB can be realized.