Mechanical Response of Assemblies Bonded with Double-Layered Pressure-Sensitive Adhesives(PSAs)

The mechanical behavior (stress-strain and creep) of double-layered pressure sensitive adhesion (PSA) was empirically investigated in this study. PSAs usually come in two laminated configurations: single-layered PSA (a layer of adhesive only) and double-layered (two adhesive layers on both side of a thin carrier layer). Both have been widely used to join parts of electronic devices such as cell phones, tablets and laptop. Therefore, the studies of the mechanical behavior of double-layered PSAs has the same importance as the studies of single-layered PSAs. The devices are expected to experience complex and harsh loading conditions during their life cycle. Compared to assemblies bonded with single-layered PSAs, assemblies bonded with double-layered PSA systems show more complex mechanical behavior, such as multiple transitions in their stress-strain and creep curves. The severity of the secondary transitions increases with the thickness of the carrier layer. The reason for these secondary transitions is hypothesized to be due to nonuniform sequential cavitation of the adhesive at each interface (with substrates or with carrier layer). This nonuniformity may be influenced by: (i) anomalies in the local surface wettability of the bonding substrate and carrier layer; (ii) variations in the flexural rigidity of the carrier layer due to variations in the thickness; and (iii) variations in the lateral constraint on the adhesive layer caused by variations in the in-plane stiffness of the carrier layer. To verify the effects of flexural rigidity of the carrier layer on nonuniform cavitation of the adhesive layer, a simplified mechanical model based on elastic material properties is developed. The resulting load-displacement curves replicated the experimentally observed multiple transition in the mechanical behavior, thus providing justification for the hypothesis.