Characterization and modeling of static and cyclic relaxation in nonconductive adhesives

Adhesive interconnections are considered to be attractive alternatives to lead or lead-free solder interconnects because of their lower processing temperatures and extendability to fine pitch applications. However, reliability issues, such as moisture-induced delamination and viscoelastic relaxation of the adhesive in both steady-state and cyclic loading, continue to pose a challenge to widespread implementation. To date, the static and cyclic relaxation characteristics of nonconductive adhesives (NCAs) are yet to be understood. This paper attempts to provide insights into this static and cyclic relaxation behavior through experimental characterization and modeling. The viscoelastic property of a typical NCA material was characterized, and a simulation program with integrated circuit emphasis (SPICE) modeling program was used to model the cyclic relaxation behavior. The modeling results were successfully validated with a series of experiments. This showed that cyclic relaxation of the adhesive can be successfully modeled using linear-viscoelastic property. The phenomenon of slower relaxation of the adhesive under cyclic loading than that in static loading suggests that accelerated reliability testing used in solder-joint fatigue durability investigations may not be directly applicable to the adhesive interconnections. A rework methodology applicable to adhesive interconnects using cyclic loading has also been proposed.