Dry film technology and performance

writer: G July 02, 2019

In the dry film coating, the polyethylene protective film is first peeled off from the dry film, and then the dry film resist is pasted on the copper clad laminate under heat and pressure. The resist layer in the dry film becomes soft after being heated, and the fluidity is increased, and the film is completed by the pressure of the hot press roll and the action of the binder in the resist. The film is usually completed on the film laminating machine. There are many types of film laminating machines, but the basic structure is basically the same. Generally, the film can be continuously attached or attached.

When continuously filming, pay attention to the alignment of the dry film on the upper and lower dry film feed rolls. Generally, the size of the film should be slightly smaller than the plate surface to prevent the resist from sticking to the hot press roll. Continuous film production is highly efficient and suitable for mass production. The three elements to be mastered when filming are pressure, temperature, and transfer speed. Pressure: The newly installed film laminating machine first adjusts the upper and lower hot pressing rolls to the axial direction, and then adjusts the pressure by gradually increasing the pressure. According to the thickness of the printed board, the dry film is easy to stick and stick. No wrinkles. Generally, the pressure can be fixed after being adjusted. If the thickness difference of the produced circuit board is too large, it needs to be adjusted. The general line pressure is 0.5-0.6 kg/cm. Temperature: It is slightly different depending on the type, performance, ambient temperature and humidity of the dry film. If the film is coated dry and the ambient temperature is low, the film temperature is higher, and vice versa, the dark room is stable. The environment and equipment are in good condition and are a good guarantee for the film.

Generally, if the film temperature is too high, the dry film image will become brittle, resulting in poor plating resistance, the film temperature is too low, and the dry film and the copper surface are not firmly adhered. During the development or electroplating process, the film is easy to warp or even fall off. Usually the film temperature is controlled at around 100 °C. Transmission speed: It is related to the film temperature. The temperature is high, the transmission speed can be faster, and the lower temperature will slow down the transmission speed. The transfer speed is usually 0.9 to 1.8 m/min.

In order to adapt to the printed board for producing fine wires, a wet filming process has been developed. This process uses a special filming machine to form a water film on the surface of the copper foil before sticking the film. The function of the water film is to improve the dry film. Mobility; repels air bubbles trapped on scratches, blisters, pits, and fabric depressions; in the process of heating and pressing the film, water acts as a tackifier for the photoresist, thereby greatly improving the dry film and the substrate. The adhesion is improved, which improves the yield of fine wires. It is reported that the pass rate of fine wires can be increased by 1-9% by this process.

Photosensitivity includes photospeed, exposure time latitude, and depth exposure. Photosensitive speed refers to the amount of light energy required by a photopolymerizable monomer to form a polymer having a certain resisting ability under ultraviolet light irradiation. When the intensity of the light source and the distance between the lamps are fixed, the speed of the light is expressed as the length of the exposure time, and the short exposure time is the speed of the light. For the purpose of improving the production efficiency and ensuring the accuracy of the printed board, a dry film with a fast speed should be selected. .

After the dry film is exposed for a period of time, the photoresist layer has been polymerized in whole or in part by development, and generally the formed image can be used, which is called the minimum exposure time. The exposure time continues to be lengthened to allow the photoresist to polymerize more thoroughly, and the image size obtained after development still conforms to the size of the master image, which is referred to as the maximum exposure time. Usually the optimum exposure time for the dry film is chosen 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 latitude.

The deep exposure of the dry film is important. At the time of exposure, the light energy is reduced by the effect of passing through the resist layer and scattering. If the transmittance of the resist layer to light is not good, when the resist layer is thick, the exposure amount of the above layer is appropriate, the lower layer may not react, and the edge of the resist layer after development is not uniform, which will affect the accuracy of the image. And the resolution, in severe cases, the resist layer is prone to warping and falling off. In order for the lower layer to polymerize, it is necessary to increase the exposure amount, and the upper layer may be overexposed. Dry film developability#e#

The developability of the dry film refers to the image quality obtained after the film is exposed, exposed and developed in an optimal working state, that is, the circuit image should be clear, and the unexposed portion should be cleaned without residue. The resist remaining on the board after exposure should be smooth and firm. The development resistance of the dry film means that the exposed dry film is resistant to over development, that is, the development time can be exceeded, and the development resistance reflects the latitude of the development process. The developability and development resistance of the dry film directly affect the quality of the printed board. A poorly developed dry film may cause difficulty in etching. In the pattern plating process, poor development may cause defects such as poor plating or poor adhesion of the plating layer. The dry film has poor development resistance, and when it is excessively developed, problems such as dry film peeling and plating plating occur. When the above defects are serious, the printed board will be scrapped.

The resolution refers to the number of lines or spaces that can be formed by the dry film resist within a distance of 1 mm, and the resolution can also be expressed by the size of the line or the absolute size of the pitch. 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 layer, the lower the resolution. When the light is exposed to the dry film through the photographic plate and the polyester film, the scattering effect of the polyester film on the light causes the light to be side-fired, thereby reducing the resolution of the dry film. The thicker the polyester film, the more severe the side light rays are. The lower the resolution. The minimum parallel line width that can usually be resolved, the first level index <0.1mm, the second level index ≤ 0.15mm.

The dry film resist layer after photopolymerization should be resistant to the etching of the ferric chloride etching solution, the ammonium persulfate etching solution, the acidic copper chloride etching solution, and the sulfuric acid-hydrogen peroxide etching solution. In the above etching liquid, when the temperature is 50-55 ° C, the surface of the dry film should be free from hairiness, leakage, warping and falling off.

In acid bright copper plating, fluoroborate common tin-lead alloy, fluoroborate bright tin-lead alloy and various pre-plating treatment solutions of the above plating, the film resist layer after polymerization should have no surface hairiness and seepage. Plating, lifting and falling off. After exposure, the dry film can be removed in a strong alkali solution after etching and electroplating. Generally, 3-5% sodium hydroxide solution is used, heated to about 60 ° C, and removed by mechanical spraying or soaking. The faster the speed, the better the productivity. The film-removing form is preferably in the form of a sheet peeling, and the peeled pieces are removed through the filter screen, which is advantageous for the service life of the film removing solution as well as for clogging the nozzle. Usually in the 3-5% (by weight) sodium hydroxide solution, the liquid temperature is 60 ° 10 ° C, the first indicator is the film removal time 30-75 seconds, the second indicator is the film removal time 60-150 seconds, the film is removed. No residue after the glue.

The dry film may become brittle due to evaporation of the solvent during storage, or may be thermally polymerized due to the influence of ambient temperature, or uneven thickness due to local flow of the resist, so-called cold flow, which seriously affects the dryness. Use of the membrane. Therefore, it is very important to store dry film in a good environment. The dry film should be stored in a cool, clean room to prevent storage with chemicals and radioactive materials. The storage conditions are: yellow light zone, the temperature is lower than 27 °C (5-21 °C is the best), and the relative humidity is about 50%. The storage period is not more than six months from the date of shipment, and those who pass the storage period can still use it. Avoid moisture, heat, mechanical damage and direct sunlight during storage and transportation.

In order to avoid leakage exposure and re-exposure during the production operation, the dry film should have a significant change in color before and after exposure, which is the discoloration performance of the dry film. When used as a mask etching, the dry film is required to have sufficient flexibility to withstand the impact of the development process, the liquid pressure of the etching process without cracking, which is the masking property of the dry film.