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80 SMT Magazine • April 2016 consisTEnT conTrol ovEr ThE sElEcTivE soldErinG ProducTion ProcEss concentration is higher at the edges of the flux deposit and less so in the center. Distribution seems to be non-homogeneous on the board. preheating process The flux solvents must be evaporated before the board can be soldered. The preheaters also heat up the assembly to activate the flux sys- tem and bring thermal heat into the assembly to facilitate soldering. Typically, two types of preheaters are used in a selective soldering pro- cess. These are IR lamps that respond quickly and are used in mixed production and forced convection heaters that can bring a lot of heat into a high thermal mass board. In the machine capability tests the pre- heating experiments proves that selective sol- dering machines can guarantee a consistent temperature on the top side of the board in the short term. For process control purposes, Figure 4: Histogram of the wet flux amount measured during a machine capability analysis. Process capability Cp=2.28 is acceptable. Figure 5: Flux spreading visual check. Solder joint positions are printed on the check paper. Visual criteria are stored at the machine.