Comment 9 for bug 1563744

> Solder mask clearance has meaning *only* for pads, not for tracks and copper zones.

I am aware of that. The solder mask clearance is the size of the purple margin around the pad, such as the pad named "Net-(U1-Pad14)" in my original screenshot. The mask applies only to that pad, nothing else. Not the zone, not the track. I understand that.

Please see my newly-attached image, containing three parts. Hopefully this will make my complaint clear.

Part A) I have indicated the solder mask clearance - the width of the purple margin is 8mil. I have indicated the zone clearance, which in this example is set to 20mil. All is safe- note how the red curving corner does not go anywhere near the purple aperture of the solder mask. No chance of a spillout here.

Before we continue - do you agree that I have correctly identified the measurements in this example that are
  solder mask clearance - the upper green arrow, measured as 0.008in (8mil)
  zone clearance - the lower green arrow, measured as 0.020in (20mil)
For the rest of my explanation to make sense it is important that I've got these correct, so please point out if they are not.

Part B) The zone clearance is now set to the same value as the solder mask clearance - 8mil. Note how the constant gap between the red parts (the copper clearance) is a constant 8mil, including an 8mil radius curve at the corner. This causes the copper of the outside zone fill to become exposed at the rectangular sharp corners of the solder mask aperture - as indicated ringed in bright green on the bottom two corners. It is this exposed copper that caused me grounding shorts when I soldered up a board, and is the problem I am trying to avoid.

Part C) The zone clearance is now set to 12mil (which is 1.5*8mil), meaning that the 12mil radius curves are now clear of the sharp corners of the purple solder mask aperture - note how the red copper zone no longer spills out from behind the solder mask aperture. 12mil is the smallest value that achieves this safety - at 11mil or smaller, the sharp corner of the mask aperture manages to just catch a little of that zone copper, risking a short.

My complaint is that as a newbie I was not aware of these rounded corners - the cause of the spillout in part B. I had thought it safe enough to simply set a value of zone clearance that is larger than the solder mask clearance and that would be good enough. I am now aware, from my mistake, that it has to be at least 1.5 times larger, before this is actually safe. I would therefore like something to help people avoid this mistake - see the list in my previous comment for my suggestions on how to do this.