PCB Tech - Introduce the advantages of PCB circuit board wet film

Introduce the advantages of PCB circuit board wet film

First, the characteristics of the wet film

1. Good adhesion and coverage. The wet film itself is a blue viscous liquid synthesized from photosensitive resin, with photosensitizers, colorants, fillers and solvents added. The pits and scratches on the substrate are in good contact with the wet film, and the wet film mainly combines chemical bonds with the substrate to make the wet film and the copper foil of the substrate have excellent adhesion. The use of screen printing can obtain good coverage., Which provides conditions for high-density thin-line PCB processing.

Excellent resolution The contact and coverage of the wet film and the substrate are good, and the contact light of the film is used to shorten the optical path, which reduces the energy loss of light and the error caused by light scattering.

This makes the resolution of the wet film generally below 25μm, which improves the accuracy of pattern production, but the actual resolution of the dry film is difficult to reach 50μm.

The thickness of low-cost wet film is controllable, usually thinner than dry film, and the packaging cost is also lower. Relatively speaking, the cost of wet film is lower. The production process speed of the wet film in the inner layer of the thin line is greatly improved, and the material cost is saved by 20% compared with the dry film.

The development speed of the wet rate is increased by 30%, and the etching rate can also increase the number of times of film loss, and can also increase the speed of melatonin, thereby saving energy, improving equipment utilization, and ultimately reducing costs.

Eliminate board edge hair

The edge of the dry film board is easy to send, and it is easy to produce film breakage, which will affect the pass rate of the board during the PCB production process. There is no film breakage or hairy phenomenon on the board edge of the wet film.

two. Main operating points for wet film applications

The material provided by the previous process (ie production of the board) for the brush board does not require serious oxidation, oil pollution, and wrinkles on the board surface. We use pickling (5% sulfuric acid) spraying to remove organic impurities and inorganic dirt, and then use a 500 mesh nylon roller abrasive brush. After brushing, the board can achieve: no oxidation on the copper surface, uniform roughness on the copper surface, strict smoothness on the copper surface, and no traces of water on the copper surface. This effect enhances the adhesion between the wet film and the surface of the copper foil to meet the requirements of the subsequent process.

The surface condition of the copper foil after brushing the board directly affects the output of PCB.

In order to meet the needs of wet film thickness, screen printing should pay attention to the thickness of the screen and the number of items (that is, the number of lines per unit length) before selecting the screen.

The thickness of the film is related to the ink flux of the metal screen. The theoretical ink transmittance (uth) is:

UTH = DW2 (w + d) 2x1000

(D--Clean sand thickness d─line diameter w─opening width) The actual ink permeability is also related to the wet film viscosity, the scraping pressure, and the speed of the scraping movement. In order to achieve uniform coverage, the scraper edge must be ground first. The thickness of the back platen mask should be controlled between 15-25μm. If the film is too thick, it is prone to underexposure, develops well, and it is difficult to control the pre-drying, which is likely to cause on-site film and difficult operation. The film is too thin to cause excessive exposure, good corrosion resistance, poor electroplating insulation, and difficult film.

For fine lines below 0.15μm, the thickness should be less than 20μm after filming.

Before using the wet film, adjust the viscosity, stir well, static electricity for 15 minutes, and keep the environment of the screen printing room clean to prevent foreign matter from falling on the surface and affecting the pass rate of the board. The temperature should be controlled at about 20°C and the relative humidity is about 50%. .

Pre-drying and pre-drying parameters use the first side to be baked at 80-100°C for 7-10 minutes, and the second side is also baked at 80-100°C for 10-20 minutes. Pre-baking is mainly to evaporate the solvent in the ink, and pre-baking is related to the success or failure of wet film applications. Pre-baking is not enough. It is easy to stick to the board during storage and handling, and it is easy to stick negative film during exposure, which will eventually lead to wire breakage or short circuit, too much pre-baking, easy to develop and unclean, and the edge of the line is jagged. Pre-drying directly affects the quality of PCB, so it is an important development process to remove the unexposed part of the wet film layer to obtain the desired circuit pattern. Strictly control the concentration of the developer (10-12G/L), temperature (30-34 degree Celsius), and the concentration of the developer is too high or too low, which will easily cause unclean development. Optimize the development speed to match the exposure, and clean the nozzle frequently to keep the nozzle pressure and distribution consistent.

Excessive development time or high development temperature will cause deterioration of the wet film surface, severe permeability or lateral erosion during electroplating or acid etching, which reduces the accuracy required for pattern production. Etching and removing film etching finally get the circuit pattern we need. The etching solution can be alkaline iron chloride, acid copper chloride and ammonia. Etching different copper foil thickness, using different etching speed, etching speed and etching solution temperature, concentration matching, always keep the etching machine nozzle, keep the pressure and spray distribution uniform, otherwise the final corrosion may be uneven and edge copper wires, affecting the PCB quality. The membrane used in the PCB factory uses 4-7% sodium hydroxide solution at 50-60°C, which is mainly used for the expansion and subdivision process of the membrane by sodium hydroxide, and the parameters in the pores of the wet membrane can also be fading. We use wet film for pattern transfer in special double-sided panels to achieve good results.