Liquid photoresist etching and pattern transfer process (2)

2. Coating

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

Screen printing is a commonly used coating method, which requires low equipment, 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, full-page printing uses 100 to 300 mesh screens, and electroplating uses 150 mesh screens. This method is welcomed by most small and medium manufacturers.

Roll coating can realize double-sided coating at the same time. It has high automation production efficiency and can control the coating thickness. It is suitable for large-scale production of various specifications, but requires equipment investment.

Curtain coating is also suitable for large-scale production, but also can uniformly control the thickness of the coating layer, but the equipment requirements are high, and can only be painted on one side before coating the other side, affecting the production efficiency.

Photo-induced coating film is too thick, easy to produce underexposure, lack of development, high pressure sensitivity, easy to stick the film; film is too thin, prone to overexposure, poor resistance to plating insulation and easy to produce metal plating film phenomenon, Slow film removal.

Working conditions: Clean room under yellow light, room temperature is 23 ~ 25 °C, relative humidity is 55 ± 5%, the workplace is kept clean, avoid sunlight and direct sunlight.

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.

5. Exposuring

After the liquid photoresist is irradiated with UV light (300-400 nm), a cross-linking polymerization reaction occurs, and the light-irradiated portion is formed into a film and hardened without being affected by the developer. The source of the exposure lamp normally used is a high-brightness, medium-pressure mercury lamp or a metal halide mercury lamp. Lamp 6000W, exposure 100 ~ 300mj/cm2, density measurement using 21 optical density table (Stouffer21), to determine the optimal exposure parameters, usually 6 to 8 levels. The use of liquid photoresist for exposure to parallel light is not critical, but its speed is not as fast as that of dry film, so a high-efficiency exposing (Drawer) should be used.

The photopolymerization reaction depends on the light intensity of the lamp and the exposure time. The light intensity of the lamp is related to the excitation voltage and is related to the lamp usage time. Therefore, in order to ensure enough light energy for photopolymerization reaction, it must be controlled by the optical energy integrator. Its principle is to ensure that the exposure time is automatically adjusted to maintain the total exposure energy when the light intensity changes during exposure. Exposure time It is 25 to 50 seconds.

Factors affecting exposure time:

(1) The closer the light is, the shorter the exposure time;

(2) The thicker the liquid photoresist thickness, the longer the exposure time;

(3) The greater the humidity of the air, the longer the exposure time;

(4) The higher the pre-bake temperature, the shorter the exposure time.

When the exposure is excessive, the astigmatism is easily refracted, the line width is reduced, and the development is difficult. When the exposure is insufficient, the development is likely to cause defects such as pinholes, hairiness, and detachment, and the corrosion resistance and plating resistance are reduced. Therefore, selecting the optimal exposure parameter is an important condition for controlling the development effect.

The quality of the film has a direct impact on the quality of exposure. Therefore, the pattern of the film is clear and there must be no halo or imaginary phenomena. Pinholes and trachoma are required. The stability is good. The film requires large black-and-white contrast: Density Dmax ≥ 3.5, DMIN ≤ 0.15 for silver salt tablets, DMAX ≥ 1.2 diazo film, DMIN ≤ 0.1.

In general, after the film has been produced, it is transferred from one process (factory) to another (factory), or stored for a period of time before it enters the yellow light chamber. This way of experiencing different environments, the dimensional stability of the film is difficult to guarantee. I think that the finished film should be directly into the yellow room, and each film should be discarded when it is used to make more than 80 boards. In this way, the micro-deformation of the graphics can be avoided, especially the micro-hole technology should pay more attention to this point.
Exposure Process Operation Precautions

(1) The exposure machine is absolutely necessary for vacuum drying and the degree of vacuum is ≥90%. Only by evacuating the film and the workpiece, can the image be distorted to improve the accuracy.

(2) When the exposure operation, if there is viscous production negative film, may be caused by insufficient pre-baking or too strong vacuum drying, etc., it is necessary to adjust the pre-baking temperature and time or check the vacuum condition of the drying.

(3) After the exposure is stopped, the board should be taken out immediately. Otherwise, the residual light in the lamp will cause excess glue after development.

(4) Working conditions must be met: dust-free yellow light operation room, cleanliness from 10,000 to 100,000, air-conditioned facilities. The exposure machine should have a cooling exhaust system.

(5) When the film is exposed, the film side of the film must be directed downwards so that it is tightly adhered to the surface of the photosensitive film to increase the resolution.