Toyobo Co., Ltd. has developed a low temperature thermosetting conductive paste together with Toshiba Corporation. Using a simple printing method and galvanizing process, this conductive paste can directly mold electric wiring on the curved surfaces of the bodies of personal computers, cell phones and other mobile devices, to make them lighter and thinner.

1. Development Background

Today personal computers, cell phones and other mobile devices require antennas that can accommodate a wide variety of transmission methods, so mobile devices are being equipped with a growing number of antennas. At the same time, manufacturers are facing demands to make their mobile devices lighter and thinner. To date, manufacturers have attached bar and plate antennas to mobile devices, or attached antennas to substrates inside the mobile devices using copper foil etching. Molding antennas directly on the bodies of mobile devices and reducing antenna size facilitates the production of lighter and thinner mobile devices, so there have been high expectations for the development of technologies that can mold electric wiring directly onto these units.

The technology of directly molding electric wiring on the bodies of devices is known as MID (Molded Interconnect Device) technology. The two methods to date are repeated injection molding and galvanizing, and the process of molding a resin mixed with conductive materials, patterning with a laser, and then galvanizing. However, both of these methods require complex manufacturing processes and have high materials costs, so while they are adopted for small parts, they are almost never used to directly mold electric wiring on the bodies of mobile units themselves.

Together with Toshiba, Toyobo has now developed the technology to directly mold electric wiring using padprinting¹ as a simple method of printing on low temperature thermosetting conductive paste to directly mold electric wiring, as well as a suitable conductive paste for this process.

*1. Padprinting is a printing method whereby paste is poured into a gravure printing plate which forms the circuit to be printed, a balloon transfer pad is pressed onto the plate lifting the paste to the pad, and the pad is then used to print the paste onto the item being printed.

2. Characteristics
(1) Large reduction in conductive paste setting temperature
The setting temperatures of conventional conductive pastes are 130ºC or higher, which may cause deformation of polycarbonate resin and ABS (acrylonitrile-butadiene-styrene) resin, which are typical resins used for the bodies of mobile devices, when the conductive pastes are hardening. The setting temperature of our new low temperature thermosetting conductive paste is just 70ºC which is lower than the heat deflection temperatures of polycarbonate resin and ABS resin, so it cannot deform the resins used for the bodies of mobile devices. This lower setting temperature also reduces the amount of energy required for hardening.

(2) Simple Method for Direct Molding of Electric Wiring on the Body of Devices
The conductive paste facilitates the direct molding of electric wiring with the simple method of padprinting the paste onto the resins used for the bodies of mobile devices, drying, and then using electroless plating.²

*2. Electroless plating is a galvanizing method that does not require electric current. Because electroless plating does not use electric current, it can be used for galvanizing materials that do not conduct electricity.

(3) Clears constant temperature equilibrium moisture test
Electric wiring molded using this newly developed conductive paste cleared the constant temperature equilibrium moisture test, which subjects materials to a 60ºC 90% RH (relative humidity) environment over a long period of time. The paste prevents the ballooning that tends to occur in the galvanizing process.

3. Future Plans
Toyobo will continue working together with Toshiba to enhance reliability toward the production and mass production of integrated mobile device body antennas. Because the setting temperature has been greatly reduced compared with conventional conductive pastes, we will also examine applications to resins used for the bodies of mobile devices which have heat deflection temperatures lower than polycarbonate resin and ABS resin.