Flow Behavior
Fluid resistance in non-Newtonian materials decreases under the application of mechanical stress. Shear thinning dynamic viscosity quantifies the specific reduction in internal friction encountered when a substance undergoes deformation at varying rates. Precise measurement requires rotational rheometers capable of maintaining controlled stress gradients while tracking torque output.
The resistance profile defines how effectively a chemical or paste transitions from a static state to a mobile liquid phase.
Material Response
Polymers and suspensions exhibit this characteristic when molecular chains align along the direction of the applied force. Viscosity drops as the shear rate climbs, allowing substances to dispense through fine apertures before recovering structural integrity upon removal of the force. Solder pastes rely on this property during screen printing to fill stencil openings completely without slumping across the board surface.
Uniformity in the relationship between stress and flow rate ensures consistent deposit volume across high density pads.
Process Limit
Thermal fluctuations change the baseline resistance and alter the point at which thinning initiates. Engineers monitor the slope of the flow curve to prevent bridge formation or insufficient wetting during the deposition phase. Data collected during these tests dictate the wiper speed and pressure settings used on automated assembly equipment.
Stability in this rheological constant governs the repeatability of fine pitch interconnects.
