[Arm-netbook] EOMA68 Computing Devices Update: Measurements and a Hypothesis

Luke Kenneth Casson Leighton lkcl at lkcl.net
Mon Dec 2 20:05:55 GMT 2019

On Mon, Dec 2, 2019 at 3:23 PM Paul Boddie <paul at boddie.org.uk> wrote:

> Hello again,
> I saw the latest update when it was issued a few days ago, with these
> details
> being particularly important:
> "PCBs are etched with acid, and when the tracks or pads are particularly
> close
> together (such as with BGA balls), not enough acid gets in to eat away the
> copper, and (in a very indirect way) the BGA pads end up being far larger
> than
> they should be. This, in turn, means that when the IC is put on the PCB
> and
> heated up, the BGA balls (which are made of solder) will spread out much
> farther, and potentially even spread so far that they contact each other
> and
> short out."
> https://www.crowdsupply.com/eoma68/micro-desktop/updates/measurements-and-a-hypothesis
> From what it says, it rather sounds like same (or a similar) problem is
> being
> encountered as the one which happened with the Ingenic JZ4775 boards:

yehyeh.  except i didn't know about the PCBs not actually matching the
gerber files, back then.

> "When the first 6 samples were done, only one was found to be working: for
> some reason the DDR3 BGA solder balls had spread and caused short
> circuits.
> One of the samples was still with the factory, and that too was found to
> have
> shorts. Mike has been extremely helpful, and this is the first time that
> he's
> been involved with BGA, where his uncle's factory has been the one that
> assembled the samples."
> http://rhombus-tech.net/ingenic/jz4775/news/EOMA68-jz4775_XRay_Photos/
> At that point in time, I don't remember any more news about resolving the
> matter, and with the campaign being finalised the priority was no longer
> to
> get these boards working.

i did actually partly get them up and running, but i'd put a 24mhz XTAL on
instead of 48mhz, and the SD/MMC wasn't having it.  i tried fixing that in
software (doubling the PLL frequencies for the SD/MMC) but couldn't get it
up and running.

with it only being single-core 1ghz MIPS32 i dropped the investigation.

But perhaps one of these situations informs the
> other, or maybe they inform each other somehow.

i'd forgotten about it, so thank you for the reminder.

> Thanks in any case for keeping us updated!

no problem paul.


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