I'm trying to understand the difference between the vias and plated through-holes in these reference Gerber files. On layer 3, I don't understand why there are so many holes (vias)?.
Why are some holes on layer 3 and the NC drill not on the other layers?
And the other holes in the NC drill that don't appear on any of the other layers, what are they? Vias? (I can see them in the final render.)
Layer 1
Layer 2
Layer 3
Layer 4
NC drill/thru-hole
PCB top render
PCB bottom render
Why are some holes on layer 3 and the NC drill not on the other layers?
I think they are there but you can't see them because copper floods over them i.e. they don't have thermal reliefs that make it easy to see them.
Layer 3 has an "island" in the middle of it, presumably for power distribution. Therefore, any of the vias that are intended to short the ground planes of the other layers together must be isolated within that island, which is why you see the extra dots there.
Don't think of the circles in Gerbers as holes, they are called antipads.
They are there to isolate that via or PTH hole (vertical electroplated copper tube) from the plane copper on that layer.
There could be just an empty 'hole' to form the antipad but instead the copper antipad is placed there to re-enforce the plating on the copper tube, with an insulating circle round it. The two entities together form the antipad.
This is done as that helps prevent the copper of the vertical plated copper tube peeling away from the fibreglass material that forms the insulating layers.
On layers where the via connects to the copper there is no antipad, so no 'hole'.
