[gta02-core] Layout - divide and conquer

Álvaro Lopes alvieboy at alvie.com
Thu Aug 13 15:59:33 CEST 2009

Werner Almesberger wrote:
> ?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)

Good. But we have other ways to avoid reflections on RF side [1], like mitered bends.

> - 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.

We can work that out, but will require some effort.

> - 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.

Can't we design a custom pad for it ?

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

Lol, not GPS expert I'm afraid :) But yes, I'll gladly do GPS layout.

I think we should come up with general layout rules first, layers specification, and then mock up some preliminary design so that everyone can have a chance to
spot problems.

> 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

I'll work on that. Won't be easy but we'll sync on IRC if needed.

> - 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.

True. But reference designs are meant for "reference" only, not guaranteed to work or fit our purpose. :P We ought to be careful about those. I wonder if
openmoko (former)people can give us a hand on this.

>   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.

Unfortunately true. But can we live with that, ie, can't we simply have slight larger pads ?

> - 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.

I'll do my best to help you on this, although it has been quite difficult for me lately.

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

I wished :) That would allow for a small USB HUB, wouldn't it ? Or we just use the extra space to avoid RF interference.

Best :)


[1] http://ezinearticles.com/?RF-PCB-Layout-Guide&id=2064253

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