Digital roll-to-roll web processing revolutionizes printed electronic production
Tension control solution for digital roll-to-roll web process enables highest quality, high throughput of printed electronics.
Source: Bosch Rexroth
One of the brightest developments in electronics is organic light emitting diode (OLED) TVs, which are attracting consumers with their eye-popping colors and super-thin designs. Unlike the components found in traditional flat-screen display technology, OLEDs use thin, flexible sheets of material that emit their own light and are produced using a technique similar to inkjet or sheet-feed printing.
Introduced to the consumer market only a few years ago, OLEDs are still relatively costly to manufacture in large sizes due to limitations in shadow-mask deposition methods, and in newer laser annealing and inkjet printing techniques. To scale up large area display production economically, printed electronics manufacturers are seeing the benefits of another production method—namely, digital roll-to-roll web processing enabled by multi-axis tension control solutions.
Originally developed for digital printing, multi-axis tension control has been found to be an excellent solution for roll-to-roll manufacturing of printed electronics, such as large area OLEDs, e-paper media, and integrated circuits. Like an inkjet printer deposits ink on sheets of paper, a digital roll-to-roll press patterns thin-film transistors and other devices directly onto large organic, flexible substrates. But unlike slower sheet-fed digital printing, the substrate in a roll-to-roll press is supplied from an infeed reel through the printing section onto an outfeed reel in one continuous inline web. An array of piezoelectric printheads deposits the ink (a conductive organic solution) on the substrate at precise locations.
It all happens on the fly. For example, digital printers using precision controls operate at speeds up to 800 ft (240 m) per minute. In roll-to-roll web processing, electroluminescent materials or other microcrystalline layers are deposited on substrate at slower speeds, on the order of 10 ft to 100 ft (3-30 m) per minute. The speed of the roll-to-roll process dramatically reduces the cost of fabrication, but several challenges must be overcome to make it pay off.
Fast speeds create big challenges
Similar to how Sunday newspaper comics require precise color registration to keep images from blurring, printed electronics require far tighter registration. Tolerances for applications such as thin-film transistors (TFTs) or OLEDs require registration smaller than 10 microns. High-speed, high-resolution cameras measure registration accuracy and provide input to the control system. To ensure that degree of accuracy, precise web tension control is required.
From the experience gained as a market leader in digital printing, Rexroth developed web tension control technology that is now being implemented to address digital roll-to-roll processing's main challenge, running at high speeds while maintaining high registration accuracy.
Web tension control
Achieving precise registration accuracy is a factor of two related variables: web tension and transport velocity.
Web transport control ensures proper uniform tension on the substrate web as it travels through the process. Because the substrate changes properties in response to force loading, changes in tension affect the stability of deposited materials. Substrate expansion causes cracks, broken traces, short circuiting, and layer delamination. Changes in web velocity in the print zone affect registration, thickness, and resolution of fine lines.
As the web travels downstream, constant tension must be maintained in each tension zone, which is defined as an isolated area in a machine where constant tension must be maintained appropriate to the process being performed in that area. A roll-to-roll press has several tension zones. Problems occur when a change is made in one tension zone and no change is needed in other areas. When tension control is coupled between all zones, a change in one creates a cascade of changes in others, impacting the stability of the entire web.
Figure 1 shows how instability affects a web traveling at 5 meters per second with two successive tension controllers for two tension zones. A command for a step change tension reduction is sent to the green zone controllers. No change is required in the upstream blue zone. But because the web is continuous, the tension disturbance is carried back to the blue zone, which causes the blue controller to compensate. In turn, this change affects the downstream green zone, sending jitter back to the blue zone. This back-and-forth jitter takes about 85 seconds to settle down. The web tension finally stabilizes in about 90 seconds. During that time, the machine is yielding waste product.
Tension adjustment challenge
In an ideal world, web instability would never occur because tension adjustment would never be needed. But tension adjustment is necessary due to several mechanical factors:
- Oscillations caused by mechanical misalignments
- Differing inertial response (lag) of mechanical elements during web acceleration
- Out-of-round unwind and tension rolls
- Slipping through nip rolls
- Over-aggressive web-guide correction
Several technical process and control issues also affect tension: tension setpoint changes, phase offset on driven rolls, tension bleed from one zone to another, and, of course, thermal effect (contraction/expansion) as the substrate passes through various processes.
The factors requiring tension adjustment cannot all be eliminated. Variance in any one factor in a zone necessitates changes in tension control and web speed. Consequently, with coupled tension zone control, jitter is inevitable in a continuous web where the controllers cause a feedback loop.
Decoupled controllers: Benefits
There is a solution. Decouple each tension zone, allowing each controller to operate independently.
This has been accomplished in digital printing applications using precision controllers incorporating a unique tension decoupling function block. As the name implies, the function block allows tension control for each zone to operate independently. As a result, tension changes can be isolated in one zone without affecting tension change in other areas.
See next page for more information and another diagram.