[gta02-core] Layout - divide and conquer

Werner Almesberger werner at openmoko.org
Thu Aug 13 14:15:36 CEST 2009

?lvaro Lopes wrote:
> I still need some PCB info for the microstrip lines, but we can start it
> and then perform the adjustments at the end.

Sounds good. I had a look at Openmoko's public Gerbers and it seems
we don't need too many more features for the footprints:

- none of the components have angled pads (unless the whole component
  is rotated)

- there's one BGA on the GPS side, so I have to add that to fped

- I couldn't find any distinct solder paste layer, so it seems that
  Openmoko put solder paste on the entire surface of each pad,
  including the center pads of QFNs.

- the ATR610 is a little quirky, with pin 5 extending from the center
  pad. KiCad doesn't have pads of that shape, so we'll have to be a
  bit creative, e.g., separate the pads in the footprint, and join
  them with a trace.

So ... how do we proceed ? Since you're the GPS expert, do you want
to do the layout of that subsystem ?

We'll have to make the footprints first, though. I don't particularly
trust the stock KiCad ones, also because they usually don't come with
a clear indication of their source or the technology they're optimized
for. (And footprints for reflow, wave, and manual soldering can be
rather different.)

So we need ...

- the part-to-footprint associations for the GPS subsystem

- from that, the list of footprints we need

- for each footprint, a reference design. Sometimes, the data sheets
  include one, but often not. Also, the data sheets don't always have
  particularly credible footprints, see the DTC123 discussion.

  I generally like the ones from NXP, so I'd try to use them as often
  as possible. One caveat: the geometry of the center pad of QFN
  varies. So vendor X's 5x5mm QFN may need a different footprint than
  vendor Y's, even if the packages are supposedly the same.

- the footprint drawing. I hope we can do them with fped. I'll make
  it my first priority to fix any issues that pop up there.

- as usual, reviews

> This will also give us a clearer idea about how much space we won by
> removing the external antenna.

Probably something like a third of the can's size :)

- Werner

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