When designing circuit boards to mill with your Bantam Tools Desktop CNC Milling Machine, there are some important things to consider. This guide is intended for people who already have basic knowledge of PCB design.
This guide covers:
- Spacing between traces and components
- Surface-mount components
- Vias
- Ground/power planes
- Traces
- Slotted/oval holes
Note: If you’re brand new to PCBs, we recommend SparkFun’s PCB Basics.
Spacing Between Traces and Components
It’s important to consider the distance between each element on your board because this spacing dictates the size of the tool you’ll use to machine your PCB. For example, if you’d like to use a 1/32" flat end mill, your traces, pads, vias, and through holes need to be greater than 0.03125" apart. You can use smaller tools, like a 1/64" or 1/100" flat end mill, which will allow you to place objects closer together. However, the trade-off is that milling with smaller tools takes longer.
The same holds true for milling your vias and through holes—the inner diameter of each hole needs to be larger than the tool you’re planning to use to drill it. If you’re unsure whether some elements are too close together, you can import your files into the software and see how it looks.
We suggest using the widest traces, through holes, and vias your design can support.
Surface-Mount Components
Likewise, if you’re using surface-mount (SMD) components, the pads need to have enough space between them for the tool to fit. Check the list of millable packages in our Surface-Mount Package Compatibility Guide to make sure the package you want to use can be milled.
If your package isn’t listed, you can determine if it can be milled in one of two ways.
First, you can load your EAGLE (.brd) file or Gerber files into the Bantam Tools software, select a cutting tool, and look to see if there are red warning lines. The red warning lines indicate that the design feature is too small to be milled with the currently selected cutting tools. Try selecting a PCB Engraving Bit 0.003", a 1/32" flat end mill, and/or a 1/64" flat end mill. If you don't see red warning lines, then the package can successfully be milled!
Alternatively, find the diagram showing the dimensions of the package you want to use. Find the smallest pad size and the smallest space between features. If this smallest size is 0.006" or larger, then the package can be milled with the Bantam Tools Desktop CNC Milling Machine.
Keep in mind that when milling features under 0.010" it's important to follow the precise workflow that utilizes:
- Removable spoilboard
- High-strength, double-sided Nitto tape
- Digital calipers to measure your PCB blank
- Sharp flat end mills and/or PCB engraving bits
Vias
If your board has vias (electrical connections between the front and back of the board), you’ll need to create a connection between them by hand when you solder the board. For this reason, we recommend minimizing the number of vias and through-hole connections you use in your board.
The simplest way to connect a via is to put a short piece of wire through the via hole and solder both ends, but you can also use via eyelets (aka rivets), which sit neatly in the holes as you solder them. Just make sure your eyelets fit your holes. A third option, which is often the fastest, is to take a long piece of tinned wire and thread it through all the vias in your board. Then solder all the vias and snip off the excess wire.
PCB software autorouters will often use through-hole pins as vias. This presents a problem when you use components that can’t be soldered easily on both sides of the board, such as pin headers. To avoid this issue, draw polygons in the Restrict layers over the areas you want the autorouter to avoid. In EAGLE, for example, these are the tRestrict (41), bRestrict (42), and vRestrict (43) layers.
Ground/Power Planes
If your file is designed with a ground or power plane, check to make sure that your polygon isolation value is set to at least the width of the smallest tool you’re going to use on the milling machine. If the isolation is set to zero, which is often the default in PCB design software, the tool won’t fit between the ground/power plane and the traces.
If your design allows for it, setting the polygon isolation to the width of your largest tool will greatly speed up the milling process. For example, if your design has a few tiny features that need to be milled with a 1/100" tool, but most of the other features can be milled with a 1/32" tool, setting your polygon isolation to 0.032" instead of 0.011" allows you to use a 1/32" tool for most of the board, cutting down your machining time significantly.
Trace Width
We suggest using the widest traces, through holes, and vias your design can support.
Slotted/Oval Holes Not Supported
The Bantam Tools software doesn’t support the G85 command, which is used to make slotted/oval holes. If you need a slot, make a row of overlapping holes that are a few thousandths of an inch larger than the tool and overlap by about half their diameter. For example, for a 1/32" (0.031") tool, make a row of 0.033" holes, with centers spaced 0.016" apart.
Ways to Increase Milling Speed
Larger end mills machine faster than smaller ones, so you can decrease your machining time by spacing out your components, traces, and holes so they can all be milled or drilled by a 1/32" or larger end mill.
Minimize the use of a 1/64” or 1/100” flat end mills by using rounded corners instead of angles. The software takes time to mill the innermost angle, where a 1/32" can’t reach, which is helpful in some situations but not here. The corner radius needs to be larger than 0.03125" for this to work with a 1/32” end mill.
To go even faster, you can customize speeds and feeds for your tooling using the Bantam Tools software’s Custom Tool Library.
DRC (Design Rule Check) Files
If you’ve never used DRC files, they help you find layout-related problems with your board, such as components or traces that are too close together.
We currently have DRC files for EAGLE, which you can download from our EAGLE guide.