CNC Milled PCBs

Cutting Paths

FlatCam is my choice for converting Gerber files to gcode. I prefer to manually place cutting paths rather than use the automatic isolation routing. Although this is time consuming, it lets me optimize the tool paths with shortcuts or using different tools for different areas. The final gcode file is much smaller, the process completes faster, and there is less wear on the tool.

Double Sided Boards

Getting the alignment right when flipping the board can be a major problem. What I do is place a via along the bottom edge of the PCB at the exact X-axis centre, and shift the imported Gerber file in FlatCam such that this X-axis centre via aligns with the coordinates (0,0). I mirror the board across the Y-axis about the point (0,0) before creating the tool paths for the underside of the board. After flipping the board, the via can be used to re-align the work origin at (0,0). Provided the x-axis is not angled, the mirrored paths should align.

Un-Plated Holes

Where electrical continuity is needed at holes between the top and bottom layers, wire such as 33AWG can be helpful. Remember that most header pins or connectors are soldered from the bottom of the baord, so if continuity is needed to the top side, thin wire should be inserted and soldered in the hole first before the connector is fitted. Or alternatively, design the board so that connectors do not need to be electrically continuous to the top layer.

Attention should always be given to ensuring extra copper space around the holes to account for alignment errors when the board is flipped. I generally use 70mil diameter pad and a 31mil hole at vias to suit 1/32 inch tool diameter. A mis-alignment of up to 10mils is typical.

Sub-Circuits

I have produced a number of sub-circuits to make it easier to add complex and tiny components to a CNC milled prototype. This also lets me re-use parts across different iterations of the design.

Challenges

Milling your own PCBs can be challenging. I've found the following problems:

• Milling PCBs can be time consuming ensuring preparing cutting paths and correcting alignment for double-sided boards.
• A simple mistake might mean the whole process has to be repeated.
• Replacing worn or broken milling bits is expensive.
• If the board is not flat, or the CNC machine does not support levelling, then you will need to cut the paths deeper which leads to excessive wear on the tools.
• Wire needs to be added to all vias and connectors where top to bottom connectivity is required.
• The board needs to be bigger to account for lower tolerances in track and via sizes.

I have enjoyed creating my own boards, but generally it is far easier to have them manufactured by board fabrication specialists when you can manage shipping and time costs.