Replacing TQFP chip with QFN chip

Community PCB Design
1

Dear Forumers!

I’ve designed a PCB for home usage and I’ve selected a most appropriate chip (PIC MCU) for the tasks to serv. When I started to design the PCB I assumed that the chip is available in TQFP 64 package, so all component placement and routing relied on this plan. Suddenly it’s reveiled that the chosen chip is not available in my country in TQPF 64 package. I can only order form the distributor in QFN 64 package.

My question is that, can I simply solder a QFN 64 chip onto the pad of a TQFP 64 chip? I think you can guess that many vias and routings are beneath the thermal pad of the QFN 64 chip. My PCB will be manufactured with high class PCB manufacturing technology, so the via’s placing will be accurate enough, and also solder mask will be top the vias and the top layer of the PCB.

Can it cause any problem in normal operation?

Thank you in advance!

2

Since the leads of the TQFP are “out” and the QFN leads are “under”, I suspect that you can’t just drop a QFN onto a TQFP land pattern even if the they are otherwise the same. Note that sometimes the two are not even the same body size or pin pitch. The Microchip doc I found does not even list QFNs beyond 44 pin so I don’t have a definitive answer for you.

The thing to do is to compare the land patterns between the two packages. Assuming you have land patterns for both packages in your library, I would put both onto a test board layout and try to overlay them. You will quickly see if they are anywhere near the same size and shape. If the pads of the QFN are totally covered by the pads of the TQFP (without shorting the thermal pad) then you are in luck. If not… well you’ll just have to see.

  • Chip