QFN (quad flatpack, no leads) and DFN (dual flatpack, no lead) are becoming more popular in new component releases. Their very small form factor allows smaller packages, better grounding, and better heat sink thermal properties compared to other SMT packages.
Most QFNs have a metal pad on the underside of the part for grounding and heat conduction. DFNs have a similar center metal pad but have leads on only two sides. Typical thickness of the QFN devices is .85mm and the body range from 3mm up to 12mm, so the packages are very small and very light. The QFN leads and ground plane conductor are flat and in the same plane on the bottom of the package. Printing solder paste 1-1 with the ground plane can cause the QFN to float during reflow, thus miss-registering the leads on the QFN and the pads on the PCB. QFN float can be controlled by reducing the amount of solder paste printed on the ground plane. Typically a 50 to 60% reduction will solve the QFN float problem. However the aperture reduction must be done judicially. A Window Pane aperture is recommended for most cases. This allows the solder paste volatiles to easily escape during reflow without moving the QFN device.