86
Figure 4-1: Thermogravimetric analysis TGA of graphite heated in air at 10 °Cmin [100].
One of the advantages of using carbon as the sacrificial layer is that carbon burns in the air producing carbon dioxide, which is not toxic in small amounts and is considered
environmentally advantageous over thin-film and silicon processes for fabricating free- standing structures.
4.2.3 Electrode Pastes
Silverpalladium AgPd pastes are commonly used in fabricating electronic components such as hybrid microcircuits, passive electronic components e.g.
multilayer capacitors, multichip modules and packaging for integrated microcircuits [94].
Silver Ag is widely used in the electronics industry for its high electrical conductivity or low electrical resistivity of 1.59 × 10
-8
Ωm. The major weakness of Ag is the electromigration effect in humid conditions under potential bias. Silver also has poor
solder leach resistance. These problems can be minimised by adding palladium Pd to Ag to make an alloy system.
Wei gh
t C han
ge
Temperature °C
87 The melting temperature for the AgPd system can be modified by adjusting the ratio
atomic of the metals in the system of the solid-solution formation. The solidus and liquidus increasing in temperature monotonically from Ag to Pd T
m Ag
= 692 C, T
m Pd
= 1552
C [94]. For low firing process at around 1000 C, a solution with 85 of Ag and
15 of Pd is used to formulate the paste. Usually the AgPd thick-film conductors are fritted with borosilicate or similar glass phases which are used to bond the metal
particles to the surface of the alumina substrate on firing.
The other alternative electrode material is gold Au. Apart from its relatively high cost, gold can be made as an excellent electrode paste. It exhibits better wire bondability and
migration resistance compared to Ag. Gold is usually added with Pt or Pd to form alloys for thick-film applications to improve solderability with SnPb solder. The properties of
the electrode materials are summarised in Table 4-1.
Although Au is better than AgPd, the trade off between the cost of fabrication and performance makes AgPd preferable as the electrode material. Conversely, due to the
use of high temperature solders to connect the thick-film terminals, Au is a better candidate for the soldering pad material.
Table 4-1: Comparison of material properties for silver, palladium and gold [94].
Metal Density
gcm
3
Melting Temperature
C Electrical
Resistivity 10
-8
m, 298 K
Thermal Expansion
Coefficient 10
-6
K Thermal
Conductivity WmK, 300
K Young’s
Modulus GPa
Silver, Ag 10.5
961 1.59
19.2 429
76 Palladium,
Pd 12
1825 10.8
11.2 71.8
112 Gold, Au
19.3 1063
2.35 14.2
317 80
