On Jun 18, 2012, at 11:18 PM, Electron wrote:

> That means pretty "big" holes, so big that the component will fall down t=
o the
> floor if the board is turned to the other side=85

I don't think I've ever assembled a professionally made PCB where the holes=
 were small enough to hold in components just because of lead friction with=
 the hole itself.  That would be a bit scary, given that you'll have drill =
tolerance plus copper plating tolerance plus tin/solder plating tolerance t=
o modify the hole size from what you thought it would be.


> Optimal pad size seems as a minimum twice the diameter of the hole, three=
 times
> maximum (if board space allows).

I like to think in terms of the "trace width" of the ring around the hole. =
 Obviously this shouldn't be any smaller than the minimum trace width suppo=
rted by the manufacturer, and should generally be significantly larger to a=
llow for easy soldering, and significantly larger again for mechanical stre=
ngth if there are heavy components or external stress (connectors) involved=
..  Since a "typical" min trace width is 0.2mm, and a typical hole is 0.8mm,=
 your "twice" allows for a ring width of 0.4mm, which is OK.  I usually fid=
dle with EAGLEs DRC parameters when I'm otherwise done, and make the rings =
as wide as I can without running into clearance issues.  OTOH, that's parti=
ally because I'm aiming at users with less soldering experience.

EAGLE defaults to 25% of hole size for the ring (1.5x hole diameter for the=
 pad), tempered by a minimum ring width=85

BillW


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