The Herschel-Bulkley-WLF viscosity model can be used for thermoset materials that show a yield stress. This model can be used in a Reactive Molding, Microchip Encapsulation, or Underfill Encapsulation analysis.
The following equation is used for determining the viscosity of these materials.:
where:
-
is the viscosity (Pa. sec.), -
is the shear rate (1/sec.), -
is the WLF viscosity, -
is the shear stress, -
-
is the temperature (K), and -
, 
and 
are data-fitted coefficients.
The Williams-Landel-Ferry model, or WLF for short, is usually used for polymer melts or other fluids that have a glass transition temperature, and is given by
where
where
-
is the degree of cure (0–1), and 
, 
, 
, 
, 
and 
(gelatin conversion) are data-fitted coefficients.
Note: Use this model for thermoset materials that show yield behavior at low shear rates. For materials that do not show yield behavior at low shear rates, use the reactive viscosity model. For underfill encapsulants, use the underfill viscosity model.