Bonding requirements in bent or curved displays
Posted By: swicn
Date: 11/08/2021
A failure mode often observed in the foldable display is delamination or debonding, which is characterized by separation of layers either near the ends of the display panel, or within the bend region.
Typically, the debonding at the end of the film stack is due to shear strain in the OCA exceeding the OCA bond shear strength, while the delamination within the bend region is the result of high interlayer tensile stresses on OCA due to local buckling.
The performance requirements of an OCA to bond layers in a flexible OLED display are quite different from a rigid, flat device. The neutral plane must be balanced to minimize strain in critical layers of the device. Thin film transistors (TFTs), thin flexible encapsulation (TFE), and touch sensors are predicted to be most vulnerable, although newer silver nanowire and metal mesh sensors are more flexible than ITO. Although manufacturers continue to improve the durability of OLED displays, publicly available literature indicates that critical components are still quite sensitive to strain.
When the form of the device requires the OLED to bend only once (during manufacturing), the cover window can be made of glass, which offers the best resistance to scratches and protects the display from impacts to the screen. However, the modulus (i.e., stiffness) of the glass is much higher than the other plastic films of the display (polarizer, touch sensor, OLED), whereas the plastic films feel a “spring-back” force due to their tendency to relax back to their original flat form. Since the glass will not move or deform, the OCAs in the curved display module must have very high adhesion to hold the plastic films in their curved state; otherwise they will pull away from the glass at the bend and leave bubbles.
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