Liquid photoresist from Aliexpress. My technology of use

There are many PCB manufacturing technologies available to hobbyists. Each has its pros and cons.
I tried the following:

At the beginning of my hobby for amateur radio, I made boards using a regular pen rod. A ball was squeezed out of the tip with a needle, and a good drawing pen was obtained. Next, a varnish of suitable viscosity was sucked in and paths were drawn with this device. Pros- necessary equipment Almost everyone has it at hand, the technology is negligible. Cons - no automation.

Then I got a laser printer and board layout programs. Experiments began on transferring the board design from a printout to textolite. There are many nuances in the technology: the quality of the translation depends on the material and structure of the paper, the temperature of the iron, the material and baking temperature of the toner, the pressure of the iron on the paper-textolite sandwich. As a result of my research, I came up with the following: HP LJ 1018 printer, we print on thin coated paper, in my case it is a gutted Upgrade magazine. We use only the original cartridge, no refills, because the toner density decreases. We sand the board with a zero polish, then transfer the print with an iron, frying at maximum, through 2 sheets of A4. And finally, under warm water, erase the paper with your finger.
Advantages of the technology: minimal time between printing and receiving the payment, no chemicals are needed, there is an incentive to read magazines. Disadvantages: instability of the technology, dependence on many factors, difficulty in obtaining large boards with small traces - they will always peel off in places, bald spots have to be retouched. With some luck, you can ruin the surface of the iron. For stability of knurling, instead of an iron, you need an expensive laminator with temperature control; ordinary cheap ones do not warm up the board, the print does not even stick.

The latest technology I mastered, which immediately showed a qualitative leap in board manufacturing - use of film photoresist...

Briefly, the technology looks like this: we make a transparent negative template with a board pattern, roll a film photoresist onto the textolite, run the sandwich through a laminator (or iron it) to fix it, put the template on the board, illuminate it with an ultraviolet lamp, peel off the lavsan and develop it.

At first glance it looks too long, but it is compensated by almost 100% result. However, to get stable results, you will have to spend a little money.

First of all, I recommend purchasing laminator. You can also roll the film with an iron, but as practice shows, due to the unevenness of the board and surface, the photoresist is often not rolled in some places and, as a result, subsequent peeling of the tracks in this place.

In METRO or AUCHAN you can buy the cheapest laminator for 800-900 rubles, but we don’t need anything better. You can also make a laminator from a stove from a printer or copier, but this is not for everyone.

Absolutely necessary Printer. I use laser, but inkjet will also work. It is highly advisable to use cartridges for laser printers only “from scratch”; refilled cartridges, due to the poorer parameters of the toner, do not provide the required contrast of the print, due to which “dancing” begins with the precise selection of exposure and development time. And this is an extra headache that spoils the joy of our hobby.

Glass for pressing. I gutted the sideboard. Quite functional.

UV lamp. I use 11W, without a ballast, for a table lamp, but it is quite possible to use ordinary ones, “a la energy saving” with a base for a standard socket.

Still needed film for printing on your type of printer, many of them are produced, for example, by Lomond. I don’t recommend plain paper using various types of “transparency agents” due to the weight negative reviews on the forums and the lack of transparency of the resulting muck for ultraviolet light.

And, of course, the photoresist himself. I used LIUXI and PNF-VShch. From chemistry you will need soda ash(in extreme cases, you can use food grade, but it will take longer to manifest itself worse) and some kind of alkali.

Photo roller for knurling

Roll of photoresist, 30 cm wide

Cut a piece of photoresist

The photoresist has one side made of lavsan (top), shiny, the other side made of polyethylene, matte (bottom).

First of all, we print the template on film

Ready template

Separately, I would like to warn you against putting just anything into a laser printer - if the film is not designed specifically for laser printing, it can melt in the oven and wrap around the shaft, as a result of which you will, at best, end up having to replace the rollers and thermal film. If you really can’t wait, try first to remove the unknown film by placing it between sheets of paper and securing everything with construction tape (not film!). If it doesn’t wrinkle and doesn’t stick to the paper, you can use it.

Next we need to roll the photoresist onto the board

We tear off the polyethylene

The workpiece floats in the bath

We melt the photoresist

Smooth with a roller

Place the sandwich in a paper box

Pull through the laminator (repeat 3 times!)

Finished workpiece

Next, we prepare a small bath of water, in which we will roll the photoresist onto the board. The bath must be washed first, because dirt, hair and other floating artifacts are contraindicated in our process - where they get in. Next, cut off the required piece of photoresist and tear off the protective cover from the corner. plastic film. You can pick it up with a needle, but I use sharpened tweezers for SMD components.
Note that film photoresist consists of three layers: transparent lavsan, through which illumination is produced, the photoresist itself and matte polyethylene protective film. So it’s the matte one that needs to be peeled off, don’t get it mixed up.
After the film is torn off, we throw the board into a bath of water, then we press the photoresist film on top of it. Gently press it against the board and smooth it out so that there are no bubbles. Then we take out what we got, put it on a cloth and roll out this sandwich with our finger (preferably with a roller, I use a photo roller) to completely remove the water from under the film. In principle, with some skill, you can roll it “dry”, but firstly, there is no dirt and dust under water (if the bath has been washed), and secondly, due to the stickiness of the photoresist, if the likelihood is that a bubble formed in the center of the board you will not end up It will be possible to remove it either with a roller or a laminator. You cannot tear off the film, so you only have one attempt at dry rolling.

Next, cut a piece of paper with a width slightly larger than the width of the board, and a length slightly longer than twice the length of the board. We fold it in half and put the board into the resulting “daw”. After this, we pull the sandwich through the laminator. You need to stretch it 2-3 times, otherwise the board does not have time to warm up properly. I don’t recommend stretching it without paper - photoresist is a sticky thing and you can’t scrape it off the laminator later.

So, we have a blank, everything is ready for exhibition

Everything is ready for illumination

Let's highlight

Side view

We put glass on top of it all for pressing. It is necessary to press, otherwise, due to unevenness, some areas during exposure may blur, become out of focus, and we will end up with a defect. We place the ultraviolet lamp at a height of 20-30 cm above the board. The distance is also important from the point of view of side illumination, since the higher the lamp, the more perpendicular the rays will fall on the template and the less light will pass from the side under the track on the template. Naturally, if you are making a board with a track width of 0.8 mm, these recommendations do not need to be followed. But if you need 0.1 mm, then any little thing can ruin the matter. Next is the lighting. I illuminate it with an 11w lamp for 5 minutes. More powerful lamps will be enough to shine for a significantly shorter time.
In principle, with a good factory template, even prolonged overexposure will not spoil the matter. But if you have a refilled cartridge or the printer produces an insufficiently dense and contrasting pattern, you will have to experiment. If you underexpose, the pattern will be washed off during development; if you overexpose, the pattern will not appear at all or, more often, there will be a coating of indelible photoresist between the tracks, which, when etching the board, will appear as tracks stuck together in a heap.

