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Research paper : Development of laser-assisted inkjet printing technology (A. Endo et al.)−11−Synthesiology - English edition Vol.4 No.1 (2011) the wide-ranging design rule from semiconductor chips to printed circuit boards (PCB). In CP nano-imprinting, fine structures can be fabricated easily by transferring the mold plate to the resin substrate, and new developments are possible. In screen printing, the desired wiring pattern can be created by printing the conductive paste onto the PCB substrate using a screen. Screen printing is used as surface mounting technology[2]. Since these process technologies use masks or molds, the 3D mounting on uneven substrate is extremely difficult.On the other hand, in the maskless process MIPTEC, the pattern can be changed easily simply by rewriting the program. Since the metal film wires formed by electroless plating are transferred by ablation with YAG or YVO4 laser to create three dimensional patterns, it is expected that this 3D mounting technology will allow high-diversity production necessary in creating structures such as 3D connectors. Another maskless process, the inkjet printing technology that is being developed in the organic electronics technology field[5] is using ink, in which the conducting nanosize metal particles are dispersed, to form the desired pattern by applying the necessary amounts in the pattern areas. The inkjet printing allows formation of patterns on uneven substrates. Moreover, recently, it has become possible with this technology to form wirings stably at about 50 m in width and this make this process suitable for the application to 3D mounting technology.A comparison of the characteristics of the process technology and the technological elements are shown in Fig. 3. The photolithography technology, which is the most practical process technology at this point, was developed heavily due to the important advantages offered by of fine-sizing, high throughput, and high yield. The mounting technologies were developed utilizing the characteristics of CP nano-imprinting technology for its fine-sizing capability, the screen printing for its high throughput, and MIPTEC for its high-diversity capability and the superiority of maskless process.In the same time, inkjet printing has other important advantages such as high-diversity, low cost, and energy and resource savings that are unseen in other process technologies, and it shows the potential to become the core of minimal manufacturing. However, there are technological issues that must be overcome, such as the low throughput needed to realize high productivity and low yield.3 Technological issues and the selection of methods to solve them3.1 Wet-spreading of ink that causes decreased wire formation speedIn the inkjet printing technology for wire formation, the wires are formed by connecting the dots, and the process factors are different from the home-use inkjet technology in which the dots are placed at even intervals. Specifically, the state-of-dot connection changes and the form of wiring pattern is greatly affected by factors such as the wire forming speed and ejecting frequency, ink viscosity and surface tension, and wet-spreading of ink onto the substrate.In the conventional inkjet printing technology, the ink that lands on the substrate spread in the planar direction, the width of the wire spread further than the diameter of the droplet even with controlled surface tension, ink viscosity, or the wetness of the substrate. For example, under the conditions of stage speed 100 mm/s and ejecting frequency of 30 kHz, when the droplet with a diameter of 15 m lands onto a substrate at contact angle of about 60°, the wire width will expand to about 50 m or several times larger than the droplet diameter[6]. To achieve fine wiring of about 10~20 m of width, it is necessary to reduce the droplet size to 10 m or less.This means that when the wire resistance is kept constant, or when the ink supply per unit length of wire is kept constant yet maintaining the throughput, the ejecting frequency depends by factor of 3 on the reduced droplet diameter.Fig. 2 Wire width corresponding to mounting position and wiring technologySemiconductor chipSurface mountingMounting positionManufacturing processSimpleComplexHigh-vacuum processHigh-vacuum processPlatingPlatingDesign ruleInkjet printingScreen printingµCP nano imprintPhotolithographyMasklessMask processProcess technologyMIPTEC1000 µm100 µm10 µm1 µm0.1 µmPCB/FPBLTCCFig. 3 Wiring technology and characteristics of the technological elementsInkjet printingScreen printingμCP nano-imprintingPhotolithography technologyHigh yieldEnergy and resource savingsReduced manufacturing procedureManufacturingcostLarge surface areaHigh throughputHigh diversityFine-sizingEnvironmental friendlinessProduction costProductivityFunctionalityMIPTEC×××××△△△△△△△△△△△〇〇〇〇〇×△△〇×△△〇〇〇〇◎◎◎◎◎◎◎◎

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