QFN float be bad
At a recent tradeshow, I received a sample part in a 3 x 3 mm QFN package. While I haven't tried yet, I'm pretty sure that, like a water bug, the part is light enough to float on a water surface due to not having enough weight to break the surface tension. But, that's not what I'm talking about.
In the middle of the part, like with many/most QFN packaged parts, is a metal contact pad. It may be there for grounding or heat conduction, depending on the specific part. The float that I'm talking about happens when we lay too much solder paste on the pcb for that center pad.
To a small extent, the height of the solder paste blop is proportional to the aperture in the solder stencil - bigger opening = taller blop. With most parts, that isn't a problem because either all of the pads are big enough so that that ratio doesn't have a first order impact, or because all of the pads are the same size and will be equally impacted.
With the QFN center pad being a much larger opening in the stencil than the signal pad openings, and the signal pad openings being in the 10 - 20 mil or less range, this blop height to width ratio begins to have a first order impact.
Acording to Amkor, the signal pads should have a standoff height of 2 - 3 mils. If too much solder is deposited in the center, the part can very easily float up beyond that height and prevent the signal contacts from contacting. To help prevent this, Amkor recommends smaller multiple openings in the stencil area for the center pad - approximately 50 - 80% pasted coverage.
Take a look at page 3 of an Amkor app note and page 5 of a Freescale app note for examples of how to cut down on the excess solder for better placement reliability. We strongly recommend that you follow these guidelines or similar guidelines published by your part manufacturer.
I won't float your boat