We've reached development

Dissolving soda ash

We show

Appeared

If you're lucky and everything works out as intended the first time, you should have a beautiful board ready for etching. But everything can go wrong (at least when I started mastering the technology, I made a lot of defects), then the board must be cleaned of the photoresist layer before the next attempt (as well as after etching). You can clean it off with sandpaper, but this is unsporting. It is better to use a solution of any alkali. I happened to have sodium hydroxide lying around, and I wash it off with it, but, for example, Mole, caustic soda and various misterproppers are quite suitable for cleaning grease from stoves. We lower the board into this solution and after a few minutes the entire photoresist film carefully peels off.

Storage

I store it in this tube

Photoresist sticks out of the tube; on the right is a plug from a bag wrapped with electrical tape.

I use a piece for this sewer pipe from the sink, which is sealed tightly on one side and plugged on the other with a removable plug made from a tightly crumpled bag wrapped with electrical tape. It’s also good to transport photoresit in such a tube without fear of crushing it along the way.

How to make your own liquid photoresist. Photoresist is a film with a sensitive layer. It is not difficult to stick it on. But the author of this video was contacted with an order to apply a pattern to a convex surface. The question arose: “how to glue photoresist to an uneven surface?”

There is a liquid photoresist that can be applied with a brush or in aerosol cans.

Since the master did not have one, it was decided to make it with his own hands. It was previously noted that the photosensitive layer is very soluble in 646 solvent. The result is a liquid that acts as a photosensitive paint.

Before you begin to dilute the photoresist, you need to remove the protection from the film. For this, ordinary tape is used. The bottom cellophane layer can be easily removed, but the cellophane layer requires a little fiddling. The video demonstrates how to do this.

When working with photosensitive materials, it is necessary to work outside of sunlight, otherwise the material may lose its properties.

Next, you need to put these pieces of photoresist in a glass jar and pour in a little solvent. The solution must be left in this state so that the photoresist film dissolves. It is advisable to shake the solution in jars from time to time to speed up the process.

As an experiment, let's try to apply a layer of liquid photoresist on the surface of a metal plate and a spoon. To do this, you will use a spray gun. After that, let’s illuminate the photoresist, develop it, and see what happens.

The result is a fairly thin layer, so you need to do the same again. The photoresist hardens quite quickly, so it does not really need any additional procedures that could speed up the hardening.

Now you need to wait a little for the layer to dry. After half an hour, the photoresist was completely dry and a bluish surface appeared on the spoon. Same thing on the plate. Now you need to take a drawing and illuminate it on a spoon and a metal plate. Let's light up a star on the spoon, cut for simplicity from ordinary electrical tape to demonstrate the technology. Let's illuminate a more complex pattern on the plate.

Many people simply put a film with a pattern and glass on top. But you can use glue, spread it on the surface and simply glue the film. It sticks very tightly and there is no need to use glass. The glue is washed off very well with water.

We will illuminate it with an ultraviolet lamp for one and a half minutes. When 1.5 minutes have passed, turn off the ultraviolet, the illumination is over. Now we will develop our drawings. After exposure, the photoresist darkened even more. Let's remove a piece of electrical tape and film. Now let’s develop the drawings in a solution of soda ash. For the indicated amount of water you need one spoon of soda ash. It appears quite quickly, from time to time you need to wash it with a brush or brush, as shown in the video. As you can see, the pattern is fully revealed on the plate. The unexposed areas were removed, but the exposed areas remained. After this, you can etch in ferric chloride or acid. By the way, if you do not rinse with running water,

Tahiti!.. Tahiti!..
We have not been to any Tahiti!
They feed us well here too!
© Cartoon cat

Introduction with digression

How were boards made in the past in domestic and laboratory conditions? There were several ways, for example:

1. future conductors drew drawings;
2. engraved and cut with cutters;
3. they glued it with adhesive tape or tape, then cut out the design with a scalpel;
4. They made simple stencils and then applied the design using an airbrush.

The missing elements were completed with drawing pens and retouched with a scalpel.

It was a long and laborious process, requiring the “drawer” to have remarkable artistic abilities and accuracy. The thickness of the lines hardly fit into 0.8 mm, there was no repetition accuracy, each board had to be drawn separately, which greatly limited the production of even a very small batch printed circuit boards(further PP).

What do we have today?

Progress does not stand still. The times when radio amateurs painted PP with stone axes on mammoth skins have sunk into oblivion. The appearance on the market of publicly available chemistry for photolithography opens up completely different prospects for the production of PCB without metallization of holes at home.

Let's take a quick look at the chemistry used today to produce PP.

Photoresist

You can use liquid or film. We will not consider film in this article due to its scarcity, difficulties in rolling onto PCBs and the lower quality of the resulting printed circuit boards.

After analyzing market offers, I settled on POSITIV 20 as the optimal photoresist for home PCB production.

Purpose:
POSITIV 20 photosensitive varnish. Used in small-scale production of printed circuit boards, copper engravings, and when carrying out work related to transferring images to various materials.
Properties:
High exposure characteristics provide good contrast of transferred images.
Application:
It is used in areas related to transferring images onto glass, plastics, metals, etc. small-scale production. Directions for use are indicated on the bottle.
Characteristics:
Color: blue
Density: at 20°C 0.87 g/cm 3
Drying time: at 70°C 15 min.
Consumption: 15 l/m2
Maximum photosensitivity: 310-440 nm

The instructions for the photoresist say that it can be stored at room temperature and it is not subject to aging. I strongly disagree! It should be stored in a cool place, for example, on the bottom shelf of the refrigerator, where the temperature is usually maintained at +2+6°C. But under no circumstances allow freezing temperatures!

If you use photoresists that are sold by the glass and do not have lightproof packaging, you need to take care of protection from light. It should be stored in complete darkness and at a temperature of +2+6°C.

Enlightener

Likewise, I consider TRANSPARENT 21, which I constantly use, to be the most suitable educational tool.

Purpose:
Allows direct transfer of images onto surfaces coated with photosensitive emulsion POSITIV 20 or other photoresist.
Properties:
Gives transparency to paper. Provides transmission of ultraviolet rays.
Application:
For quickly transferring the outlines of drawings and diagrams onto a substrate. Allows you to significantly simplify the reproduction process and reduce time s e costs.
Characteristics:
Color: transparent
Density: at 20°C 0.79 g/cm 3
Drying time: at 20°C 30 min.
Note:
Instead of regular paper with transparency, you can use transparent film for inkjet or laser printers, depending on what we will print the photomask on.

Photoresist developer

There are many different solutions for developing photoresist.

