So, I have been slowly coming down the learning curve on milling copper printed circuit boards. I have lots to learn.
In the past, I have fabricated lots of small PCB for my electronics projects. When I do it myself, I have used the toner transfer method to prepare and etch the board. Also, I have designed a number of circuits and had them milled professionally in the US and China. DIY with toner transfer takes a lot of time and getting the pattern to stick to the copper is often problematic. Ordering fabricated boards requires shipping time and is more expensive, although still fairly cheap.
Recently, I bought a low-cost Genmitsu 3018 CNC mill with a view to routing and drilling my own boards just for fun.
My first step was to learn some new software. FlatCAM is a wonderful program to convert a PCB design into G-Code for CNC milling. Candle GRBL Controller is a simple CNC control program I have learned to use. More on these later.
After building a height map to level the PCB surface, it was time for me to carve away some copper. My cutting tool is a tiny V-shaped bit that can make cuts only 0.1 millimetres wide. But since the bit is V-shaped, the width of the cut increases as the bit cuts deeper into the copper. So I designed a test pattern (upper right) containing some pads and traces typical of a small circuit design.
Each pad is just under 2 millimetres in diameter, 1.7 mm to be exact. The copper traces between the pads are even smaller, 0.5 mm in width. The process uses isolation routing. This means, I remove copper on both sides of the pads and traces, so that the desired circuit feature is isolated. So, you make a PCB my removing the copper you don’t want, leaving only the pads and traces.
Milling Copper – Figuring Out Cut Depth
My first adventure was learning how deep to cut into the copper surface. Many web sites suggest a depth of 0.1 mm. But I found this is far to deep for use on my copper boards. Typically, I know that 1 ounce copper on a PCB is only 0.035 mm thick, at least nominally. So a cutting depth of 0.05 to 0.07 mm should be enough to clear copper and reach into the underlying fiberglass layer.
Above right, you can see my early experiments milling to depths between 0.050 to 0.065 millimetres. A continuity test with my VOM demonstrated that only the 0.065 cut was deep enough to properly isolate the copper pads and traces. Also, good news in that my caliper measurements showed that the pads and traces were properly sized, compared to my original board design.
This is pretty precise work. This depth of cut is roughly the size of the diameter of a human hair!