Optiplex 5070 SFF replacement motherboard

For a direct fit, NO.  But, if you are good with tools and don't mind the cosmetic looks, you can try as they are very similar in shapes with some differences of ports locations.  Just note that the total cost of gathering parts may exceed the cost of a complete (used) system.  Because you will need to upgrade virtually everything else but the case.

OptiPlex 5070 SFF with system board:

OptiPlex 5090 SFF with system board:

the cpu and cpu socket for 5070 is not compatible with 5090

5070 LGA 1151 Intel 8th/9th gen

5090 LGA 1200 Intel 10th/11th gen

if you want to replace a bad 5070 mobo w a 5090, you also need a new cpu

also, Dell make sure you cannot do this easily.

the psu for 5070 and 5090 have different connectors for motherboard.

5070: 6+4 pin

5090: 6+4+4 pin

so your 5070 psu probably would not work for 5090 sff board.

forget it.

@Chino de Oro​ Thank you for the info. I can see that the screw pattern and location of both machines are very different indeed.

@redxps630​ Thank you, I understand the CPU bit. But I think Chino's answer is better.