Adhesives as a thermomechanical stress source-comparing silicones to epoxies

VTT Electronics has developed a capacitive microphone that is manufactured using silicon micromachining. The movement of the bending polysilicon membrane changes the capacitance of the microphone. The low stress polysilicon membrane is very sensitive, and therefore the microphones can also be used to study packaging induced stresses. We have used flip chip bonded microphones to compare the thermomechanical stresses caused by two silicones and two epoxies. The packaged microphones have been subjected to a temperature cycle between -40°C and +60°C. During this cycle, we measured their capacitance as a function of temperature as well as frequency. From these measurements, it is possible to obtain the effect of the packaging related stresses. We estimate that these thermomechanical stresses result in a capacitance change of about 20 fF/MPa. In this study, one of the silicones gave the best results. This silicone has a low value of modulus of elasticity and not too high a value of thermal expansion. On the other hand, the hardening of this silicone (i.e. increase in modulus of elasticity) could be seen already at temperatures around -10°C