Processing a web of media in roll-to-roll fashion can be an advantageous and low-cost manufacturing approach for devices or other objects formed on the web of media. An example of a process that includes web transport through an additive printing system is roll-to-roll flexographic printing.
Co-planar wave guide circuits and touch screens are two examples of electrical devices that can be manufactured using a roll-to-roll additive flexographic printing process. For example, a capacitive touch screen includes a substantially transparent substrate which is provided with electrically conductive patterns that do not excessively impair the transparency—either because the conductors are made of a material, such as indium tin oxide, that is substantially transparent, or because the conductors are sufficiently narrow such that the transparency is provided by the comparatively large open areas not containing conductors. For capacitive touch screens having metallic conductors, it is advantageous for the features to be highly conductive but also very narrow. Capacitive touch screen sensor films are an example of an article having very fine features with improved electrical conductivity resulting from an additive printing system.
U.S. Patent Application Publication 2014/0295063 by Petcavich et al. discloses a method of manufacturing a capacitive touch sensor using a roll-to-roll process to print a conductor pattern on a flexible transparent dielectric substrate. A first conductor pattern is printed on a first side of the dielectric substrate using a first flexographic printing plate and is then cured. A second conductor pattern is printed on a second side of the dielectric substrate using a second flexographic printing plate and is then cured. The ink used to print the patterns includes a catalyst that acts as seed layer during a subsequent electroless plating process. The electrolessly-plated material (e.g., copper) provides the low resistivity in the narrow lines of the grid needed for excellent performance of the capacitive touch sensor. Petcavich et al. indicate that the line width of the flexographically-printed microwires can be 1 to 50 microns.
Flexography is a method of printing or pattern formation that is commonly used for high-volume printing runs. It is typically employed in a roll-to-roll format for printing on a variety of soft or easily deformed materials including, but not limited to, paper, paperboard stock, corrugated board, polymeric films, fabrics, metal foils, glass, glass-coated materials, flexible glass materials and laminates of multiple materials. Coarse surfaces and stretchable polymeric films are also economically printed using flexography.
Flexographic printing members are sometimes known as relief printing members, relief-containing printing plates, printing sleeves, or printing cylinders, and are provided with raised relief images (i.e., patterns of raised features) onto which ink is applied for application to a substrate. While the raised relief images are inked, the recessed relief “floor” should remain free of ink.
Although flexographic printing has conventionally been used in the past for the printing of images, more recent uses of flexographic printing have included functional printing of devices, such as touch screen sensor films, antennas, and other devices to be used in electronics or other industries. Such devices typically include electrically conductive patterns.
To improve the optical quality and reliability of the touch screen, it has been found to be preferable that the width of the grid lines be approximately 2 to 10 microns, and even more preferably to be 4 to 8 microns. In addition, in order to be compatible with high-volume roll-to-roll manufacturing processes, it is preferable for the roll of flexographically printed material to be electrolessly plated in a roll-to-roll electroless plating system. More conventionally, electroless plating is performed by immersing the item to be plated in a tank of plating solution. However, for high volume uniform plating of features on both sides of the web of substrate material, it is preferable to perform the electroless plating in a roll-to-roll electroless plating system.
Flexography is a form of rotary web letterpress, combining features of both letterpress and rotogravure printing, which uses relief plates comprised of flexible rubber or photopolymer plates and fast drying, low viscosity solvent, water-based or UV curable inks fed from an anilox roller. Traditionally, patterns for flexographic printing plates (also known as flexo-masters) are created by bitmap patterns, where one pixel in bitmap image correlates to a dot of the flexographic printing plate. For instance. pixels arranged in a straight line in the bitmap image will turn into a continuous straight line on the flexographic printing plate. For flexographic printing (also known as flexo-printing), a flexible printing plate with a relief image is usually wrapped around a cylinder and its relief image is inked up and the ink is transferred to a suitable printable medium.
Flexographic printing plates typically have a rubbery or elastomeric nature whose precise properties may be adjusted for each particular printable medium. In general. the flexographic printing plate may be prepared by exposing a UV sensitive polymer layer through a photomask, or using other preparation techniques.
Application of flexographic printing as additive means has advantages. However, printing fine lines is difficult for many reasons. In some examples, the flexographic substrate may be too flexible, therefore, fine line patterns and small isolated dots are easily distorted making it difficult to maintain the quality of the fine printed lines and patterns.
In recent years, designs of electronic devices incorporating touch screen sensors have been using less area for electrical connection of the touch screen sensors to their controllers. This requires the use of thinner electrical bus lines with narrower spacing between each bus line. Printing these thin lines with narrow spaces can be difficult due to their susceptibility to damage in the flexographic printing plate and the potential for over-inking of the print features on the plate resulting in electrical short circuits.
Further, designs of electronic devices incorporating touch screen sensors have been using increasingly thinner materials, including thinner glass substrates, thinner touch sensor substrates, and thinner adhesive layers. Printing on thin substrates can be more challenging due to their tendency toward wrinkles and dimensional changes under tension.
U.S. Pat. No. 9,067,402 to Bielak describes the difficulty in printing small dots in a halftone application using flexography. Bielak solves this problem using additional UV exposure of scaffold dots to produce a raised floor around fine dots to provide sufficient mechanical support to prevent poor printing of the fine dots.
U.S. Patent Application Publication 2015/0122138 to Van Ostrand et al. describes several modes of failure to print fine lines. Van Ostrand et al. provides a change in the printed pattern at the junction of fine lines to protect the integrity of the lines and prevent unwanted printing at the junction locations.
U.S. Pat. No. 9,063,426 to Ramakrishnan et al. describe the use of support structures formed between the lines of a conductive mesh to prevent waves from being introduced into the printed lines.
Electrical devices of low visibility printed on a transparent substrate often include feature groups of parallel thin lines that carry charge or current cooperatively such that the lines are thin enough to be difficult to see, but the group of lines have enough charge carrying capability to provide the required electrical conductivity. A new failure mode has been observed in the printing of groups of parallel lines such that the outermost lines in either the in-track or cross-track direction (relative to the processing or web-transport direction) fail to print properly and show ragged or missing ink traces, thereby rendering the lines incapable of carrying current or charge and decrease the collective conductivity.
Another failure mode for printing fine lines results from the width of the features on a flexographic printing plate being of similar width as the cells of the anilox rollers used to provide uniform ink coverage to the flexographic printing plate. Due to the compressibility of the features on the flexographic printing plate, when outer features of a set of features are brought into contact with the anilox roller, under-inking can occur due to edge deformation, resulting in print defects that result in reduced or broken electrical connection.
There remains a need for a method to reliably fabricate an array of micro-wires by printing a set of fine lines using a flexographic printing system without breaks or other artifacts.