液態金屬顆粒墨水於多層可撓式印刷電子之應用

Abstract

隨著科技的發展，穿戴式裝置的輕量化需求漸增，多層可撓式印刷電子在近年被廣泛討論。對於多層可撓式電子而言，導電線路必須克服在變形時產生的機械破壞，並且需在垂直通孔形成穩定的導通線路。液態金屬(liquid metal)由於具有高拉伸與高導電性，因此在應用於可撓式電子上具有相當大的潛力。但液態金屬的高表面張力以及低表面潤濕性使得材料分散性與貼附性差，導致其無法直接圖樣化。將液態金屬製備成懸浮顆粒能夠有效改善上述的困難，進而能製備成導電線路印刷用的墨水。除此之外，在製備成多層立體結構時，為了避免如線路轉折、重力等因素破壞導電通路，因此需要以高固含量墨水使顆粒厚實堆疊。然而，乾燥後的液態金屬線路必須經過後續的機械燒結以聚結顆粒，對於非平面結構可能造成不均勻燒結的問題。因此，提高固含量的穩定懸浮墨水與燒結過程改進便成為製備液態金屬墨水的關鍵技術。 為了製備多層可撓式電子線路，本研究利用穩定懸浮的高固含量液態金屬顆粒墨水實現可撓式基材的平面印刷以及多層結構中導通的孔洞結構。在30wt%的固含量下，使用乙二醇(ethylene glycol)作為墨水溶劑，並添加奈米纖維素(cellulose nanofiber)，提升墨水懸浮穩定性以及與基材貼附性。透過直寫式印刷於剛性或柔性基板的良好表現，展現墨水優異的印刷能力。此外，藉由蒸發誘導燒結(evaporation-induced sintering)，免去液態金屬顆粒後續機械燒結的步驟，封裝後導電率可達到105 S/m。在經過1000次的彎折測試後，電阻值可維持不變，呈現其優異的撓曲能力。最後，利用PET層壓與液態金屬墨水填孔技術，製備出多層印刷電路板和NFC天線，展示多層液態金屬可撓式印刷線路的應用性以及可撓式電子元件的發展潛力

With the development of technology and the increasing demand for lightweight wearable devices, multilayer flexible printed electronics have been widely studied in recent years. To fabricate multilayer flexible electronics, the mechanical damage of conductive circuit during deformation must be overcome and a high stability of conductive circuit in the vertical vias is required. Liquid metals have great potential for flexible electronic applications because of its’ high surface tension and high conductivity. However, the high surface tension and low surface wettability of liquid metals lead to poor material dispersion and adhesion, making it difficult to directly pattern. The suspension liquid particle inks can effectively overcome the above difficulties, and can further be used to prepare inks for conductive circuit printing. Furthermore, in the formation of multi-layer 3D structure, in order to avoid negative factors, such as line turning and gravity, to destroy the conductive path, a thick stack of particles has to be formed with high solid content ink. Nevertheless, the circuit must undergo subsequent mechanical sintering to coalesce the particles, which may cause uneven sintering for non-planar structures. Therefore, increasing solid content and enhancing sintering process are critical to manufacture liquid metal ink. In this study, in order to prepare multi-layer flexible electronic circuits, high solid content liquid metal colloidal inks with high stability have utilized to achieve 2D printing and 3D conductive via structure in multilayer flexible structures. By using ethylene glycol as the ink solvent and adding cellulose nanofiber, suspension stability and adhesion to the substrate are enhanced at 30wt% liquid metal ink. The excellent printability of ink is demonstrated by the good performance of direct printing on rigid or flexible substrates. In addition, evaporation-induced sintering eliminates the need for subsequent mechanical sintering of liquid metal particles and achieves a high electrical conductivity of 105 S/m. After 1000 bending cycles, the resistance remains constant, presenting its excellent flexibility. Finally, multi-layer printed circuit boards and NFC antennas are prepared by the PET lamination and liquid metal ink hole-filling technology, showing the applicability of multi-layer liquid metal flexible printed circuits and the development potential of flexible electronics.