If my understanding is right via in rules apply to pad not the drill. There are three pad types in the rules SMD, PAD and VIA but all drill holes are treated the same. That being said try this. Setup a split mixed layer in the stackup. Go to setup via padstack in menu. There shuld be an antipad tab, click on that. Click the circle anti pad and check "Pad size relative to drill size". Type in the 8 mils. Pretty sure this will get you what you want. I have used defined antipads to get copper into BGAs but never tried to make vias unique.
Thanks, WIlliam, I will give that a try.
I think you're right, it would normally be pad-to-copper clearance, but I
should have also mentioned that the layout has enabled Remove Unused Pads
in Tools > Options > Split/Mixed Plane.
So for vias not connected to the plane copper, the pad-to-copper clearance
seems to not apply on those plane layers. Instead it seems to be the
drill-to-copper clearance that wins out if its value is larger than the
To use your method with antipads, I think I would also have to disable Use
Design Rules for Thermals and Antipads in Tools > Options > Split/Mixed
Plane. Am I correct? Current practice at our outside layout house appears
to have this setting enabled. I wonder what disabling it might do to the
other plated holes if the outside layout house didn't define antipads for
Forgot about that setting. Here is what I think I remember. Default aplied first. Padstack specific overrides default. Use default switch in tools ignores padstack overides with deefault settings. I think I got this right. Maybe someone else can confirm.