[Tinkerphones] GTA04A5: good news and bad news
H. Nikolaus Schaller
hns at goldelico.com
Thu Feb 23 08:09:57 CET 2017
> Am 22.02.2017 um 15:49 schrieb H. Nikolaus Schaller <hns at goldelico.com>:
> But now the bad news.
> -1.: Only 12 out of 36 (*) GTA04A5 boards we
> have produced, are booting to Linux. The others either have a
> short circuit under the OMAP (which is the general PoP soldering
> problem we already know since the first generations of the
> GTA04) or the OMAP shows no response or does not recognise the
> µSD card for unknown reasons.
Because we are a community to share experiences and some people
expressed (unfortunately not here) the question how Nokia was
obviously able to build the N900 or N9 and doubt the warpage
effect, let me write down some of the thoughts:
* we do not know how much scrap others do or did produce (such info
is usually not shared with the public), so our 30% yield may
be equally good. Or may be worse. Since it is >0 cases you only
see the successful results in the wild.
* most likely, they were allowed to scrap 1000 units at a total value
of let's say 300k€ to learn the process and then they were able to
build 1 million units at high yield. We simply don't have that
much money for experimentation with the production process. I.e.
we want to win champion's league but can't afford to do the complete
* there are subtle variations between the chips. Basically the OMAP
is a tiny 0.1mm PCB where the Silicon die is sitting in the middle.
The thickness of this PCB has a big influence because it is soft outside
of the Si-chip and the exact size of the Si-chip also has an influence
because it makes this PCB stiff in this area. TI did shrink the DM3730
compared to the OMAP3530 and made the PCB thinner. So it is a completely
different thing wrt. thermo-mechanical effects.
* the PoP-RAM chip sits on top and has a hard plastics case and does
not warp, but modifies the way the lower chip gets heat and how far
it can move until the upper side of the Si-die touches the bottom of
the PoP-RAM and starts to lift it off. Even the exact material of the
BGA balls of the PoP-RAM can have an influence how much they "pull upwards"
the DM3730 carrier PCB. Or press down the Si-Chip onto the PCB.
* due to this special construction the BGA balls under the OMAP die
behave differently than the BGA balls where no die is above. The effect
is that we either have shorts or open joints in the inner or outer ring.
Finding a compromise between Scylla and Charybdis, where we have neither
shorts nor open joints, is the key to high yield. It looks (really looks
in X-ray) as we simply have not yet found the best compromise and now ran
out of money and components during this search.
* it looks as if the iMX.6-PoP chips use a hard plastics case for the
processor - so that it can't warp at all and all BGA balls on the lower
and upper end have the same mechanical background. So one could say we
fight with a design weakness of the DM3730 package design. To be fair it
must be said that the PoP stack of the DM3730 appears to be thinner than
the iMX.6 which might be crucial to squeeze the PCB into a smartphone
case without making bulges.
* and not to forget: we don't have 0% yield, like it would happen with
a simple software or hardware bug. Those are usually show-stoppers and
when fixed, it goes perfect afterwards. We have 30% yield. This means that
production is possible, but just too expensive for us to find and eliminate the
final disturbing factors. So it is mainly a financial limitation we have.
If the community can afford to pay 1500€ for each GTA04A5, we can start over
tomorrow. So one could even develop a crude view that the bug is that the
community can't afford what we are doing. Well, I have no idea how to fix
such a bug... Would be nice to learn.
* others failed as well with this or similar combinations. For example
the OpenPandora was initially planned to be built in the US by the same
production line as the BeagleBoard was built. But if you ask Pandora owners,
they will tell you about the pain when it initially failed.
* we then suggested to ED to go to the production company we now have used as well,
and the OpenPandora 600MHz (OMAP3530) was successfully built. When they switched
to the 1GHz Pandora some problems came back again, but were solved again. What we
also have to consider is that during this time people have changed and machines
were upgraded which is a process that usually looses some knowledge or has to
be regained for a modified situation.
* for the GTA04A3/4 we also had to fight the same issues at a different production
site. And after a long learning curve it worked better. You may ask why we didn't
choose them again. Well, they knew about the low yield problem and they would have
asked for much more money for trying production again without warranty...
So it is like soccer. You know all rules, you know the dimensions of the goal, the
ball and the shoes, the playground and that the grass is green. You also know your
players - but making a goal is not at all guaranteed by this knowledge. It needs
training and luck.
Hoping this big bunch of experience helps other projects to succeed,
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