Altium 0402 Resistor Footprint

a: 0.55 b: 0.58 I: 0.40

The PADS library parts seems to be the closest match. However, the PADS b: dimension is slightly large because there is no metal on the side of a resistor terminal and the package Width Tolerance is only +/- 0.05 mm. 

The part is 1.00 mm long with a 0.50 mm gap (between two 0.25 mm lead widths). 

The Altium I: dimension of 0.20 mm is too close and the heel joint would be 0.15 mm. Way out of line and I don't know why the assembly shop would recommend that one. 

The Library Expert Pro heel joint is 0.05 mm to cover the terminal tolerance. 

Download the Free (no license required) Library Expert Pro and use the Altium interface to produce an accurate footprint and High Quality 3D STEP model - 

Library Expert Pro actually shows you the package and terminal outlines on the pad so you can visually see where the solder is going to flow. And the body and terminal are translated to Altium on a mechanical layer. 

a: 0.58     b: 0.60    I: 0.35

Not sure why it is different from your results but not a long way off.

Also are the rounded corners (set to 25% of pad width) preferred to square corners?

Thanks

Using the numbers from the datasheet you posted the link to I do indeed get the same numbers as you.

Vishay has a lot of versions of the CRCW datasheet.

Here is the one I am using that matches the P/N I am using: http://www.vishay.com/docs/28773/crcwce3.pdf

The tolerances are different and it is dated 02-Nov-17.

The one you posted the link to is dated 18-Nov-10

Also we have Stackpole as a second source, fortunately their dimensions match Vishay except the tolerance on the termination width is 0.15mm instead of 0.1mm for Vishay.