Design Considerations for the Bantam Tools Desktop PCB Milling Machine

The Bantam Tools Desktop PCB Milling Machine is an excellent tool for making and prototyping circuit boards. However, there are some important things to consider when designing your board. This page is intended for people who already have some knowledge of PCB design. If you’re brand new, we recommend SparkFun’s PCB Basics. We’ll start with the design rule list, and then go into detail about each design consideration.

Design Rule List

The Othermill Pro and Bantam Tools Desktop PCB Milling Machine have the same design rules.

• Minimum trace width: 6 mil for Othermill Pro, 10 mil for Othermill
• Minimum trace spacing: 6 mil for Othermill Pro, 10 mil for Othermill
• Minimum via/pad annular ring thickness: 6 mil for Othermill Pro, 10 mil for Othermill
• Minimum drill size: 16 mil (or smaller with non-standard bits)
• Minimum SMD pad size: 6 mil for Othermill Pro, 10 mil for Othermill
• Polygon isolation value: 1 mil larger than the largest tool you want to use
• Route traces to through-hole pins on the bottom side only (holes are not plated)
• Slotted/oval holes are not supported

Spacing Between Traces and Components

When designing circuit boards to mill on the Bantam Tools Desktop PCB Milling Machine, Othermill, or Othermill Pro, it’s important to consider the distance between each element of your board because the spacing between elements dictates the size of the tool you’ll need to mill them.

For example, if you’d like to use a 1/32" Flat End Mill, your traces, pads, vias, etc. 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 smaller tools take longer.

The same holds true for 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 as to whether some elements are too close together, you can test it by importing your files into the software and seeing how it looks.

The spacing between traces is too small for the chosen end mill.

By selecting a smaller tool, now the chosen end mill can fit in the spacing.

If you’ll be 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 the 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.  

1. You can test if it will work by loading your .brd file or Gerber files into our software, selecting a cutting tool, and looking to see if there are red warnings 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 a 1/64" Flat End Mill. If you don't see red warning lines, then the package can successfully be milled!

2. Find the diagram showing the dimensions of the package you want to use. Find the smallest pad size, and find 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 PCB Milling Machine and Othermill Pro. If the smallest size is 0.010" or larger then the package can be milled with the Othermill. 

Keep in mind that when milling features under 0.010" it's important to use follow a precise workflow, which includes using high-strength double-sided tape, the high-precision collet set, using digital calipers to measure PCB blank thickness, and using sharp end mills. 

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 will not 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" will allow you to use a 1/32" tool for most of the board, which is much faster than using a 1/100" for the whole board.

Trace Width

The minimum trace width we suggest is 6 mils (0.006") for the Bantam Tools Desktop PCB Milling Machine and Othermill Pro, 10 mils (0.010") for Othermill because thinner traces are more likely to delaminate from the board. We suggest using the widest traces your design can support.

Likewise, annular rings for through holes and vias should be at least 6 mils for Bantam Tools Desktop PCB Milling Machine and Othermill Pro, 10 mils for Othermill (outer diameter minus inner diameter).

Slotted/Oval Holes Not Supported

The 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 (i.e. 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 mill faster than smaller ones, so you can increase milling speed by spacing out your components, traces, and holes such that they can all be milled or drilled by a 1/32" or larger end mill.

Minimize the use of 1/64" and 1/100" 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.

Still want to go faster? Check out our Custom Tool Library guide to learn how to change the feeds and speeds.

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 the EAGLE page.

If you’ve made DRC files for your favorite software, please email them to us at support@bantamtool.com and we’ll post them here!