Projected Capacitive Touch Panel
Optical Bonding
Color TN
UB LCD
 
 
Introduction
With touch panel attached to LCM, the display suffers not only brightness going down but also the deteriorating in display quality. Especially the reflection of ambient light causes the dark image being not as dark as it supposed to be.
By adding the process of Optical Bonding, we can reduce the brightness loss and avoid the influence of ambient light.
 
Principle
Reflection appears when light propagates through media with different optical indices. The reflection energy is around 4% of incident light at each air-glass interface. As touch panel attached to LCM with air gap, two additional air-dielectric interfaces contribute to light reflection.
Optical bonding technique fills the air gap with Optical Clear Adhesive or OCA. While the optical index of OCA is designed to be very close to that of glass, light reflection at those two interfaces is minimized. Since less ambient light is bounced off the interfaces with optical bonding, the dark image remains as dark as it should be and the contrast ratio as well as the color saturation are restored back to the level when touch panel was not there. Likewise, back light of LCM experiences less reflection loss when it propagates through the interfaces with OCA, and the brightness off the touch panel is up as if the TP was not there.
Optical bonding is implemented by either liquid or sheet form OCA. Attachment process is performed in the air with liquid OCA and either UV or thermal curing process is needed after attachment. On the other hand, vacuum condition is necessary when sheet form OCA is applied. The high cost of both processes is mainly due to un-recoverable yield loss issue. The countermeasure regarding re-work process is already developed in URT.
 
 
 
 
Figure 1, Display contrast ratio and color saturation are both up with optical bonding in the present of ambient light.
 
 
  
 
Figure 2, Display brightness is up with optical bonding.