Liquid photo resist in PCB pattern transfer and its manufacture (3)

Coating operation should pay attention to the following aspects

1) If the coating layer has pinholes, the photoresist may be unclear, wash with acetone and replace the new resist. It may also be that particles in the air fall on the surface of the board or the board surface is not clean due to other reasons. It should be carefully inspected and cleaned before coating.

2) If the photocoating film is too thick during screen printing, it is because the number of meshes is too small; if the film is too thin, it may be caused by too many meshes. If the coating thickness is not uniform, thinner should be added to adjust the viscosity of the resist or adjust the speed of the coating.

3) Try to prevent the ink from entering the hole while applying the film.

4) Regardless of the method used, Photoimageable cover coating should achieve uniform thickness, no pinholes, bubbles, inclusions, etc., and the film thickness should reach 8-15 μm after drying.

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

6) Wash hands with soap after work.

3. Pre-curing

The pre-baking means that the surface of the liquid photoresist film is dried by heating and drying to facilitate contact exposure and development of the film to produce a pattern. Most of this process is performed in the same chamber as the coating process. Pre-baking methods are most commonly used in both drying tunnels and ovens.

Generally, oven drying is used. The first-side pre-baking temperature on both sides is 80±5° C. and 10 to 15 minutes. The second-side pre-bake temperature is 80±5° C. and 15 to 20 minutes. This pre-and post-bake pre-bake makes the degree of pre-curing of the wet film on both sides different, and the effect of the development is also difficult to ensure complete consistency. Ideally, both sides are coated simultaneously and pre-baked at a temperature of 80±5°C for about 20-30 minutes. This double-sided pre-curing at the same time and can ensure that the two-sided development of the same effect, and save man-hours.

It is important to control the temperature and temperature of the pre-baking. If the temperature is too high or the time is too long, the development is difficult, and it is difficult to remove the film; if the temperature is too low or the time is too short, the drying is not complete, the film has pressure-sensitive properties, and the negative film may be easily exposed due to poor exposure, and the film may be easily damaged. Therefore, pre-baking is appropriate, development and removal of film is faster, and the graphics quality is good.

The operation should pay attention to

(1) After pre-baking, the plate should be subjected to air-cooling or natural cooling before exposure of the film.

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

(3) If an oven is used, it must be equipped with blast air and constant temperature control so that the pre-bake temperature is even. And the oven should be clean, no impurities, so as not to fall on the board, damage the film surface.

(4) After the pre-baking, the film coating development time is not more than 48 hr, and when the humidity is high, it is exposed and developed within 12 hr.

(5) The requirements for different types of liquid photoresist are different. Read the instructions carefully and adjust the process parameters such as thickness, temperature, and time according to the production practice.

4. Positioned (Fixed Postion)

With the continuous expansion of high-density interconnect technology (HDI) applications, 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, moving pin positioning, fixing pin positioning and other methods.

Visual positioning is to use a Diazo film to pass through the pattern to align with the hole in the printed board, and then expose it with adhesive tape. The diazo film is brown or orange and translucent, which can ensure good repositioning accuracy. Silver film can also be used in this method, but it must be made of transparent positioning plate in the film to locate.

The movable pin positioning system includes a photographic film punch and a double-round hole out-positioning device. The method is: firstly aligning the front and back two bottom film films, and using the film punch device to arbitrarily punch two positions outside the effective pattern. Holes, any one to take a drilling program, you can use a drill hole drilling, printed boards metallized holes and pre-plated copper, you can use the double-round hole out positioning locator positioning exposure.

Fixed pin positioning is divided into two sets of systems, a set of fixed photo plates and another set of fixed PCBs. By adjusting the positions of the two pins, a coincident alignment between the photo plate and the PCB can be achieved.

(to be continued)

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