What are the reasons for poor welding of QFN packaging in SMT processing?

Generally, the main reasons that affect the bad welding of QFN package are: PCB board design defects, pad pollution, QFN device welding end pollution, QFN device damp, steel mesh design defects, poor solder paste printing, reflow soldering temperature curve settings. When QFN welding is found to be poor, the problems can be analyzed from these aspects.