Page 380 - Handbook of Adhesives and Sealants

336 Chapter Nine

whereas the base polymer provides the elastic nature. The effect of
tackifiers on cure characteristics and end-properties must be under-
stood since the effects can be significant. 19

9.3.7 Modifying electrical and thermal
conductivity
The electrical and thermal conductivity properties of adhesive systems
can also be modified by formulation composition. Unmodified poly-
meric resins are natural insulators and do not exhibit electrical con-
ductivity. There are certain applications, notably in the electrical and
electronics industry, where electrically conductive adhesives provide a
significant value. One application involves the use of conductive ad-
hesives as an alternative for wire or circuit board soldering.
Similarly, most unmodified polymeric resins have very low thermal
conductivity. There are certain applications where high levels of ther-
mal conductivity are important in adhesives. One example is the at-
tachment of heat sinks that must be tightly bonded to power electronic
components.
Appropriate fillers have been used to produce adhesive with high
thermal or electrical conductivity for specialized applications. It
should be noted that, regardless of the adhesive system itself, electri-
cal or thermal conductivity is improved by minimizing the adhesive
bondline and by minimizing the organic or non-conductive part of the
adhesive.
Electrically conductive adhesives owe their conductivity as well as
their high cost to the incorporation of high loadings of metal powders
or other special fillers of the types shown in Table 9.5. Virtually all
high performance conductive products today are based on flake or pow-
dered silver. Silver offers an advantage in conductivity stability that
cannot be matched by copper or other lower cost metal powders. 14
Where elastomeric properties are required, silver filled epoxy and sil-
ver filled silicone rubber systems are commercially available. Conduc-
tive carbon (amorphous carbon or fine graphite) can also be used in
conductive adhesive formulations if the degree of conductivity can be
sacrificed for lower cost.
Figure 9.6 shows the mechanism for current flow through a metal
filled polymer. If enough metal particles are added to form a network
within the polymer matrix, electrons can flow across the particle con-
tact points making the mixture electrically conductive.
The metal powder filled adhesives conduct both heat and electricity.
Some applications, however, must conduct heat but not electricity. For
example, power electronic devices and other heat generating compo-
nents are bonded to heat sinks and other metal surfaces. Here the