(Invited) Conformal Printing for Advanced Electronic Devices

Abstract

Different electronic devices are ubiquitous in our daily lives. These devices keep on improving daily, and their evolution contributes to the enrichment of our lives. However, two factors remain, that is, little or no progress at all has been introduced in the field of electronics for a long time. First, the use of a printed circuit board (PCB) to drive electronic-device circuits has not been improved. The PCB remains thick and hard. Second, the PCB is enclosed in a cabinet for finished products. Generally, device engineers and product designers design electronic circuits and cabinets, respectively. In this case, engineers emphasize that they cannot change the circuit size and arrangement of the lead-out wiring, whereas designers insist that they must realize an attractive cabinet beyond the restriction of the circuit designs.To overcome this issue, conformal printing using a soft blanket has attracted much attention as a technique that can form wires/electrodes on objects with complex shapes. Although the cabinets of electronic devices often have a curved surface, printing that uses a soft blanket can simply and easily form patterns on these electronic devices. At least, wires/electrodes can be formed on a cabinet with a complex shape. Among the many printing methods, the screen-printing-based technology is the most practical. The wires/electrodes of various commercially available electronic devices are fabricated using this printing method. Thus, we are developing screen-printing-based conformal-printing techniques such as screen-offset and screen-pad printing.Screen-offset printing involves screen printing onto a blanket made of silicone and transfer printing from the blanket to a substrate [Fig. 1(a)]. In this process, the organic solvents in conductive ink are absorbed by the silicone. Therefore, collapse of the patterns due to cohesion failure during transfer printing can be prevented. In addition, solvent absorption prevents ink from smudging caused by its fluidity, which subsequently means that fine patterns can be formed compared with those obtained using the conventional screen printing. The width of the patterns formed by screen-offset printing can be as fine as 10 µm, whereas the minimum width obtained by screen printing in mass-produced products is approximately 40–50 µm.Furthermore, the surface free-energy of silicone is low, indicating that screen-offset printing can form patterns even on an adhesive. For example, a material sticks to a screen mask when conventional screen printing is employed. Although heat-applied soldering is often employed for electronic-component mounting, non-heated interconnection can be performed by fixing the electronic components on a screen-offset-printed adhesive substrate using an alignment of component and substrate electrodes.Another feature of the screen-offset printing is that the silicone blanket can deform along the shape of a substrate owing to its softness, which indicates that the conductive ink can be transcriptionally formed on the surface of an object with a complex shape. For example, we developed printed wire-bonding techniques using screen-offset printing [Fig. 1(b)]. A 200-µm-thick Si chip with face-up-type electrodes was arranged on a PCB. Using screen-offset printing, the conductive-ink patterns could climb over the cleaved sidewall of the Si chip because silicone can deform along the steric structure. Thus, wiring connections between the PCB pads and Si-chip pads were successfully performed.Furthermore, screen-offset printing can properly form conductive patterns on textiles [Fig. 1(c)]. For example, plain-woven cotton, which is often used for clothes such as shirts, has a bumpy and porous surface, implying that forming wires/electrodes without breaking is very difficult. However, the silicone blanket can deform along the textured surface. In addition, dried ink caused by organic-solvent absorption of the blanket can bridge the air gap. This wiring technique can contribute to the e-textile technology as a platform for wearable electronics.Applying screen-offset printing on a surface with a large step on the order of millimeters is difficult. To overcome this problem, we need to employ a much softer and thicker blanket to drastically deform along the substrate surface. In contrast, screen printing ink onto such a softer blanket becomes difficult. To address this issue, we have developed screen-pad printing, which is composed of the following: (i) screen printing to a normal blanket, (ii) picking ink to a soft pad from the blanket, and (iii) transcriptionally forming ink patterns on a substrate from the pad. Patterns can be formed even on bumpy surfaces with a large step owing to the soft pad, although the number of processes increases.Common objects generally do not have a flat surface but have a complicated shape. We are currently attempting to fabricate such object-sensor devices by forming functional-material patterns using our developed conformal-printing technique. Figure 1