thermal conductivity values give a good indication
of the level of heat transfer expected but relying
on these values alone will not necessarily result in
the most efficient heat transfer.
Thermal resistance, measured in K m
2
/W, is
the reciprocal of thermal conductivity. It takes into
account the interfacial thickness and, although it
is dependent on the contact surfaces and pres-
sures applied, some general rules can be followed
to ensure thermal resistance values are kept to a
minimum. For example, a metal heat sink will have
a higher thermal conductivity than a heat transfer
compound used at the interface and it is important
that only a thin layer of this compound be used;
increasing thickness will only increase the thermal
resistance in this case. Therefore, lower interfacial
thicknesses and higher thermal conductivities give
the greatest improvement in heat transfer. In some
cases, however, using a material with a higher bulk
thermal conductivity could be to the detriment of
contact resistance and thus, no improvement will
be accomplished.Table 1 provides some indication
of the differences between thermal management
materials and how the combination of properties is
more important than a single value alone.
Another concern with using bulk thermal con-
ductivity values alone for product selection is that
there are a number of different techniques avail-
able. Significant variations in thermal conductivity
values for the same product can be achieved by
using different test methods or parameters. This
can result in bulk thermal conductivity values that
appear very high when quoted but in use have a
dramatically reduced efficiency of heat dissipa-
tion. Some techniques only measure the sum of
the material’s thermal resistance and the material/
instrument contact resistance. Whichever is used,
it is essential that products are compared using the
same method to obtain bulk conductivity values
and in all cases the products should be tested in
the final application for a true reflection of effective
heat dissipation.
Another important factor in product selection is
the application of thermal management materials.
Whether an encapsulation compound or an inter-
face material; any gaps in the thermally conductive
medium will reduce the rate of heat dissipation.
For interface materials, the viscosity of a product
or the minimum thickness possible for application
will have an effect on the thermal resistance and
thus a highly thermally conductive, high viscosity
compound that cannot be evenly spread onto the
surface may have a higher thermal resistance and
lower efficiency of heat dissipation when compared
to a lower viscosity product with a lower bulk ther-
mal conductivity value. For encapsulation resins,
this could be expressed in a similar way; the higher
the viscosity, the more difficult it is for the resin to
flow evenly around the unit and therefore, air gaps
are formed in the potting compound reducing the
rate of heat dissipation. It is essential that users
address bulk thermal conductivity values, contact
resistance, application thicknesses and processes
in order to successfully achieve the optimum in
heat transfer efficiency.
With rapid advances in the electronics industry
and, more specifically, LED applications, it is im-
perative that materials technology is addressed to
meet the ever-demanding requirements for heat
dissipation. Some companies, including Electro-
lube, have developed specific technologies to
improve the ability to process thermal manage-
ment compounds. These technologies have been
transferred to encapsulation compounds, providing
products with higher filler loadings and therefore
improved thermal conductivity combined with im-
proved flow. In addition, Electrolube manufactures
conformal coatings and encapsulation resins in opti-
cally clear formats for applications where protection
of the entire LED is required, again affirming the
importance of continually developing formulated
chemical products to meet the rapid and demand-
ing requirements of this popular technology.