It is recommended to develop using a “liquid glass” solution. His chemical composition: Na 2 SiO 3 * 5H 2 O. This substance has a huge number of advantages. The most important thing is that it is very difficult to overexpose the PP in it; you can leave the PP for a non-fixed exact time. The solution almost does not change its properties with temperature changes (there is no risk of decay when the temperature increases), and also has a very long shelf life - its concentration remains constant for at least a couple of years. The absence of the problem of overexposure in the solution will allow increasing its concentration to reduce the time of development of PP. It is recommended to mix 1 part concentrate with 180 parts water (just over 1.7 g of silicate in 200 ml of water), but it is possible to make a more concentrated mixture so that the image develops in about 5 seconds without the risk of surface damage due to overexposure. If it is impossible to purchase sodium silicate, use sodium carbonate (Na 2 CO 3) or potassium carbonate (K 2 CO 3).

I haven’t tried either the first or the second, so I’ll tell you what I’ve been using without any problems for several years now. I use a water solution of caustic soda. For 1 liter of cold water 7 grams of caustic soda. If there is no NaOH, I use a KOH solution, doubling the concentration of alkali in the solution. Development time 30-60 seconds with correct exposure. If after 2 minutes the pattern does not appear (or appears weakly), and the photoresist begins to wash off from the workpiece, this means that the exposure time was chosen incorrectly: you need to increase it. If, on the contrary, it quickly appears, but both exposed and unexposed areas are washed away; either the concentration of the solution is too high, or the quality of the photomask is low (ultraviolet light passes freely through the “black”): you need to increase the print density of the template.

Copper etching solutions

Excess copper is removed from printed circuit boards using various etchants. Among people doing this at home, ammonium persulfate, hydrogen peroxide + hydrochloric acid, copper sulfate solution + table salt are often common.

I always poison with ferric chloride in a glass container. When working with the solution, you need to be careful and attentive: if it gets on clothes and objects, it leaves rusty stains that are difficult to remove with a weak solution of citric (lemon juice) or oxalic acid.

We heat a concentrated solution of ferric chloride to 50-60°C, immerse the workpiece in it, and carefully and effortlessly move a glass rod with a cotton swab at the end over areas where copper is etched less easily, this achieves a more even etching over the entire area of ​​the PP. If you do not force the speed to equalize, the required etching duration increases, and this eventually leads to the fact that in areas where copper has already been etched, etching of the tracks begins. As a result, we don’t get what we wanted at all. It is highly desirable to ensure continuous stirring of the etching solution.

Chemicals for removing photoresist

What is the easiest way to wash off unnecessary photoresist after etching? After repeated trial and error, I settled on ordinary acetone. When it’s not there, I wash it off with any solvent for nitro paints.

So, let's make a printed circuit board

Where does a high quality PCB start? Right:

Create a high-quality photo template

To make it, you can use almost any modern laser or inkjet printer. Considering that we are using positive photoresist in this article, the printer should draw black where copper should remain on the PCB. Where there should be no copper the printer should not draw anything. Very important point when printing a photomask: you need to set the maximum dye flow (in the printer driver settings). The blacker the painted areas, the greater the chances of getting a great result. No color is needed, a black cartridge is enough. From the program (we will not consider programs: everyone is free to choose for themselves - from PCAD to Paintbrush) in which the photo template was drawn, we print it on a regular sheet of paper. The higher the printing resolution and the higher quality the paper, the higher the quality of the photomask. I recommend no lower than 600 dpi; the paper should not be very thick. When printing, we take into account that with the side of the sheet on which the paint is applied, the template will be placed on the PP blank. If done differently, the edges of the PP conductors will be blurred and indistinct. Let the paint dry if it was an inkjet printer. Next, we impregnate the paper with TRANSPARENT 21, let it dry and the photo template is ready.

Instead of paper and enlightenment, it is possible and even very desirable to use transparent film for laser (when printing on a laser printer) or inkjet (for inkjet printing) printers. Please note that these films have unequal sides: only one working side. If you use laser printing, I highly recommend dry running a sheet of film before printing - simply run the sheet through the printer, simulating printing, but not printing anything. Why is this necessary? When printing, the fuser (oven) will heat the sheet, which will inevitably lead to its deformation. As a consequence, there is an error in the geometry of the output PCB. When producing double-sided PCBs, this is fraught with a mismatch of layers with all the consequences And with the help of a “dry” run, we will warm up the sheet, it will be deformed and will be ready for printing the template. When printing, the sheet will pass through the oven a second time, but the deformation will be much less significant checked several times.

If the PP is simple, you can draw it manually in a very convenient program with a Russified interface Sprint Layout 3.0R (~650 KB).

At the preparatory stage, it is very convenient to draw not too cumbersome electrical circuits in the also Russified sPlan 4.0 program (~450 KB).

This is what the finished photo templates look like, printed on an Epson Stylus Color 740 printer:

We print only in black, with maximum dye addition. Material transparent film for inkjet printers.

Preparing the PP surface for applying photoresist

For the production of PP they are used sheet materials with applied copper foil. The most common options are with copper thicknesses of 18 and 35 microns. Most often, for the production of PP at home, sheet textolite (fabric pressed with glue in several layers), fiberglass (the same, but epoxy compounds are used as glue) and getinax (pressed paper with glue) are used. Less commonly, sittal and polycor (high-frequency ceramics are used extremely rarely at home), fluoroplastic (organic plastic). The latter is also used for the manufacture of high-frequency devices and, having very good electrical characteristics, can be used anywhere and everywhere, but its use is limited by its high price.

First of all, you need to make sure that the workpiece does not have deep scratches, burrs or corroded areas. Next, it is advisable to polish the copper to a mirror. We polish without being particularly zealous, otherwise we will erase the already thin layer of copper (35 microns) or, in any case, we will achieve different thicknesses of copper on the surface of the workpiece. And this, in turn, will lead to different etching rates: it will be etched faster where it is thinner. And a thinner conductor on the board is not always good. Especially if it is long and a decent current will flow through it. If the copper on the workpiece is of high quality, without sins, then it is enough to degrease the surface.

Applying photoresist to the surface of the workpiece

We place the board on a horizontal or slightly inclined surface and apply the composition from an aerosol package from a distance of about 20 cm. We remember that the most important enemy in this case is dust. Every particle of dust on the surface of the workpiece is a source of problems. To create a uniform coating, spray the aerosol in a continuous zigzag motion, starting from the upper left corner. Do not use the aerosol in excess quantities, as this will cause unwanted smudges and lead to the formation of a non-uniform coating thickness, requiring a longer exposure time. In summer at high temperatures environment Re-treatment may be necessary, or the aerosol may need to be sprayed from a shorter distance to reduce evaporation losses. When spraying, do not tilt the can too much; this leads to increased consumption of propellant gas and, as a result, the aerosol can stops working, although there is still photoresist in it. If you are getting unsatisfactory results when spray coating photoresist, use spin coating. In this case, photoresist is applied to a board mounted on a rotating table with a 300-1000 rpm drive. After finishing coating, the board should not be exposed to strong light. Based on the color of the coating, you can approximately determine the thickness of the applied layer:

• light gray blue 1-3 microns;
• dark gray blue 3-6 microns;
• blue 6-8 microns;
• dark blue more than 8 microns.

