Impact of adhesive rheology on stress-distortion of bonded plastic substrates for flexible electronics applications

For fabrication of flexible electronics using standard microelectronics toolsets, a temporary bond–debond method has been developed that requires minimization of the distortion of bonded flexible substrate and bow of bonded system (flexible substrate-adhesive-carrier) during processing. To elucidate the critical parameters of the adhesive used in the bonding that control the stress (bow) and distortion, adhesives with different viscoelastic properties are examined systematically. By blending a high modulus adhesive into a low modulus adhesive, the storage modulus, loss modulus and loss factor of the adhesive can be tuned by orders of magnitude. Detailed examination of the impact of these three rheological parameters on the stress and distortion of bonded system reveals that the relative viscoelastic flow properties of the bonding adhesive to that of the bonded flexible substrate are directly correlated to bow and distortion. When the loss factor of the adhesive is less than that for the plastic substrate, precise registration of layers during photolithography is observed. These results provide insight into the rheological parameters critical to the adhesive formulations for the temporary bond–debond method in the fabrication of flexible electronics.