I designed my first PCB when I was 17 years old. I worked for Great Plains Research and Development and we had a project involved with the flight-loading computer for the B1B bomber (nothing top secret, I promise). I had to design a 32-kbyte static RAM memory card for the computer.
It involved four static-RAM chips and an address decoder. This simple board took me a week to design because of the way we worked back then. Instead of computers and Gerber files, I started by drawing the circuit lines with red and blue pencils: red for the top and blue for the bottom. Then I used clear Mylar film and black tape to go over the hand-drawn lines to make a positive image at a scale of 2:1. A quick-turn PCB took two weeks and cost thousands of dollars. Today, I can create a ready-to-use PCB in a matter of hours. That just amazes me.
I thought my process and equipment might interest many of you, so I decided to write about them. I use three tools for rapid prototyping: a PCB-design package, a program to convert Gerber files to G-code, and a CNC mill with a special engraving bit.
For more than a decade, companies such as T-Tech and LPKF have offered specialized versions of CNC mills for PCB prototyping, but they are expensive and outside the range of most personal budgets, and definitely too expensive for me.
Last year, I bought an brand-new, inexpensive general table-top CNC mill on eBay for around $800. Unlike the T-Tech ad LPKF machines, this Chinese-made CNC has no integrated software, so it doesn't offer a straightforward way to mill PCBs. My CNC mill works with a control package called Mach3, which is powerful and inexpensive.
PCB design package generally create Gerber files and NC drill files. A Gerber file includes a set of instructions used to create an image of the PCB layout on a plotter manufactured by Gerber Technology, hence the name "Gerber file." The NC drill file comprises a list of drill tools needed to bore holes in a PCB and a list of drill locations.
But the Mach3 software does not directly process Gerber files. Instead, it executes G-code, used by many numerically controlled machine tools. So I had to write or find a program to convert the Gerber files to G-code programs for my milling machine.
Some PCB packages have software plugins that create a G-code program -- mine does not. After looking for quite awhile, I found a program called CopperCAM. It does exactly what I need. Figure 1 shows a PCB design loaded into CopperCAM.
Setup is easy. I load the Gerber files for the top and bottom layers of the PCB and the NC-drill file for the drill point. Then I select one of the traces on the top or bottom layer that represents the outline of the board. This trace becomes the routing parameter for the board.
Two software tools within CopperCAM create the engraving paths for removing copper. The first CopperCAM tool (Figure 2), outlines the traces to separate them from the remaining copper. But the "dead copper" remains on the PCB. To remove it, I use the second CopperCAM tool (Figure 3).
Finally, the CopperCAM software generates the G-code programs. I usually create four of them: top engraving, top drill, bottom engraving, and outline routing. Then I load the G-code files into Mach3 and mill the board.
The milling and drilling operations need at least three milling tools:
- A 25- or 30-degree engraving bit (Figure 4, left).
- A a routing bit (Figure 4, middle); I use a eighth-inch flat-end mill bit.
- Special PCB drill bits (Figure 4, right) created specifically for PCB drilling operations.
The topic of quick-turn prototyping PCBs deserves its own book, but I hope this abbreviated article helps you understand what you could do. If you would like to see this process in action, please view this YouTube video. Figure 5 shows my finished board, populated with parts and inside its prototype plastic enclosure.