Chemical Reaction
Oxide reduction on copper pads during reflow soldering is initiated by the thermal decomposition and mobilization of acidic components in the solder paste. Flux activation occurs when the temperature of the assembly rises to a level where the activators, typically organic acids or halides, liquefy and begin to react with surface oxides. This reaction converts metal oxides into soluble salts that are swept away from the joint area.
The cleaned metal surface then becomes highly wettable, allowing the molten solder alloy to form a metallurgical bond with the underlying copper. This process is essential for overcoming the natural oxidation that occurs during circuit board storage and handling before the assembly stage.
Thermal Process
Reaching the temperature threshold for this chemical mobilization is a primary step in the thermal profiling of printed circuit board assembly. If the oven ramp rate is too slow, the activators may exhaust their chemical potential before the solder reaches its liquidus point. Conversely, a rapid temperature increase can vaporize the solvent carrier prematurely, causing the flux to splatter and create solder balls.
The temperature window must be carefully tuned to align with the thermal profile of the specific paste alloy.
Oxide Removal
Incomplete mobilization of these acidic compounds leaves active residues that can cause long-term electrochemical migration or board corrosion. This chemical process must conclude before the molten solder begins to solidify, ensuring that all residues are encapsulated or volatilized. The threshold is typically governed by the thermal characteristics of the rosin or synthetic resins.
Testing the effectiveness of this process is routinely done through solderability evaluations and surface insulation resistance tests.
