Have you ever thought of making a project that includes a circuit, but you ended up ordering the circuit or building it on a breadboard/solderboard? This method can help you make custom circuit boards yourself with simple components you can get in every electronics store, and a FDM 3D printer (no CNC mills or routers are required)

The general idea is to use the 3D printer firmware that can accept Gcode commands to draw our circuit on a copper clad and then etch the clad and drill it to get our circuit.

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Supplies

FDM 3D printer with Marlin

PLA filament in any colour you'd like

socket head screws (see step 1 for amounts and types)

Fine permanent marker

FeCl3 solution

single sided copper clad

Computer

painter's tape

optional:

heat inserts (see step 1 for amounts and types)

soldering iron (for heat insert)

Step 1: Turning the Printer Into a Plotter

I have seen some projects where the mask is an actual 3D printer plastic layer, but they seemed a bit sketchy, and not all FDM printers have proper settings and capabilities to do such things. Instead we are going to attach a permanent marker to our print head and use it to plot our circuit, and then etch the copper clad, as the plot protects the copper under it.

first we need to make an mount that will allow us to fix the permanent marker to the printer head. Most printer heads have a way of mounting a BLtouch (or other sensors). I suggest looking it up for your own printer, as the mounting is going to be very similar in this case. My printer (BIQU B2) has 4 mounting holes threaded for M3 and it came with a mount for a BLTouch and screws. I used three of the mounting holes, to avoid over constraining the part. If you trust your measuring skills and the accuracy of the 3D printer, you can use 4 holes.

depending on your printer, you may need different types screws. You also need at least one screw to fasten the marker to the mount. I used an M3x9 I had for that purpose. You may also use a headed threading insert to make your mount more durable and wear resistant, but I did not, and if I ever strip the threads I will just reprint the mount.

I used a permanent marker for plotting, you may use a PCB drawing pen. The general mount profile is described in the attached drawing, but you do need to modify the design to fit your pen, screw and print head. after assembling the mount on the printer and checking the fit of the pen, you may advance to the next step (do not mount your pen yet)

Step 2: Designing the PCB

Design your desired PCB. I used KiCad 8.0, it was easy to learn and use. Once you have your PCB schematics and layout, export only edge cuts and front copper layer as gerber files (.gbr files).

Step 3: Producing the Gcode

for this step use only FlatCam 8.5. Later versions will not work.

Insert both gerber files into the software. Select one of the files and in the selected tab set tool diameter of -0.5mm, width to 4, pass overlap should depend on the size of your footprints (larger footprints require more passes to cover them fully), a travel Z of 40mm and a cutting z of 35mm. FlatCam is designed for CNC engravers, and setting a negative tool size will make it draw the PCB instead of milling anything but the PCB. A cutting height of 35 is comfortable for setting up the pen in the mount, and a big difference between cutting and drawing will ensure the edge of the pen or marker does not touch the clad when traveling.

Click Generate geometry, you should see some red lines on your PCB (if they are outside the PCB outline, make sure you use a negative tool diameter). Then go to the options tab and repeat the same selections as before and click Generate Gcode and export the file to a removable drive or a memory card (depending on what your printer can use, mine can use both).

Repeat this step for the other file to get two .gcode files.

Step 4: Setting Up the Printer

Level your bed in the traditional way and clean your nozzle from any plastic stuck to it. If your printer has a filament runout sensor, insert a piece of filament to disable it.

Attach the copper clad to your printer, it should be a bit larger than your final PCB and in the approximate location that you set in the KiCad layout file (clean the clad with acetone beforehand). You can run the file for several times without the marker to make sure it is placed correctly. Use some painter's tape to fix it in place, just make sure you don't put tape over your intended PCB.

Set the printer head height to 35mm and move it to be above your copper clad. Set the marker inside the mount so that its tip touches and draws on the clad and tighten the screw to secure it in place, then lift the head by 5mm and zero X&Y. Clean your copper clad again to remove the dot you just made.

Step 5: Plotting the Circuit

Run the file from the memory card or removable drive. Your printer uses the same code to know where to go as a CNC cutter, and thus it can read the commands. If a printer refuses to run due to certain unknown commands in the file, open the .gcode file in the Text Editor and erase said commands (they usually mean the direction or speed of the spindle, motion commands are identical). If your marker does not draw well enough:

your bed might not be leveled correctly or the clad is not flat with the bed (you will notice only certain areas are not drawn well)
the height of the marker is too high, set it up again like shown in step 4.
the size of your tool in FlatCAM is too small, enlarge it by 0.1mm and try again

Picture shows the circuit plotted on a piece of paper.