Membrane-based thermal flow sensors on flexible substrates

• Flexible flow sensor on polyimide foil with functional layers on a membrane.
• Integration on nonplanar surfaces by soldering or conductive adhesives.
• Flow separation and reattachment detection possible when integrating on airfoils.

We investigated the fabrication, integration and resulting characteristics of a thermal flow sensor on flexible substrates with functional layers (heater, thermopiles for temperature measurement) being located on a membrane. The sensor was fabricated by realizing all high temperature deposition and structuring processes on a silicon wafer first followed by a transfer of the functional layers onto a flexible polyimide substrate. An integration of the sensor by flip chip soldering and conductive adhesives has successfully been demonstrated.The flexible flow sensor presented here is a further development of a standard membrane-based thermal flow sensor using silicon substrate with similar design of the functional layers. Thus, it has given us the chance to make a direct comparison between a flexible and non-flexible approach. The sensors’ characteristics have been experimentally evaluated in terms of response to fluid flow, response time and minimum detectable flow rate and compared with an existing similar silicon-based design. It has turned out that the membrane-based flexible flow sensors integrated by soldering showed expected characteristics of a calorimetric flow sensor. A second – not membrane-based – flexible flow sensor approach being integrated by conductive adhesives showed differences in the response to fluid flow which can be explained by the changed heat distribution. Sensitivities (at zero flow) being measured have values between 0.51 mV/m/s (soldered) and 20.1 mV/m/s (bonded by adhesives) compared to 30.4 mV/m/s (silicon substrate).Finally, an application is presented showing the successful integration of the flexible flow sensor on an airfoil without restricting its structural health. A wind tunnel measurement has been done demonstrating capability of the flexible flow sensor concept for detection of boundary layer separation and reattachment on the overflowed surface.