Author Topic: M.2 Connector with bigger (off-spec) board  (Read 1501 times)

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Offline PatchedUp

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M.2 Connector with bigger (off-spec) board
« on: June 04, 2016, 02:26:28 pm »
Hi all,

first post so be gentle  ;D - I've been lurking around the forum for some time though!

I am laying down the groundwork for a project that uses reasonably small daughterboards (~80x45mm) which I was originally going to simply use header pins to connect to the main board. However as the project has progressed I started to consider other ways to connect - one of the main reasons for this is I want to be able to use the same daughterboards in a backplane type add-on as well as on the main board so mounting horizontally or vertically would be nice as well as the connectors being cheaper in the long run.

One of the more preferred methods would be to use the M.2 connectors found in a lot of newer hardware like SSDs and Wifi boards. I can envisage one problem with this however, the M.2 spec only has boards going as wide as 30mm (although as long as 110mm). I could swab the board layout around to be more aligned with the M.2 style  (ie. 45mm wide 80mm long) however I would prefer not to if I could get away with it.

There won't be many components on the daughterboards - the heaviest would consist of 2 DIP24's, 2 DIP8, a DIP14 and a few SOIC's with a few SMD aluminium caps and the rest would be 0603's. The board will be entirely 5V with less than 500mA current and carry digital as well as analogue signals.

My main concern is with the board thickness (M.2 spec's 0.8mm thickness) and stress on the board edge connector. Unlike a usual M.2 connection, the user would be connecting/disconnecting more often but not a crazy amount, probably less than 100 times. The far side of the PCB would still be screwed onto the main board as with any normal M.2 SSD etc.

Would the board edge be up to this task and would there be an excessive amount of stress on the rest of the PCB? I know there have been a few topics discussing flex of 0.8mm boards and the general conclusion was it depends what's on it but generally it should be okay.

I'm not against using any other connector (or indeed retaining the header pins) but the M.2 solution is neat and cheap.

Thoughts? Entirely possible, risky or ludicrous?!

Offline stmdude

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Re: M.2 Connector with bigger (off-spec) board
« Reply #1 on: June 04, 2016, 02:36:19 pm »
Very doable.

Myself, I use 204-pin SO-DIMM connectors in the same way (system on a module things), and sometimes I use 1X PCI-express sockets to hold daughter-boards.

It works fine for development work anyways. Don't know if I'd want to ship a commercial product like that though.. :)

As for the flex of 0.8mm PCBs. Yep, they can flex, but it's most likely not a problem for you. Remember, a PCB is a fiber-glass sandwich construction with epoxy as the glue. It's pretty damn stiff, even at 0.8mm.

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Offline PatchedUp

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Re: M.2 Connector with bigger (off-spec) board
« Reply #2 on: June 04, 2016, 02:44:16 pm »
Excellent, thanks! A nice quick reply!

I had considered SO-DIMM as well but decided that M.2 would suit my needs slightly better, glad to hear it works well though and good to know that the flex wouldn't be an issue either. I will progress down this route currently and prototype it using cheap boards and test the connection to destruction to see how well it fares.

Didn't want to waste any more time if it was going to be a dead-end but if it's looking good from a theory point of view there is only one way to find out!

Offline TiN

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Re: M.2 Connector with bigger (off-spec) board
« Reply #3 on: June 05, 2016, 02:13:47 pm »
As with any edge-connector, you will required to have gold plating on the contacts, or oxidation will quickly render in unreliable connections. A thing to consider.

And if you want to go with edge connectors, why not use PCI-Express ones? They are sturdy, with good spacing and feature size and easy to source. And to get horisontal direction you can design simple right-angle board with connector on it, to act as an adapter.

Other thing is that M.2 slot is not that heavy duty (much less than mentioned SO-DIMM) and any serious mechanical force (e.g. PCB drop on the floor) can cause damage/contact issues, especially given heavy DIP packages and such (why use DIP and SMT connector?).
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Offline PatchedUp

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Re: M.2 Connector with bigger (off-spec) board
« Reply #4 on: June 05, 2016, 06:30:48 pm »
To give a bit more background, the daughterboards will have legacy chips on them (hence the mix of huge DIPs and more manageable SOIC/TSSOPs).

Yep, was planning on having a hard gold finish (ENIG for testing though) for durability.

The main reason I wanted to use edge connectors was they were simple plug and play connectors that would provide a bit of mechanical support. I did also look at PCI-E but the lack of "clean" right angle solution put me off - I guess I could use a ribbon or something to connect from the main board to a chassis mounted PCI express connector and then plug it in that way.

Hit a snag with the M.2 anyway - the tallest board clearance is fine for the chips but the through-hole pins are too long...

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