The Flux Coefficient (FC) value is a measurement relating to the dielectric properties (ion mobility / conductivity and polarizability) of the flux. When the FC is well outside of normal bounds, then it is more likely for printing and reflow defects to occur.
When the FC is above the upper red limit, the flux is less conductive than what is usual for that solder paste. This can occur when the solder paste is measured in an environment with a low ambient humidity and may cause incorrect paste viscosity or inconsistent print volume.
When the FC is below the lower red limit, the flux is more conductive than what is usual for that solder paste. This can occur when the solder paste is opened cold or expired and may cause incorrect paste viscosity or solder balling.
Note: what constitutes a normal / usual measurement is different for each solder paste and for different applications and there is no singular "correct" control limit. It is highly recommended to tailor the control limits to your specific process. Please contact Insituware for assistance.