Whenever you mill a PCB, you need to choose tools that are the appropriate size for the features on the board. Choosing tools that are too large can result in some areas not being milled, while choosing tools that are smaller than necessary can increase milling time. The software’s unmillable area warnings help you identify when you may need to choose a smaller tool.
Bridged traces and skipped holes are two of the challenges inherent in milling PCBs. This guide helps you understand, recognize, and avoid bridged traces and skipped holes.
What are bridged traces?
Traces are the “wires” on the surface of a circuit board that connect electrical components together. The milling machine makes traces from FR-1 by milling away the copper that surrounds the traces — this isolates them electrically from the other components and features of the PCB. Precisely connecting components to each other in a specific configuration is the purpose of a PCB; traces are the wires that make this possible. PCB design files specify the width of traces, as well as the distance between traces (often called “trace-and-space”) to ensure that components are connected correctly.
By inspecting the software’s 3D preview and looking for copper connections that shouldn’t exist, you can see that the traces on the left are bridged and the traces on the right are properly isolated.
If two or more traces are connected to one another but aren’t supposed to be connected, they’re called bridged traces. Bridged traces generally occur because of a manufacturing or design defect. At best, bridged traces can cause circuits to work incorrectly, and at worst they can cause damage to components.
Although we’re using the word “traces,” the concept of bridging also applies to pads and vias — any part of the PCB that conducts electricity has the potential to be bridged. To keep things simple, in this guide, we use the phrase “bridged traces” to mean “bridged traces, vias, or pads.”
What are skipped holes?
Holes, sometimes called drills, are openings on a circuit board used to connect wires and through-hole components. They’re also sometimes used for vias that connect the two sides of a double-sided board together.
If the diameter of a hole is narrower than the selected milling tool, it can’t be milled without damaging other features of the PCB. In this case, the software will not mill the hole. This can result in a non-functional PCB.
What causes bridged traces and skipped holes?
Trace bridging is caused by milling your PCB with a tool that is too large. If you’ve selected a tool that is too large to fit between traces, the software will not mill that area. In other words, the software only attempts to mill regions that are large enough to mill with the tool you’ve selected.
Similarly, if the diameter of a hole is narrower than the selected milling tool, it can’t be milled without damaging other features of the PCB. In this case, the software will not mill the hole.
Identifying Bridged Traces
Import your board and select the milling tools. In the 3D preview, look for the smallest features on your board. If those features are not shown, they won’t be milled.
Trace bridging in action: On the left, an Otherduino is configured with a 1/16" flat end mill, and on the right, with a 1/32" and 1/64" flat end mill. Many traces on the left show bridging. The bridged trace warning is shown in the plan setup panel; bridged trace indicators appear on the 3D preview.
When looking at the software's preview image, you may be warned about potential unmillable areas. Look for the warnings in the plan setup panel.
If unmillable areas are detected, you’ll also see red outlines on the 3D preview. These lines show you where the bridging is expected to occur — the regions of copper that should be cleared but aren’t are outlined. If you’re having trouble seeing whether the areas will be milled or not, you can try selecting “Increase preview resolution” in the View menu.
Preventing Unmillable Areas
The simplest way to prevent unmillable areas is to select a smaller tool. For instance, if you’re using a 1/32" flat end mill, try adding a 1/64" flat end mill as well. If you select more than one tool, the software will use the smaller tool to mill the details and the larger tool to clear the bulk of the material. For bridged traces, you might also try using an engraving bit instead of a flat end mill, as engraving bit isolation milling is a fast way to mill precise PCBs.
If you don’t wish to use a smaller tool, you’ll need to modify your PCB design so that there’s more space between the traces or so the holes are larger. Make sure that the space between each feature and the diameter of each hole is at least as large as the diameter of the tool you wish to use.
What if I get an unmillable area warning but don’t see bridging?
We’re continually refining the algorithm that the software uses to find bridged traces. As a result, false positives are possible. If the bridged trace warning or indicator outlines appear when traces aren’t bridged, please let us know. Email us at email@example.com and include your design files. We’re happy to help!