This paper presents a comprehensive methodology to model the static temperature-humidity (TH) ageing test (85°C/85%RH over 1000 hours) of flip chip on flex interconnections with non-conductive adhesives (NCAs). Nonconductive adhesives, being a special form of conductive adhesives, are chosen, as they allow bringing the pitch further down. The methodology combines experimental techniques for material characterization, finite element modeling (FEM) and model validation. A non-conductive adhesive (NCA) has been characterized using several techniques. The thermomechanical properties and the moisture properties were obtained for the NCA. A temperature dependent viscoelastic constitutive model was also obtained for the NCA. The viscoelastic model was defined by the Prony series expansion. The shift factor was approximated by the Williams-Landel-Ferry (WLF) equation. Finite element modeling has been performed to analyze the flip chip interconnects on flex with the NCA under process condition and reliability ageing conditions. The viscoelastic constitutive relation has been used to model the NCA in ageing modeling. An integrated process-ageing modeling methodology has been developed to combine the thermo-mechanical stress and hygro-mechanical stress, followed by stress relaxation analysis. To verify the finite element models, the static TH ageing test (85°C/85%RH) were also performed. The contact resistance was monitored with high measuring resolution during the accelerated test. The simulation results are good agreement with the experimental results. The approach developed in this paper can be used to provide guidelines with respect to adhesive material properties, assembly process parameters and good reliability performances.
Finite element modeling of the stress-strain state of waxy compacts
