How to make a printed wiring board - PCB
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Making a printed circuit (wiring) using the Heatless Toner Transfer Method
The “cold” toner transfer method is a lesser-known method besides the “hot” toner transfer method, which most often uses an iron or a laminator.
In the case of the “cold” method, a heat source is not required to transfer the toner, but a mixture of certain substances.
With this technique we can create PCBs, single-plated and double-plated, which do not require very small distances between traces (< 0.3 mm). With some experience we can also create 0.1 mm traces using the cold transfer technique. There are several methods to create PCBs, but this is the best method in relation to the quality of the wiring traces and the cost of materials/difficulty level.
STEP 1 – Designing and printing the wiring on paper
The design process is assisted by DipTrace, which through the "Preview" function, provides us with all the printing facilities, including the "mirror" mode, for the printed sheet, in front of the printed wiring, that is, the "Top" side. This must have this mirror effect, to correspond functionally, electrically, with the "Bottom" side, the lower face of the wiring, which is printed directly, at the bottom, that is, the back. Pay attention to "Print Scale: 100%". Usually it is printed in pdf, the scaling is preserved very well.
If we have a PCB circuit that is not already edited with the "mirror"/"horizontal flip effect" mode, we can do it very easily in a pdf or photo editor.
Printing Top and Bottom on paper, on the transfer sheet is done only with a laser printer, on photo paper or glossy cardboard. The print settings for the best transfer results are: Print Quality - maximum, Black Contrast - maximum (Rich black / Super black), Resolution of at least 300 DPI, CMYK mode - enabled, Economy mode - disabled.
STEP 2 - Transferring toner to the printed circuit board
Printing transfer sheets on the board is an operation that requires a lot of attention.
The copper foil on the circuit board is usually oxidized. This oxide resists the corrosion process. Finely sand the copper foil until the oxide film is removed. Wipe it well and degrease with isopropyl alcohol/acetone/technical alcohol, without touching it with your hand afterwards. Now the circuit board is ready to transfer the toner onto it.
To prepare the transfer solution, it would be ideal to use pure Acetone (99% purity) together with Isopropyl Alcohol (99% purity), in a ratio of 8/3 (8 parts alcohol + 3 parts acetone), but if necessary we can replace pure acetone with diluted acetone (commercial nail polish remover solution, ~60% purity), nitrodiluant, toluene or xylene. Isopropyl alcohol can be replaced with industrial ethyl alcohol. The proportions of the solution change depending on the substances used.
Very good results were obtained using diluted acetone (60%) together with isopropyl alcohol (99.6%) and after some tests we arrived at a ratio of 1/1, 5 ml of acetone with 5 ml of isopropyl alcohol.
Using a syringe, we apply the acetone + isopropyl alcohol solution over the copper surface of the board and quickly spread it over the entire surface; the acetone evaporates very quickly. Then we place the paper containing the printed wiring over the surface of the board, of course the side with the traces.
We overlap and perfectly center the transfer paper over the board without pressing on it and wait about 10 seconds, then using a piece of parquet we start pressing perpendicularly and strongly, 2-3 minutes on our wiring to make the toner adhere to the copper, eventually we add some more solution over the transfer sheet, to wet it well. The procedure is identical for both sides.
We let the PCB dry and then we immerse it in a tray with warm water. In about 10 minutes the paper will soften and we will be able to remove it easily without affecting the traces. If small interruptions or "short circuits" occur, they can be touched up with a good quality permanent marker, Edding or Schneider.
During those 5-10 seconds, the acetone reacts with the toner, making it “sticky”, and the alcohol in the solution prevents the toner from completely dissolving.
For printed wiring smaller than 80x80mm, a laminator (without heating) can be used instead of manual pressing; the PCB is passed through the laminator a maximum of 2 times. For a PCB larger than 80x80 mm, I recommend fixing it between two plywood sheets, without unevenness, pressing with a quick vice.
STEP 3 - Corroding the wiring
ATTENTION! Use protective equipment consisting of safety glasses, gloves resistant to corrosive substances and a work coat before handling and using the chemicals used! Keep a safe distance from sources of heat and/or fire before using flammable substances! Do not throw used chemicals down the drain!
The most commonly used corroding agent is Ferric chloride, it is cheap, it can be found at almost any specialty store and with 0.5l we can etch up to 5-10 PCBs depending on their size. To speed up the corroding process, it can be slightly heated in an enameled container, approx. 40 `C.
For low-complexity wiring it can be used successfully, all you have to do is pour a small amount of ferric chloride into a plastic container, preferably a tray, then immerse the PCB in it and wait about 20-30 minutes.
Personally, I gave up on ferric chloride because of its “aggressiveness” in etching for medium to high complexity wiring, plus it irreparably stains any material it encounters except ceramics and glass. I decided to try the etching method with sodium persulfate. I did a test on a small but medium complexity double-plated wiring and was impressed by the quality of the etching.
I used this etching technique for about 12 PCBs and I was not disappointed with the results. Sodium persulfate is a solid white substance, crystallized, odorless, similar to table salt, it is legally available for sale in many online stores and costs around 30 lei/500 grams.
The etching solution is prepared from 200 grams of sodium persulfate per 1 liter of warm water, 40-45 `C, leaving about 20 minutes for all the "salt" to dissolve.
I recommend using a transparent container, so that we can see through it the corrosion stage of our pcb, preferably a glass one but you can also use a harder plastic one, so that we can keep the etching solution warm. The etching takes place in about 15-20 minutes, depending on the size of the PCB, but also on the temperature of the solution.
Personally, I made a special mini-aquarium for etching from 4 mm thick glass, in which I insert an aquarium heater. I canceled its internal thermostat to be able to heat the etching substance above 32 degrees Celsius. And a pump recovered from a decorative fountain to stir the solution in order to accelerate the etching process. The etching takes place in about 15-20 minutes, depending on the temperature of the solution and the size of the PCB.
STEP 4 - Final stages
After the wiring is corroded, it is washed under a stream of cold water to remove all the solution from it. Using a paper towel or a piece of cotton wool soaked in thinner or acetone, remove all the toner from the board. The completed wiring is washed with water and detergent, then left to dry.
Now the necessary holes can be made in the board.
To protect the wiring from oxidation, we can cover it with a thin film of a diluted solution of rosin with isopropyl alcohol or acetone.