label: PCB,PCB assembly,PCB production
The earliest PCB pattern transfer material using wet film production process, with the use of wet film technology and PCB requirements to improve the disadvantage of the wet film also revealed, mainly polyethylene in long production cycle, uneven coating thickness, coating board needle and debris too difficult hole after developing the film. Led to the subsequent wet film production problems more difficult process control together, eventually dry the wet film is replaced.
With PCB assembly technology development, as well as PCB lines and line spacing is getting smaller, accuracy and density requirements are raised, the traditional problems in such dry PCB pattern transfer process, mainly reflected in the resolution does not dry further increase (mainly caused by dry coating film of polyester), can not meet the needs of specific PCB production, the needle and the substrate, pits, scratches affect the effect of the film, resulting in the finished trachoma, gap, disconnection. The wet film is advanced to address these problems , providing a way.
One wet film's features
1 adhesion, good coverage
Wet film itself is composed of a photosensitive synthetic resin, a photosensitive agent is added, colorants, fillers, and the solvent viscous blue liquid. Pits on the substrate, the contact with the wet film is scratched portion is good, and mainly by the action of the wet film and the substrate to the adhesive bond, so that wet- film copper foil and the substrate is excellent good adhesion, screen-printing a good coverage can be obtained, which is a high -density fine lines to provide PCB processing conditions. Table I can be explained by the wet adhesion of the film:
Table a dry, wet film adhesion test comparison table
Coating type modules come off ratio (%)
6 * 6mil 7 * 7mil 8 * 8mil 9 * 9mil
Dry 0.06
0.01 0.02 0
Wet film 0000
The data in Table I show that the adhesion of the wet film is much better than dry, fine lines with respect to the production of the wet film is much better.
2 excellent resolution
Wet film and the substrate contact, good coverage, and contact exposure using film, reducing the optical path, can reduce the loss of light, the light scattering caused by the error. This resolution of the wet film is 25μm or less in general, to improve the precision of the graphic production, where the actual resolution is difficult to achieve in the dry 50μm.
3 low cost
Wet film thickness controllability, generally thinner than the dry, packaging costs are low , the cost is relatively low in terms of wet film. Wet film in the fine lines of the inner process of making the pass rate is greatly improved, compared with the dry material cost savings of 20%. Wet developing faster 30%, the etch rate can be increased by 10-20%, increase the speed of faded film, thus saving energy, improve equipment utilization, and ultimately reduce the cost.
4 Elimination board edge scared
Easy scared paste dry plate edge, easy to produce films broken in the production process will affect the board pass rate, the board's edges are not printed on the wet film membrane broken and scared phenomenon.
Two wet film process
Our company according to its own conditions, using a variety of wet film , the final selection of the Beijing Rio Tinto wet film. Its inner process procedure is as follows:
Brush plate (substrate pre-treatment) → → → drying screen printing exposure → development → → to film etching
General two-panel process:
Brush plate (substrate pre-treatment) → → → drying screen printing exposure → development → Plating → → etched to film
Three operating points when applied wet film
1 brush plate
Materials provided by the former process (ie production board) requires the board without serious oxidation, oil, crease. We use acid (5% sulfuric acid) spray, to remove organic impurities and inorganic contaminants, and then a 500 mesh nylon brush roller brush grinding. Brush plate to achieve: no surface oxidized copper, the copper surface is uniformly roughened copper surface smoothness with strict, but also copper surface water mark. This effect enhances the binding strength of the wet film and the copper foil surface , in order to meet the requirements of the subsequent process step. Brushing the surface condition of the copper foil after the PCB directly affect the yield.
2 Screen printing
To achieve the desired thickness of the wet film before printing screen to choose, pay attention to the thickness of the mesh ( ie, the number of lines per unit length ) mesh. Through the same amount of ink film thickness on the screen, the theoretical amount of ink through the ink (Uth) as follows:
Uth = Dw2 \ (w + d) 2 × 1000
(D ─ net sand thickness d ─ diameter w ─ opening width)
The actual amount of ink still wet through the membrane viscosity, squeegee pressure, squeegee movement speed related, to achieve uniform coverage, scraper mouth to brake wear well. After printing mask plate to control the thickness between 15-25μm, thick film underexposure prone developing bad prebaked difficult to control, resulting in easily field sheet is difficult to handle. Too easy to produce thin film overexposed, good corrosion resistance, insulation when poor plating, too difficult to film. Manufacture of fine lines 0.15μm or less, the film should be less than 20μm.
Wet film before use should be fine viscosity and stir evenly, still fifteen minutes, screen room to keep the environment clean, so as not to affect the external surface of the debris fell on the board pass rate, the temperature should be controlled at about 20 ℃, relative humidity at 50% to the vertical.
3 Pre- bake
Using a first surface of the pre-bake oven in parameter 80-100 ℃ 7-10 minutes and the second surface are 80-100 ℃ bake for 10-20 minutes. Prebaking the main solvent in the ink is evaporated, the success or failure prebaking the wet film applications. Pre- bake inadequate storage, handling process easy to stick plate, easy to stick the film exposure, ultimately resulting in disconnection or short circuit; over pre- bake, easy development without a net, by the jagged edges of the lines. Prebaking directly affect the quality of the PCB, so important in the normal operation of the wet film thickness often tested and adjusted according to changes in the ambient temperature of the oven parameters, always check the oven and blast circulation system is good. Exposure to the board as soon as possible after drying, preferably not more than 12 hours.
4 . Exposure
Exposure under ultraviolet light, the light of the wet film of the monomer molecules in the absorption of light energy produced by the polymerization process. The selection of high power exposure, exposure time and to reduce the accumulation of heat, to ensure stability and reduce exposure pattern sticky film. To maintain exposure class clean room to avoid the debris caused by trachoma adhesion, gap, break in the layout, making the exposure scale to adjust the exposure time to avoid excessive exposure or lack of exposure, the final stages to control the exposure of 6 - between 8.A printed board with the exposure at the same exposure position as possible, try to ensure that a board with the same energy tricky. Excessive exposure, etch resistance, anti- plating better, but there is not ideal to film and graphic line narrowing (using positive) or expand (using negative) , underexposure, resulting in poor development, poor corrosion resistance, line edge hairy, line spacing increases or decreases in the etching could easily lead to a short circuit or disconnected.
5. Developing
Development is not exposed to the wet film is partially removed to obtain the desired circuit pattern. Strictly controlling the concentration of the developer (10-12g/l), temperature (30-34 ℃), developer concentration is too high or too low are easy to create development without a net. Developing speed optimized to match it with the exposure, frequent cleaning of the nozzle, so that consistent pressure nozzle and distribution. The development time is too long or the developing temperature, the surface of the wet film will cause deterioration of permeability or serious side etching in an acid etching or plating, reducing the production of graphic accuracy.
6 etching and stripping
Finally obtain a desired etching circuit pattern, the etching solution can use basic ferric chloride acid copper and ammonia. Different thickness of the copper foil to be used in etching the etching rate different etching rate but also the concentration and temperature of the etching solution to match, regular maintenance etching machine nozzle, the liquid ejection pressure and maintaining a uniform distribution, or both, and may finally result in losses from the edge copper, affecting the quality of the PCB. I used to film 4-7% sodium hydroxide solution at 50-60 ℃, sodium hydroxide layer mainly subdivided expansion process, control parameters can be wet film hole faded. We use wet film made in a special double-sided pattern transfer to achieve good results.
Source://www.seekic.com
