Improved mounting of strain sensors by reactive bonding

Aim of this work is to improve the bond between a strain sensor and a device on which the strain shall be determined. As strain sensor, a CMOS-integrated chip featuring piezoresistive sensor elements was used which is capable of wireless energy and data transmission. The sensor chip was mounted on a standardized tensile test specimen of stainless steel by a bonding process using reactive multilayer systems (RMS). RMS provide a well-defined amount of heat within a very short reaction time of a few milliseconds and are placed in-between two bonding partners. RMS were combined with layers of solder which melt during the bonding process. Epoxy adhesive films were used as a reference bonding process. Under mechanical tensile loading, the sensor bonded with RMS shows a linear strain sensitivity in the whole range of tested forces whereas the adhesive-bonded sensor has slightly nonlinear behavior for low forces. Compared to the adhesive-bonded chips, the sensitivity of the reactively bonded chips is increased by a factor of about 2.5. This indicates a stronger mechanical coupling by reactive bonding as compared to adhesive bonding.