A quantitative analysis of the stress relaxation effect of thermoplastics in multilayer substrates

All plastics can be divided into two groups called thermoplastics and thermosets, and each group exhibits very different behaviors. Unlike thermosets, thermoplastics are empirically known by the viscoelastic behavior of the thermoplastic layers; the stress generated by building subsequent layers can be significantly relieved in a multilayer structure. In this study, we quantitatively investigated the stress relaxation effect of thermoplastics in lamination-based multilayer substrates. As an increasing number of layers were laminated using either thermoplastic or thermoset adhesives, the maximum bow values were measured layer-by-layer using a laser profilometry during thermal cycling. When analyzed through an analytical model, the measured thermal behavior of thermoplastics exhibited as much as 70% stress relaxation in the multilayer structures, and highly contrasted with that of thermoset adhesives. The result provides an important design guideline that thermoplastics can be used as a stress relaxation layer in lamination-based multilayer substrates.