Interface Separation
Fluid-solid boundary anomalies describe the localized loss of adhesion between a flowing complex fluid and the solid surface of the container or measuring tool. The term wall slip refers to the formation of a low-viscosity layer of liquid next to the solid boundary, causing the bulk fluid to slide rather than shear. This phenomenon is particularly common in highly filled suspensions like solder paste, where the metal particles are pushed away from the wall.
It leads to inaccurate viscosity measurements and poor material control if left unaddressed.
Measurement Error
Rheometer tests using smooth metal plates frequently suffer from this flow anomaly. The occurrence of wall slip during testing yields an artificially low viscosity reading, which can mislead engineers about the printability of the solder paste. To prevent this error, testing protocols call for serrated or textured plates to physically grip the paste.
Stencil Release
In stencil printing operations, the behavior of the paste at the aperture wall determines the success of the transfer process. While wall slip is undesirable during rheological testing, a controlled degree of slip at the stencil walls can actually improve paste release during the separation of the stencil from the board. If the paste adheres too strongly to the metal aperture walls, it will stay in the opening instead of depositing on the circuit pad.
Optimizing the surface finish of stencil apertures helps balance this release behavior to achieve consistent paste deposits.
