PCB cutout specification

Question

I want to specify a square cutout in my PCB, to fit a sensor module.

The cutout must be very tight, that is it should only be 100μm wider than the sensor on all sides, as it is used to hold the sensor.

But as to my knowledge, the manufacturer cannot make sharp edges because the cutout is made by milling, so the corners will be round.

My corners don't need to be sharp, only the cutout dimensions need to be correct. So how is the correct way of during this? Should I just define a hole in each corner?

Answer 1

One way to handle this is to place a drilled out hole at each corner. Then the milling corner is not a problem. Your cut out corner then ends up looking like the image below. Another alternative is to have the route go past the corner as shown in the second image.

I used a cutout that looked like this for a snap in plastic switch component:

The above style of the cutout was also used on another board design to accommodate a plastic screw standoff that had a rectangular snap-in profile so it would not spin as a security screw was installed.

Answer 2

You can define a hole in each corner (hole must be bigger radius than the routing tool) or modify the routing at the corners to remove all the material (extend the routes in one axis or add a stub route that cleans out the corner at a 45° angle). Called a 'dog bone' for obvious reasons.

For example (from here):

If you use the hole method you probably want to specify an unplated hole. The above diagram also shows you a possible size of hole and position.

Edit: The above page is sub-optimal as is extending the routed line if you want to minimize the volume of material removed beyond the required amount.

I think the minimum material removed would be a hole (must be drilled since it is of radius less than R) with center at -R/2, -R/2 from the edge and diameter \$\sqrt{2}R\$

Comparing the figure of merit of area of unnecessary material/radius squared (lower numbers are better):

Extend routed line: 1.57

Circle of radius R: 0.571

Pefhany method: 0.285

It's also possible (from some vendors- you can ask) to do drilled slots which can be very small radius.

Answer 3

I think you need to talk to your board vendor. Find out what size routing tool they use, find out whether they would prefer you to give them actual tool paths or just the final shape of the cutout. Find out what precision they can work to.

Then look at the details of your sensor, are it's corners also rounded slightly or do you need to do things like Michael and Sephro described to clear the corners of your rectangle.

Finally draw the lines on a "mechanical layer" in your gerbers and send instructions on how to interpret them with your gerbers to the board house.