Is KiCAD expecting the final hole diameter or the drill diameter to be entered via the Pad Properties dialog in the Footprint Editor? Or, what is the best known practice?
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Eurocircuits state that it is the finished hole size
This is one of the key points to check with your PCB maker
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It’s not a matter of what KiCAD expects - the important thing is, what is expected by the fabricator who will etch and drill the board.
The overwhelming majority of fabricators expect you to specify the FINISHED size of the hole. The last time I encountered a fabricator who wanted the drill size was at least 15 years ago (perhaps 20). From the perspective of manufacturing philosophy it should be no other way. You, as board designer, have a requirement create a certain hole size. But you have no knowledge of the processes used to create plated-through holes, much less any control over them. The fabricator will do the initial drilling, pre-plating, plating, and after-plate finishing. If he has these processes under control he can reliably predict the finished hole size much better than you can guess the outcome. A fabricator who tells you to specify the initial drill size is saying, in effect, that he is not using well-controlled processes.
Having said that, keep two things in mind.
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In any manufacturing process, there are tolerances. In this case there are two significant tolerances. The first is the tolerance of the (finished) hole diameter. I believe an uncertainty of 3 or 4 mils (0.1mm or a bit less) is currently the typical tolerance for a “standard” board. Then there is a tolerance on the hole’s placement - the deviation of the hole’s actual center point, from the specified center. I’d allow another 3 or 4 mils if hole location is critical. (This is often absorbed into the spec for minimum annular ring, or covered under a general spec on feature locations.)
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Your specified hole sizes may be mapped into the fabricator’s “standard tool rack”, especially on quick-turn or short-run orders. While any nominal hole size is theoretically possible, the fabricator is typically set up to produce only 10 to 20 specific hole sizes. Some fabricators will round your hole sizes up to the next standard size. Of course, this guarantees that the holes will be at, or a few mils larger, than specified. Others will convert your sizes to the nearest standard size, which may be either larger or smaller than specified, by a few mils. A few fabricators may require you to change your drawings, and only use the sizes available from his standard tool rack.
Dale
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