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Can a small PCB be used as a SMD?
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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:

--- Quote from: Nusa on August 01, 2019, 12:56:45 am ---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,

--- End quote ---

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.


--- Quote from: Nusa on August 01, 2019, 12:56:45 am ---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.

--- End quote ---

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.


--- Quote from: Nusa on August 01, 2019, 12:56:45 am ---Naturally, fix it properly if you need to order more main pcbs.

--- End quote ---
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:

--- Quote from: Nusa on August 01, 2019, 12:56:45 am ---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.

--- End quote ---

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.
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