Rheological Property
Fluid resistance measured under specific shear conditions defines how a material flows when external force is applied during the stencil printing process. The apparent viscosity of a solder paste decreases as the squeegee speed increases due to the shear thinning nature of the flux and powder suspension. This behavior allows the paste to liquefy for aperture filling and regain its shape once the force is removed.
Measurement Protocol
Accurate readings require a temperature controlled environment and a spindle viscometer that replicates the shearing action of a printing stroke. Because the material is non-Newtonian, the apparent viscosity is not a single fixed number but a curve that tracks with the rate of mechanical deformation. Technicians record these values at multiple shear rates to verify that the paste batch meets the consistency requirements for high speed assembly.
Deviations from the expected curve often indicate that the flux has begun to dry out or that the material has reached the end of its shelf life.
Printing Effect
Proper flow control ensures that the paste does not bleed under the stencil or stay trapped within the aperture walls. High apparent viscosity prevents bridging on fine pitch pads by maintaining the vertical walls of the deposit.
