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Topic: Via size vs drill size? (Read 3913 times) previous topic - next topic

Via size vs drill size?


I am using Eagle and in it you can specify via size in which the hole is described as drill hole. The pad around is then the amount of Cu covering the surface (or so it seems).

In DangerousPCBs you specify that finished hole size should be 12mil. Is this the Drill size or the VIA size? I read somewhere that VIA size is 4mil smaller than drill size (as it is filled with electroplated Cu).

So I am confused and I don't understand how this can be made in Eagle. There are many scenarios:

1) Drill size is 12mil and VIA size is 8mil. Thus I can have a 2mil pad around the 8mil VIA hole in Eagle and this is fine for ordering (even if Eagle specifies this as a 8mil drill).

2) VIA size has to be 12mil and drill size is 16mil. Thus I can have a 2mil pad around the 12mil VIA hole in Eagle. Eagle specifies this as a 12mil drilled VIA.

3) The pad has to be more than 2mil, for example 3mil. Thus I can specify a VIA size of 8mil and pad of 3mil. The drill size is 8+4=12mil and the pad covers 3mil around the 8mil via hole, thus everything fits inside a 8+3+3=14mil circle.

4) The VIA size is 12mil (and drill 16mil) with 3mil pad around. Everything fits inside a 12+3+3=18mil circle.

5) The drill size is 12mil (resulting in a 8mil via), but one needs a 3mil pad around it, everything fits inside a 12+3+3=18mil circle.

So what is it? And is it the drill size that shows up in Eagle or the VIA size (e.g. after filling the drilled hole with Cu metal)?

Re: Via size vs drill size?

Reply #1
The "drill size" in Eagle is actually the finished size of the hole (after plating with Cu). You shouldn't care about the tool size that is necessary to drill the hole at all, this will be done by the board house. The restring (the width of copper around the finished hole) for vias is determined by the design rules. For Dirty PCBs, it is 25% of the finished hole size, with some min/max value.