In 1940, two brothers opened a walk-up restaurant featuring a simple menu of hamburgers and french fries offered in disposable paper wrappings. Their goal was to serve food quickly, efficiently, and less expensively than a sit-down restaurant, and their name has become synonymous with fast food ever since. You might have heard of them, their name was McDonald.
Just as the goal of fast food is to quickly provide you with something to eat, the goal of a PCB prototype is to quickly get you a working version of your design. The problem though is that PCB prototypes have traditionally been available only from outside manufacturers, or complicated do-it-yourself etching kits. Thankfully this situation has now changed as low cost milling machines have opened up another option for designers. We’ll take a look here at CNC prototyping, and how it may be a better choice for your next PCB project.
What is PCB CNC Prototyping
As you know, CNC milling machines are already used in the PCB manufacturing process to route, or cut, around the board outline. They can also be used for cutouts in the board itself. Taking this a step further, the milling can also be used to remove copper from the surface of a circuit board in a process known as isolation milling. The milling will remove areas of copper on the surface of a copper covered substrate in order to create standard circuit board features such as pads, traces, fills, and vias. Once completed, you will have a hardware prototype to work with.
Prior to CNC prototyping, a circuit board would either have to be breadboarded, etched in-house, or sent out to a manufacturer:
Breadboard: Creating a circuit on a breadboard has been the standard method for circuit development used by hobbyists, students, and engineers alike. Breadboarding, however, does have some drawbacks when it comes to replicating faster and more complex circuits. Milling a PCB prototype though can create a working circuit board relatively quickly, and structured more like the eventual production circuits than a breadboard.
Etched: Another method of producing a quick and simple prototype is to etch it yourself. There are kits to do this, but no matter what you are going to be dealing with chemicals and the complexities of the etching process. CNC milling, however, can be accomplished in a typical office setting without exposure to hazardous chemicals and will produce higher-quality boards for less money than etching.
Out-source manufacturing: Sending out a board to be fabricated will take time and each board will have a higher unit cost then if you were to mill it in-house.
It is important to note that PCB CNC milling is restricted to two-layer boards without plated thru-holes. You will either have to have the boards platted, or simply solder the holes while stuffing the board. The CNC process is not intended to replace the mass production of PCBs through the standard etching processes. But for a quick and inexpensive prototype, a CNC produced board can offer you a lot of benefits.
CNC prototyping of your small PCBs can be a quick an inexpensive fabrication solution
How Can This Type of Prototyping Help My PCB Design?
Although you can use outside CNC prototyping services to create a milled PCB, you also have the option of purchasing your own CNC milling machine. A quick search of one the larger online shopping services shows that a simple desktop CNC machine that can produce PCBs, will go for between two and three hundred dollars. With copper covered substrates being as inexpensive as they are, this can result in a huge savings for you when producing a prototype. It also releases you from the requirement of working with in-house etching processes which are messy and require achieving the right balance of chemicals for the best results.
Here are some of the benefits that you can expect to get with your own CNC setup:
One machine will do it all; the isolation milling to create the board features, the regular milling around the board outline, and the hole drilling.
Turn around is much quicker when you don’t have to wait for an outside manufacturer to fabricate and then ship the board back to you.
The process is repeatable once you set the design up.
You won’t have the expense and mess of stocking, mixing, and using the chemicals of the etching process.
Like anything though, there are some trade offs with using the CNC rapid prototyping process to fabricate a board:
Once you design your board, you will have to set up the milling machine for that board. This is not a difficult step, but it will have to be done.
The milling process is not instantaneous, it will take a little time to remove all of the copper that needs to be eliminated.
There will be copper and fiberglass debris from the milling process.
Drill and milling bits do wear down with use and will have to be replaced. These can be expensive.
You can’t specify design spaces on the board that are smaller than your drill bits, or the accuracy of the CNC machine. This requires some forethought during design, which can be accomplished easily by setting your design rule spacings to the correct values.
Once you understand the process of a CNC machine however, these caveats will become second nature to your work. The next important step now is understanding how to use your design tools to drive the CNC process.
With a full set of design rules & constraints, you can configure your design for CNC prototyping
What Type of Tools Will Drive PCB CNC Milling Machines?
As long as your PCB design tools will create standard Gerber and drill files, you will be set up for producing the GCODE formatted files needed for the CNC milling machine. This step can be easily accomplished with third party open source software that will accept standard photoplotter Gerber files as input. Once you are working within this software, you will have options to assign different tool paths for milling, cutouts, or isolation milling. You will also assign parameters that will define cutting depths and drill sizes, and then generate GCODE for the CNC machine.
As a PCB designer, you will want the best PCB design system for all of the different design rules and setup options available to you to produce manufacturing data for CNC prototyping. Allegro PCB Designer is a CAD system whose tools will perfectly fill this need. Allegro features a full and comprehensive set of design rules and constraints that will give you complete control over all of the spacing requirements you need for your next prototype board.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.