QFN CAD Libraries
In previous posts (here and here), I've talked about QFN layout issues and solder stencil openings. Here is an example of a the library that follows the component manufacturer's and our recommendations.
Next is the solder-mask area (inverted). Note that the solder mask leaves the entire center copper pad area unmasked. I've seen a few boards where the designer tried to solve the problem by reducing the solderable area by reducing the center opening size in the solder-mask. What that will tend to do is just make the part less stable and cause it to tilt to one side or the other, almost ensuring a non-functional board.
Last is the solder paste layer. This is pretty much a best case example. The solder paste stencil will have a series of small openings in the center, each about the same size as the signal pads on the outside edge of the chip. This gives about 50% paste coverage in the center and will result in even solder distribution and a finished part with a reliable mechanical and electrical connection all around.
If your part libray doesn't look like this, consult with the part manufacture layout guidelines first, but you will likely need to modify the component library before sending the board out for fab and assembly. Some QFN parts require a custom copper pad layout in the center too so you may need to adjust that part of the library as well.
Paste well, my son