The underfill viscosity model, which is a modification of the Herschel-Bulkley-WLF viscosity model for reactive materials, is used specifically for underfill encapsulants.
The underfill viscosity model includes an additional term, 
(thickness), as described below: 
where
-
is the viscosity (Pa. sec.) - is the cavity thickness (meter)
-
is the viscosity at a reference thickness 
, and -
and 
are data-fitted coefficients.
The viscosity at the reference thickness can be obtained from the following equation: 
where
- is the viscosity (Pa s)
-
-
is the shear rate (1/s)
-
-
-
is the temperature (K) -
is the degree of cure (0–1), and -
, 
, 
, 
, 
, 
, 
, 
and 
(gelation conversion) are data-fitted coefficients.
Note: For general thermoset materials, use either the Herschel-Bulkley-WLF viscosity model or the reactive viscosity model.