Designing in Eagle CAD
I started with downloading the Circuit Playground Express board file from github. Using Autodesk Eagle software, you can open the design and inspect components, traces and the pin layout. From there I referenced the size and positions of the pads. I then created a simple layout with the exact same pin layout and fitted two 1x7 headers in the center of the board. I made sure the spacing between the headers fit a standard breadboard.
If you're new to eagle, I suggest watching a few tutorials on youtube to get familiar with the user interface and features.
CPX EAGLE Library
Once I laid out the traces on the board, I wanted to create a reusable object of the CPX pin layout so that I could easily create new shields and adapters. As a single object, it's much easier to place it into a new design instead of recreating it each time. The library can be download and added to EAGLE by opening it and adding it to a board design. It features the "device, package and symbol" configuration so it'll properly connect to nets in the schematic layout.
Feel free to download the library and use it in your projects!
Design for CNC Milling
Since I plan to mill this on the Bantam tools desktop CNC, theres a few design considerations. I suggest reading through the articles on the Bantam Tool's resource pages. They have some good tips on using vias, ground planes, trace and space widths and a handy design rule check files for eagle.
I tend to design the PCB with a single cutting tool in mind. Using a large tool can make faster cuts which can speed up the machining time. The 1/32in flat end mill is my goto tool that can do all of the traces, holes and board outline if the traces and components are spaced accordingly. In this design, I decided to make the traces thicker than usual (~39mil, 1mm, ~0.039in). This way theres less chance of the trace breaking or getting torn from excessive stress.
The pads are custom sized circles placed on Layer 17 (the pads layer) with 3mm diameter holes. Circle objects in EAGLE need a width and radius to match the pads on the CPX. A 1mm width and 2.2mm radius fit the pads pretty nicely.
- Using oblong header pins for the bigger size to make soldering easier.
- Got creative with lines to make custom emoji face
- Simplified paths for traces
BantamTools Software
The software natively supports eagle board files so you can easily add files. First, choose Double-sided FR-1 from the material dropdown. Under the size dropdown, choose the Standard option if you're using the precut material from Bantam Tools. You can optionally set your own if you have custom sized stock.
Double sided tape is the recommend method of securing the material to the spoil board. Under placement, you'll want to adjust the Z-axis input field to match the thickness of the double-sided tape you'll be using. Most double-sided scotch tape is around 0.10mm thick. This option offsets the material away from the spoil board to accommodate for the thickness of the tape.
Under Plans, click Open Files. Browse and select your board file to add it. Once opened, the board will be placed on the spoil board allowing you to rotate around it in 3D. Now we can reposition it and select our milling tools.
Under the placement dropdown of the .brd file, you can set the position of the board anywhere on the material. I suggest offsetting it by 1-2mm on both the X and Y-axis so all of the edges are cut nicely.
Parts to mill, keep "top" selected and deselect the holes and outline options because we'll be doing those on the bottom side of the PCB. It's best to take those for the last operations.
Choose the 1/32in flat end mill for the tools to mill. This will be cutting all of the operations.
By default, the trace depth is 0.15mm ( this is how deep the tool will be cutting into the material). The trace clearances (The spacing between the traces) is set to 0.15mm as well. These options can be changed under the Advanced drop down apart of the board file.
Inspect and Analyze
What you see if what you get. The 3D rendering in Bantam Tools does a great job of showing you how the traces will look once milled. If your design has any intersections or issues they'll be highlighted in red along with a message of how to resolve them.
This is a great way to spot any issues or mistakes that aren't apparent in EAGLE. My process is to check all traces in Bantam Tools, find issues, tweak the traces in EAGLE, then hit the refresh button in Bantam Tools to get instant feedback. Since I'm using a single tool, the 1/32in end mill can be a bit too big to get in tight areas so I needed to rework the traces so they have proper clearances. Being able to see changes update in Bantam Tools very quickly is really nice!
Multiple Copies
You can add as many copies of the same board file as you'd like. Just click Open Files button and select the same .brd file. I was able to fit three boards on one piece of FR-1 board (127mm x 101mm). You'll have to manually type in the X and Y axis of each board to reposition them. Each .brd file contains it's own placement, parts to mill, milling tools and advanced options. Be sure to keep track of each and double check they're consistent.
