Board to Board Connectors: mechanical stability and reflowing

This post is related to Controlled Impedance Board-to-Board (antenna), but as it’s a new topic I have broken it out to make finding information in the future easier (I generally like one question/topic per thread! reading that thread will provide more context :slight_smile:

@ToyBuilder the image you show here (Controlled Impedance Board-to-Board (antenna) - #4 by ToyBuilder) is very close to what I have in mind - though the boar-to-board I was considering was this: (I don’t need many pins, likely power, serial and a few GPIO for control signals) - I was taking inspiration from the CM4.

I had a couple of concerns and maybe you can help me out

  1. the CM4 uses two connectors which provides some mechanical stability; in your design you use an edge connector and a retainer; any advice? As I say, I anticipate only needing one connector, and I am looking for as low a profile as possible; maybe use two connectors anyway, get the stability and route some extra functionality? a low profile standoff/retainer?

  2. although I am no where near producing anything, I am looking ahead about things that might be an issue. I anticipate probably having the main board assembled for me, as it will be generic and the part that has the most volume. The module/daughter boards though will vary, so I am thinking I will assemble myself - I was inspired by this short from adafruit on prototype assembly (Adafruit ItsyBitsy RP2040 prototype assembly timelapse - YouTube) and thought to do the same (if i can keep the board to < 30mm x 30mm!) - thinking about assembly order - as the reflow plate will do single sided, if i can keep the vast majority of parts on the top side, then use the reflow plate to solder the top side; then use hot air to reflow the B2B connectors on the bottom side?

  3. again, i was looking at the CM4 for how they route to the connector from the top side, and not sure if i was expecting anything else, but its a LOT of via’s - there must be a ton of work that goes into routing most of that through vias, esp. with signal integrity of high speed lines? Fortunately I am not looking at anything like that at this stage… maybe if i ever get there USB would be something I might look at.

I didn’t design the Blues Wireless Notecard – just the carrier for it – the Notecard has components on both sides of the board which are big/heavy enough that it probably made sense to not use a mezzanine connector. I think they are mechanically more vulnerable to damage and unmating under shock.

I would urge caution about using two mezzanine connectors. Many of them are not intended to accommodate alignment issues.

Samtec discusses this issue.

Depending on the volume you are expecting to see and the complexity of your board, deciding to go for hand assembly might be something you will regret later. People often overlook how many hours you will be sitting at your workbench in a bad sitting position which will generate back pain/fatigue.

SMT assembly is cheap these days if you can fit within the constraints of a service like JLCPCB.

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yes, that was a real concern actually; especially if hand assembling

yep, for sure - i am only looking to do a couple prototypes; but even then JLCPCB might be a better option