Rheological Prediction
Non-Newtonian fluid dynamics provide the mathematical framework for characterizing materials that require a threshold force before initiating flow. The herschel-bulkley model combines the properties of a bingham plastic with power law behavior to describe the shear stress response of solder pastes or viscous adhesives. It calculates the stress through the addition of a yield stress component to the product of a consistency index and the shear rate raised to a flow behavior index power.
This formulation remains accurate for substances displaying shear thinning or thickening characteristics once the applied force exceeds the internal static resistance.
Solder Characterization
Verification of printing performance during surface mount assembly requires precise knowledge of how these pastes behave under stencil apertures. Engineers utilize this calculation to determine the viscosity gradient of specific chemical mixtures before high speed deposition processes. Accurate mapping of the yield stress ensures the paste remains stable inside the aperture during the squeegee stroke while maintaining the ability to release onto the pads without smearing.
The application of this standard allows for the reduction of bridging defects and insufficient volume counts on complex board architectures.
Process Verification
Rheometer data from controlled stress sweeps generate the points necessary to fit the constitutive equation and isolate the yield point from the plastic viscosity. Comparison of the output against expected batch performance constants determines the viability of a material for a specific printing profile. Testing procedures involve ramping the shear rate at a constant temperature to isolate the consistency index from the flow index.
Deviation from established performance limits signals potential degradation in the binder system or moisture absorption. A high consistency index predicts poor flow into narrow aperture widths during the print cycle.
