Agitation Cycle
Conditioning of solder paste on the stencil surface must occur prior to active printing to bring the rheological properties to a stable state. Before the first board of a batch is run, dynamic knead strokes agitate the paste back and forth across the metal foil without depositing any material. This pre-run exercise lowers the viscosity of the material to its working level.
Viscosity Adjustment
Solder paste exhibits thixotropic behavior that causes it to soften when subjected to mechanical shear. The execution of dynamic knead strokes breaks down the internal gel structure of the paste binders, ensuring that the material flows easily into the finest stencil apertures. Without this initial conditioning, the first few printed boards of a production run often exhibit low deposit volumes.
This preparation step ensures process uniformity from the very first board and eliminates the waste of scrapped assemblies.
Temperature Effect
Mechanical action on the stencil also generates localized heat that slightly raises the temperature of the paste. Overdoing these dynamic knead strokes can lead to excessive softening, causing the paste to slump or bleed after deposition. Line operators must balance the number of strokes against the ambient factory temperature to maintain paste integrity.
