Contact through silver particles in ink

Modern cars are full of sensors. The optimum quantity of air in the intake tract of a combustion engine is regulated by thermoelectric flow sensors, for instance.

They measure which quantities of a gas or a liquid flow in a particular direction. Another application for sensors like these is in medicine, where they regulate tiny quantities of drugs.

These thermoelectric sensors depend for their correct function on the right contact: The measuring sensors, consisting of a silicon wafer and a membrane, are embedded in a printed circuit board. So that the necessary current can flow between the contacts of the sensor and the printed circuit board, a conductor path has to be created – experts speak of ‘contacting’. Researchers at the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research IFAM in Bremen are working on a special technique: “Up to now, contacting was usually done with wire bonds – thin wires, that is,” explains IFAM project manager Christian Werner. “But wire bonds stick out, and thus impair the flow behavior of the gases and liquids.

That can affect high-precision measurements.” The researchers have therefore developed a new technique: INKtelligent printing®. What is different about this technique is that the researchers print the conductor paths instead of wiring them. This is basically a contactless aerosol printing method. The secret lies in the ink: “The suspension contains nano silver particles in a special solvent,” says Werner. “This enables us to print extremely thin-layered conductor paths.” Subsequent thermal treatment activates the electrical conductivity of the paths.

The researchers have tried and tested these conductor paths together with colleagues from the Institute for Microsensors, -actuators and -systems IMSAS in Bremen. Altogether, the engineers have solved one of the main problems of thermoelectric sensors. In contrast to wire bonds, which have an overall height of 1 to 1.5 millimeters, the printed conductor paths are a mere 2 to 3 micrometers high, or almost five hundred times thinner than wire bonds.

This enables the sensors to make far more accurate measurements. Fraunhofer researchers will be presenting the novel technology platform INKtelligent printing® at the Sensor and Test fair in Nuremberg from May 6 to 8 (Hall 7, Stand 331).