In printed electronics, conductive inks printed on substrates are heated and cured to form electronic circuits. When low-heat-resistant substrates and parts are to be used, it becomes necessary to heat required portions only, using selective heating technologies. [2]

Microwave heating is one of the selective heating technologies, as is Photonic Curing process technology already introduced by SDK. Microwave heating is suited for printing on low-heat-resistant plastic films and for roll-to-roll continuous printing. Furthermore, microwave can penetrate into a substrate, enabling the formation of conductors inside a substrate and the bonding of internal parts.

In the conventional microwave heating, there is a problem of spark discharge involving conductive inks that contain metallic materials. SDK has developed a new technology, jointly with Professor Yuji Wada, of Tokyo Institute of Technology, and The National Institute of Advanced Industrial Science and Technology (AIST) to control spark discharge by separating microwaves magnetic field and electric field and applying the fields separately in heating [3].

In our newly developed silver/carbon hybrid conductive ink optimized for this technology, carbon particles (which do not develop a phase change during heating [4]) have been added to silver particles. This step ensures uniform curing of the ink under microwave heating, preventing a non-uniform distribution of conductivity. Conventional silver inks tend to develop, upon absorption of microwave, a non-uniform phase change due to melting, resulting in non-uniform distribution of conductivity.

This development forms part of the New Energy and Industrial Technology Development Organizations (NEDO) research project on "Development of Both Metal Thin Film Fabrication Technology with Microwave and Its Patterning Technology," coming under "Research and Development for Nanotech and Advanced Materials Applications."

In the wiring of electronic circuits, silver-containing conductive adhesives are generally used to ensure conductivity. However, silver tends to cause migration due mainly to halogen components contained in peripheral materials. SDK has developed, jointly with Professor Katsuaki Suganuma, of Osaka University, a halogen-free conductive adhesive, enabling substantial reductions in migration.

This adhesive can also be used with the newly developed microwave-heating equipment. The adhesive can be printed on a plastic film, and subjected to heating and curing with devices being mounted on the substrate.

The adhesive is based on the epoxy resin manufacturing technology [5] developed jointly by SDK and AIST as part of research projects in the areas of "Development of Fundamental Technologies for Green and Sustainable Chemical Processes; Development of Innovative Chemical Process-Product with Less Waste Emission; and Fundamental Technology Development of Innovative Oxidation Process." These projects were sponsored by the Ministry of Economy, Trade and Industry in 2008, and by NEDO in two stages, namely, in the 2009-2011 period and in 2012. Resin formulations have been optimized for conductive adhesive applications.

Printed electronics-the technology to manufacture electronic circuits through printing-is expected to be increasingly used as an efficient production method for electronics, electric appliances, and transport machinery. SDK will continue to work to develop and commercialize materials for printed electronics through collaboration with outside partners.