Viscosity model for underfill encapsulation

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.