Dimensional stability of substrates for printed electronics

Abstract

Substrates for flexible displays must retain registration between multiple process steps. Although polyester substrates offer cost and handling advantages, their dimensions are affected by tension, temperature and humidity. This paper will summarize the understanding of viscoelasticity, thermal and hygroscopic expansion, shrinkage, and their anisotropy in polyester. Thermomechanical and Dynamic Mechanical Analysis data are particularly useful to understand and predict substrate behavior.

These effects have been mitigated in sheet processing by laminating the film to a glass sheet: however, a thermal cycle produces bowing distortion that can make subsequent steps impossible. A model for the development of bow from differential thermal expansion, and its relaxation and setting-in by the viscoelastic polymer layer properties, will be outlined. The bow direction and trends are predicted well but the absolute accuracy is not perfect. The model is useful in developing thermal and constraint conditions during fabrication.

Finally, key points for maintaining dimensional stability and preventing wrinkles during roll to roll processing will be summarized. It is particularly important to control tension during drying steps at elevated temperature: high levels can obliterate the benefits of expensive "heat-stabilized" substrates.