Design rules

Set the via and drill hole size to something you can drill and rivet.

• minimum clearance: 0.3mm
• minimum track width: 0.8mm

The minimum track with is a bit wider than SMD footprint pads for SOT-23 and SOT-8, but those can be used without issue.

Panellisation

Optionally, you can panellize several boards to etch several designs on one piece of feedstock. To do that:

• Create a new separate kicad project.
• open the board editor and choosing "Append board".
• Group everything in the board (right click selection, group) and select 'create from selection' 'create array'.

Generate SVG files

Kicad export settings

Generate bitmaps

I created this bash script I call prepare.sh that uses the ImageMagick 'convert' command line tool to do the boring work of inverting, mirroring and centering the design on the pcb feedstock. Somehow the svg's from KiCAD don't seem to render correctly in LightBurn.

#!/bin/bash
set -ex
echo "$1 board size x (mm)"
echo "$2 board size y (mm)"
echo $3 - input
echo $4 - output
border_cm=0.05
x_mm=$1
y_mm=$2
infile=$3
outfile=$4

echo "$cm - substrate size (rectangle)"
dpi=3000
echo $dpi - dpi
x_pixels=$(python -c "print(int(round($x_mm* 0.1* $dpi / 2.54)))")
y_pixels=$(python -c "print(int(round($y_mm* 0.1* $dpi / 2.54)))")
border_pixels=$(python -c "print(int(round($border_cm* 1.0* $dpi / 2.54)))")
echo pixels: $y_pixels $y_pixels

#render as bitmap
inkscape "$infile" --export-filename="exported.png" -d $dpi
#remove transparency
convert exported.png -format png -background white -alpha background -alpha off nontransparent.png

#invet colors
case $infile in
      (*_Cu*)
        echo image should be inverted
        convert nontransparent.png -channel RGB -negate inverted.png
      ;;
      *)
        #do nothing
        cp nontransparent.png inverted.png
      ;;
esac

convert inverted.png -gravity center -extent ${x_pixels}x${y_pixels} -background white "$outfile"
case $infile in
      (*-B_*)
        echo image should be mirrored
        convert "$outfile" -flop "$outfile"
      ;;
esac

#cleanup
rm exported.png
rm nontransparent.png
rm inverted.png

This wrapper script then calls the previous script for each svg file:

#!/bin/bash
set -ex
for file in *.svg
do
    ./prepare.sh 51 71 $file "${file%%.*}".png
done

Create a LightBurn project

• Create a contour layer and draw a rectangle the size of the board

• Set the contour coordinates

  • Important here is that you do this so that the laser head will move out of place when it is done so you can access the board area easily. I use x=80, y=110.

• Create a handles layer with cutouts to grab under the board to remove it (note the half circles on my cardboard photos).
• Switch to a new layer by clicking the colored layer numbers on the bottom side of the screen
• Drag a png to the application canvas

• In the image/layer settings

  • disable bidirectional scanning
  • set DPI to 300
  • set image mode to threshold
  • set speed to 10
  • set power to 100%
  • you can click set defaults to avoid setting these every time.

• Repeat the steps for the png images for F_Cu and B_Cu (and F_Mask, B_Mask if you will use soldermask)
• Align all layer images by setting the XPos and YPos to be consistent with the contour rectangle

lightburn layers

Prepare the machine for engraving

• Cut a piece of cardboard
• Fix the cardboard on the worksurface, I used these 3d printed pins enlarged in PrusaSlicer at 138% size to hold the cardboard onto my honeycomb bed.
• Laser-cut out the shape of the PCB stock and handle holes out of the cardboard.

Engrave the board

• Make sure that in the cuts/layers sidebar, the only output switch is the layer you want to cut/engrave
• engrave side 1
• flip the board
• engrave side 2 (mirrored vertically by the script)

Note that with these settings, the example board of a few square centimeters takes ~20minutes. There is room for experimentation to optimize these settings.

lasering in progress

Etch the board

Drill a hole in the board to hang it in the etching tank.

etched front side

etched back side

Design rules

For text I used 1.5mm font size, 1.0mm was barely readable. If you want to make sure lines are readable, Use a 0.2mm width. If you use soldermask paint that hides tracks, it's best that you also export the 'Fab' layers which contains outlines of components. However the KICAD footprints overlay the lines on top of pads sometimes. You can try modifying the footprints and removing those if you tweak the footprint to make the lines thicker.

Lightburn design preparation

• Add the silkscreen image to the project

• Double check the design rules enforced that no silk screen got on the solder pads.

  • You could also use the 'boolean subtract' function to fix this in lightburn

• right click the image and select Trace image.
• On the Cuts/Layers sidebar, select 'Fill' mode for this layer

• Double-click the layer in the Cuts/layers list to bring up the 'Cut settings editor'

  • Set speed to 5 mm/sec
  • Set Max Power to 30%
  • Disable bi-directional fill

Painting and drying

Apply painters paper tape to the board and engrave the image.

Applying paint

Next we add the acrylic paint, and spread it using a spatula.

Applying paint

I put it on my Prusa 3D printer heatbed at 85 degrees for 15minutes, but I'm not sure what the perfect drying process is.