Inkjet wiring advances 'desktop factory' goal

Seiko Epson Corp. has developed multilayer wiring boards by using its inkjet printing technology to print metal wiring patterns directly onto the layers. The simplified production process could eliminate the need for large and costly clean rooms and for expensive photolithography systems, Epson researchers said.

A factory using the Epson approach could be squeezed into a facility one-tenth to one-hundredth the size of present facilities, said Akio Mori, general administrative manager of the production engineering and development division at Epson. "The desktop factory is still a dream, but it's our final target," he said.

The boards were created as part of a three-year project started in June 2003. Epson intends to establish volume production using the inkjet technology in the next 18 months. "We hope we can offer samples of the wiring board by 2007," said Kenji Wada, GM of the KN promotion project at Epson's production engineering and development division.

The company is also exploring other possibilities for its inkjet technology and thus far has attempted to apply it to the production of 40-inch OLEDs and to LCD color filters. "The technology can be applicable to various fields, but the printed-wiring board is the application closest to practical use," said Mori. "In the future, more-advanced applications, such as forming organic TFTs with the inkjet printing method, are in our sights."

Epson developed the multilayer technology for its inkjet printers. In the technique, a multilayer actuator head (Mach) fires micro-ink droplets via a mechanical force generated by piezoelectric elements that change shape when voltage is applied. Unlike thermal inkjet printing, in which heat generates bubbles to push out the ink droplets, the approach does not involve heating.

To form the 20-layer wiring boards, the Epson R&D team used the Mach heads, a newly developed insulator ink and a conductive ink containing a dispersion of silver microparticles measuring from several nanometers to several tens of nanometers in diameter. The silver particles were coated with an organic material to prevent them from cohering to each other. They were then baked at 150°C to 200°C to remove the organic material.

The process is said to be simpler than conventional approaches. An insulator layer is printed on a substrate. Then metal wiring patterns are drawn on the insulator layer and baked to form the first wiring layer. Next, metal posts are formed to connect the first wiring layer to the second and an insulator layer is printed. Those steps are repeated to form the number of layers desired.

By contrast, the conventional process used to form fine-pattern multilayer boards is a subtractive process. In the conventional approach, a metal layer and resist layer are formed, and then photolithographic and development steps are performed in which the layers are etched and the resist material removed. The process requires costly photomasks and generates waste in the form of the removed resist and the etched-out metal.

By directly drawing wiring patterns on the substrate, the inkjet printing method could reduce energy and material requirements and eliminate waste, Mori said.

Epson's 20-layer wiring board measures 20 x 20mm and is 200m thick. The silver wiring lines placed on the layers are 50?m wide and 4?m thick. The metal posts printed in the insulator layers provide 2,480 electrical connections between layers. Consequently, a continuous wiring line measuring 5m long was formed on the prototype board, Epson said. The research team confirmed that the wire carries electricity.

Epson opted for silver nanoparticles rather than the copper used on conventional wiring boards because "copper is easily oxidized," Wada said. "Silver is more practical for nanoparticles."

But the electrical resistance of the wiring formed by the nanoparticles exceeds that of bulk-metal wiring by a little less than 10x, according to Epson. "Thus our wiring board will not necessarily replace all wiring boards," Wada said.

In the future, Epson expects to use the inkjet printing technology to embed components such as bare semiconductor dice, condensers, resistors, passive components and even power supply units onto wiring boards.

Epson's R&D project is subsidized by the New Energy and Industrial Technology Development Organization, an independent Japanese government agency.

- Yoshiko Hara

EE Times