Introduction to dry film technology and performance

Printed circuit manufacturers all hope to use dry films with good performance to ensure the quality of printed boards, stabilize production, and improve efficiency. In recent years, with the rapid development of the electronics industry, the precision density of printed boards has been continuously improved. In order to meet the needs of printed board production, new dry film products have been continuously launched, and the performance and quality have been greatly improved and enhanced.

When laminating dry film, first peel off the polyethylene protective film from the dry film, and then paste the dry film resist on the copper clad board under heating and pressure. The resist layer in the dry film softens after being heated, and its fluidity increases. The laminating is completed with the help of the pressure of the hot press roller and the action of the binder in the resist. Laminating is usually completed on a laminating machine. There are many types of laminating machines, but the basic structure is roughly the same. Generally, the laminating can be continuous or single.

When laminating continuously, attention should be paid to aligning the dry film on the upper and lower dry film feed rollers. Generally, the size of the film should be slightly smaller than the board surface to prevent the resist from sticking to the hot pressing roller. Continuous laminating has high production efficiency and is suitable for mass production. The three factors to be mastered when laminating are pressure, temperature, and conveying speed. Pressure: For a newly installed laminating machine, first adjust the upper and lower hot pressing rollers to be parallel to the axis, and then gradually increase the pressure to adjust the pressure. According to the thickness of the printed board, adjust it to make the dry film easy to paste, stick firmly, and wrinkle-free. Generally, the pressure can be fixed and used after it is adjusted. If the thickness of the circuit board produced is too different, it needs to be adjusted. The general line pressure is 0.5-0.6 kg/cm. Temperature: It varies slightly according to the type, performance, ambient temperature and humidity of the dry film. If the film is coated relatively dry and the ambient temperature is low and the humidity is low, the laminating temperature should be higher, otherwise it can be lower. A good and stable environment in the darkroom and intact equipment are good guarantees for laminating.

Generally, if the film temperature is too high, the dry film image will become brittle, resulting in poor plating resistance. If the film temperature is too low, the dry film will not adhere firmly to the copper surface. During the development or electroplating process, the film is prone to warping or even falling off. Usually the film temperature is controlled at around 100°C. Conveying speed: related to the film temperature. The higher the temperature, the faster the conveying speed. If the temperature is low, the slower the conveying speed. Usually the conveying speed is 0.9-1.8 m/min.

In order to adapt to the production of printed circuit boards with fine conductors, a wet lamination process has been developed. This process uses a special lamination machine to form a layer of water film on the surface of the copper foil before laminating the dry film. The function of the water film is to: improve the fluidity of the dry film; remove bubbles trapped in scratches, sand holes, pits and fabric depressions; during the heating and pressurizing lamination process, water has a viscosity-enhancing effect on the photoresist, which can greatly improve the adhesion between the dry film and the substrate, thereby improving the qualified rate of fine conductors. It is reported that the qualified rate of fine conductors can be increased by 1-9% using this process.

Photosensitivity includes photosensitivity, exposure time tolerance and depth of exposure. Photosensitivity refers to the amount of light energy required for the photoresist to undergo a polymerization reaction under ultraviolet light to form a polymer with a certain corrosion resistance. When the light source intensity and lamp distance are fixed, photosensitivity is expressed as the length of exposure time. Short exposure time means fast photosensitivity. In order to improve production efficiency and ensure the accuracy of printed boards, dry films with fast photosensitivity should be selected.

After the dry film is exposed for a period of time, after development, the photoresist layer has been fully or mostly polymerized, and generally the formed image can be used. This time is called the minimum exposure time. If the exposure time is further extended, the photoresist will be polymerized more thoroughly, and the image size obtained after development will still match the image size of the base plate. This time is called the maximum exposure time. Usually, the best exposure time for the dry film is selected between the minimum exposure time and the maximum exposure time. The ratio of the maximum exposure time to the minimum exposure time is called the exposure time tolerance.

