QFN solder paste stencil guidelines
To better illustrate the proper way to make your solder paste stencil for QFN parts, as I discussed in the post just before this one, I went to the back room and took a couple of photos of good and bad.
This is essentially what happens with too wide an opening for the center pad on the QFN. The part high-centers and never gets the opportunity to contact the signal pads. In some case, the part will cock a little sideways and contact some of the signal pads but not all.
This is what a worst-case stencil would look like. Note how much surface area that the center pad has compared to the row of side cut-outs. Actual size for this part is 5 x 5 mm. By default, we typically reduce the size of such cut-outs already, but in a case like this, it is difficult to reduce it enough and still get even paste distribution. The proper option is to segment to solder stencil area.
This is one example of a recommended practice. We would actually suggest even a little less coverage. The basic idea is that you distribute a lower quantity of solder over a broader area. You reduce your chances of high-centering and other problems associated with large paste areas, such as outgassing and spattering.
Just make sure that the openings match your copper layer underneath the stencil openings. Some parts require that the copper pad be segmented also. Be sure that your stencil openings only fall above the copper and not over any solder-mask or bare-board sections.