Is it feasible to put solder mask cured on the top of tinned pads?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.