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I am trying to build a PCB board that I found on GitHub. In a rash we ordered few sets of the PCBs without inquiring about the parts availability. Turns out one of the parts is not available in all the parts websites (including Ebay). It is QFN-28. However, I think I can find the chip in TSSOP28 package.

Is it possible to design (or find elsewhere) an adaptor PCB that will mount (solder) to QFN-28 footprint and then lead out the pins to TSSOP28 footprint on top? This is similar to this question, however, my new chip is bigger than the original footprint. I have done PCB to PCB soldering before but it has been with 0.1" or 0.05" half-cut through holes. QFN is much trickier. Luckily the area around the QFN footprint is free so fitting a bigger board won't be a big issue.

One idea I had is to design a small rectangular PCB with QFN-28 on one side and TSSOP28 on the other side and then SMT solder, but I'll have no way of knowing if it's correctly soldered.

The other idea is to have the same but with square QFN28-sized cutout so I can inspect the soldering job.

Does this make sense? Or should I just redesign the board slightly to accommodate a different chip? Unfortunately I did not design the board myself, but just downloaded the schematics and premade gerber files, so recompiling might not be trivial.

FYI, it is a 16-bit IO expander (MCP23S17), and I could not find pin compatible.

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  • \$\begingroup\$ I have my doubts that this surfboard could work, but I've seen stranger things work as well. The bottom would be QFN28, which is small. The top is TSSOP28, which is likely to be larger. Therefore you would have a blind land-grid array type of thing going. The risk of soldering problems is high. Also, and especially lately, I make sure the BOM is purchased and on its way before committing the PCB. Availability has been such a huge problem. \$\endgroup\$
    – Smith
    Commented Aug 25, 2022 at 3:38
  • \$\begingroup\$ QFN28 is a horrible package to work with, so I wouldn't recommend any dirty patches - they will simply not be reliable. \$\endgroup\$
    – Lundin
    Commented Aug 25, 2022 at 13:12
  • \$\begingroup\$ It is not impossible to edit gerber files manually but is there space for 8 x 10 mm.. \$\endgroup\$
    – Jens
    Commented Aug 27, 2022 at 0:29
  • \$\begingroup\$ Yes there is space luckily. While we were in the process of redoing the board the authors uploaded a new version of the project that makes no use of this chip so we are just going with the new design! Thank you. \$\endgroup\$
    – Swetko
    Commented Sep 19, 2022 at 4:18

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