Shear Rate
Rate at which the speed of a fluid changes across a given distance perpendicular to the direction of flow determines the internal stress within a material. In the gap between a squeegee and a stencil, the velocity gradient dictates the shear thinning behavior of the solder paste. A steep change in speed results in lower viscosity, allowing the paste to enter small apertures easily.
Flow Characterization
Laminar flow conditions inside a dispensing needle or near a stencil wall create distinct layers of movement. The velocity gradient is highest near the stationary surface and lowest in the center of the flow stream. Because solder paste is non-newtonian, the relationship between this change in speed and the resulting force is not constant.
Understanding this profile helps in the design of nozzles that prevent the separation of solder balls from the flux. Engineers adjust the print speed to maintain a profile that ensures consistent deposition.
Fluid Limitation
Boundary layer effects and the roughness of the stencil walls can alter the theoretical profile. If the velocity gradient becomes too high, it may damage the organic modifiers in the flux. Stable production relies on maintaining this parameter within a narrow range.
