[Arm-netbook] Schematic and PCB layout CAD files

[Paul Boddie]

On Friday 31. May 2019 23.49.57 Luke Kenneth Casson Leighton wrote:
> On Saturday, June 1, 2019, Paul Boddie <paul at boddie.org.uk> wrote:
> > 
> > The inner dimensions are actually interesting for practical reasons, which
> > was why I was asking about the range of PCB sizes.
> 
> If there is no user facing connector, there is the possibility of a range
> of length.
> 
> If it is ok to stick the PCB out the end of the Card just like WIFI and Eth
> PCMCIA used to do, length is limited only by practicalities.

Yes, this makes sense.

> Otherwise, there is no "range": the casework really does define the PCB
> size to around a 0.1 mm tolerance or less.

It was mostly needing to know how wide a PCB might need to be. Obviously, one 
can play it safe by keeping the width well within 54mm.

[...]

> > There was a run of cards done at one point (or more). Did anyone actually
> > do anything with those cards at the time? I seem to remember confusion
> > about engineering boards, people having one kind of board and not the
> > other, and so on. Did they all end up in people's desk drawers or
> > something?
> 
> Pretty much.

Do you have any reflections on that? Was there a realistic expectation that 
people might develop software or hardware to take advantage of those boards? 
Could more have been done to accelerate the initiative with this access to 
actual hardware?

[...]

> > Again, it was interesting to see this with regard to what kind of PCB
> > sizes would actually be produced.
> 
> 78.1 x 47.3 is what is required for the litkconn casework, and there is no
> wiggle room on that.

Right.

[Amphenol 95622-004LF]

> > When you note that 1.5mm is useless, do you mean that within a housing
> > (casework), a 1.5mm board takes too much space from, say, 3.3mm (Type I)
> > or 5.0mm (Type II), and that a thinner board is needed?
> 
> Both.

[...]

> They are. This was described several times on this list and in at least one
> update.

Knowing your policy on information existing only on the list, I had hoped to 
see more information on the wiki, but it is useful to get more details again 
now.

> The stainless steel is 0.1mm thick.  That leaves 4.8mm.
> 
>  The SDMMC sits 1.9mm above the PCB, as does the mid mount USB OTG.
> 
> The 2.2uH Inductors were also very very specifically chosen to fit under a
> 1.9mm threshold, they are 3.2 x 3.2 mm and are quite expensive compared to
> cheaper wire wound inductors which come in around the 2.5mm height mark.
> 
> Also the SoC is around the same height, plus various large capacitors (1206
> 10uF) and one large diode have all had to be special item "Low Profile"
> orders, all 1.9mm or lower.

I must admit that I don't have a wide experience of SoC heights, with the only 
one to hand here being an Ingenic JZ4780 that doesn't really seem very tall.

> So now we are down to 2.9mm.
> 
> On the underside the *MID MOUNT* Micro HDMI and USB OTG connectors sit
> 1.6mm below the PCB.
> 
> No components above that height are permitted, it has meant a couple of
> redesigns, moving some large 0805 capacitors topside.
> 
> These underside capacitors, particularly under the CPU and DRAM, are
> absolutely essential for stabilising power during peak loads, and they
> clearly cannot go on TOP because they have to be extremely short tracks.
> Being NEXT to the SoC would NOT be okay. They HAVE to go underside, as
> close to the BGA pin where power is drained as physically possible.
> 
> Aside from the Micro HDMI and OTG mid mount connectors there is nothing
> over a 1mm height. Still, that is enough.
> 
> 2.9 minus 1.6 is 1.3mm
> 
> That leaves a mere TENTH of a millimetre spare clearance if using a 1.2mm
> PCB.
> 
> Can you see that for this type of design a 1.5mm PCB would be completely
> impractical?

Yes, it mostly makes sense.

> If on the other hand there were no mid mount connectors it MIGHT be
> possible.
> 
> Anyway this is why Litkconn header and casework were selected because it
> has all 68 PCMCIA pins on TOP instead of a twin pair staggered in height.

Are such headers (I wasn't aware of the terminology) easy to find? Searching 
for PCMCIA headers in the mainstream channels, even using convenient sites 
like Octopart, is excruciating.

[...]

> > This is what I found, but I wondered why it wasn't mentioned on the
> > specification pages, or why 55mm appears in the diagrams. I did find two
> > pages with the 85mm x 54mm x 5mm dimensions on the Rhombus Tech site,
> > however:
> If you see any that are wrong please do correct them.

The diagrams are the challenge since they are not readily editable. However, 
they possibly need reworking, anyway, partly due to their vintage.

[...]

> > Again, not having any experience with the casework, it would be
> > interesting to know what the margins are, especially if ports would be
> > exposed.
> 
> 0.1mm would be an acceptable tolerance.

Right. Good to know, thanks.

[...]

> > Yes, so my query was mostly motivated by a lack of familiarity with the
> > variables unknown to me, which are mainly related to how bulky the
> > casework is.
> 
> The case is very very specifically tied to the connector.
> 
> It's no good ordering random headers and hoping that the plastic and metal
> case will fit it.
> 
> The ends of the connector for example may have a different shape from
> another manufacturer part.
> 
> Bottom line, it is basically absolutely critical to get a matched set of
> casework and connector.

Yes, I understand that although the connector should be standardised, how the 
"non-mating" side of the connector fits into the housing might be wildly 
different from one kind of connector/header to the next.

> The casework forms the basis of fitting in the rails.
> 
> No casework, the bare PCB can be misaligned on insertion, not just
> horizontally by a couple of pins, it can even be misinserted by an entire
> row.

This seems pretty unfortunate, really. It does seem that the Amphenol 
connector tries to guard against this - as well as I interpret the drawings - 
and I wonder if others also do so. But I wouldn't really know about these 
kinds of assembly problems.

Do you think that, ultimately, some other connector standard would be more 
accessible? That is, easier to develop for without substantial tooling 
investment.

> I deliberately overordered litkcon cases and headers, as a just in case.
> And the AMP socket and rails, too. Meaning, some can be ordered from Mike
> direct, if you really need them.
> 
> And overordered somewhat on the JAE mid mount micro HDMI Type D, after
> going through FOUR redesigns due to HDMI Connectors going EOL, sigh

Yes, very frustrating.

Paul