On copper, the coating color may have a greenish tint.

The thinner the coating on the workpiece, the better the result.

I always spin coat the photoresist. My centrifuge has a rotation speed of 500-600 rpm. Fastening should be simple, clamping is carried out only at the ends of the workpiece. We fix the workpiece, start the centrifuge, spray it on the center of the workpiece and watch how the photoresist spreads over the surface in a thin layer. Centrifugal forces will throw off excess photoresist from the future PCB, so I highly recommend providing a protective wall so as not to turn workplace to the pigsty. I use an ordinary saucepan with a hole in the bottom in the center. The axis of the electric motor passes through this hole, on which a mounting platform is installed in the form of a cross of two aluminum slats, along which the workpiece clamping ears “run.” The ears are made of aluminum angles, clamped to the rail with a wing nut. Why aluminum? Low specific gravity and, as a result, less runout when the center of mass of rotation deviates from the center of rotation of the centrifuge axis. The more accurately the workpiece is centered, the less beating will occur due to the eccentricity of the mass and the less effort will be required to rigidly attach the centrifuge to the base.

Photoresist is applied. Let it dry for 15-20 minutes, turn the workpiece over, apply a layer on the other side. Give another 15-20 minutes to dry. Do not forget that direct sunlight and fingers on the working sides of the workpiece are unacceptable.

Tanning photoresist on the surface of the workpiece

Place the workpiece in the oven, gradually bring the temperature to 60-70°C. Maintain at this temperature for 20-40 minutes. It is important that nothing touches the surfaces of the workpiece; only touching the ends is permissible.

Aligning the top and bottom photomasks on the workpiece surfaces

Each of the photo masks (top and bottom) should have marks along which 2 holes need to be made on the workpiece to align the layers. The farther the marks are from each other, the higher the alignment accuracy. I usually place them diagonally on the templates. Using a drilling machine, using these marks on the workpiece, we drill two holes strictly at 90° (the thinner the holes, the more accurate the alignment; I use a 0.3 mm drill) and align the templates along them, not forgetting that the template must be applied to the photoresist the side on which the print was made. We press the templates to the workpiece with thin glasses. It is preferable to use quartz glass as it transmits ultraviolet radiation better. Plexiglas (plexiglass) gives even better results, but it has the unpleasant property of scratching, which will inevitably affect the quality of the PP. For small PCB sizes, you can use a transparent cover from a CD package. In the absence of such glass, you can use ordinary window glass, increasing the exposure time. It is important that the glass is smooth, ensuring an even fit of the photomasks to the workpiece, otherwise it will be impossible to obtain high-quality edges of the tracks on the finished PCB.

A blank with a photomask under plexiglass. We use a CD box.

Exposure (light exposure)

The time required for exposure depends on the thickness of the photoresist layer and the intensity of the light source. Photoresist varnish POSITIV 20 is sensitive to ultraviolet rays, the maximum sensitivity occurs in the area with a wavelength of 360-410 nm.

It is best to expose under lamps whose radiation range is in the ultraviolet region of the spectrum, but if you do not have such a lamp, you can also use ordinary powerful incandescent lamps, increasing the exposure time. Do not start illumination until the lighting from the source has stabilized; it is necessary for the lamp to warm up for 2-3 minutes. The exposure time depends on the thickness of the coating and is usually 60-120 seconds when the light source is located at a distance of 25-30 cm. The glass plates used can absorb up to 65% of ultraviolet radiation, so in such cases it is necessary to increase the exposure time. The best results are achieved when using transparent plexiglass plates. When using photoresist with a long shelf life, the exposure time may need to be doubled remember: Photoresists are subject to aging!

Examples of using different light sources:

UV lamps

We expose each side in turn, after exposure we let the workpiece stand for 20-30 minutes in a dark place.

Development of the exposed workpiece

We develop it in a solution of NaOH (caustic soda) see the beginning of the article for more details at a solution temperature of 20-25°C. If there is no manifestation within 2 minutes small O exposure time. If it appears well, but useful areas are also washed away you were too clever with the solution (the concentration is too high) or the exposure time with a given radiation source is too long or the photomask is of poor quality the printed black color is not saturated enough to allow ultraviolet light to illuminate the workpiece.

When developing, I always very carefully, effortlessly “roll” a cotton swab on a glass rod over the places where the exposed photoresist should wash off; this speeds up the process.

Washing the workpiece from alkali and residues of exfoliated exposed photoresist

I do this under the tap with regular tap water.

Re-tanning photoresist

We place the workpiece in the oven, gradually raise the temperature and hold it at a temperature of 60-100°C for 60-120 minutes; the pattern becomes strong and hard.

Checking the development quality

Briefly (for 5-15 seconds) immerse the workpiece in a ferric chloride solution heated to a temperature of 50-60°C. Rinse quickly with running water. In places where there is no photoresist, intensive etching of the copper begins. If photoresist accidentally remains somewhere, carefully remove it mechanically. It is convenient to do this with a regular or ophthalmic scalpel, armed with optics (soldering glasses, magnifying glass A watchmaker, loupe A on a tripod, microscope).

Etching

We poison in a concentrated solution of ferric chloride at a temperature of 50-60°C. It is advisable to ensure continuous circulation of the etching solution. We carefully “massage” poorly bleeding areas with a cotton swab on a glass rod. If ferric chloride is freshly prepared, the etching time usually does not exceed 5-6 minutes. We wash the workpiece with running water.

Board etched

How to prepare a concentrated solution of ferric chloride? Dissolve FeCl 3 in slightly (up to 40°C) heated water until it stops dissolving. Filter the solution. It should be stored in a cool, dark place in sealed non-metallic packaging in glass bottles, for example.

Removing unnecessary photoresist

We wash off the photoresist from the tracks with acetone or a solvent for nitro paints and nitro enamels.

Drilling holes

It is advisable to select the diameter of the point of the future hole on the photomask such that it will be convenient to drill later. For example, with a required hole diameter of 0.6-0.8 mm, the diameter of the point on the photomask should be about 0.4-0.5 mm in this case the drill will be well centered.

