Page 89 - MEMS Mechanical Sensors

78 Mechanical Sensor Packaging

Table 4.4 Thermal Expansion Coefficients of Common Packaging Materials
–6
Application Material TEC (10 /°C)
Die Si See Table 4.5
GaAs 5.7
Lead frames Copper 17
Alloy 42 4.3–6
Kovar 4.9
Invar 1.5
Substrates/constraints Alumina (99%) 6.7
AIN 4.1
Beryllia (99.5%) 6.7
Pyrex 7740 3.3
Adhesives Au-Si eutectic 14.2
Pb-Sn 24.7
Pb glass 10
Ag loaded epoxy 23–40 1
Thermoplastic 30–54 1
RTV silicone 300–800 2
1
Below glass transition temperature.
2
Above glass transition temperature.
Source: [48].

Table 4.5 Thermal Expansion Coefficient of Silicon Versus Temperature
–6
Temperature (°C) TEC (10 /°C)
–53 1.715
7 2.432
27 2.616
127 3.253
427 4.016
Source: [49].

Pressure sensing die

Silicon
intermediate

Soft adhesive

Package Glass sphere spacers
Figure 4.15 Soft adhesive die mount with glass spacers.

A commercial low absolute pressure sensor has also been successfully packaged
using soft adhesives for use in space applications and in particular a mission to Mars
[50]. The application requires the sensor to survive shocks of up to 100,000g, oper-
ate in temperatures as low as –80°C with fluctuations of 50°C and resolve 0.05 mbar
over a 14-mbar range with an overall accuracy of 0.5 mbar. Given the size and