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.