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In the production of printed circuit boards, both industrial and individual, light-sensitive varnishes are often used. One of the best is POSITIV 20 photoresist varnish from KONTAKT CHEMIE, easy to use, easy to remove, highly sensitive and relatively inexpensive.

When working with this varnish, the image is exposed directly from the photomask-positive, without making intermediate negatives. A 200 ml aerosol can usually cover 4 m2 of copper foil.

POSITIV 20 photoresist varnish can be stored for a year at a temperature of 8 to 12°C. Do not overcool POSITIV 20.

How, without experience, to make a board yourself? Consider the main stages of this process.

The surface of the foil to which the photoresist will be applied must be absolutely clean and free of grease. After removal of oxides and impurities, copper acquires a bright pink color. Then the board blank is washed in a large amount of water to remove the remnants of the detergent composition and abrasive particles. In the future, the working surface of the board blank should no longer come into contact with other solvents (acetone, alcohol), it should not be touched by hands.

After washing, the workpiece is dried with warm air from a hair dryer. Lacquer should be applied immediately after drying, so that an oxide film does not have time to form on the foil. POSITIV 20 does not have to be applied in complete darkness - in the liquid state, the photoresist is insensitive to light. Work can be carried out in diffused lighting, excluding direct sunlight or bright light on the surface of the workpiece. It is also important that the place of work is free of drafts and dust.

The photoresist is applied at room temperature, so it must be removed from storage 4-5 hours before use. Place the board blank on a horizontal or slightly inclined surface and spray the compound from an aerosol can from a distance of about 20 cm. To create an even coating, spray the compound in a continuous zigzag motion starting from the upper left corner. Avoid overspray as this will result in streaks and uneven coat thicknesses. In the future, this will require an increase in exposure.

To reduce the loss of photoresist, it is permissible to spray the aerosol from a shorter distance. When spraying, make sure that the nozzle of the package is always located above the container in order to avoid wasting propellant gas. Otherwise, the aerosol can stop working when there is still photoresist left in it.

The thickness of the resulting layer can be approximately estimated by its color - light gray-blue corresponds to a thickness of 1 ... 3 microns, dark gray-blue - 3 ... 6 microns, blue - 6 ... 8 microns and dark blue - more than 8 microns. On light copper, the color of the coating may have a greenish tinge.

After spraying the photoresist, the board blank should be immediately transferred to dry in a dark place. As the coating dries, the degree of photosensitivity of the coating, especially to ultraviolet (UV) rays, increases.

In the absence of special equipment, the photo-varnish layer can be dried at room temperature for at least 24 hours. To speed up the process, the workpiece is placed in an oven or thermostat. If a household heater, such as an electric grill or a hair dryer, is used for drying, make sure that there is no light from outside and from the heating element. Raise the temperature slowly. At 70°C it is enough to dry for 20 minutes. Heating the workpiece above 70°C may damage the photolayer.

The stock of dried board blanks before exposure should be stored in a dark, dry and cool place.

The original image of printed conductors used for transfer to foil must be carefully prepared and retouched, otherwise all its imperfections will affect the quality of the copy. It is important that the pattern is contrasting, and the dark areas are completely opaque. Folds and folds of the original must be excluded. The basis of the photomask - film or paper - must transmit UV rays, but the paint must not.

Some publications place on their pages drawings of printed circuit boards specifically designed for the described technology. - the reverse side of such drawings is left blank. After processing the page with TRANSPARENT 21 spray from KONTAKT CHEMIE, the paper becomes transparent to UV rays, i.e. suitable for direct copying of the drawing directly from the page onto the board blank. TRANSPARENT 21 eliminates the tedious copying of board drawings.

The photomask is tightly pressed against the photoresist layer of the workpiece and intensely illuminated. The time required for exposure depends on the thickness of the photoresist layer on the workpiece and the intensity of the light. Since POSITIVE 20 varnish is sensitive to UV rays, it is advisable to use 300 W mercury or quartz lamps for exposure. Satisfactory results are obtained by an ordinary incandescent lamp with a power of 200 W at a distance of about 12 cm from the object. Before the start of illumination, the lamp is heated for 2 ... 3 minutes.

Exposure time with a mercury lamp from a distance of 25...30 cm usually does not exceed 1...2 min. Of course, you can also use bright sunlight rich in ultraviolet (exposure time - 5 ... 10 minutes).

To press the photomask to the workpiece, it is better to use a sheet of organic glass, since ordinary glass absorbs up to 65% of ultraviolet radiation, which will require a corresponding increase in exposure time. When using a long-term stored photoresist, the exposure time will also have to be increased (for a storage period of up to a year, approximately twice).

If there are small details on the original, then in order to maintain their dimensions on the foil and obtain even contrasting edges on the narrowest elements, the photomask should be applied to the photoresist with the side where the drawing is applied before exposure. This in some cases makes it necessary to produce an intermediate drawing in a mirror image.

Exposed blanks can be developed in diffused daylight. Developer composition: 7 g of powdered caustic soda NaOH per liter of cold water. The workpiece is placed in a vessel with a developer and the solution is stirred. For a properly exposed photoresist layer 4–6 µm thick, the development time in a fresh solution usually does not exceed 0,5–1 min. maximum - 2 min. The temperature of the developer should be within 20...25°C.

The developer completely removes the photoresist from the exposed areas of the coating from the workpiece. Do not keep the workpiece in the solution longer than necessary for development, otherwise it will begin to act on unexposed areas that are not intended for etching. If the exposure time was too long or the ink used to make the drawing was not UV opaque, the image of the conductive tracks will appear for a while, but will then be removed by the developer.

After removing the workpiece from the solution, rinse it thoroughly in running cold water. Wash your hands thoroughly after handling the caustic soda solution.

The POSITIV 20 photoresist lacquer layer is resistant to acidic solutions containing ferric chloride FeCl3. ammonium persulfate (NH4)2S2O8. hydrochloric and hydrofluoric acids. Copper foil boards are recommended to be etched in a solution of ferric chloride with a concentration of 35 ... 40% at a temperature of about 45 ° C. To speed up the etching, the solution is slightly stirred. The end point of the process is determined visually.

After the end of etching, wash the board blank with soapy water and clean it from the remnants of the photoresist that is no longer needed. This operation can be performed with a piece of cloth soaked in any organic solvent, such as acetone.

In addition, when working with printed circuit boards, FLUX SK10 (preparation of printed circuit boards for soldering) and URETHAN 71 (protective acrylic varnish) are used.

Author: Electronic components, Moscow

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