Selective Solder Nozzle Geometry and Thermal Preheat Window Boundaries
Optimal selective soldering requires balancing nozzle orifice dimensions with precise top-side preheat windows to achieve Class 3 vertical barrel fill.
Nitrogen Blanketing Purity
Atmospheric Specification
Concentration of inert gas maintained within the soldering chamber or around the nozzle prevents the oxidation of molten alloy surfaces during assembly. High nitrogen blanketing purity reduces the formation of dross and improves the wetting characteristics of lead-free solders. This chemical environment is measured in parts per million of residual oxygen.
Oxidative Prevention
Molten alloy reacts with atmospheric oxygen to form a skin of oxides that impedes the flow and adhesion of the metal to the PCB pads. When nitrogen blanketing purity is maintained at levels such as 50 to 500 parts per million, the surface tension of the solder wave remains low. Low surface tension facilitates the drainage of excess solder and helps prevent bridges between fine-pitch components.
It also extends the life of the flux by preventing premature evaporation or charring.
Supply Requirement
Implementation of this inert atmosphere usually involves either bottled liquid nitrogen or on-site membrane generators. Total gas consumption scales with the size of the solder pot and the frequency of board transitions. Maintaining the seal integrity of the machine housing is necessary to prevent ambient air from diluting the internal environment.
Excessive oxygen levels result in increased maintenance costs for dross removal.