Milling breakout PCBs

Components continue to come in smaller sizes, which makes it harder and harder to prototype with unless you also buy the vendor’s evaluation module or breakout board.  With the right tools, a bit of patience, and a few tips you can build your own basic breakout boards.

The primary tool I use is a 3040T CNC mill. I purchased it on eBay for around $650.  I’ve upgraded it with a better spindle and stepper controllers, but I could get the same results with the original setup for these projects.

I use the free version of Eagle PCB tools to design my breakout boards.  There’s a free CAM tool for taking your designs and outputting machine code.  Here’s where things get interesting though.

When the tool generates the cuts, it wants to place the center of the tool at the edge of the trace, assuming the tool diameter is 0.000″ For a perfect tool, this would be great, but unfortunately you won’t be able to find a tool like this.  You will find tools that have relatively fine tips, but anything below about 0.010″ is going to be unreliable.

 

Milling with a 15-degree engraving bit.

When creating your PCB, with fine-pitch part you will want the tool to run in the areas between the traces, and not necessarily mill out each trace individually.  I’ve managed to do this by lying about the size of the trace in Eagle.

For example, a 0.008″ trace might need to be 0.020″.  This seems crazy, right?  But in the end, this causes the tool to run in between each trace, rather than cutting away a majority of what you wanted to be that 0.008″ trace.

Breakout board and module that will be connected to it.

The best bits I have found are Kyocera 2-flute 15-degree engraving bits.  Search eBay for the seller “drillman1“.  Their store happens to be a few miles from me, but either way they seem to have the best prices.  You’ll spend $8-15 on a bit which seems a lot compared to the Chinese equivalent you’ll also find online, but every time I’ve tried using one of those the tip literally breaks off within 30 seconds of starting to mill copper due to their brittle carbide composition.  Do yourself a favor and buy a good tool that will last awhile, you’ll save yourself a lot of frustration.

Fine pitch at 30x magnification

Another variable is cutting depth.  Unfortunately boards are not perfectly flat, which means you’ll end up cutting to different depths across the board.  The photo below shows the case where the top was higher than the bottom, so the tops were cut through, while the bottom still has a small amount of copper where it was engraved.  Once the copper gets that thin, it generally will tear off at the engraved spots though, so cleanup is pretty easy.

Boards are never perfectly flat, resulting in undercut areas

There are fancier tools that will account for this, but with what I have I try to find a zero point near the center, so I don’t cut too deep, which would result in cutting traces too thin, but do end up with some edges not cut completely though.  It’s better to err on the side of cutting shallow rather than deep, since you can always take more away, but it’s harder to add something back.

Mach3 Milling Software

X and Y movement rate also matters a lot.  If your CNC machine has some backlash or slosh, you will want to run the cuts slowly in order to allow the bit to keep up with the commanded position. If you run too quickly, you’ll end up with traces not very straight, and worst case accidentally cutting some portions of a trace too thin.

Assembled breakout board for a GPS module. You can see the first attempt below the assembled circuit.

Finally, it still comes down to trial and error.  I usually end up milling a portion of a design and then making some adjustments and starting again.  The most common changes are to the trace width and cutting depth.

With a bit of patience you can mill your own breakout boards.  Others have built fixtures for mounting their PCBs, which allows them to create 2-layer PCBs as well.

 

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