Lightweight, durable construction materials and optimized aerodynamics are examples of 'green' potential in aircraft construction. In order to exploit this potential, the aviation industry needs reliable measurement methods that provide insights into the behavior of materials and record loads down to the nanometer. To close this knowledge gap, scientists at the Fraunhofer Institute for Reliability and Microintegration IZM in Berlin are developing a type of sensor skin that is stretched over the wings of an aircraft, measures various values and evaluates them live.

The focus of the study is the plastic that is used as standard for painting aircraft: thermoplastic polyurethane (TPU). The researchers' task was to construct test strips of the material with integrated electronic components such as sensor systems and later test them under various loads. When used in reality, such a sensor coating can provide information on the temperature, air pressure, oscillation and vibration of the wings. For this purpose, the resistance of the sensor strip to chemicals such as de-icing agents or kerosene must be tested in advance. Aircraft manufacturers gain a detailed pool of data from such comprehensive sensor technology in the outer skin, from which conclusions can be drawn about the wear and tear or longevity of the installed material. Furthermore, kerosene consumption could be optimized through live evaluation of environmental parameters and flow resistance during the flight.

Although the Fraunhofer team was able to use its expertise in the field of assembly and connection techniques for stretchable electronics as a basis, it was no easy task to make the plastic with miniaturized sensors usable for aeronautics. Initially, the TPU is available in the form of a loose film, which makes it complicated to process. The crux of the matter is to avoid damaging either the sensitive components or the flexible material during processing. In addition, high functionality must be guaranteed with an extremely thin overall structure of 200 µm. The substrate thickness is also decisive for the weight, which must be as low as possible. In the first step, the plastic was characterized so that relevant parameters such as temperature-related expansion or elasticity were known. These findings were in turn incorporated into further simulations, which predicted specific weak points and the service life of the TPU under mechanical and thermomechanical stress. The data obtained was also used to derive ideal process parameters such as temperature and pressure settings for film lamination and soldering of the components.

The researchers created the circuit pattern for the stretchable sensor module using lithography and etching processes that are standard in PCB manufacturing. Once the components had been assembled and soldered, the team led by Dr. Stefan Wagner and Joao Alves Marques demonstrated two methods of protecting the components from external influences: firstly, using so-called glob tops, which are also made of polyurethane and seal microelectronic components as a hardening potting compound, and secondly, integrating thin chips directly into the interior of the substrate using flip-chip assembly. One advantage of the selected technologies and materials is that the TPU offers flexibility as a circuit carrier with integrated sensor modules, which is of great importance for use in air traffic. The flexible substrate can conform to the wings and at the same time protect the built-in electronics.

Now that the test strips have been successfully tested against mechanical and chemical influences by industry partner Airbus Central C&T, follow-up projects are conceivable. With the applied assembly and connection technology, for example, the aim is to integrate the sensor technology not only in strips but also in planar modules that can be realized within an area of up to 60 cm x 60 cm. In order to ensure a certain degree of self-sufficiency from the flight electronics, it is of great interest to investigate the integration of the entire evaluation electronics in such TPU sensor mats. This would allow data to be collected independently of aircraft resources and communication to be carried out completely wirelessly via radio or Bluetooth.