It is advisable to use drills coated with tungsten carbide: drills made of high-speed steels wear out very quickly, although steel can be used for drilling single holes of large diameter (more than 2 mm), since drills coated with tungsten carbide of this diameter are too expensive. When drilling holes with a diameter of less than 1 mm, it is better to use a vertical machine, otherwise your drill bits will break quickly. If you drill with a hand drill, distortions are inevitable, leading to inaccurate joining of holes between layers. The top-down movement on a vertical drilling machine is the most optimal in terms of the load on the tool. Carbide drills are made with a rigid (i.e. the drill fits exactly to the hole diameter) or a thick (sometimes called "turbo") shank that has a standard size (usually 3.5 mm). When drilling with carbide-coated drills, it is important to firmly secure the PCB, since such a drill, when moving upward, can lift the PCB, skew the perpendicularity and tear out a fragment of the board.

Small diameter drills are usually fitted into either a collet chuck (various sizes) or a three-jaw chuck. For precise fixation, fastening in a three-jaw chuck is not the best the best option, and the small size of the drill (less than 1 mm) quickly makes grooves in the clamps, losing good fixation. Therefore, for drills with a diameter less than 1 mm, it is better to use a collet chuck. To be on the safe side, purchase an extra set containing spare collets for each size. Some inexpensive drills come with plastic collets; throw them away and buy metal ones.

To obtain acceptable accuracy, it is necessary to properly organize the workplace, that is, firstly, to ensure good lighting of the board when drilling. To do this, you can use a halogen lamp, attaching it to a tripod to be able to choose a position (illuminate the right side). Secondly, raise the work surface about 15 cm above the tabletop for better visual control over the process. It would be a good idea to remove dust and chips while drilling (you can use a regular vacuum cleaner), but this is not necessary. It should be noted that the dust from fiberglass generated during drilling is very caustic and, if it comes into contact with the skin, causes skin irritation. And finally, when working, it is very convenient to use the foot switch of the drilling machine.

Typical hole sizes:

• vias 0.8 mm or less;
• integrated circuits, resistors, etc. 0.7-0.8 mm;
• large diodes (1N4001) 1.0 mm;
• contact blocks, trimmers up to 1.5 mm.

Try to avoid holes with a diameter of less than 0.7 mm. Always keep at least two spare drills of 0.8 mm or smaller, as they always break just at the moment when you urgently need to order. Drills 1 mm and larger are much more reliable, although it would be nice to have spare ones for them. When you need to make two identical boards, you can drill them simultaneously to save time. In this case, it is necessary to very carefully drill holes in the center of the contact pad near each corner of the PCB, and for large boards, holes located close to the center. Lay the boards on top of each other and, using 0.3mm centering holes in two opposite corners and pins as pegs, secure the boards to each other.

If necessary, you can countersink the holes with larger diameter drills.

Copper tinning on PP

If you need to tin the tracks on the PCB, you can use a soldering iron, soft low-melting solder, alcohol-rosin flux and coaxial cable braid. For large volumes, they tin in baths filled with low-temperature solders with the addition of fluxes.

The most popular and simple melt for tinning is the low-melting alloy “Rose” (tin 25%, lead 25%, bismuth 50%), the melting point of which is 93-96°C. Using tongs, place the board under the level of the liquid melt for 5-10 seconds and, after removing it, check whether the entire copper surface is evenly covered. If necessary, the operation is repeated. Immediately after removing the board from the melt, its remains are removed either using a rubber squeegee or by sharp shaking in a direction perpendicular to the plane of the board, holding it in the clamp. Another way to remove residual Rose alloy is to heat the board in a heating cabinet and shake it. The operation can be repeated to achieve a mono-thickness coating. To prevent oxidation of the hot melt, glycerin is added to the tinning container so that its level covers the melt by 10 mm. After the process is completed, the board is washed from glycerin in running water. Attention! These operations involve working with installations and materials exposed to high temperatures, therefore, to prevent burns, it is necessary to use protective gloves, goggles and aprons.

The operation of tinning with a tin-lead alloy proceeds in a similar way, but the higher temperature of the melt limits the scope of application of this method in handicraft production conditions.

After tinning, do not forget to clean the board from flux and thoroughly degrease it.

If you have a large production, you can use chemical tinning.

Applying a protective mask

The operations with applying a protective mask exactly repeat everything that was written above: we apply photoresist, dry it, tan it, center the mask photomasks, expose it, develop it, wash it and tan it again. Of course, we skip the steps of checking the quality of development, etching, removing photoresist, tinning and drilling. At the very end, tan the mask for 2 hours at a temperature of about 90-100°C - it will become strong and hard, like glass. The formed mask protects the surface of the PP from external influence and protects against theoretically possible short circuits during operation. It also plays an important role in automatic soldering: it prevents the solder from “sitting” on adjacent areas, short-circuiting them.

That's it, the double-sided printed circuit board with mask is ready

I had to make a PP in this way with the width of the tracks and the step between them up to 0.05 mm (!). But this is already jewelry work. And without much effort, you can make PP with a track width and a step between them of 0.15-0.2 mm.

I did not apply a mask to the board shown in the photographs; there was no such need.

Printed circuit board in the process of installing components on it

And here is the device itself for which the PP was made:

This is a cellular telephone bridge that allows you to reduce the cost of services by 2-10 times mobile communications for this it was worth messing with the PP;). The PCB with soldered components is located in the stand. Previously, there was an ordinary charger for mobile phone batteries.

Additional Information

Metallization of holes

You can even metallize holes at home. To do this, the inner surface of the holes is treated with a 20-30% solution of silver nitrate (lapis). Then the surface is cleaned with a squeegee and the board is dried in the light (you can use a UV lamp). The essence of this operation is that under the influence of light, silver nitrate decomposes, and silver inclusions remain on the board. Next, chemical precipitation of copper from solution is carried out: copper sulfate ( copper sulfate) 2 g, caustic soda 4 g, ammonia 25% 1 ml, glycerin 3.5 ml, formaldehyde 10% 8-15 ml, water 100 ml. The shelf life of the prepared solution is very short; it must be prepared immediately before use. After the copper is deposited, the board is washed and dried. The layer turns out to be very thin; its thickness must be increased to 50 microns by galvanic means.

Solution for applying copper plating by electroplating:
For 1 liter of water, 250 g of copper sulfate (copper sulfate) and 50-80 g of concentrated sulfuric acid. The anode is a copper plate suspended parallel to the part being coated. The voltage should be 3-4 V, current density 0.02-0.3 A/cm 2, temperature 18-30°C. The lower the current, the slower the metallization process, but the better the resulting coating.

A fragment of a printed circuit board showing metallization in the hole

Homemade photoresists

Photoresist based on gelatin and potassium bichromate:
First solution: pour 15 g of gelatin into 60 ml of boiled water and leave to swell for 2-3 hours. After the gelatin swells, place the container in a water bath at a temperature of 30-40°C until the gelatin is completely dissolved.
Second solution: dissolve 5 g of potassium dichromate (chrompic, bright orange powder) in 40 ml of boiled water. Dissolve in low, diffused light.
Pour the second into the first solution with vigorous stirring. Using a pipette, add a few drops of ammonia to the resulting mixture until it becomes straw-colored. The emulsion is applied to the prepared board under very low light. The board is dried until it is tack-free at room temperature in complete darkness. After exposure, rinse the board under low ambient light in warm running water until the untanned gelatin is removed. To better evaluate the result, you can paint areas with unremoved gelatin with a solution of potassium permanganate.

