Flow Resistance
A rheological property determines the resistance of a fluid to flow under extreme shear stress conditions. During stencil printing, the high shear viscosity of a solder paste dictates how easily the material flows through the squeegee-induced shear field. High squeegee speeds generate shear rates exceeding one thousand reciprocal seconds, which forces the thixotropic paste to transition from a semi-solid to a highly fluid state.
If this property is too high, the paste will not flow through the apertures. Rheometers measure this behavior by rotating a cone or plate at high speeds to simulate the action of a squeegee blade. These measurements establish a benchmark for paste behavior during high-speed printing operations.
Process Impact
Stencil aperture filling relies on the paste dropping to a predictable viscosity during the squeegee sweep. Excessive high shear viscosity causes insufficient stencil filling and leads to starving solder joints. When the paste is too thick under shear, it behaves like a solid blocks the stencil.
Squeegee friction increases, which leads to premature stencil wear.
Viscosity Monitoring
Capillary or rotational rheometer testing is utilized to measure the fluid behavior at defined shear rates. Formulations that do not exhibit the correct high shear viscosity are rejected before they reach the production floor. Maintaining this metric ensures that high-speed printers run continuously without requiring manual paste additions or frequent blade adjustments.
Consistent viscosity preserves the life of printer blades and stencils.
