Review of wound roll stress in roll-to-roll manufacturing of flexible electronics

Abstract

Flexible electronics has been widely used in many main-stream applications like flexible display, thin-film solar cell, RFID tags, flexible sensors, and so on. Roll-to-roll (R2R) is a kind of method by which flexible substrate can be processed in a continuous way. This kind of method saves time, cost, reduces delay time and increases efficiency, throughput, performance, etc. The typical R2R system mainly consists of unwinding unit, feeding unit and winding unit, where deviation rectification control, tension control, vibration control and position control are needed. By taking full advantages of the deformation characteristics of flexible electronics, the R2R process has become one of the most effective ways in manufacturing flexible electronics. The winding unit is an essential part in the R2R system and the control of the wound roll stress in winding process is one of the main factors affecting the quality and productivity of flexible electronics. A tiny wound roll stress will cause difficulties in storage and transportation. The substrates or the electronic circuits mounted on it may be broken when the wound roll stress is too large. This paper begins with the discussion of common issues and the influence of wound roll stress in winding process. Currently, the web tension and its lateral position are two factors that mainly affect the wound roll stress. The wound roll stress will increase with the augment of the web tension. While a variation of lateral position can cause non-uniform wound roll stress in cross machine direction. Subsequently, the modelling methods and solution methods based on elastic mechanics, Laplace transformation, discrete modelling and finite element method are generalized. The four models are called elastic winding model, viscoelastic winding model, pseudo winding model and FEM winding model respectively. The elastic model is derived to predict wound roll stress of the elastic web. The viscoelastic model is developed to study how the wound roll stress changes with time after winding. In the pseudo model and the FEM model, the effect of thickness variation along the width direction is considered. Then this review analyses the effects of winding speed-induced inertia force, web deformation and its inhomogeneous thickness, temperature, air entrainment, structure of the nip roller and the core shaft on the wound roll stress. The taper tension control and constant tension control methods for wound roll stress and engineering applications in RFID tag packaging, seal lamination and flexible OLED manufacturing are also presented. Finally, this paper forecasts the key problems need to be further studied for the wound roll stress in winding process. The study of wound roll stress may focusing on the following aspects: (1) Conducting wound roll stress studies that considering the effect of thickness variation in machine direction; (2) usually we consider the wound roll as an axisymmetric cylinder, so it is necessary to develop a winding model to deal with the helical structure of the wound roll; (3) when deriving winding models, its necessary to consider the discontinuous contact between layers.