Chemical Dissociation
Decomposition of the liquid carriers and binders in solder paste during the heating cycle allows the flux to activate and the metal to melt. Vehicle breakdown is the process where the solvents evaporate and the resins chemically transform to prepare the surfaces for wetting. It occurs primarily during the soak and pre-heat phases of the reflow oven.
Successful transition from a paste to a liquid metal requires the timed removal of these organic components.
Reflow Sequence
Temperature ramps must be controlled to prevent the volatile ingredients from boiling too violently and causing solder balling. During vehicle breakdown, the flux cleans the oxides from the copper pads and the solder spheres. If the breakdown occurs too early, the flux will be spent before the solder reaches its liquidus temperature.
Delayed breakdown causes trapped gases to create voids within the completed solder joint. The chemistry of the vehicle is engineered to provide a specific boiling point and activation window. Monitoring the oxygen levels in the oven helps maintain the integrity of the chemical transition.
Residue Property
Exhaust systems in the reflow oven remove the fumes generated by this chemical reaction to prevent contamination of the circuit board. Vehicle breakdown leaves behind a residue that may be conductive or corrosive if not properly managed. In no-clean processes, this residue is designed to be benign and remain on the board.
For high-reliability applications, the remnants are removed using aqueous or solvent cleaning systems.
