Printed circuits made with the silk screen technique

Note: The development time of the silk and drying of the cards exposed here, changes depending on the quality of the ink, the type of bulb used and the distance of the light, when exposing the cards. We recommend doing tests until you find the times, that you delay, both the development and the drying of the paintings, according to the things you get in your country.

materials

Silk No 90 and No 120 with its respective frame or frame -T300
90% universal national emulsion or green atlas and developer 10%
Solvent (activator) or PREGASOL EP 3
Floral foam (oasis)
1 glass of 4 or 5 millimetres with the same dimensions than the interior of the frame.
Anti- solder paint ( UV solder mask )
White and green UV paint or metal or polyethene ink (timpes) screen line
1 plastic spatula (squeegee or squeegee) (acrylic waste)
Hypochlorite or Varsol to yield the ink
Ferric chloride
Thinner
Resin Rosin
Caustic soda
Domestic chlorine
Plastic container suitable for bathing cards
UV or photoflood light bulb
Cloth and bast pieces

NOTE: The original inks used in the printed circuit board manufacturers are obtained in the store's www.xenoa.co.uk

The screen printing is a very old technique, which allows you to print images on any material. Basically, it is transferring an ink through a tempered silk mesh into a wooden frame. The silk has been previously treated with an emulsion that blocks the passage of ink in areas where there will be no image, leaving free the area where the ink will pass. This printing system is used to print many quantities, without losing definition.
The technique of production of printed circuits with screen printing is used industrially, since you can get very good quality prints at very low cost. Once the silk frames or more known as frames have been revealed, multiple copies of the same design can be made. This allows the serial production of printed circuits. Although it is still a manual procedure, this technique is one of the most used, since it allows obtaining jobs with the necessary quality and presentation, for the realization of electronic prototypes and/or applications in the industry.

The frame

Screen Printing frame

The frame is a wooden frame built with strips of 3 x 3 centimetres, perfectly square, which supports a fine silk, very warm and without wrinkles, which when placed on a flat surface, supports evenly. The silk should be well tempered and without wrinkles. This silk is glued to the frame using a stapler, reinforced with masking tape and glue. Three racks must be made; one for the printed circuit with silk of 120 threads, since the more detail is required, the number of threads must be greater. The second frame is for the mask of components and the third for the mask antisolvent ( solder mask UV ), this must be 90 threads. In this tutorial, for convenience and practicality, we use a 120-thread frame to make all three prints.

Applying the emulsion

Applying the emulsion

After having the frames ready, in a room where there is not too much light, with the help of a thin spatula, 9 parts of an emulsion are mixed with 1 of bichromate or developer, until a uniform mixture is obtained. It should be mixed slowly because if bubbles are made, the development can be damaged. Once a uniform mixture is obtained, it is spread throughout the length of the silk, using an emulsifier, as shown in the photograph, three layers of emulsion are applied; One layer on the back side of the frame and two on the front. The emulsion should be applied uniformly on the silk, taking into account that there are no spaces without emulsion or bubbles.

If you do not get an emulsifier, you can use a squeegee, which is nothing more than a spatula of the width of the frame, with the rubber edge. You can make the spatula by recycling the leftover edges of the thick acrylic sheets.

Then we dry the silk with the emulsion using a hair dryer that helps to minimize the drying time.  Remember that the emulsion is photosensitive, so the process should be done as quickly as possible.

To see the photo bigger and be able to observe in detail click on it.

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Once the emulsion is dry, the acetate, or in this case the acetates, are placed with the printed designs on the silk, on the back side of the frame and fixed with transparent adhesive tape. All the acetates go backwards, that is to say; on the opposite side to what the printed one will look like, when it is done. Be very careful in this procedure, since it depends on this that the printed circuits are good or touch repeat the work, losing material and time.

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To make the acetates with the design of the printed circuit, the mask of components and anti-solder mask, it must be printed in a place where they have a machine for burning photomechanical plates. It is not useful to do it in a laser printer because if it is not a very good quality printer, the drawings remain pored.

Development of the emulsion

Development of the emulsion

Now we turn the frame, we put a thin foam that can be jumbo of 3 millimetres, then we put a glass that covers the entire silk and to finish we put weight on it and thus avoid a separation between the acetates, silk, foam and glass.
All this goes on a development table that we can build with a wooden drawer, which we place in the bottom, about 50 centimetres from the upper glass, a reflector, with a UV light bulb or UV light. This can be a 250W street reflector bulb.
We expose the set of; frame, acetate and glass to light for 13 minutes, no more no less. In the case of not having a reflector, the silk can be exposed to the light of a lamp or high-intensity focus, using a bulb ( Photoflood ) at 15 centimetres distance, for a time of 2 and a half minutes. Be careful with the heat.

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After exposing the frame to light, we immediately cover the silk and take it to a source of water under pressure, to wipe it on both sides. After a few seconds, it is observed how the silk reveals the pattern of the printed circuit, as the emulsion falls from the areas where the light did not enter through the covering provided by the acetate. The development is identical, according to the design.

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Once the silk is revealed, we dry it with the hairdryer for 10 minutes and it will be ready to print as many cards as we want. The drying of the emulsion is distinguished because the gloss decreases on drying.

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We proceed to cut the phenolic plate to the size of the printed circuit, taking into account to leave a small edge of about 5 millimetres that serves as margin when it comes to printing. When the plate is bakelite, it must be heated before cutting to facilitate cutting and prevent splitting. In the photo, we see a semi-industrial shear with which the material is cut. If you do not have one of these, you can use a very sharp chisel and a metal ruler and mark several times until the card cuts.

Printing the clues about Bakelite

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We place the frame on a flat, hard and uniform surface, fixing it with hinge presses, which allow it to be easily lifted without it running out of position. We place the phenolic plate exactly below the circuit board drawing with the tracks and we adjust it with adhesive tape or by placing some stops made of leftover bakelite pieces, so that when printing it does not run out of place. We place the frame on top of the plate, leaving it at least 5 millimetres up from it, you can use a small piece of wood. Then we apply the UV ink or in its defect, polyethene paint, on the part above the silk, making a path in the part before the drawing, and then trace with the spatula ( Rasero or Raclette), the design of the printed circuit on the surface of the cards. This process is not easy the first time and you will have to practice it until the form is perfect.

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Observe how the print should look. It must be impeccable, without shifting or stains that damage the form.
After making all the desired impressions it is necessary to clean the silk of the accumulated ink, since otherwise the silk would be covered, spoiling it. For this, we use varsol with a tow and clean the silk. If you want to eliminate the printed circuit of the silk and enable it for another design, use chlorine or thinner to remove the circuit embodied in the silk.
Now we proceed to let the cards dry before proceeding with the development.

If you want to send printed circuits of good quality, such as those we use on our website, and do not want to do them yourself, we recommend the company Xenoa Ltd , which makes very good prints.

1 Response

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