The problem with soldering QFN packages is that they float, skew and tilt on the 'domed' melted solder during the SMT re-flow process. This floating, skewing and/or tilting will lead to solder bridging or un-solders. These defects are hard to detect until the assembly reaches test. Troubleshooting these elusive defects can be very time consuming and costly leading to rework, replacement parts, possible late deliveries and eventually the high cost of a re-design.
SMTA - Surface Mount Technology Association is a non-profit international association of companies and individuals (totalling 4,000) involved in all aspects of advanced electronics assembly, surface mount and related technologies. Membership Info | SMTA Certification | Bookstore