Enhance Your PCB Design with Chamfers and Fillets

PCBs typically come in square or rectangular formats. This works fine most of the time. However, in my humble opinion, 90º-only designs lack a bit of style. They could be prettier, stronger, and perhaps even a tiny bit safer with... As they say on the Internet, THIS ONE WEIRD TRICK!

Or maybe two tricks, e.g. the addition of chamfers and/or fillets!

What are chamfers and fillets?

Fillets and chamfers break, or smooth out, internal or external edges. More specifically:

• Fillet - rounds an interior or external corner so that instead of a sharp transition between two surfaces, it's more gradual. Pronounced "fill it" – not like a cut of meat.
• Chamfer - adds a second intermediate angle to a corner. This creates two new corners at reduced angles compared to the previous single feature.
• Chamfer + Fillet - after a chamfer is applied to a part, fillets can be added to further even out the transition.

You may also hear the terms break, bevel, round, and radius for these operations, which may have slightly different meanings depending on the context.

Why use chamfers and fillets? [in PCBs and beyond]

While chamfers and fillets are typically optional in the world of PCBs, there are several reasons why you might want to employ these techniques.

• Strength: Consider two surfaces attached at a 90º angle that are being pushed apart. The weak point here is the internal corner, which basically acts as the start of a crack that can expand (AKA propagate) into your thing. Smooth things out with a fillet, or add a chamfer to divide up the stress. A crack becomes much less likely.

• Safety/longevity/smooth operation: If you have a sharp external angle on something, it can poke into someone – or something – and cause damage. Such sharp angles are also subject to wear over time, and if you need to plug a PCB into a slot – e.g. an edge connector or similar – a non-90º edge can help smooth out the process.
• Looks: I much prefer the look of a nicely angled/curving part versus a simple 90º block design. Go ahead and add a chamfer or two to your next PCB. I suspect your job satisfaction will go up .5 - 1% for the day. Maybe your boss will even notice your pretty PCB work and give you a raise! Don't hold your breath.

How to implement PCB fillets and chamfers in KiCAD

KiCad includes simple tools for adding fillets and chamfers to your design. You can also draw your board outline with these and other features in mechanical CAD (MCAD) packages like Fusion 360, then import them. I go over both methods below.

Note that while these tools will work for other layers, our discussion will primarily refer to Edge.cuts in the PCB Editor.

How to use KiCad's built-in fillet tool

In the PCB Editor, select two or more lines that come together to form an angle (which doesn't have to be 90º). Right-click, select Shape Modification > Fillet Lines. Type in the fillet radius that you desire, press enter, and Voila! You now have a fillet between the lines at the specified radius.

How to use KiCad's built-in chamfer tool

Select two or more lines that come together to form an angle. Right-click and select Shape Modification - Chamfer Lines. Type in your chamfer radius and press enter. As with the fillet tool, you can select more than one intersection. You can also fillet lines after they've been chamfered for a rather elegant-looking part... Or the potato PCB outline shown below:

How to import your board outline with KiCad [fillets, chamfers, more]

First, prepare the vector graphics file that you will use as your board outline. This will depend on the CAD or drawing package, and the screencap below shows a simple DXF export in Autodesk Fusion 360.

In the KiCad PCB Editor, select File > Import > Graphics. Select the exported DXF file and modify other parameters as needed. Hit OK and place the board outline in the Editor.

If Group imported items is checked, the board outline will act as one unit when selected. If you want to make the segments independent later, right-click on the object(s) and select Grouping > Ungroup Items

You now have a nicely chamfered and/or filleted (or whatever other featured) board outline on which to place components!

The MCAD export option gives you the most versatility, and it is especially helpful if you need your geometry to match up with other elements of your design. On the other hand, there's a bit of complication involved.

Notably, Fusion 360 integrates ECAD and MCAD into one package, which allows you to keep everything updated without the same export/import steps shown here. I know and like KiCad, so I'm sticking with my unintegrated MCAD/ECAD combo, but it's something to consider when starting out.

Round out those sharp corners!

If you want to make your PCBs to work – and look – just a little bit better, don't forget to add fillets and/or chamfers. With the advent of quick-turn automated PCB manufacturing, all it costs you is a little bit of design time!

Thanks for reading!

Thanks for reading my biweekly PCB Friday column! I'm looking forward to sharing more PCB-related knowledge and insight here, and I hope you'll follow along for the journey. You can find my more semi-technical musings at TechAdjacent.io, or email me at hi@jeremyscook.com if you have any suggestions!

You can also read the addendum below 👇 for another fillet/chamfer option.

Addendum: Old way to make fillets and chamfers in KiCad

I didn't know about KiCad's built-in fillet and chamfer tools when I started this article (thx for the tip agassichan), and originally wrote up the older procedures for adding them. I included instructions below in case they are helpful for someone. If nothing else, you can appreciate the good work KiCad is doing to incrementally make its PCB design tools better and better!

Add fillets in KiCad [old procedure]

Starting from a rectangular PCB, set your grid to a convenient setting (e.g. 1mm) and select the Draw Arcs tool. Place your cursor inside the corner to be filleted by X and Y distances equal to the radius of the fillet and left-click. Move in the X or Y axis to the start of your arc – which will move in a clockwise direction from this point – and left-click. Move the cursor to the end point of the arc and left-click again. Press escape to deselect everything.

With the arc complete, move each straight side to the appropriate starting points of the arc. Left-click on one line, then left-click and hold the appropriate little white box that pops up. Move this to the corresponding endpoint of the arc and release the mouse button to deposit the endpoint in place. Repeat for the second line.

The series of pictures above shows the sequence. Internal fillets are made in the same way, except the center of the arc is on the outside of the PCB material.

Add chamfers in KiCad [old procedure]

Select the Draw Lines tool, then move the cursor to the point at which you'd like to start your chamfer. Left-click, then move the cursor to the end point of the chamfer (often the same distance from the existing edge at 90º from the first line) and left-click. Move the previously existing lines to the corresponding ends of the chamfer line as outlined in the fillet section above.