Flood fill gotcha using V-Cuts

[gwideman]

When creating the single base PCB, you may define fill zones using an outline that's the same as the board outline, relying on the zone's clearance settings to reduce the zone away from the edge of the board.

When Kikit copies that board to the panel, Kikit preserves the current state of the fill, so it's initially fine.

However, if you selected for Kikit to use VCuts, there are no longer Edge.Cut lines to repel the fill zone, so if you then cause Kicad to recalc the fill on the panel, the fill runs right up to the actual fill zone boundaries you drew.

The moral of the story is: If you intend to use VCuts, then draw a fill zone boundary that's inside the board edges by the desired amount, and don't rely on the zone's clearances at the board edges.

[yaqwsx]

Could you open an issue with an example? KiKit should not refill your zone ever (for exactly these reasons). I briefly checked the source code and I see no way it can refill existing zones on the PCB. The only zones that are refilled are the zones added by KiKit.

Having a concrete example will help us to track down the problem.

[yaqwsx]

Oh, I misunderstood your comment. Just to clarify; the intended workflow for KiKit is to have a valid board to start with and do not refill the zones for mousebites - as they are NPTH holes they should not yield any electrical connection. So they should be relatively safe to add. If there is a use case for that, we can add a post-processing step that will refill all zones on the panel.

[gwideman]

Yes, I indeed came to the conclusion that you were assuming that refill would not be run again after panelizing.

I do not think it is safe to NPTH drill and snap through flood fill regions, as that leaves copper exposed around the outside of the board. Imagine a 4-layer board and the two inner layers are power and ground planes, which, after snapping the mouse bites, are exposed. And then the board is mounted in a metal chassis. Or for that matter, the fill regions' copper foils might fray at the snapped edges of the mouse bite, and short to each other.

So when using mouse bites I think we do need to refill. I don't know whether Kikit should trigger that automatically, or just provide instructions to refill manually (or by script).

In the v-cut case, the problem is that once Kikit removes the Edge.Cuts for individual boards, then Kicad no longer shrinks zone fills away from the edges of the board. So running refill on the panel results in fill up to the edge of the board. I don't think it's worth trying to get Kikit to remediate that, as it could be tricky. Again, this might be decently well solved just by including in the instructions that PCB designers need to be wary of drawing fill zones up to the edge of the board, when using V-cuts.

For those of us who like things simple:

1. During design, always draw zone fills with boundaries that are away from the edge of the PCB by the desired clearance. (Do not rely on Kicad's fill zone edge margin feature.)
2. Always run refill after panelizing.

[yaqwsx]

The later case with V-Cuts can be solved easily - KiKit can add a clearance region around V-cuts (that basically behaves the same as a board edge). Just specify clearance: 0.2mm in cuts.

[gwideman]

The later case with V-Cuts can be solved easily
Are you saying that's already implemented? That's pretty cool.

[yaqwsx]

Yes, it is. See the following image from examples: