Thixotropic Rebuild Kinetics Governing Fine Pitch Stencil Print Yields
Thixotropic rebuild kinetics dictate stencil release and deposit stability, where tuning separation speed to viscosity recovery maximizes fine pitch print yield.
Knead Stroke
Rheological Activation
Preparatory motion of the squeegee across a stencil without depositing material ensures the solder paste reaches the correct viscosity for printing. This knead stroke shears the paste to break down the internal structure of the flux and metal suspension. It prevents the first few boards in a production run from having insufficient volume due to stiff material.
Proper conditioning of the paste is required after the machine has been idle for more than twenty minutes.
Thixotropic Recovery
Internal friction generates a slight temperature rise that further aids the flow characteristics of the medium. During the knead stroke, the paste rolls in a uniform cylinder ahead of the blade. This rolling action mixes the heavier solder particles with the lighter chemical vehicles.
A successful cycle results in a smooth, creamy consistency that easily fills fine-pitch apertures. Operators typically program two or three cycles to achieve the target shear-thinning effect. If the paste is too cold, more cycles are needed to reach the operating window.
Process Interval
Overworking the material can lead to excessive thinning and subsequent slump after printing. The knead stroke must be balanced against the environmental humidity to prevent moisture absorption during the agitation. Once the paste is conditioned, the printer transitions to the production phase.
Effectiveness of the conditioning expires if the stencil remains unused for an extended period.