1 - Photoengraving¶

This tecnique is used to transfer the pattern of the circuit to a board with two cooper layers using UV light. The circuit for the two copper layers, front and back, has to be printed in transparencies and atached in a way that you can put inside the board. The light is going to react with the resin that covers all the board except the zones just under the desired circuit. The following picture shows the transparecies attached together and the UV machine in the Department of Electronic Technology (DTE) in University of Vigo. Since the black ink of our printer is not black enough for the UV we print two transparecies by side (therefore we use 4 in total).

The next step is to switch off the lights of the room and remove the protective film (blue film in the previous picture) of the board. The light can not be switched on until the end of the process in order not to damage the photosensitive resin. Put the board inside the transparencies. Handle it with care in order not to touch, as much as possible, in order not to pollute the copper.

After that, put the board in the insolator and program 150 seconds of light.

When the photoengraving has finished, the insolator can be openned and the board taken outside the transparencies. Do not switch on the light yet.

2 - Developing¶

In order to eliminate all the resin that have reacted with the UV light, a developing solution is used. There are many commercial developers. The most common ara bags with small stones that are diluted in water.

Put the solution in a plastic tray. The plastic doesn´t react with the quemical developer. The quantity has to be enough to cover all the board. It is very important to keep the lights switched OFF during this step.

Once you have the tray full, put the board inside until you see the design of the circuit. It is crucial do this step correctly but leaving the board in the developing solution a time above the necessary has not bad consecuencies to the process. In the UviSpace’s PCB’s manufacturing process a time of 5 minutes was used. After that time the lights can be switched on. The board should look something like this:

During the development process it is important not to leave the board on the floor of the tray because the solution has to touch all the parts of the board. It is also recommended to move the tray during the process and generate some waves.

With the appropiate plastic gloves, the board can be rub few times to help the developer remove the resin.

To finish with this step, the board can be softly cleaned with tap water.

After a visual checking, if a development error is detected, the board can be introduced back again in the tray.

The development solution can be used again to process other boards or can be stored back its bottle. Developing solutions can be reused several times.

3 - Etching away the copper¶

In order to etch away the copper of the PCB, a quemical solution has to be done and put in a plastic tray. The plastic doesn´t react with this quemical solution.

Classical recipe:

• 40 % of H₂O
• 30 % of H₂O₂
• 30 % of HCl (concentration 20%)

In the DTE laboratory a more concentrated HCl concentration is used, therefore the recipe is updated to:

• 55 % of H₂O
• 30 % of H₂O₂
• 15 % of HCl (concentration 40%)

The order of the mixing process is very important and is the order that appears in the reciepe. That is, firstly put the H₂O in the plastic tray, after that put the H₂O₂ and finally the HCl.

The following picture shows the H₂O₂ used (left) and the process of mesauring the volume needed using a graduated cylinder (right).

The following picture shows the measurment of the HCl in the graduated cylinder (left) and the process of filling the tray (already with water and H₂O₂) with HCl (right).

Put the board in the tray and wait until the solution removes the cooper. It is important for the PCB not to touch the bottom of the tray as it can damage the resin on top of the cooper and the solution can remove cooper from where it was not suposed to. You can use a piece of plastic for this task like in the following picture:

Do this process under a extractor hood as the gas emanated by the process is toxic. When all the traces seem to be clearly separated from each other the board can be removed from the solution and cleaned with water. At this moment the PCB can be visually checked. In case any of the traces were wrong it could be re-introduced in the quemical solution to continue the etching process.

When the board is ready it should be cleaned with water and some soap to stop the chemical reaction. At this step the board should look something like this:

The quemical solution has to be deposited in the proper quemical container for recycling. Do not spill the mixture through the sink.

The copper is still protected to the ambient conditions for a resin layer. It has to be removed only when the components were ready to solder in order to not allow copper to rust.

4 - Drilling component holes and vias¶

In general terms, the appropiate moment to drill all the holes of the PCB is before soldering the components. In the moment the PCB has no components, nothing can be damaged.

5 - Soldering components¶

Now it’s the turn of placing components but before doing that, the resin layer that protects copper has to be removed. This is quite an easy process. Using acetone and a piece of paper towel, remove all the resin from the PCB. Be careful and not scratch the copper surface.

The board is now ready to recieve all the components and vias. First of all, solder the smaller or more complicated component as small SMD integrated circuits with many small pins. Secondly, solder the vias. Introduce some wires and solder one side of the board. After that, solder the other side, and finally cut the surplus wire. Finally, solder the rest of the components both sides.

6 - Varnishing¶

After soldering all the components and test that the board correctly worksy, it is time to to protect the copper from the ambient. To do this, in the DTE, specific barnish spray was used. Be aware the varinish is not conductive so protect the parts of the design that you need a contact like probe poits, etc. This can be made using paper tape.