Roll-to-roll coating is often proposed as a logical route for production of high-volume, flexible printed electronics as it poses several benefits, including high line speeds and a continuous process, lowering manufacturing costs. Manufacturers already produce touch sensors as input devices to displays using roll-to-roll processes with extremely good yields and economics. Similarly, both active and passive circuitry are produced using roll-to-roll processes, as are thin film batteries, solar cells and displays of many types, including conventional LCDs, OLEDs, and bistable structures like the eInk/Kindle. The concept of a full device featuring input (touch), output (display), power, circuitry, and recharge seems ripe for combination in whole or part.
The continuous process and high line speeds of roll-to-roll multi-layer coating enable production of value-added technologies at meters-per-minute rates. The streamlined process afforded by roll-to-roll coating equipment already physically built and staffed also results in decreased time-to-market and capital for new products.
While high-precision continuous layers are common, the emergence of manufacturing patterned layers is a key driver for using roll-to-roll coating for printed electronics. Successfully creating a product with both continuous and patterned layers requires a number of complex processes and techniques – from encapsulation to isolation to passivation, light management, registration, integration and beyond. Techniques for patterning a moving web with high precision and resolution include gravure, microgravure, industrial inkjet, photoresist techniques, and others.
Choosing the right coating method for a specific product is crucial. The most common methods for roll-to-roll coating of printed electronics include gravure, slot coating, inkjet, roll coating, slide coating, rotary screen printing and curtain coating, but there are a host of others. Each of these processes consists of a complex list of variables in fluid, intended result and coating method that can radically affect the final outcome. Below is a general overview of some of the techniques, as well as the most common advantages and disadvantages associated with each method.
- Gravure: Key equipment for gravure coating includes the engraved cylinder, doctoring system and a solution delivery system.
o Coating thickness: 1 – 10 μm wet laydown; coating uniformity: 1 – 2%
o Solution types: aqueous and solvent-based solutions, <1 to 100 cP
o Speeds: 150 to 1000 fpm
o Advantages: thin wet coatings, uniformity, patterning and high line speed
o Disadvantages: open solution delivery, little to no theory/modeling, doctoring and engraving specification
- Slot Coating: Key equipment for slot coating includes the die/bar/hopper, vacuum system and solution delivery system.
o Coating thickness: 10 – 250 μm wet laydown; coating uniformity: 2 – 5%
o Solution types: aqueous and solvent-based solutions, 1 to 10,000 cP
o Speeds: 50 to 500 fpm
o Advantages: large thickness capability range, large range of acceptable rheologies, modeling capability, pre-metered solution delivery, two-layer capability and relative simplicity
o Disadvantages: minimum ~ 10 micron wet thickness, only gross patterning (patches and stripes) and low-moderate line speed
- Slide Coating: Key equipment for slide coating includes the die/bar/hopper (multi-slot), vacuum system and solution delivery system.
o Coating thickness: 8 – 250 μm wet laydown; coating uniformity: 2 – 5%
o Solution types: aqueous and solvent-based solutions, 5 to 1,500 cP
o Speeds: 300 to 1,000 fpm
o Advantages: multi-layer capability, moderate to high line speed and pre-metered solution delivery
o Disadvantages: formulation complexity, die expense and only continuous
- Curtain Coating: Key equipment for curtain coating includes the multi-slot hopper, edging and air stabilization, and precise solution delivery.
o Coating thickness: 20 – 200 μm dried thicknesses; coating uniformity: < 2%
o Solution types: aqueous and solvent-based solutions, 2 to > 600 cP
o Speeds: 400 fpm to 2000 fpm
o Advantages: multi-layer capability, very high line speed, large gap over substrate, and pre-metered solution delivery
o Disadvantages: formulation complexity, only continuous, die expense and difficulty with edges
With so many variables involved in roll-to-roll coating processes, choosing the right coating technique and product design is a complex process. After discussion about external capabilities and experience, many turn to contract manufacturing to achieve their objectives. In addition to accessing knowledge and problem-solving expertise in coating processes, contract manufacturing offers several other distinct advantages as it provides a low capital approach, a fully variable manufacturing capacity, and improved time-to-market. Contract coating partners like Carestream feature diverse process expertise and established roots in multilayer coating. From product and process development to engineering and manufacturing, Carestream delivers a unique skill-set to help customers deliver revolutionary, high-quality products on time and on budget.