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I've had trouble finding a guide to explain PCB vias to me. Primarily, I don't understand why the drill layer (44 in eagle) is the same size for vias of differing diameters.

In the image below, I have vias with different diameters (in mm) lined up. The (white) drill layer does not vary in diameter with the vias. I've tried changing various parameters of the vias and in the DRC file, but neither seem to change the drill diameter. I'm especially confused that for the 1.4mm via, which is greater than the drill diameter. What does the drill layer tolerance represent? How can it be changed and when should it be modified?

Vias of differing diameters in mm

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It looks to me as if the white drill layer could be used to produce a "drill drawing" with different symbols (various crosses and circles) used to indicate the drill size. The size of the symbols has no relation to the actual drill size.

Eagle should produce a drill file that will specify the actual drill sizes and hole locations - that will be what the board shop will use when making your board.

(I don't think I've ever produced a drill drawing for any board I've designed.)

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The white layer is just symbols, which are unique for each drill size. It's for making a diagram showing the drill sizes and locations, for human use.

For machine use, you will have an NC drill file that lists the hole locations for each tool size. It's human readable ASCII, but in g-code. Below is a (partial) example of a drill file created in Orcad. The first tool is a 0.028" drill for vias:

%

T2C0.028F200S100

X010600Y036700

X012400Y036400

X013600Y039120

X014300Y029860

X014600Y036200

X016575Y037225 (and so on)

The first line after the % indicates the tool number (T2), the diameter (0.028), the feed rate (200ips) and the spindle RPM (100,000). Both the latter will likely be changed by the PCB manufacturer to fit their machine.

Each line after that represents one 0.028" hole, the numbers after the X and Y are the hole locations relative to the origin normally.

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