Do people use the default library footprints for DFN and QFN chips? The ones in the Housings_DFN_QFN library?
The reason I ask is because I was initially very happy that they are all there, but then noticed that they all have thermal pads designed in, with a huge stencil opening. I already discovered the hard way that this is not a good idea and that a hatched pattern is much better, limiting the amount of paste applied in the middle.
Moreover, some QFN chips I use do not have a thermal pad, in which case this opening does even more harm.
So, what do people do? Do you just design your own footprints for everything, or is there another library out there somewhere that I don’t know about?
Generally I’d advise a custom footprint for any package that has a thermal slug. There doesn’t seem to be a standard for the size and shape of these slugs so each manufacturer’s recommendation for paste window seems to be different. I’ve had serious problems with a window size error of half a millimetre so best to go with manufacturer recommendation.
If it has no thermal pad I guess it should be a fairly standard footprint, in which case you should be able to reuse it, but I still make my own or at least check the measurements against the manufacturer’s data sheet. It takes a while but very rarely results in bad boards.
Sigh. I guess you’re right. I will have to make my own.
Still, it bugs me that we all duplicate so much effort — everybody keeps making their own footprints from scratch. I wish manufacturers provided good, verified, easy to use footprints.
TI kind of provides footprints — they have this weird converter program, and you can supposedly convert their data files into whatever format you need, but in practice I found this is unusable, at least with Eagle. Farnell kind of provides footprints in that you can download an Eagle script file and with a weird procedure import and use it.
I think we could do better. And at the very least, I think a set of QFN packages without thermal pads would be a nice addition to the standard KiCad libraries — I’ll see if that can be done programmatically.
The big pads are divided into smaller to limit amount of solder paste. It’s not good idea to create QFN without thermal pads by removing it because it’s possible that recommended size of pads in QFN without thermal pad differs. I think that these QFN/DFN may be usefull but usually should be adjusted to users needs by editing sizes of pads. These QFN/DFN land patterns were generated according to datasheets except for thermal pad solder paste pattern which is usually not specified and technology dependent (user should set correct solder paste ratio in pads properties).