Reliable and Robust Integration of SAW Strain Sensor Through Reflow Soldering

Surface acoustic wave (SAW) sensors represent a prominent technologi-cal avenue for quantifying mechanical stress-induced deformations. Nonetheless, the integration of these microelectromechanical systems (MEMS) sensors remains a complex challenge. Consequently, a common approach within a multitude of research involves the utilization of an adhesive formulation. However, this mode of integration continues to exhibit deficiencies in terms of measurement repeatability, particularly when confronted with harsh environments characterized by pronounced mechanical and thermal stresses. In this context, the utilization of backside reflow soldering for SAW sensor integration stands as a notably dependable alternative to epoxy bonding. In this letter, we present the results of our feasibility study concerning the reflow soldering of a SAW sensor onto a binocular copper load cell to measure static strains and a ferromagnetic steel sheet to measure dynamic strains resulting from magnetostriction. To use reflow soldering, a nickel/copper electrodeposition on the ferromagnetic steel sheet was carried out, demonstrating the potential to apply this technique to various materials. The results have indicated that reflow soldering enhances the transmission of static and dynamic mechanical stresses to the SAW sensor, exhibiting a coefficient threefold greater than that achieved through simple adhesive bonding. Furthermore, this integration technique demonstrated the long-term stability of the measurement.