Rheological Viscosity Ratio
Mathematical ratio comparing solder paste viscosity at low shear rates to viscosity measured at high shear rates defines fluid behavior under stress. The shear thinning index quantifies how readily paste thins out during printing and recovers shape after deposition. High index values indicate paste that flows easily under squeegee pressure while maintaining crisp print definition on component pads.
Formulators measure viscosity across standardized shear rate steps to calculate this dimensionless stability metric.
Paste Printing Kinetics
Squeegee movement across stencil surfaces applies mechanical shear stress that drops paste viscosity rapidly. Fluid paste fills small aperture cavities completely without requiring excessive down-force from the printing head assembly. Post-print recovery restores high static viscosity, preventing printed deposits from slumping together before component placement.
Maintaining precise rheological ratios prevents print defects like bridging, tailing, and incomplete aperture release.
Print Quality Boundaries
Low shear thinning ratios result in stiff pastes that paste-block fine apertures and leave incomplete print deposits. Excessive shear thinning prevents rapid viscosity recovery, causing printed paste deposits to spread laterally and form solder bridges. Quality assurance protocols test paste batches to confirm index values remain within specification limits over extended shelf lives.
Stable rheology ensures consistent print volume deposition across long automated production runs.
