Re: gEDA-user: QFP fan-out tips

[DJ Delorie <dj@xxxxxxxxxxx>]

> What does "high frequency" mean?  I'm in the <50MHz range.

I'm not an expert, but I think that's still medium frequency.  I run
in the 20-35MHz range.

> been trying to use big vias for my high-current connections, but maybe
> that's just a waste of board space?

Consider that vias have thinner walls than traces, but 3.14 times the
"width".  I think as long as your drill size and annulus are at least
as big as your trace width, you should be OK.  You can also use
multiple vias; my alarm clock uses three vias in a row to make "wide"
connections to the ground and power planes from the power supply.

> Speaking of vias -- Anybody have any experience putting them right
> under an SMD pad?

Don't unless your assembler knows how to deal with them.  They *do*
wick solder.

> but rather doing under 0603 or 0805 capacitor pads, to provide the
> connection to the plane layer.  That would certainly save me some
> board space.

If you're thinking of doing that "to save space", first consider using
something smaller than 0603.  You can also put the via adjacent to the
pad, so that the annulus overlaps it.

> I also have a couple of parts with exposed thermal pads, and the
> datasheets recommend vias underneath the pad to conduct heat to the
> other layers.  But won't this cause wicking of the solder to the
> underside of the board?

Yes, but that's different - the specs usually tell you where to put
the paste, and usually you just don't put paste where the vias are.

> Is there any way to get a little ring of soldermask around a via
> that's in the middle of a pad?

Not easily, in pcb.  What you'd end up doing (besides patching pcb ;)
is building your thermal pad from multiple pads, each with a mask
opening smaller than the pad, so you end up with a grid of mask lines
forming a grid of open squares of copper.  You'd then put the vias at
the mask grid intersections.

But I don't think wicking is as big a deal with thermal pads; it's
mostly signal pads that are a problem, especially with BGAs.

> DJ -- you mentioned using 0402 caps.  How tweezer-friendly are
> those?  I find I lose maybe 10% of 0603 parts as they fly out of the
> tweezers and across the room.

You need a much gentler touch on the tweezers.  I rarely lose 0603's,
0402's aren't *that* much smaller.  0201's I have to be careful with,
and 01005's have been named "quantum parts" due to their tendency to
just vanish.


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