Can a small PCB be used as a SMD?

[kizmit99]

Hi all - odd question here...

I've just had a 4 layer PCB manufactured that has a surface mount HDMI connector on it (driven from a Cyclone 10LP FPGA)...
Unfortunately when assigning pins to the FPGA I missed the fact that true LVDS outputs can only be assigned to the left and right IO blocks and I used a top IO block for the HDMI outputs.  So, I need to use LVDS-E-3R outputs instead, which required 3 external resistors to produce the proper outputs.

Instead of re-spinning the main PCB I'm wondering if I could make a small 2 layer adapter PCB that has the original HDMI footprint mirrored on the bottom, the necessary traces and additional resistors, plus a new connector footprint on the top. 
I'm thinking this adapter could then be soldered to the original board in place of the original connector.  Effectively I would be trying to sandwich two PCBs together with overlapping mating pads between the two boards (not using castellated holes around the edges).

Is this something worth pursuing, or is it doomed to failure from the get go?

[Nusa]

I'd think that making an adapter cable (which could have a small pcb between the two connectors if that's required) might be easier, assuming that's the only problem with your first-run pcbs. You probably are going to do whip up a protoboard version to test your first unit anyway.

Naturally, fix it properly if you need to order more main pcbs.

[kizmit99]

I'd think that making an adapter cable (which could have a small pcb between the two connectors if that's required) might be easier,

Yeah, my first thought was a small adapter board with a male HDMI connector, the necessary resistors and the female connector.  In fact, I ordered some of the male connectors from the only place I could find *any* - aliExpress (should be here sometime around thanksgiving I would guess , good news is that I think it cost me a whopping $3 for 10 of them)
But then I got to thinking about this approach and I'm wondering if it could possibly work?  It seems feasible to me, sort of like a small 12 pin bga (without the grid, or balls)...  I'm guessing the adapter PCB would probably only cost a few dollars (shipped) from OSHPark, being it's so small.

assuming that's the only problem with your first-run pcbs. You probably are going to do whip up a protoboard version to test your first unit anyway.

In some ways this is actually the third revision of this design (so a lot of it has already been shaken out), but I have added a bunch of stuff (including the HDMI output) so in other ways it's really a first prototype.  In any case, I do expect I'll find other problems, at the moment I'm hoping they'll be things I can work around without re-spinning the board.

Naturally, fix it properly if you need to order more main pcbs.

Totally agree, if I end up redoing it I would make the correct fix (either assigning the proper FPGA pins, or adding the external resistors).
This exercise is really just trying to figure out if there would be a (really cheap) way of making a half-way decent (looking) bodge to correct this problem.

Thanks for the thoughts! 

[langwadt]

I'd think that making an adapter cable (which could have a small pcb between the two connectors if that's required) might be easier, assuming that's the only problem with your first-run pcbs. You probably are going to do whip up a protoboard version to test your first unit anyway.

Naturally, fix it properly if you need to order more main pcbs.

yeh, soldering the fine pitch of smd hdmi connectors is usually painful enough

[KL27x]

Yeah, you can solder pcb to pcb. Check out DC-DC converter modules. They sell them completely assembled onto PCB-as-component.

[NorthGuy]

Is this something worth pursuing, or is it doomed to failure from the get go?

I've done this several times. Put solder on both PCBs, add flux, put them together and then heat with an air gun. Verify the soldering with DMM. Then solder the components to the top PCB. If the pitch is 0.5mm or bigger this is easy. With 0.4mm pitch there could be not enough solder, especially if solder mask is thick, so you may need to add some solder paste.

[langwadt]

Is this something worth pursuing, or is it doomed to failure from the get go?

I've done this several times. Put solder on both PCBs, add flux, put them together and then heat with an air gun. Verify the soldering with DMM. Then solder the components to the top PCB. If the pitch is 0.5mm or bigger this is easy. With 0.4mm pitch there could be not enough solder, especially if solder mask is thick, so you may need to add some solder paste.

yeh, but the way the pins are it is real easy for solder to wick up between the pins and it is pain to get out

[ejeffrey]

You could find out what adafruit is using for this product: https://www.adafruit.com/product/3548 or just buy one and desolder the connector.

[graybeard]

I have seen it done with sawed through vials as the solder connections

[KL27x]

The PCB fabs sometimes call them castellated thru holes. They might cost you extra.

[kizmit99]

Thanks guys...

Yeah, you can solder pcb to pcb. Check out DC-DC converter modules. They sell them completely assembled onto PCB-as-component.

I've done this several times...

Good to hear.  I'll consider pursuing this approach. 

I have seen it done with sawed through vials as the solder connections

Yeah, I was trying to avoid castellated holes, just to keep the price of the adapter boards down.  Afterall, at some point (pretty quickly in fact) it'll end up cheaper just swallowing the cost of a redoing the main board...

You could find out what adafruit is using for this product: https://www.adafruit.com/product/3548

Very interesting find.    That looks like the same connector I found.  I'm pretty sure it's meant to mounted through-hole vertically, but they's slipped the pins over the edge of a PCB.  I had wondered if that would be possible, apparently it will be.  That approach would make for a very decent, compact, little adapter board.  Mounting it through-hole would have ended up with a right-angle adapter, which for some reason I wasn't liking...  If the parts I ordered get here in any kind of reasonable timeframe I may go with this approach instead (although the idea of trying the SMD PCB is still intriguing to me.