SMD Pad Roundness

[sdalling]

I'm working on upgrading my BOM tool and one of the issues I have with rendering are smd pads that have a roundness parameter. I just need some clarification on how roundness works since the documentation does not really mention it. 

All smd pads are by default rectangular(general sense here, four sided polygons). So make a a rounded shape, like an oval or a circle, roundness can be set. Roundness is a measure of how "round" a corner is. For example 0 states that the corner should have a point, while 100% states that it should be perfectly round. Is this correct? If so, what kind of geometric property defines this shape.

[Yura.Ivanov.VLuki · Accepted solution]

Curve radius.

A right angle with vertex O is rounded with an arc of radius R in the following sequence:

1. From top About carry out an arch set radiusomr, to crossing with sides of angle in points A and B (interface points) ;
2. Center of rounding point О1 must be at the geometric location of points equidistant from the sides of the corner, that is on a bisector АОВ and is defined by the intersection of the radius arcs R, posted from blend points А and B;
3. Carry out an arch АВ radius R and center О1.

[joe.althaus.2]

I found myself with the same question and the "Solution", while illuminating, didn't really help me understand the Roundness parameter for SMD pads. Here is what I learned. The roundness parameter is the percentage of half the side of the pad size that is taken up by the radius/filet as compared to the original dimension on that side ( for non-square pads, the shorter size drives this and the same feature is applied to the longer side).

It is typical for datasheets to specify a radius rather than a percentage on the landing footprints for parts. This is where the above answer helps. The distance that the fillet feature takes up on the side of the pad is actually the radius that you want to specify. So if you have a SMD pad with a 5mm short side and you want it to have a 0.625mm radius, then you specify the Roundness parameter to be 25% or 0.625mm/(5mm/2).