[gta02-core] Board parameters update

Rene Harder rehar at saweb.de
Sun Aug 30 23:17:32 CEST 2009

According to our financial situation right now, it's might be worth to
tweak some board parameters to reduce production cost.

I think we should try to stay as much within common standard specs as
possible because the goal of gta02-core is to create a prototype and we
are not going for production right away. Custom specs always means that
our board has to be processed individually which adds up to higher cost.

So here are a few comments where i think we can cut costs.

Werner Almesberger wrote:
> Here's an update of the number of components we'll need and the structure
> of the PCB:
> - PCB size is about 10 x 6 cm2.
> - PCB has 8 layers, 1 mm overall thickness. (*)
> - Minimum trace width is 4 mil, with a 4 mil clearance between traces
>   and between traces and pads.

Is 4 mil really necessary? Most fabs go down to 5-6mil as min.
Trace/Space width. I believe we can find a fab who can make boards with
4mil but that require a more precision fabrication process as well makes
it electrical and optical test absolutely necessary. Although most fabs
will require an electrical test for board with equal or more than 6 layers.

> - Surface finish is ENIG

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.

> - For the board structure, we need input from the PCB factory. Openmoko
>   used a core of FR4 to which layers of PP 1080 65% and PP 1078 61% were
>   added.

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.
> - PCB has components on both sides, about 10% on one, 90% on the other.
> - Total number of components on the PCB will be about 400.
> - Approximately 140 of the components on the PCB are unique.
First of all I've not much experience with SMT assembly fabs, so if I'm
wrong please correct me.

Are we planning on population all components in a fab? 140 unique
components, that's quite much and I think this means an increase of
setup costs for the placements. So maybe it's worth to solder some
unique components our own, if that would reduce the costs.

Another thing, for the assembly fab we would need a square/rectangle
board so we should keep in mind that we need to panelize the board with
an additional outer frame.

> - There are about 20 components that don't have a standard chip
>   package (connectors, buttons, shields, etc.)
> - Largest components are a 27 x 18 mm2 SIM/SD combi-cardholder, and a
>   22 x 22 x 3 mm3 BGA module.
> - Largest chip is a 14 x 14 mm2, 0.5 mm pitch 332-FBGA (SC32442)
> - Smallest component is 0402.
> (*) I'd like to try to make this work with only 6 layers, but I'm not
>     sure if that's possible. Our circuit is considerably simpler than
>     Openmoko's, with a greatly simplified memory bus and GSM in a
>     module, so there is hope.
6 layer would be good to cut costs, every additional layer increases
production cost.

For solder stop mask and silk screen i think they don't make much of a
difference in production cost usually you have to pay a small setup fee
for the whole batch if it's not included.

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.

Just a an example a board with our size, 6 layer, 6 mil Trace/Space, all
holes platted through, quantity 15 pcs. is about $80 USD/ per board
(board outline routing is only rectangle though)

For a board with our current specs 8 layer, ENIG, burried and blind
vias, we would need to get a custom quote (which usually needs a final
board layout). But i do think Werner's estimation is not far away from
the real cost so we would end up around $250-300/per board.

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.


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