Improved homemade photoresist:
First solution: 17 g of wood glue, 3 ml of ammonia aqueous solution, 100 ml of water, leave to swell for a day, then heat in a water bath at 80°C until completely dissolved.
Second solution: 2.5 g potassium dichromate, 2.5 g ammonium dichromate, 3 ml aqueous ammonia solution, 30 ml water, 6 ml alcohol.
When the first solution has cooled to 50°C, pour the second solution into it with vigorous stirring and filter the resulting mixture ( This and subsequent operations must be carried out in a darkened room, sunlight is not allowed!). The emulsion is applied at a temperature of 30-40°C. Continue as in the first recipe.

Photoresist based on ammonium dichromate and polyvinyl alcohol:
Prepare the solution: polyvinyl alcohol 70-120 g/l, ammonium dichromate 8-10 g/l, ethanol 100-120 g/l. Avoid bright light! Apply in 2 layers: first layer drying 20-30 minutes at 30-45°C second layer drying 60 minutes at 35-45°C. Developer 40% ethyl alcohol solution.

Chemical tinning

First of all, the board must be picked out to remove the formed copper oxide: 2-3 seconds in a 5% solution of hydrochloric acid followed by rinsing in running water.

It is enough to simply carry out chemical tinning by immersing the board in an aqueous solution containing tin chloride. The release of tin on the surface of a copper coating occurs when immersed in a tin salt solution in which the potential of the copper is more electronegative than the coating material. The change in potential in the desired direction is facilitated by the introduction of a complexing additive, thiocarbamide (thiourea), into the tin salt solution. This type of solution has the following composition (g/l):

Among those listed, the most common are solutions 1 and 2. Sometimes it is proposed to use as a surfactant for the 1st solution detergent"Progress" in the amount of 1 ml/l. Adding 2-3 g/l bismuth nitrate to the 2nd solution leads to the precipitation of an alloy containing up to 1.5% bismuth, which improves the solderability of the coating (prevents aging) and greatly increases the shelf life of the finished PCB before soldering components.

To preserve the surface, aerosol sprays based on fluxing compositions are used. After drying, the varnish applied to the surface of the workpiece forms a strong, smooth film that prevents oxidation. One of the popular substances is “SOLDERLAC” from Cramolin. Subsequent soldering is carried out directly on the treated surface without additional varnish removal. In particularly critical cases of soldering, the varnish can be removed with an alcohol solution.

Artificial tinning solutions deteriorate over time, especially when exposed to air. Therefore, if you have large orders infrequently, then try to prepare a small amount of solution at once, sufficient to tinning the required amount of PP, and store the remaining solution in a closed container (bottles of the type used in photography that do not allow air to pass through are ideal). It is also necessary to protect the solution from contamination, which can greatly degrade the quality of the substance.

In conclusion, I want to say that it is still better to use ready-made photoresists and not bother with metalizing holes at home; you still won’t get great results.

Many thanks to the candidate of chemical sciences Filatov Igor Evgenievich for consultations on issues related to chemistry.
I also want to express my gratitude Igor Chudakov."

So, for work we will need the following materials and tools:
1. Foil fiberglass.
2. Film photoresist.
3. Small needle.
4. Soda ash.
5. Ultraviolet lamp for a standard 220V socket.
6. Film for inkjet printer.
7. Computer, inkjet printer, program for layout of printed circuit boards.
8. Eraser.
9. Scissors.
10. Glass 4 mm thick.
11. Plastic container.

First, we need to make a photomask, through which we will subsequently expose our future printed circuit board.
We open the program for creating a printed circuit board, everyone has their own preferences, some like P-CAD, some like Eagle, but I myself prefer to make boards in Sprint-Layout 4.0. The file of the future printed circuit board was opened in the program.

We checked that there were no jambs of undone chains and other things. Conducted, there is nothing in doubt, you can move on.

Click on the button and a window for printing will open.

Since the photoresist is negative (white areas on a black field), you need to check the appropriate box next to the negative option, and also disable layers that are unnecessary for printing.

You should also think about whether to display a mirror image during output or not, since the film for an inkjet printer has only one working side and this side will need to be subsequently applied to the workpiece in order to increase the contrast and eliminate side illumination. I usually draw on layer F2, and place the inscriptions on layer M1, so in my case I don’t need to mirror anything.

Now press the button and the printer settings window opens and set the maximum print quality and contrast

As a result of these actions, we tell the printer to pour more toner into those areas that will be black. Next, click on the OK button in the color settings window, and the OK button in the printer properties.
Thus, we created a photo template, set up the printer, and the whole thing is ready for printing.
We take a pack of

We take the resulting template and carefully examine it; the black fields should not have any gaps. If the contrast of the black fields is not sufficient, you can use this tool - Density Toner Optical Density Enhancer

Now let's start preparing and apply it to it

From the previously prepared fiberglass, we cut out a small piece, ideally with an allowance of approximately 3-5 mm, larger on each edge than the dimensions of our future board.

We take an eraser and carefully go over the entire surface of the foil fiberglass. This is necessary in order to remove all fingers, dirt, etc., and also to ensure a good fit of the film photoresist. After we have gone over the fiberglass laminate with an eraser, we blow off all the remaining dust particles and the remains of the gum. You can’t wash all this with acetone or some kind of solvent, the photoresist won’t stick, if you can’t blow something off, then go over it with a clean cloth. Without touching the cleaned surface of the workpiece with your hands (holding by the ends is allowed), place it on the table and cut off a piece of photoresist with scissors. (The author used domestic photoresist, we recommend
use one that is significantly superior to ours in many respects)

After cutting it off, take a needle and pick up the matte film from the matte side and move it about 0.5 centimeters without touching the adhesive layer on the photoresist itself with your fingers.

Holding a piece of the removed film with your fingers, place it on the edge of the fiberglass workpiece and level it with your fingers with moderate pressure so that the film sticks properly.
After the film has stuck to the edge, we place the fingers of the right hand on the right side under the film, on the piece of matte film that was removed at the beginning.

Now, slowly, with your right hand, pull out the matte film approximately 2-3 mm at a time, while at the same time pressing and leveling it over the surface of the foil with the fingers of your left hand. There is no point in rushing here because the better you press it, the better it will lie on the surface of the fiberglass foil.
After we have smoothed the entire film, we cut off the excess and get fiberglass coated with film photoresist.

We take scissors and cut out our photo board to the required size.

