[gta02-core] Board parameters update

Rene Harder rehar at saweb.de
Mon Aug 31 17:17:40 CEST 2009

Werner Almesberger wrote:
> Rene Harder wrote:
>> I think for a prototype a regular chemical or HAL tin finish might be
>> sufficient or are the any specific reasons why we need ENIG.
> I have to admit that I'm not an expert on surface finishes. I did a
> bit of Web research a while ago and found the following:
> - HAL/HASL: cheap and easily available but with a relatively uneven
>   surface. A potential problem for fine-pitch BGA. (There, we need
>   better than 99.9% accuracy, or each of the boards will end up with
>   some random flaw or another.)
> - OSP (Organic): inexpensive and with great surfaces, but unstable
>   and demanding on process parameters. With OSP, I would mainly be
>   worried about test pads oxydizing and becoming unsolderable. It
>   probably also constrains the choice of SMT fabs. OSP seems to be a
>   good choice when optimizing the board cost over large quantities.
> - ENIG (Nickel-Gold): the standard process for anything demanding.
>   Old and well understood. Boards can take some abuse, such as heat,
>   storage, or oxygen. Needs two process steps and is more expensive
>   than the rest.
> - Immersion Silver: a relatively new coating that competes with ENIG.
>   Less expensive but susceptible to oxydation. Suitable for
>   fine-pitch packages.
> So ENIG seems to be the safest choice, particularly if we assume that
> we may have to do significant rework. HASL doesn't seem to be suitable
> for small-pitch, or it's at least risky. OSP looks more like the thing
> you consider for mass production. Silver may be a possibility. I'm not
> sure how delicate the handling is, though, and how quickly oxydation
> would become a problem with test pads and unpopulated footprints.

Hmm not sure here,  I would use ENIG for production because it's a
reliable and stable process for many years now. So you probably will not
end up with a lot of faulty or useless boards.

The chemical deposition of organometallic compounds (OSP) is becoming
more and more popular, and you'll find it in prototype spec quite often
these days. However I have no idea about long time stability (have never
used it myself)

>> I guess 1080 woven-glass with 65% FR4 epoxy laminate is pretty much
>> standard everywhere so shouldn't be too difficult to get. About 1078
>> prepreg im not sure, haven't seen it anywhere in the specs so this might
>> be only possible within custom specs.
> The board is quite thin, only 1 mm. That may complicate it somewhat.
> Going to 6 layers should bring us closer to standard thicknesses.
> Besides that, I think we can adapt the layout to a wide range of
> parameters.

Indeed, that makes things more complicated However, there are companies
who have boards with an overall thickness of 0.31" or 0.40" for 6
layer[1] or 0.48" for 8 layer[2] in their standard specs. Sure you can
build you own board with an individual stack-up [3], but that means a
custom design again.

>> A serious problem are blind and buried vias though. They are our killer
>> for any standard specs, if I'm not mistaken they require a huge amount
>> of extra steps between every lamination process which is impossible to
>> do within standard spec. So best would be to stick to PTH vias if that's
>> not possible our cost will be probably more than doubled.
> Hmm, I'm not sure we'll be able to pull this off with through vias.
> Also, don't they increase the EMI risk by spreading signals all over
> the place ?

Actually neither do I and It probably will get really messy around the
BGAs ;-)
Well, yeah they'll make a swiss cheese out of our image planes, which
results in an increase of their impedance.

>> At the end, i think we should consider that this will be *only* a
>> prototype and not a usable phone, so we don't need a lot of gimmicks
>> which are not absolutely required.
> Yes, but on the other hand, we wouldn't want to use a very fragile
> processa that introduces problems we could have avoided. There's this
> moment when you try to power up a new board and nothing works. It
> really helps then if you don't have to suspect every single solder
> joint to be a traitor to the cause ;-)

Sure, i don't suggest that we have to use a fragile or instable process.
I just think that it's worth to check what features are really necessary
and what's their benefit over different/similar technologies.

[1] http://www.4pcb.com/media/031-%206%20Layer.pdf
[2] http://www.4pcb.com/media/048%208%20layer%20copy.pdf
[3] http://www.4pcb.com/media/PrePreg%20Thickness%20chart.pdf

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