For hand soldering QFN packages, I've had success with putting a very large via (to GND or whatever your pad is attached to) right in the middle of the pad that I can stick a soldering iron tip through.
After drag soldering the perimeter pads, the via lets me solder down the center pad. I'm sure it's not 100% reliable but for a small number of hand soldered parts, it's worked quite well.
For reflow, I typically aim for 50% paste coverage of the center pad, broken up into at least 4 squares/rectangles (more for larger parts). This prevents the part from floating, lets gases from flux escape, and results in more uniform paste application. Very large apertures in the stencil can result in uneven paste.
Philip
On 3/12/15 5:50 PM, Jason Barnett wrote:
> dispensing the paste in a grid of 4 to 8 squares
I've seen those on stencils, but I didn't know why they were done that
way. Makes sense. But I think you're talking about problems that are
light years ahead of where I'm operating, at least. I count myself
lucky when I can boil off the flux, suck out the excess solder, and only
have to fix a couple of pins!
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