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Bench Talk for Design Engineers

Bench Talk


Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

Milling About: CNC Routing a Circuit Board Mike Parks

Figure 1: A milled circuit board


Breadboards are fantastic for experimenting with a circuit design and testing components. Eventually, you will want to move to a more robust solution for a circuit that is intended to escape the confines of laboratory and bravely venture into the real-world.


I have tried and used many solutions throughout my engineering career and weekend tinkering; each with their own set of tradeoffs and end results. The first solution many of us would turn to was perfboard, which is a relatively inexpensive and simple solution for a first generation off-breadboard prototype. Perfboard provided an opportunity to fine-tune one’s wire-wrapping and soldering skills.


Chemical etchant based circuit boards were the next option for someone looking for more professional looking boards. The cost would be significantly more than perfboard since there is an initial investment for a computer and CAD package to create the PCB design. You would also need a laser printer, a clothing iron, and the consumables for the printer as well as the etchant itself. The process was dirty and potentially hazardous, but the results were relatively solid.


Fast forward to today and many circuit capture and PCB design tools, such as MultiSIM BLUE, have built-in features that let you send off a design to a PCB manufacturer. For a relatively low cost and a wait of a few weeks, you can have professional boards in your hand ready to be mated with electronic components.


As mentioned before, each of these options has a series of tradeoffs:


1.     Cost: Perfboard being very inexpensive; chemical etchant requires significant upfront costs, and PCB manufacturers are typically somewhere in the middle.

2.     Professional Appearance / Reliability: No doubt utilizing a PCB manufacturer will yield the best results aesthetically and functionally, assuming the design is sound.

3.     Speed: Perfboard and etchant are both quick turnaround; whereas there can be significant wait times involved for professional PCBs to be delivered.


Figure 2: An example of a desktop CNC machine

This has led me to begin experimenting with using a CNC machine to mill circuit boards. What is a CNC machine you ask? First, CNC stands for Computer Numerical Control which is a fancy way of saying a machine that is controlled via a computer that sends numerical commands to control the machine. You can think of a CNC as a machine that performs the inverse of the more in vogue 3D printers which employ additive manufacturing techniques which adds a small amount of material, layer-by-layer, to bring an object into existence. In contrast, CNC machines are subtractive - starting with a large block of raw material that is slowly removed by a quickly spinning router, with a milling bit tailored for the material being milled.


I am a few board designs in, and so far the results have been surprisingly good when utilizing FR4 copper clad boards. In most applications, I limit myself to boards no larger than 4” by 6” and I am constrained by the fact that you can only build boards with two signal layers. After experimenting with a variety of design parameters such as trace widths, I have found that I can reliably mill traces down to 20 mils.  This means when I am looking for parts on Mouser that I can utilize surface mount components as small as a 1206 package for resistors and capacitors. However, I seem to have better reflow results when I go a bit bigger with 1210 (3225 metric) or 2010 (5025 metric) footprints. To be sure, this is not the end-all-be-all solution. Professional PCBs still have the advantage of many more signal layers, tighter trace widths, smaller components, and silk screening for easier to understand board layouts.


On the other hand, milling a PCB is fast. No waiting weeks for board delivery from an overseas manufacture, and unlike chemically etching a PCB I don’t have to manually drill holes for through hole components. I can just let the CNC machine drill the holes for me after switching out the milling bit for a drill bit. In just a few minutes, I can take a circuit board from one’s and zero’s on a computer to having a functional board in my hands. For those of us who have spent years in our favorite CAD tools designing PCB, perhaps the best news is that there is little change to the workflow needed to get a milled board versus using the toner transfer method. Instead of printing the circuit you simply need to run the vectorized image of your circuit board through a tool that converts SVG image files into the gcode cutting paths that are spoken by CNC machines.


While a professional design studio with capital can buy a CNC machine pre-built for a few thousand dollars, a Maker can build their own using components you can find here at Mouser, including stepper motors and an Arduino that can serve as the CNC “brains”. There is even a CNC control shield that makes wiring together the H-Bridge motor controllers very easy. Really, the electronics are really that simple. The software needed to run a homebrewed CNC machine is open source and available for free. Furthermore, I have found building a CNC machine is much more forgiving of a task than building a 3D printer that has much more sensitivity to things like bed leveling and tight Z-axis control.


So, for what specific use cases have I found milling a PCB useful? I am personally enamored by the notion that I can create a much more robust prototype that looks better and is more tolerant to real-world handling than a breadboard-based prototype. I am building quite a few interface boards so that I can connect many external components to a DIP package microcontroller. Another side benefit is that by actually building the board at home I can ensure that the electrical connections are correct before I send my Gerber files off to be manufactured for a final product.


Bottom line, milling your own PCBs is pretty cool. It’s not for everyone, but for those looking to find a happy middle between homebrewed and professional you might just find pretty amazing results with milled circuit boards. Of course your results will vary based on skill and tools available, thus if at first you don’t succeed, experiment with different component sizes and techniques. 


Have you ever milled circuit boards?  What tips and tricks do you know? Share them with us by leaving a comment down below.

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Michael Parks, P.E. is the owner of Green Shoe Garage, a custom electronics design studio and technology consultancy located in Southern Maryland. He produces the S.T.E.A.M. Power podcast to help raise public awareness of technical and scientific matters. Michael is also a licensed Professional Engineer in the state of Maryland and holds a Master’s degree in systems engineering from Johns Hopkins University.

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