The Powder Reactivity Coefficient (PRC) represents the interaction between the flux activator and the powder in a solder paste. Due to the formulation differences between pastes, water soluble pastes are more conductive and active than no-cleans, causing the powder-flux interface to be more visible in the Electrochemical Impedance Spectroscopy (EIS) measurement by the Vision MARK-1. Therefore, the PRC is typically always tracked in water soluble pastes.
The reaction between the powder and flux activator is diffusion-limited, meaning that the rate of the reaction is defined by the flux activator. If the powder-flux reaction is occurring more readily, the powder-flux curve in the impedance spectra will appear small, reflected by the PRC being small. If the powder-flux reaction is slowing down, the powder-flux curve will appear large, reflected by the PRC being larger. The PRC correlates well with viscosity and the probability of solder balling after reflow.