The reflow soldering profile must be matched to the specific requirements of the solder paste as well as the thermal thresholds of the PCB assembly. It is essential that accurate profiling be achieved and thus use of an accurate data recording system as well as a board that has been instrumented properly is imperative. Lead-free solder pastes do not change the basic principles of solder reflow. They do, however, significantly reduce the size of the thermal process window, and hence make reflow a more critical step in the assembly line, affecting total product quality yield.
It should also be noted that most solder defects originate in the solder deposition stage, either through improper printer parameters, damaged or incorrectly designed stencils, or less than adequate solder paste. A legendary study done a number of years ago by Hewlett-Packard revealed as much as 60% of solder defects in PCB assemblies were caused by such means. Subsequent experiences by the author with users is consistent with this figure.
The initial design of the board, particularly pad geometries and solder mask parameters, also have a profound effect on the quality of soldering accomplished. Good design for manufacturability is a fundamental to achieving low solder defects. Finally, the quality of the materials being soldered is also of primary importance. Besides having a high-quality (and qualified) solder paste, parts and boards with good solderability are also required. Control of the procurement and storage of parts and PCBs is essential.