may be 40, 50 or 100 µm. On the
other hand, it is possible to create
fillers with maximum particle sizes
achieved, which plays an important
role in realizing good heat transfer.
The particle size of standard fillers
Many adhesive chemistries are
available today, such as epoxies,
silicones, polyurethane and poly-sulfide-based materials. Typically,
the bulk thermal conductivities for
these systems falls in the 0.1 to 0.2
W/(m•K) range, essentially making
them behave as thermal insulators.
To make these materials transfer
heat, i.e., act as heat dissipaters,
various fillers can be used depending on the application. Fillers play
the critical role of increasing the
thermal conductivity of these chemical systems. Potentially, the thermal
conductivity can be increased by
10 to more than 20×, depending
on the type of filler and the chemistry used to blend the filler. A few
examples of fillers are aluminum oxide, aluminum nitride and silver and
graphite based fillers (see Figure 1).
Aluminum oxide and aluminum nitride increase thermal conductivity
while providing electrical insulation.
Silver and graphite based fillers
boost heat conduction while providing electrical conductivity. The material surrounding the filler prevents
oxygen penetration and the formation of oxides. The compounds are
formulated and mixed in a way that
ensures air is not entrapped and
voids are minimized, to give maximum particle-to-particle contact
and maximize the surface area. This
ensures the best thermal conductivity performance, as well as either
electrical conductivity or insulation.
The particle size of the filler has
a significant impact on the thickness of the bond line that can be
Fig.1 Insulative and conductive
Al2O3 Silver Ag
Low Outgassing • Cryogenically Serviceable •
Chemically Resistant • Superior Bond Strength •
High Temperature Resistant
Thermally Conductive Epoxy Adhesives
The following are some practical, commercial examples of adhesives and potting compounds currently offered.
A thermally conductive adhesive that performs well in many high frequency,
power applications is silver-filled epoxy EP3HTSDA-1. This NASA-rated, low outgassing product, which has been used in vacuum systems, is both a thermally and
electrically conductive adhesive system.
Particularly useful for die attach applications, an electrically insulating adhesive
is Supreme 3HTND-2DA (see Figure S1). It is a one component epoxy with an
extremely fast cure time at 125°C.
MB600S (see Figure S2) is a one part, sodium-silicate based material that can
be used to coat devices in high frequency applications to achieve EMI shielding
up to 18 GHz.
EP21TCHT-1 is a widely used aluminum oxide filled insulating adhesive, even
in cryogenic applications.
A graphite-filled epoxy system, EP75-1 can be used for bonding and sealing.
Supreme 18TC is a specially formulated one component epoxy with the largest particle size of just 10 to 15 µm. The small particle size enables very thin bond
lines, increasing heat transfer and lowering thermal resistance.
EP121AO (see Figure S3) is a potting compound that has good flow properties
and cures at an accelerated rate at temperatures around 150°C. It is also thermally
conductive and has superior resistance to high temperatures.
EP30TC has a low viscosity and is good for high temperatures. It is a very stiff,
high modulus material, which is key in many applications. Even though it is a two
part chemistry, it can be pre-mixed and packaged in a specified amount in a frozen
syringe, to speed the production process.
Fig. S1 Supreme 3HTND-2DA is a
fast curing, no mix, insulating epoxy
system for die attach applications.
Fig. S2 MB600S is a silver containing,
aqueous based, sodium silicate system
used as a coating in applications where
the highest level of EMI shielding is
Fig. S3 EP121AO is a thermally
conductive, electrically insulating epoxy
resin system for potting, encapsulation,
coating and sealing.