B5.1 - Integrating Sensors Into Mechanical Structures

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
B5 - Sensor-Packaging
J. Wilde, T. Fellner, R. Zeiser - Albert-Ludwigs-Universität Freiburg -IMTEK- (Germany)
294 - 299


The conventional technology to integrate sensors into mechanical structures utilises mechanical attachment of completely packaged sensors. The reason is that the protection of the device must be assured and that the specifications can be verified on the device level. In the case of structure-integrated sensors we will regard transducers for mechanical properties like force, stress or strain, which are mounted directly on a component. This allows much more compact solutions and reduced packaging effects. Two main types exist, one is a foil type like a strain gage and the second is a chip-like element. Both concepts are investigated and their specifics will be discussed.
In conventional packaging it is one of the principal aims to avoid the transfer of mechanical stress into a device as cross-sensitivities will affect sensitivity, offset and linearity. In the case regarded here it is the target to maximise the stress in the transducer as it is the characteristic property. Besides, sensitivity and offset must not be affected by the operating environment like temperature or humidity.
As it is well-know from strain gages, foil-type transducers are well-suited as they exhibit low inherent stiffness. Unfortunately conventional strain gages will not be suited well for autonomous operation due to their low resistance and high power consumption. To that purpose we explore new concepts for thin-film based high-resistance as well as capacitive strain gages. These sensors have been characterised extensively and the operating characteristics showed good potential for application in wireless sensing.
On the basis of these results, packaging concepts for strain gages with integrated instrumentation circuits and furthermore for wireless operation have been developed.
In the case of bulk-type transducers the principal question of sensor mounting was investigated for the case of SAW devices. Based on Finite-Element simulations the technologies for mounting the chips were evaluated with respect to their influence on offset and sensitivity. Die attachment techniques like adhesive bonding, soldering, brazing, glass bonding or eutectic bonding were regarded. Based on these results, proposals for assembly, packaging and interconnection are made.
Also, several examples for structure-integrated sensors for control applications as well as for structural health monitoring will be presented.