QFN parts (also known as MLF or micro lead frame) used to cause a lot of problems a few years ago, as evidenced by the number of blog posts covering the subject.

Can I use my own blog as cited evidence to justify my conclusion? Doing so is probably bad form, but I’m doing it anyway. Interestingly, if you look up “citations” in Wikipedia, the entry (as of this writing) has a note indicating that the article on citations has insufficient online citations. Hmmm.

Anyway, it seems that the industry in catching up with the proper manufacturing methodology for use of the technology. It’s important enough though that it bears repeating now and then. The key to successful QFN and DFN manufacturing really is in the solder paste stencil pattern. Consult the data sheet for the part, but if you can’t find the datasheet or if it doesn’t cover the stencil layer, use the window pane technique, or “segmenting” for the stencil layer when you’re making the library part for your CAD software.

If you leave the full thermal pad area fully open, you’ll most likely end up with too much solder in that area. The part will ride higher than it should and may very well float too high for all of the pads on the  side to connect. See the top  part on the above illustration.

Shoot for 50 – 75% paste coverage by segmenting the stencil as in this illustration on the left here. That’ll ensure that the center pad and the side signal lands will be at the same level. You’ll get much better yields and reliability.

Duane Benson
The strangest sight I’ve ever seen
2 buffaloes, 2 buffaloes, buffaloes on my lawn.

http://blog.screamingcircuits.com/