Thermomechanical and viscoelastic behavior of a no-flow underfill material for flip-chip applications

No-flow underfill is an alternative material technology for packaging high-speed flip-chip assemblies in microelectronics industry. In this study, the thermal expansion behavior of a cured no-flow underfill material was examined by thermomechanical analysis (TMA), and the dynamic mechanical behavior was investigated by dynamic mechanical analysis (DMA) at a fixed frequency of 1 Hz. In addition, because the no-flow underfill material is polymer-based and its mechanical properties are influenced by both temperature and time, it is important to consider its viscoelastic behavior. This was accomplished by conducting the time-temperature superposition (TTS) experiments using DMA. From the TTS results, master curves were constructed for both the storage (E′) and the loss moduli (E″) as a function of frequency at a pre-selected reference temperature. The shift factors along the frequency axis were also determined as a function of temperature, and they can be fitted using the Williams-Landel-Ferry (WLF) equation. Based on the master curves for E′ and E″, one can obtain the relaxation modulus, E(t,T), as a function of time and temperature. The measured thermomechanical and viscoelastic properties of the no-flow underfill material provided crucial material properties for accurately modeling the package stress.