[Arm-netbook] Schematic and PCB layout CAD files

[Paul Boddie]

On Friday 31. May 2019 17.06.14 Luke Kenneth Casson Leighton wrote:
> On Fri, May 31, 2019 at 3:47 PM Paul Boddie <paul at boddie.org.uk> wrote:
> > Also, the A10 news page has mentions of these dimensions from several
> > years
> > ago:
> > 
> > http://rhombus-tech.net/allwinner_a10/news/
> > 
> > However, the PC Card documentation indicates that the actual housing of
> > such a card is only 54mm wide, so I don't see how existing PC Card
> > housings would accommodate such a PCB.
> 
>  that's what cost USD $15k when the engineer we paid didn't read the
> spec properly.  i told him to source one of the caseworks, measure the
> INNER dimensions of the area as defined by the plastic, and use those.

The inner dimensions are actually interesting for practical reasons, which was 
why I was asking about the range of PCB sizes.

[...]

> > I do remember that EOMA68 cards (maybe of an earlier generation) were
> > produced.
> 
>  yes.

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?

> > Then again, looking at the A10 news page, there is a picture of a
> > PCB layout from 2013 with dimensions of 78.1mm x 47.3mm, although it isn't
> > completely clear what the screenshot is really showing.

Again, it was interesting to see this with regard to what kind of PCB sizes 
would actually be produced.

> > Another thing that I wondered about is the width of the board when
> > accommodating a board edge connector like the Amphenol 95622-004LF, which
> > seems to be a low-cost and readily-available connector. It seems that the
> > board edge needs to be less than 50.8mm across because such connectors
> > enclose the contact area on each side.
> 
> https://cdn.amphenol-icc.com/media/wysiwyg/files/drawing/95622.pdf
> 
> that's a really weird connector.  it appears to be a socket, however
> it is one that fits on the *edge* of a PCB, of a very specific height
> (am having difficulty working that out from the diagram).  it's
> probably requiring 1.2mm PCB however that is guess-work.
> 
> no wait, Cross Section C, it's 1.5mm, and that also tells us it's the
> PCMCIA *header*.

Yes, my interpretation of Section C-C is that 1.5mm is the minimum board 
thickness, with the usual 2.2mm connector.

> btw, 1.5mm is useless because the clearance on TOP components is
> nowhere near enough.

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?

Computer card heights/thicknesses mentioned here, by the way:

https://www.elinux.org/Embedded_Open_Modular_Architecture/EOMA68/FAQ

I could understand 3.3mm being difficult, with 0.9mm left on each side of the 
board, but 5.0mm should leave 2.6mm (minus casework) on one side. Then again, 
I know the margins can be pretty small with these things.

> > I am only really asking these questions because I have been looking at
> > making some footprints and other resources, and at least the fundamental
> > board dimensions should be an obvious thing to discover, but I just
> > didn't see them mentioned as prominently as I had thought they would be.
> 
>  the PCB has to fit inside the casework, and the casework's *external*
> dimensions are required to conform to PCMCIA.

Yes.

>  PCMCIA, is, obviously
> (https://www.google.com/search?q=PCMCIA+card+dimensions) 5.0 x 85.6 x
> 54.0 mm

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:

http://rhombus-tech.net/allwinner/a31/orders/
http://rhombus-tech.net/freescale/iMX6/orders/

>  however the reason why there's no prominent mention of the *inner*
> dimensions is because, obviously, they're critically dependent on what
> *casework* is chosen, *NOT* repeat *NOT* on a hard spec.
> 
>  it is *PCMCIA case size conformance* that is the hard requirement,
> *NOT* the actual PCB size.

Yes, it was really for an indication of what kind of PCB sizes result from 
these hard requirements that prompted me to ask. That and a need to question 
why there were boards made that were wider than the apparent external width of 
the card housing.

>  example: the PCB size (its length) will also critically depend on the
> PCMCIA header dimensions.  if the header is N mm deep, then obviously,
> if you make a PCB that is exactly 85.6 mm long, it will stick out the
> end of the PCMCIA casework by N mm, won't it?

Yes, so my impression is that a PCB using the Amphenol part referenced above 
would give up 6mm of the total board length budget, yielding a 79.6mm PCB, 
perhaps minus whatever any rear part of the casework might require. Again, not 
having any experience with the casework, it would be interesting to know what 
the margins are, especially if ports would be exposed.

>  so this is why you don't see "PCB dimensions" mentioned as a fixed
> quantity anywhere in the EOMA68 specification, because the PCB
> dimensions must be *CALCULATED*, based on the PCMCIA *PARTS* that are
> selected for use in production.

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.

Paul