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I made a batch of PCBs with a QFN package IC. I accidentally connected the pad to GND even though it should be either left floating or connected to some signals else. I am glad that I have requested a few samples before I made the PCB fab house assemble all of them.

It would be very expensive to re-order the boards, and there are vias-in-pad connected to internal layers. So I think that disconnected the IC's exposed pad from the PCB's pad with additional solder mask should be a viable solution.

As long as I know that the pads are tinned with HASL. Would you suggest whether my next few actions are feasible or not:

  1. I request the fab house to mask the pads and assemble the boards.
  2. For my received sample boards:
    1. I desolder all the affected QFN chips
    2. remove all the solder with the copper braid
    3. clean with isotropic alcohol.
    4. Then either apply solder mask paint cured by UV light or stick Kapton tapes to cover the pad.
    5. Finally re-solder the parts with heat gun.
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    \$\begingroup\$ Anything you stick between the pad and the chip is also going to raise the signal "pins" off the board by the same amount. \$\endgroup\$
    – Dave Tweed
    Commented May 16, 2023 at 19:50
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    \$\begingroup\$ How many units are you ordering, and are you confident in soldering the QFN yourself? \$\endgroup\$
    – LordTeddy
    Commented May 16, 2023 at 21:28
  • \$\begingroup\$ Depending on the board design and how many you have, another possibility may be to scrape the copper off the top layer so the pad is there and solders to the chip but doesn't connect to anything. (The fab house certainly will not do this; scraping copper off is an at-your-own-risk type of thing) \$\endgroup\$ Commented May 16, 2023 at 21:37
  • \$\begingroup\$ "or stick Teflon tapes to cover the pad." I never knew you could do that. How do you get Teflon tape to stick? \$\endgroup\$
    – jy3u4ocy
    Commented May 17, 2023 at 4:33
  • \$\begingroup\$ @LordTeddy There are 20 units ordered. And I am not that confident to solder QFN myself. They are surrounded by the through-hole parts, which makes them even harder to work with IMO \$\endgroup\$
    – ONLYA
    Commented May 17, 2023 at 9:49

3 Answers 3

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The fab house is unlikely to be able to mask the exposed pad. You could ask but prepare to be disappointed.

If this is a small run, have the boards assembled, but leave the QFN unpopulated (change the BoM to mark it as DNI). If the fab is purchasing the parts for you, have them ship the unassembled QFNs to you with the board. Make sure also to send an updated Gerber file for the solder paste stencil to remove the openings for the QFN so that the pads don't end up with reflowed solder on them. This will avoid the unnecessary desoldering and cleanup.

For manually assembling the QFN, you'll want to cover the exposed pad in a heat resistant lacquer that is as thin as possible so as to avoid raising the QFN pads off the copper too far. Ideally something as thin as the existing solder mask. You could use Kapton tape (polyimide) but that may be too thick (though would be a better choice than Teflon tape), there are places that will sell 25 micron thick stuff (including adhesive) which might be ok.

Once covered, you could then use some low temperature solder paste (optionally applied with a tiny stencil just the size of the QFN if there is enough space around it) and either a hot plate or air or IR heater or reflow oven. The lower temperature paste avoids risking desoldering the existing joints.

For larger runs, it is probably more economical to simply respin the boards. Doing anything nonstandard like asking the assembly house to start covering pads is probably going to cost more than remaking the boards.

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  • \$\begingroup\$ Thank you! How about the solder mask paint? I think this could be a better option than the kapton tape. \$\endgroup\$
    – ONLYA
    Commented May 17, 2023 at 9:54
  • \$\begingroup\$ @ONLYA if you can paint it on thinly and cure it, solder mask paint should work fine. \$\endgroup\$ Commented May 18, 2023 at 9:45
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Can't you lift and scrape off the pad with a knife for your received boards? You can also take a small twist drill and manuall drill out vias or traces connecting the pad to GND.

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You could consider buying polyimide adhesive dots which are available in 1mil thickness, however once you add silicone adhesive they're more like 2.5mils.

For assembly, you should talk to your assembly house or assembly people. For small quantities, since the coplanarity specification for QFP is +/-4 mils, I suspect this will work, perhaps with a bit of extra attention to the stencil printing parameters to get a decently thick paste layer. Contraindications would include very small parts eg. 0201 on the same side of the board which require relatively thin paste.

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