Now we are ready to start exposing the photoresist through the template.
Let's take it, for example I use this one

It’s simple and economical, and most importantly, there’s no need to add anything extra. This is an energy-saving UV light lamp for a standard 220V socket.
We place fiberglass with a film photoresist applied to it on a flat surface, and on top there is a film with a printed template, the side on which was printed towards the photoresist, what is this for and why did I say this earlier.

We press the whole thing on top with glass taken from the shelf with books.

And on top of all this, I usually put two boxes with compacts, this provides even better pressing of the photomask to the board and determines the distance at which the ultraviolet lamp is removed from the surface.
It is very easy to select the time and distance for a specific lamp. We take a small piece of fiberglass and apply photoresist to it. Then we make a template on which we write the numbers 1,2,3,4,5,6,7,8, etc., this will be the time in minutes. We put the lamp on, turn it on, take some opaque material, for example, another piece of fiberglass, and gradually move it at the indicated intervals, gradually covering the parts with the numbers. After that, we develop it and look at the result. Where it is the best, then we leave time for this distance.

After this, turn on the lamp for 10 minutes.

While our lamp will shine for 10 minutes and form our board, let's go to the bath and prepare a solution for developing the photoresist.
Pour 0.25 liters of water (half a 0.5 liter juice bottle) into a plastic container of a suitable size to accommodate the board, the temperature of the water does not matter, I pour it straight from the tap. We take out a bag of soda ash from the shelf. ( If you don’t have soda ash on the shelf, then you need to put it there first, and only then take it out. If you don’t have a shelf, then you don’t have to read further - nothing will work anyway. Note Kota)

Take a teaspoon and scoop some baking soda into it, then thoroughly stir it in the water until all the lumps dissolve.

After all the soda has dissolved, we wait for the end of exposure, as we remember, before we had it for 10 minutes. As soon as the time is up, we remove the glass and our photo mask. We take the fee and go to the bathroom, not forgetting to take a needle with us.
Arriving in the bathroom, carefully use a needle to pick up the second (transparent) film and remove it.

After removing the second film, we put the board in a plastic container with diluted soda and wait about 30 seconds. After this time, the pattern begins to appear, future tracks are visible, and in those places where tracks should not be, the photoresist dissolves. Now we take an unnecessary toothbrush and start running it over our board in order to speed up the process of washing off the photoresist from areas we don’t need.
An indicator that the photoresist has washed off where it is needed, the surface of the copper is light and shiny, as before gluing the photoresist.
After we have washed off all the unnecessary photoresist and left the required one, we remove the board from the soda solution and rinse it under running water. This is done in order to wash off the developing solution from the surface of the board. After rinsing it under running water, set it aside and pour out the developing solution we don’t need.

Now the only thing left to do is pour the ferric chloride solution into another plastic container and poison it. After pickling, take it out and rinse again under running water, this time to wash off any remaining ferric chloride.
That's the whole simple process, after which we get a high-quality printed circuit board.

Thus, we made a printed circuit board, the photoresist that we needed is visible on it. All that remains is to remove it. We take a cotton swab, soak it in acetone, and first blot the entire surface of the board, then blot it with three. After about 1-2 minutes, the photoresist begins to slide off in pieces; we completely wipe off the entire photoresist. Then, as usual, we tin, drill holes, cut, align and solder the components.

Possible defects in the output after developing the photoresist:
1. Photoresist completely dissolves in soda - insufficient exposure time or large distance to the lamp.
2. The photoresist is not washed off anywhere - a transparent photomask in dark areas, as a result of which ultraviolet rays pass through them and expose things that should not be exposed.
3. The photoresist is not washed off anywhere, but in those areas where it should be washed off it is slightly cloudy, the pattern is visible, and the pattern is clear - a transparent pattern in dark areas, but in this case it is much darker than in the previous version.
4. The photoresist has washed off as it should, but the tracks are wider than on the photomask, this is especially noticeable on those tracks that pass between the pins of the microcircuits (sticking), for example, on the photomask the track when measured with a 1 mm ruler on the board 1.2-1 .5 mm - insufficient pressure of the photomask to the surface of the workpiece; this can also happen when the fiberglass laminate itself is curved, so I recommend paying attention to its evenness when purchasing, since I myself have encountered the curvature of the fiberglass laminate being sold more than once.

Let me remind you that earlier in this blog we talked about the manufacture of printed circuit boards using LUT. This good method, but with its own limitations. For example, if you hold the iron a little too long, the toner will flow and closely spaced tracks will stick together. That is, if you decide to use SMD chips, the method becomes practically unusable. Someone successfully solves this problem by purchasing, in addition to an already expensive and space-consuming laser printer, a laminator. But I decided to go a different route and try an alternative method to LUT. This method involves using film photoresist.

Note: Just as laser ironing technology is often abbreviated to “LUT,” film photoresist technology is often abbreviated to “photoresist” or “FR.”

Shopping list

To make printed circuit boards using film photoresist we will need:

• Suddenly, film photoresist. Literally everything depends on the quality of the photoresist. I used Ordyl Alpha 350 photoresist and highly recommend using it. There is also Ordyl Alpha 300, which, judging by the reviews, is also good. What is the difference between 300 and 350, alas, remains a mystery to me.
• Transparent film for printer. For laser or inkjet, depending on what kind of printer you have. I used A4 laser film Lomond 0703415.
• Ultraviolet light bulb. In theory, any will do, as long as it fits the socket of your table lamp. It is better to take an energy-saving one so that it lasts longer. The UV energy saving bulb I use is called Camelion LH26-FS.
• Soda Ash. You need very little, 100 g will last you a very long time.
• A clean cloth that absorbs water well, a clean sponge and dishwashing detergent. Available in any home, and also sold in any hardware store.
• Optional - a piece of plexiglass. Instead, any other fairly clean, scratch-free glass will do. For example, glass from a bookshelf. I used plexiglass size 30 x 40 cm and 2 mm thick.
• Flux, ferric chloride, acetone or its equivalent, glass or plastic dishes, and so on. Everything regarding etching the board and subsequent steps is no different from LUT.

Having everything listed in your hands, you can get down to business!

Fun fact! Plexiglas is also called plexiglass, acrylic glass, metaplex, and also in other words. It's all the same thing.

Process description

The first board you make using FR will be special. With its help, you will not only test the entire process from start to finish, but also determine very important parameter— the required exposure time of the photoresist under a UV lamp.

Open EAGLE, or whatever you use to design boards, and enter the numbers from 0001 to 0020 into the column. The thickness of the lines of the numbers should be approximately the same thickness as you usually make the tracks, or a little thinner. Then we print the resulting board in negative. In EAGLE, to do this, go to File → CAM Processor, in Device select PS_INVERTED, in File indicate the path to the .ps file in which you want to save the result, select the desired layers and click Process Job. Then we view the resulting .ps file, for example, using Evince, and print it on transparent film, for example, using lpr.

