Predicting Cure Evolution and Thermal Endurance of a Highly Filled Epoxy Underfill for Advanced Packaging

By Ryan Giang ^1,2, Ran Tao ^1,3,  Elena Moukhina ⁴, Stian K. Romberg ^1,5, Zois Tsinas ^1,6, Amanda L. Forster ¹, Tae Joon Cho ¹, Polette J. Centellas ¹, Christopher L. Soles ^1
1 National Institute of Standards and Technology, Gaithersburg, Maryland, USA 
² University of California San Diego, La Jolla, California, USA 
³ North Carolina State University, Raleigh, North Carolina, USA 
⁴ NETZSCH- Gerätebau GmbH, Selb, Germany 
⁵ University of Tennessee, Knoxville, Tennessee, USA 
⁶ Theiss Research, La Jolla, California, USA

Abstract

Epoxy underfill materials are critical to advanced semiconductor packaging by enhancing mechanical integrity, redistributing thermomechanical stresses, and improving solder-joint reliability in flip-chip and other fine-pitch interconnect architectures. Their performance is strongly influenced by cure evolution during processing and thermal stability during service, making quantitative evaluation important for process optimization and long-term reliability assessment. The cure kinetics and thermal degradation behavior of a highly-filled epoxy underfill were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and diffusion-incorporated kinetic modeling. Isoconversional analysis showed that the apparent activation energy varied with conversion and increased sharply at high conversion, indicating multi-step cure behavior and diffusion control. A two-step modified Kamal–Sourour model incorporating diffusion control in the second step improved prediction of conversion after vitrification and was used to predict cure evolution under a designed oven cure schedule, which achieved nearly complete cure. Cryomilling improved TGA reproducibility without causing detectable chemical changes. Analysis following the ASTM E1641/E1877 protocols revealed a strong temperature dependence of estimated thermal endurance based on a 5% mass-loss criterion under nitrogen. Together, the results provide a quantitative framework for predicting cure evolution under practical thermal schedules, evaluating late-stage diffusion effects, and estimating degradation-based thermal endurance for highly-filled industrial thermosets.

Keywords: epoxy underfill, cure kinetics, isoconversional analysis, vitrification, diffusion-controlled reaction, thermal endurance, differential scanning calorimetry, thermogravimetric analysis, advanced electronic packaging

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