Chemical Depletion
Active acid concentrations in paste and liquid vehicles undergo thermal degradation when exposed to high temperatures during the preheat phase of soldering. The occurrence of flux exhaustion means that the chemical cleaning agent has been fully vaporized before the alloy melts. This leaves the metal surfaces exposed to oxidation.
Defect Induction
Solder wetting ceases to function correctly when the metal surfaces are left unprotected against oxidation during the liquid phase. Without active flux, the molten alloy gathers into ball shapes instead of spreading across the surface of the pad. This behavior leads to dry solder joints, head-in-pillow defects on ball grid arrays, and heavy solder balling on the mask.
Exposed copper pads that remain unsoldered after the reflow cycle are susceptible to corrosion and fail mechanical shear testing. This failure mode typically arises during long thermal profiles or slow conveyor speeds.
Profile Management
Adjusting the thermal ramp rates and shortening the soak zone duration prevents the pre-liquation loss of active cleaning compounds. Industrial reflow profiles must limit the time the paste is held above the activation temperature of the flux before the actual liquidus zone begins. Correct profiles ensure that sufficient active ingredients remain to clean the joint area right up to the moment of alloy solidification.