Fun fact! There is also a positive photoresist. But, as far as I know, it is usually liquid and is only used in factories. Film photoresist is always negative and requires printing the board in negative.

To achieve better results in subsequent steps, the film should be placed with the toner side down. It is easy to determine which side of the film the toner is on, since the film shines in the light, but the toner does not. You may need to print the .ps file as a mirror image. If you are printing via lpr, this is done by passing the -o mirror option. Or simply check the appropriate box in EAGLE when generating the .ps file. However, at first you don’t have to worry about all this, since the film is quite thin.

When printing negative, quite a lot of toner is used. Give it some time to dry. Then cut the negative to a size that suits you using scissors.

The result will look something like this:

We take fiberglass, preferably without any special oxide on it. I just found an unnecessary piece of the right size, which I didn’t cut very evenly at the time. Fiberglass standard size 5 x 10 cm is also quite suitable.

Then take a clean sponge and wash the fiberglass in warm water using dishwashing detergent. I used Fairy, but any product should work. The goal is to wash off all the dirt and grease from your hands. You cannot use acetone or its analogue for this! You can rub with the hard side of the sponge, but not too much. When everything has been washed off, wipe the fiberglass on a clean cloth:

It goes without saying that from now on we will not touch pure copper with our fingers.

We cut off enough film photoresist by eye so that it can cover all the copper. We quickly put the rest of the roll back into the packaging and put it in a dark place so as not to expose it to light. The photoresist is covered with film on both sides. If you look closely, a glossy film is used on the outside of the roll, and slightly matte on the inside. We pick up the matte film with our fingernails, tweezers, or, best of all, a piece of electrical tape (you are unlikely to be able to pick up the glossy film at this stage anyway) and glue the photoresist to the copper, as shown in the following photo:

If you choose to use a photoresist other than Ordyl Alpha, it may be a different color.

We peel off about half a centimeter of the film, carefully press down and smooth the photoresist, peel off the next half centimeter, and so on until we have covered all the copper with photoresist. It is very important to glue the photoresist properly, without air bubbles, creases, and so on. The quality of the future board directly depends on this. If you are not in a hurry, after this step you can put the board under a press for a couple of hours. The result will be at least no worse from this. However, you can do it without a press.

Addition: There is an alternative, the so-called “wet” method. The entire matte film is removed from the photoresist at once, and it is applied to fiberglass laminate in water. Then the future board is dried a little, wrapped in paper and passed through a laminator a couple of times at a temperature of 120 degrees. As an inexpensive laminator, we can recommend the FGK-120 model. Subjectively, this method is faster, more pleasant and more reliable, but it additionally requires a laminator.

Next, we place the negative on a photoresist. Let me remind you that ideally you should place it with the toner side down. This way there will be less distortion when transferring the drawing. Place a piece of plexiglass on top (or glass from a bookshelf, or whatever you decide to use). If you are not sure about the cleanliness of the glass, you should first wipe it on both sides with a damp, clean cloth or napkin for cleaning monitors. We put something heavy in the corners of the glass. I used dumbbell plates, but you can use books or anything else. We cover all the numbers on the negative with something completely opaque. I used another piece of fiberglass, but a notepad or piece of plywood would work just as well. Above all this we put a lamp with a UV light bulb screwed into it.

Important! Looking at ultraviolet light is not good for the eyes. I don't recommend doing this for too long, and ideally I recommend wearing appropriate safety glasses.

The result will be a design like this:

Let's note the time. We move the fiberglass, thereby revealing the number 20. We wait exactly one minute. We move the fiberglass again. Now the numbers 20 and 19 are open. And so on, we open one number per minute. As a result, each digit will be exposed for the corresponding number of minutes. After exposing number 1 for one minute, turn off the lamp.

Based on which numbers transfer best, we will determine the optimal exposure time. Exposure time depends on the photoresist and UV bulb used, the height of the desk lamp, and a number of other factors, so it is different for everyone. It goes without saying that when manufacturing future boards, there will be no need to cover the negative with anything. You just need to turn on the lamp for a certain number of minutes.

Now we pick up and peel off the second film of photoresist. It will be easier to pick it up if you use scissors to cut the photoresist exactly to the size of the fiberglass:

Note that the numbers are already visible on the photoresist. This is a characteristic property of Ordyl Alpha photoresist. It’s very convenient - you can immediately tell whether it worked or not. If you are using a different photoresist, it may still be the same color at this stage.

We take glass or plastic dishes. Preferably a clean one, and not one in which you etch copper with ferric chloride. Pour warm tap water and dilute one teaspoon of soda ash in it. Place the workpiece in the resulting solution and let it lie there for about a minute. Then we take the fiberglass by the ends and gently rinse it in the solution until we wash off all the excess. Then we rinse the workpiece under a (weak!) stream of tap water.

Result:

As you can see, my optimal exposure time turned out to be approximately 15 minutes. When making boards with very thin traces, it is better to play it safe and expose for 20 minutes.

Then we etch the board in ferric chloride, as usual (UPD: or better, using hydrogen peroxide with citric acid). To remove photoresist, use acetone or its equivalent. I personally use a product called Degreaser 65. In the end, I got the following:

It is worth noting that as exposure time increases, the photoresist becomes increasingly difficult to clean.

The remaining steps, such as tinning and drilling holes, are no different from the LUT already discussed earlier. Now that we have figured out the optimal exposure time, we can make a real board. So, I just made a board for electronic dice using film photoresist.

Conclusion

Let's consider the advantages of the method. The main advantage is that you can safely use all sorts of TQFP44 (for example, ATmega32U4) and not be afraid that all the tracks will stick together due to an overexposed iron. You can use any printer, be it laser or inkjet. Finally, one negative can be used an unlimited number of times.

The main disadvantage is the limited shelf life of the photoresist. An online store delivered me a roll that expires in four months. Perhaps he will cope with his task excellently even after this period, I don’t know that yet. But to buy film photoresist, it still makes sense to walk to an offline store. To all this it is worth adding that in order to use photoresist, copper on fiberglass laminate should not be heavily oxidized. Finally, specifically in EAGLE, when exporting a board to the .ps format, in some places the tracks may turn out to be a little shorter or a little longer. EAGLE is unlikely to screw up your payment, but it can easily be made to a slightly different size. You need to be careful.

Overall, if you want to use one method of making PCBs at home, I would recommend film photoresist. This is a more universal method, and subjectively it is more pleasant than LUT. Please note, however, that FR is somewhat more complicated, and may not work the first time.

Which method do you prefer - LUT or FR?

Addition: As it turns out, expired photoresist also works, but requires twice as much exposure time. Otherwise, it will be completely washed off when placed in a solution of soda ash. In addition, for better adhesion of the old photoresist to the copper, it makes sense to heat it with a hairdryer (if you don’t have a laminator).

Addition: You may also be interested in articles