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In the recommended layout for USB4500 by GCT (a mid-mount USB-C receptacle) there are these unexplained circular cut-outs on the PCB edge. What are these for? Are they needed? There are no dimensions available, so I don't know how to reproduce them.

Recommended PCB layout for USB4500

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2 Answers 2

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The cutouts are specified because the milling cutter has a non-zero radius so cannot cut a sharp inside corner.

Extending the cut ensures that there will be no interference. This is a standard technique in machining (for example to machine a pocket in a mold that will accept a sharp-cornered rectangular insert). Of course there are ways such as EDM to make a sharp-edged pocket, but that wastes time and money if it is not required. Just as you could file the inside corners of a few boards if you had to.

The optimal dimension will depend on what cutter size your PCB fab prefers to use, hence they don’t make a suggestion that could be wrong. In any case, the minimum cut extension to fit a square corner would be the radius of the cutter, and they've also rounded the exit. Scaling they have assumed a cutter of around 0.6mm diameter, considerably smaller than the default of some PCB houses (eg. 2 mm Ø for Eurocircuits).

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    \$\begingroup\$ Note that in general, when inside corner like that are needed, it is considered good design practice to radius the corner a bit. This serves to reduce stress concentrations that can lead to cracks at the corner. \$\endgroup\$
    – SteveSh
    Commented Apr 1 at 11:35
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    \$\begingroup\$ @SteveSh There's no option not to have a substantial radius with milled boards. With punched boards do you have a guideline for corner radius of punched rectangular holes? I don't recall worrying about it with high-volume hot punched paper-based phenolic substrates (the punch and dies will have a bit of radius anyway). \$\endgroup\$ Commented Apr 1 at 12:36
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    \$\begingroup\$ In the CNC world, these features are generally known as "dog bones" or "t-bones". \$\endgroup\$
    – jkej
    Commented Apr 1 at 22:22
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There's nothing in the 3D model that clashes with the PCB without the cut-outs: -

enter image description here

I think those cut-outs might be trying to tell you that you need to ensure there is some clearance to components or tracks that might be placed on the PCB or, that a regular square hole in the PCB may clash if fillets are applied (as you would expect with a milling machine).

If you still have doubt contact the supplier. Or, just add a 0.5 mm radius hole like below: -

enter image description here

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