Rheology Study of Wafer Level Underfill

Abstract

Summary: As a special epoxy resin material used in the electronics packaging, wafer level underfill (WLU) is studied using a chemorheology approach to provide a fundamental understanding of its reaction dependent rheology behaviors. In this work, the relationship between the molecular weight ($\overline M _{\rm w}$) and the viscosity of the epoxy resins at fixed temperatures has been established. Subsequently an Arrhenius‐Erying equation was used to fit the relationship between viscosity and temperature for given molecular weight epoxies. By combining the two relationships, the viscosity could be modeled as a function of temperature and molecular weight. To obtain the viscosity change of the WLU during the reflow process, the molecular weight change of the underfill was calculated from the degree of curing through the kinetics modeling. A semi‐empirical model was developed to predict the viscosity of the underfill as a function of time and temperature during the curing process. Modeled predictions were compared with experimental data under isothermal and ramping temperatures during curing experiments. The model showed good agreement with the experiments in the early stage of curing reaction. The critical viscosity of the underfill for solder wetting was obtained by wetting experiments, which can be used as the criteria to determine the flowability of the wafer level underfill.Data fitting of the viscosity model at isothermal condition.magnified imageData fitting of the viscosity model at isothermal condition.

Sigmoidal Chemorheological Models of Chip‐Underfill Materials Offer Alternative Predictions of Combined Cure and Flow

“…Plot representing the experimental data of the original manuscript and the model fit. [26] represents both the initial, low viscosity region as well as the asymptotic plateau of dynamic viscosity, where the rate of polymerization declines due to viscous constraints on the reactive end-groups. The experimental data matched the sigmoidal model up to a threshold viscosity of about 1 Â 10 4 Pa Á s. lnh ¼ À42:54 þ 3:27lnM w þ 8608 1 T…”

“…[26] The chemorheology experiments were originally carried out in a TA Instruments AR1000 rheometer using a parallel plate geometry stress rheometer at fixed curing temperatures (150, 160, 170 and 180 8C) to simulate the heating profile conducted in underfilling.…”

Cure versus Flow in Dispersed Chip‐Underfill Materials

“…Model sigmoidal cure parameters based on Equation (2) for the underfill resins in ref. [12] allowing some degree of controlled sedimentation to occur in chip-underfills. The inherent insulation resistance of the resin could be enhanced if there was a small zone of clarified resin near the top of the underfill zone.…”

“…[12] The chemorheology experiments were originally carried out in a TA Instruments AR1000 rheometer using a parallel plate geometry stress rheometer at fixed curing temperatures (150, 160, 170 and 180 8C) to simulate the heating profile conducted in underfilling. Datasets from published rheology experiments [12] were subsequently inputted into Microcal Origin TM , which has a fourparameter sigmoidal plot function included.…”

“…[12] The chemorheology experiments were originally carried out in a TA Instruments AR1000 rheometer using a parallel plate geometry stress rheometer at fixed curing temperatures (150, 160, 170 and 180 8C) to simulate the heating profile conducted in underfilling. Datasets from published rheology experiments [12] were subsequently inputted into Microcal Origin TM , which has a fourparameter sigmoidal plot function included. The model incorporates as variables the initial and final viscosities, which are functions of the resin formulation, the corresponding network densities in the cured state, and two kinetic parameters, which are functions of the initiation and propagation steps associated with polymerization.…”