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• Commercial availability of functional materials and infrastructure
• Utility in short run lengths
• Quick changeover/make ready systems
There are however some challenges. The following properties have to be appropriately optimized for offset lithography and flexography.
• Particle sizes distribution in the inks
• Solvent evaporation rate
• Rheological properties
• Substrate surface energy
• Printing speed
The major high volume printing processes explored in this work are offset lithography and Flexography. These two important printing processes are discussed briefly below along with the ink considerations for such processes.
1) Lithography
Lithography (or offset lithography) is a process that relies on two dissimilar wetting characteristics to produce an image. It may use photographic processes to make negatives. A schematic of the process can be seen in Figure 2
The plate making process: The lithography plate is a flexible aluminum or plastic printing plate. Modern printing plates have a brushed or roughened texture and are covered with a photosensitive emulsion. A photographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to light. After development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image. The plate is then chemically treated so the positive image is receptive to printing inks. The plate is affixed to a drum on a printing press. Rollers apply water (in form of the fountain solution), which wets the non-image areas of the plate, and ink, which adheres to the positive image areas. If this image were directly transferred to paper, it would create a positive image, but this would wet the paper as well. Instead, the plate rolls against a drum covered with a rubber blanket, which squeezes away the water and picks up the ink. The paper rolls across the blanket drum and the image is transferred to the paper. Because the image is first transferred, or offset to the rubber drum, this reproduction method is known as offset lithography or offset printing.
The advent of desktop publishing made it possible for type and images to be manipulated easily on personal computers for eventual printing on desktop or commercial presses. The development of digital imagesetters enabled print shops to produce negatives for platemaking directly from digital input, skipping the intermediate step of photographing an actual page layout. The recent development of the digital plate-setter eliminates film negatives altogether by exposing printing plates directly from digital input. [20]
The pros: This process has excellent control of registration and resolution in printing of circuit patterns. The commercial printing presses can be used for printing electronics. Conductive lithographic inks are also commercially available. Low ink volume is required for any given pattern. With the advent of waterless/UV inks, there is a substantial development potential in lithography. Waterless lithography makes use of purely of the difference of surface energy of the image area and non-image areas for ink transfer to image areas. With current printing developments, circuit components of up to 25 m lines/gap have been achieved and thinner lines/gaps are being aimed for.
The cons: Offset lithography has a very high start up costs. The single printed ink layers are very thin (~1 micron). This makes multiple passes imperative especially in obtaining sufficient thickness for conductivity in ink films. The Start up waste on very short runs is high. The lithographic process requires stringent control on the ink rheology. Thus the inks have to be highly viscous. Therefore most available commercial inks are resins loaded with conductive materials like silver/copper particles or flakes or carbon black. It is difficult to formulate organic inks meeting such requirements [21]. Also ink drying speed and water balance in the printing process have to be optimized well.