[Gta04-owner] GTA04A5/Letux 2804: status update

H. Nikolaus Schaller hns at goldelico.com
Thu Feb 25 14:22:35 CET 2016


Hi all,

Am 19.02.2016 um 18:50 schrieb H. Nikolaus Schaller <hns at goldelico.com>:

> Hi all,
> I have promised to give more regular status updates on this project.
> 
> 1. WLAN 5 GHz
> from the discussion I got the impression that there is a majority who
> wants 5 GHz. It is just replacing the WL1835MOD with WL1837MOD
> and one chip antenna with a different one.
> 
> So we can decide between at the last possible moment, which is when
> components are ordered.
> 
> There is also some discussion about new FCC rules. Well, we already have
> the problem that we don't have source code for the firmware of the module.
> 
> Rather, we have to download a firmware blob from Texas Instruments to
> use the module. And I assume that the boot loader of the module does
> some CRC check, so nobody (and we as well) are able to modify the firmware
> that is certified. Since the modules already have a FCC precertification,
> this is sufficient to fulfill all new rules that nobody can modify the radio
> spectrum.
> 
> It is not nice because it is against our idea of software/firmware freedom,
> but to be honest, we are a too small project to change the world. We can
> only fight where we can, and that is the application processor running
> free and open software (Debian, Replicant, QtMoko etc.).
> 
> 2. Time plan
> I had planned to make the 1GB RAM soldering experiments this week,
> but the tool to help getting a tiny amount of solder paste on the chips did
> not yet arrive. Looks as if it comes mid of next week.

The tool we have designed was manufactured and did arrive in time. But it
does not yet work as expected. So we will have to get a modified variant.

Some of you may be interested in what we are doing here, so let me explain
a little:      

The key task is to rework an existing GTA04A3 or A4 board with the 1 GByte
RAM chip before we can have all GTA04A5 produced with that one.

The good thing is that we can take a fully populated GTA04A3/A4 board and
already can remove the OMAP and the PoP chips. This is a process which
works fine without damages.

The other way round is the difficult one: to solder a new chip onto an existing
PCB where there is already some solder on the copper pads. Initial experiments
were disappointing. The chip did swim on the solder flux and was soldered
after moving several 100µm to the side.

On a fresh PCB we use a laser cut stencil which allows to print a very tiny
amount of solder paste on each of the contact points of the OMAP3 BGA.
This solder paste "glues" the chip at the right position and there is not too
much flux and therefore the chip does not swim away during reflow soldering.

But since we already have components on the boards for rework, it is
impossible to use the stencil printer and print solder paste onto the PCB.

So the idea is to print the solder paste on the OMAP chip (it is like if you
want to glue two things together: it usually doesn't matter on which part
you put the glue on).

This is where our specially designed tool comes into the game. It should
allow us to to place the OMAP CPU chip (which is just 12 mm  x 12 mm)
in the stencil printer and keep it in place.

Basically it works, but what does not yet work is to fine-adjust the stencil
so that the tiny openings (ca. 300µm diameter) are exactly over the OMAP
BGA balls. So we get the solder paste printed at the wrong locations.

You may wonder how others are doing such rework. The answer is: they
rarely can do it with such fine-pich and package-on-package chips (like the
OMAP3 is) at all. It is a task that many SMD repair houses decline to do.
And unless you have miniaturized systems like smartphones, chips are
much bigger, less critical in placement, reballing and resoldering.

There are even DIY videos on youtube. But that is for 1mm pitch (and not 0.4mm).
Or they have a very expensive rework station with laser optics, vacuum nozzles
and 4D positioning system. And everything in precision for 100µm.

This is where we hope the redesigned tool will be better. Next week.

BR,
Nikolaus

> 
> 3. Letux Kernel
> is getting more stable again as we test it now on the Pyra hardware and
> the GTA04. This reveals some tiny subtle bugs. Nothing really important,
> but for the future (e.g. an issue in the touch screen driver).
> 
> BR,
> Nikolaus
> 
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