The depth exposure of the dry film is very important. During exposure, the light energy is reduced by passing through the resist layer and the scattering effect. If the light transmittance of the resist layer is not good, when the resist layer is thick, if the exposure of the upper layer is appropriate, the lower layer may not react. After development, the edge of the resist layer is not neat, which will affect the accuracy and resolution of the image. In severe cases, the resist layer is prone to warping and falling off. In order to polymerize the lower layer, the exposure must be increased, and the upper layer may be overexposed. The development of dry film #e#

The developability of dry film refers to the image effect obtained after the dry film is applied, exposed and developed in the best working state, that is, the circuit image should be clear, and the unexposed part should be removed cleanly without residual glue. The anti-corrosion layer left on the board after exposure should be smooth and solid. The development resistance of dry film refers to the degree to which the exposed dry film can withstand overdevelopment, that is, the degree to which the development time can exceed. The development resistance reflects the tolerance of the development process. The developability and development resistance of dry film directly affect the quality of printed circuit board production. Poorly developed dry film will bring difficulties to etching. In the graphic electroplating process, poor development will cause defects such as failure to plate or poor bonding of the coating. The poor development resistance of dry film will cause problems such as dry film shedding and electroplating penetration when overdeveloped. If the above defects are serious, the printed circuit board will be scrapped.

The so-called resolution refers to the number of lines or spacings that can be formed by the dry film resist within a distance of 1mm. The resolution can also be expressed by the absolute size of the lines or spacing. The resolution of the dry film is related to the thickness of the resist film and the thickness of the polyester film. The thicker the resist film, the lower the resolution. When the light passes through the photographic plate and the polyester film to expose the dry film, the light is scattered by the polyester film, causing the light to be side-shot, thereby reducing the resolution of the dry film. The thicker the polyester film, the more serious the side-shot of the light, and the lower the resolution. The minimum parallel line width that can usually be resolved is <0.1mm for the first-level indicator and ≤0.15mm for the second-level indicator.

The dry film resist after photopolymerization should be resistant to etching by ferric chloride etching solution, ammonium persulfate etching solution, acid copper chloride etching solution, and sulfuric acid-hydrogen peroxide etching solution. In the above etching solutions, when the temperature is 50-55℃, the dry film surface should not have hair, leakage, warping and shedding.

In acid bright copper plating, fluoroborate ordinary tin-lead alloy, fluoroborate bright tin-lead alloy and various pre-treatment solutions of the above electroplating, the polymerized film resist should have no surface hairiness, infiltration, warping and shedding. The dry film after exposure can be removed in a strong alkaline solution after etching and electroplating. Generally, a 3-5% sodium hydroxide solution is used, heated to about 60°C, and removed by mechanical spraying or immersion. The faster the film removal speed, the more conducive to improving production efficiency. The film removal form is preferably in the form of sheet peeling, and the peeled fragments are removed through the filter, which is beneficial to the service life of the film removal solution and can also reduce the clogging of the nozzle. Usually in a 3-5% (weight ratio) sodium hydroxide solution, the liquid temperature is 60±10°C, the first-level indicator is a film removal time of 30-75 seconds, the second-level indicator is a film removal time of 60-150 seconds, and there is no residual glue after film removal.

During storage, the dry film may become brittle due to the volatilization of the solvent, or may produce thermal polymerization due to the influence of the ambient temperature, or the thickness may be uneven due to the local flow of the resist, which is the so-called cold flow. These seriously affect the use of the dry film . Therefore, it is very important to store the dry film in a good environment. The dry film should be stored in a cool and clean room to prevent storage with chemicals and radioactive substances. The storage conditions are: yellow light area, temperature below 27℃ (5-21℃ is the best), relative humidity of about 50%. The storage period is no more than six months from the date of leaving the factory. Those that pass the inspection after the storage period can still be used. During storage and transportation, it should be avoided from moisture, heat, mechanical damage and direct sunlight.

In order to avoid missed exposure and re-exposure during production operations, the color of the dry film should change significantly before and after exposure, which is the color change performance of the dry film. When used as a mask for etching, the dry film is required to have sufficient flexibility to withstand the impact of liquid pressure during the development process and etching process without breaking, which is the masking performance of the dry film.