Printed electronics consist of products such as electronic circuits, sensors, and devices that are manufactured by means of printing technology. The wiring of the circuit boards used in such electronic devices is produced by printing, dispensing with the need for procedures such as light exposure and etching that are required in the regular semiconductor manufacturing process. This offers the potential for saving energy and reducing the use of chemicals, and this method is therefore attracting attention as an environmentally friendly manufacturing process.
In Japan, there has been particular focus on the use of inkjet printing technology in the development of this field, and it is starting to come into use in areas, such as the formation of color filters for liquid crystal displays, as well as for devices with large surface areas such as displays, sensors, and batteries.
Inkjet printing involves the ejection of tiny ink droplets from the inkjet printhead, causing the required volume to be placed on the substrate at the right location so as to create a pattern that follows the design of the wiring pattern. The important factor is the accuracy with which the ink is placed on the substrate.
Inkjet printing is used in Organic Light Emitting Diode (OLED) display patterning, thin-layer coating of OLED lighting, and the manufacture of high added-value displays for smartphones that require a high degree of precision, meaning that inkjet printheads capable of jetting ultra-fine droplets reliably and accurately are required. Customers were also looking for high-speed ejection performance capable of handling mass production processes.
One picoliter (one trillionth of a liter) is an ultra-small volume equivalent to one fifty-millionth of a single eyedrop (which contains around 50 µl), and a 1-pl ink droplet is around 12 µm in diameter. Inkjets eject several tens of thousands of these ultra-fine ink droplets from each nozzle every second.
The finer the ink droplets, the tinier the fluctuations constituting noise in the droplet formation process, which makes high-precision jetting difficult. Consistent jetting is also difficult to achieve with low-viscosity inks.
The bend-mode inkjet printhead developed by Konica Minolta is capable of the consistent jetting of 1 pl of ink at a speed of 6 m/s, even with low-viscosity inks (0.95 mPa·s, 33.8 mN/m). The picture on the right is a series of photographs of ink being jetted from the printhead, taken using strobe lighting, which show that 1-pl droplets (approximately 12 µm in diameter) are being consistently jetted.
Even when ink is ejected from all 128 individual nozzles at once, it is placed with great precision in a straight line. The variation in angle between the 128 nozzles when a solvent analog ink (3 mPa·s, 28.6 mN/m) is jetted at a droplet speed of 6 m/s is less than 1º, a similar accuracy to hitting a 10-cm target from a distance of 10 m. The variation in speed between the ink jetted from all 128 nozzles is also only around 3%.
As shown in the illustration below, in the bend-mode inkjet printhead, the ink is pushed out by the deformation of a plate-shaped piezoelectric element (vibrating plate).
When a voltage is applied to the piezoelectric element, a pull-push drive operates to jet the ink, and the projection of ultra-fine droplets requires that it move rapidly with a small displacement. Konica Minolta further modified its bend-mode inkjet printhead by making the diaphragm circular in shape, which both shortens the natural frequencies of the printhead and helps to improve drive efficiency.
Konica Minolta also optimized the flow channel design of 1 pl using PSPICE (equivalent circuit analysis) and ANSYS (structural analysis). For example, if droplet ejection is unstable, the ink that should constitute a single droplet splits up to form satellite droplets, reducing the accuracy with which it is placed. To resolve this problem, the company used fluid analysis to study the droplet ejection process and optimized the nozzle shape in an effort to stabilize droplet ejection.
Inkjet printheads might be described as an embodiment of microfabrication technology. Konica Minolta’s inkjet printheads are around 500 µm thick, and the nozzle aperture diameter is only around 10 µm, about the same fineness as a capillary blood vessel.
The manufacture of inkjet printheads utilizes a technology known as micro electro mechanical systems (MEMS). This involves the formation of a tiny mechanical system on a substrate such as silicone or glass, requiring sophisticated micron-level microfabrication technology. Konica Minolta has used this MEMS manufacturing technology for the high-definition, high-precision fabrication of nozzles and ink channels, which affect the characteristics of printhead ejection.
By combining the company’s own piezo fabrication technology with MEMS manufacturing technology to produce compact, high-performance actuators that are mounted on the printheads.
Konica Minolta’s proprietary automated precision assembly technology is used to bond plates fabricated using MEMS manufacturing technology with high precision and without contamination by foreign substances, to complete the MEMS printhead chip.
It is the availability of these reliable microfabrication technologies that enables the ink ejected by 128 nozzles to land in a straight line with great accuracy.