kicad-developers team mailing list archive

[Lorenzo Marcantonio <l.marcantonio@xxxxxxxxxxxx>]

On Sat, 28 Aug 2010, Alex G wrote:

> > a hole of 3.2mm. I know the pad is 3.2mm, but when generating drill
> > files, the hole size will be the size of the hole, not that of the
> > drill. That's not a big problem when drilling manually, but when using a
> > CNC, especially with auto-tool-change, it can give the engineer quite a
> > few headaches.

In drill files the tool specified is *always* the finished hole, no
exceptions. This is because PCB drills usually are actually cutters and special
compensation applies. And of course there is plating which is a process
variable, too. Buried and blind vias are another kind of hole too, but
anyway you just state the *finished* hole size.

In conventional processes you can ask for holes from 0,4 mm up in 0,1 mm
steps; large holes (typ >6mm) are usually milled using
a circular path (often during the final cut, if not plated). Smaller
holes are done using microvia processing with laser evaporation
(usually).

For these reasons it's the manufacturer job to 'compensate' tool size
(and tolerances are stated in their process description). So if you want
a 3.2 mm hole, just state a 3.2 mm tool, the board manufacturer will do
whatever it would be done to achieve it. It's a CAM issue, not a CAD
one!

> > The recommended hole size varies between manufacturers, but all
> > datasheets that I've consulted "stated" "compatibility" with 3.2mm
> > holes. (Amphenol: 3.2+- 0.126; Harting: 3.1+-0.1, etc).

I agree. It's actually an imperial hole size but since boardlock is
usually elastic or a fixating screw/rivet, there is a good tolerance.
Slimline connectors are more picky...

> Referring exactly to the pad size for an M3 nut without washer.
> The general rule I use is
> Max component size + Screw in hole tolerance + Other tolerance (like
> washer to screw) + some random value (0.2 mm :) )
> The following would amass to (M3 screw in 3.2mm hole)
> Screw only:	6.4mm
> Nut:		6.8mm
> Washer		7.4mm
> The last one doesn't exactly follow the formula above; if it did, the
> full-body-condom--safe pad size for an M3 washer would be 7.8mm (a 3.4mm
> ID washer fitting over an M3 thread add the 0.4). I don't think anyone
> will need as much torque to justify a washer, so 6.8mm should be fine.

There is a standard for these too... you can use the LP calculator,
which gives hole/pad/clearance for mechanical fixturing:D

Of course these are sizes absolutely not criticals and often compatible
connectors (i.e. Tyco vs Molex) states different sizes but are actually
interchangeable...

> > I would also rename the library to dsub, or conn_dsub. Then the Molex
> > Micro-Fit library I'm dying to work on would be conn_microfit. Anyway,
> > semantics :) .

I have a whole series of picoflex, spox and phoenix terminal blocks already
done, if you want them. They should be on my bzr on launchpad. No idea
if they fit your lib conventions, I have mine.

> > > Yes, you're right - there are _way_ too many
> > > connectors out there to put them all in one big lib :-)

I use a lib for each series (picoflex, ampmodu,combicon, minispox) and the
assorted ones in the connect lib. When there is a sufficient amount of
similar connectors in misc I tear them out in a dedicate lib.

The advantage with some connector series is that you can generate easily
a whole series with a script (microfit is one of these).

> > > The pad size and position on the Al SMD Caps (C-AEC-*)
> > > is pure guesswork, as I couldn't find any land pattern
> > > info :-(, so they definitely needs double-checking ...
> I'll try to see if I can get this info from suppliers.

Panasonic gives them. Also there are two 'standards' for Al SMD caps
sizes: the panasonic one (a letter optionally followed by a number) and
the 'diameter x length' one. They actually are interchangeable, but you
need to build some cross reference table; in the pana packages the
letter states the body size/diameter and the default height. The number
is more-or-less the length (with an arbitrary approximation:D). In the
diameter x length system the diameter implies the body size, so you need
the datasheet anyway... (for pin size, too)

Also the length doesn't influence pad layout so you only need, for
example, an H layout instead of different H17, H23 and so on (exception:
the 3d shape would be different if you use them, but the layout is the same)

And, as usual, the LP calculator give you the layout given capacitor
sized :D

--
Lorenzo Marcantonio
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