Holes without copper, and slight drill offsets

[shabaz]

Hello,

I had a question about non-plated holes. Ordinarily, I set the hole type to NPTH, and then set the circular pad size and the hole size to be identical.

However, what happens if the pad size is larger? How does the PCB manufacturer know from the Gerber files, that the hole is to be non-plated in that case? I thought the PCB manufacturer would look to see if there was copper right up to the edge of the hole, and if so, then it would be through-hole-plated, but this is an assumption from me, and perhaps I’ve very wrong. Just curious how that information is conveyed in the Gerber files.

The reason I’m curious, is because I’m wondering what happens (in the case that NPTH and same sizes were set for the pad and hole), that the manufacturer’s drill is accidentally very slightly offset due to tolerances. Would a sliver of copper appear? If so, I guess the pad setting should be set slightly smaller than the hole size? Or is this over-thinking it?

Many thanks.

[paulvdh]

KiCad generates two drill files. one for PTH and one for NPTH.
This makes the drills independent from the pad size.

I normally never use NPTH holes, even for mechanical stuff.

PTH holes are a standard part of PCB manufacturing and therefore “free”.
Non plated holes are a separate step during PCB production and sometimes cost extra, or it is a hassle during ordering.

And I really do not care whether a hole is plated or not when I want to drive a screw in it.
PTH holes apparently have a higher tolerance of their diameter (because of varying plating thickness), but that’s not a problem for me.

[shabaz]

Hi Paul,

Thanks for the info, I too normally almost always use PTH. I guess that settles it, the absence of plating or the presence of plating is determined if the NPTH box is set or not, but in the scenario that NPTH is chosen, then from what I can tell the pad should either be set larger than the hole (if a non-plated pad is desired), or the pad should be set smaller than the hole. If the pad is set to the same value as the hole, then I think there’s a risk of a sliver of copper remaining on the copper clad surface if there is a manufacturing tolerance on the drill size or position.
However, if the user sets the pad size to be smaller than the drill hole, a warning appears:
Warning: Pad drill will leave no copper or drill shape and pad shape do not overlap.
I’m not sure if there is a low-pri feature request here, to just have a ‘bare drill hole’ or similar option, where no pad size needs to be entered.
I’m not going to raise such a request because I can live with the warning, but am happy to raise it if anyone feels it is an issue.

"

[paulvdh]

If you do not want copper around your NPTH, then just turn off the copper layers.

Leaving thin slivers of copper around your hole due to tolerances is indeed very bad. They can break off and create short circuits.

[chris9]

just a remark: Non plated holes are sometimes necessary for fixation holes (typically 2 diagonal holes) for InCicuitTest to guarantee the contact needles positioning.