Download Thermally Conductive Adhesives Keep Things Cool

TECH SPOTLIGHT
Thermally Conductive
Adhesives Keep
Things Cool
Thermally conductive adhesives
and potting compounds are an important
weapon in the war against heat raging within
today’s powerful electronic devices
Master Bond Inc. 154 Hobart Street, Hackensack, NJ 07601 USA
Phone +1.201.343.8983 | Fax +1.201.343.2132 | [email protected]
TECH SPOTLIGHT
Thermally Conductive
Adhesives Keep Things Cool
Thermally conductive adhesives and potting compounds are an important
weapon in the war against heat raging within today’s powerful electronic devices.
Electronics are hot right now—literally. Chip makers have
significantly upped the ante on microprocessor power
and density over the past decade. And these powerful
microprocessors are shoehorned into tiny mobile and
embedded devices that make thermal management all
the more difficult. Design engineers charged with keeping
these devices cool have a big challenge on their hands.
How big a challenge? Today’s microprocessors already
have to dissipate up to 100 W. And that number is bound
to go up as chip makers continue to increase the operating
frequency and number of CMOS devices within each
package. In fact, semiconductor industry projections
have microprocessor power dissipation densities rising
to 200 W/cm2 over the next decade or so. What’s more,
modern electronics packaging techniques will exacerbate
the thermal issues caused by rising power dissipation
requirements. For example, some of the new space-saving
system in a package (SiP) designs stack chips and other
electronic components on top of one another, which can
make it more difficult to remove heat.
Smart engineers have long taken a systems approach
to thermal management, employing a wide variety of
active and passive cooling measures to get the heat away
from sensitive electronic components. Thermal interface
materials, which fill in the air gaps between thermal transfer
surfaces, are one of the key cooling measures. Among
these materials are the thermal greases applied between
power dissipating and cooling components, such as
microprocessor and its heat sink. But there’s a lot more to
thermal interface materials than greases.
The line-up of thermal interface materials also includes a
variety of thermally conductive adhesives and related potting
compounds—including one- and two-component epoxies,
eXtra BONd LiNe CONsideratiONs
While engineers should pay attention to the bond line
characteristics in any structural bonding application,
thermal adhesives make that attention all the more
important. One oft-forgotten consideration in these
applications is the particle size of the thermally
conductive fillers.
In some cases, the size of the filler particles can be
the limiting factor in bond line thickness (BLT). If the
particles, for example, are 40 µm across, the bond line
can’t be thinner than that without some reduction in
the structural strength of the bond. In ordinary bonding
applications, the effect of fillers on BLT would not
necessarily matter because optimal structural bonds
often favor thicker bond lines. Thermal applications,
however, require relatively thin bond lines for the sake
of heat transfer. Thus, the particle size of the conductive
fillers can contribute to a trade-off between optimal bond
strength and optimal heat transfer.
This problem pops up less and less as suppliers develop
more advanced fillers. For instance, Master Bond has new
proprietary fillers that measure just 3 µm across, rather
than the more typical 40 µm. These smaller fillers enable
a better balance between bond strength and the thin
bond lines required for the best heat transfer. Since it’s
difficult for engineers to find out how large the particles
are in a given adhesive product, the best thing to do is
discuss your desired BLT with Master Bond’s technical
service engineers, who can flag any size conflicts during
the adhesive selection process.
Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | [email protected]
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Check Out The Latest Films
Not all thermally conductive materials come in tubes
and cans. Thermally conductive epoxy films are an
increasingly popular application option—and for good
reason: Films improves the uniformity of heat transfer.
Depending on the type of film chosen, uniform bond
lines as thin as 0.003 inches are easily achievable. With
traditional adhesives in electronic assemblies, the bond
lines are hard to control and can vary between 0.003
to 0.006 inches.
Right: Master Bond’s line-up of thermal interface materials
Films can also be die cut to intricate shapes that closely
match and fully cover an electronic component’s thermal
surfaces, which again has a beneficial effect on heat
transfer. And unlike thermal greases, films don’t flow or
run as temperature and assembly forces increase.
includes one- and two-component epoxies as well as
silicones and solvent-based compounds.
silicones and solvent-based compounds. Also available are
thermally conductive epoxy films that address some of the
most common application issues.
From a design standpoint, thermal adhesives have a lot
going for them because they do double duty: Not only do
they help manage heat but they also bond components
to create electronics assemblies. Likewise, thermal potting
compounds improve heat transfer of the components
they encapsulate, and they also work to protect those
components from shock, vibration and other environmental
threats.
Here’s a closer look at the wide range of thermal adhesives
and potting compounds and what they can do in your
application.
Balance Of Properties
In general, it’s a good idea to think of adhesives and related
compounds not in terms of a single property but in terms
of how well they balance a variety of desirable properties.
That’s true with thermal management applications too.
What many engineers don’t realize is modern adhesive
chemistry can combine thermal conductivity with other
useful or even essential properties. To take a couple of key
examples from just the epoxy family, Master Bond offers
one- and two-part thermally conductive products that have
been certified to NASA’s low outgassing specifications.
Products that meet USP Class VI biocompatibility
standards are pending as are grades that meet aerospace
specifications.
Despite these benefits, films have in the past had
some trade-offs in the ease-of-use department.
If the films need to be cut into intricate shapes,
there’s more upfront engineering work than non-film
adhesives. For instance, engineers may have to design
and commission a die. What’s more, most thermally
conductive films require cryogenic storage and heat
cure temperature requirements that make them more
complicated to use. The trade-offs, however, are
diminishing as new films come on the market. Master
Bond has recently introduced one such film. Called FL
901 AO, it comes in many pre-cut shapes and can be
stored at room temperature or in an ordinary refrigerator.
Thermally conductive products offer a host of desirable
physical and mechanical properties too. Among them are
chemical, moisture and temperature resistance—with the
latter ranging from cryogenically serviceable grades to
products able to withstand temperatures in excess of
500° F. Grades that do a good job in thermal cycling
applications are also available as are grades optimized
for shock and vibration resistance. Thermal products are
finally available with many different moduli—from flexible
to rigid—as well as different viscosities and cure rates.
(See table for list of popular grades and their properties).
As for electrical properties, the vast majority of thermally
conductive products are formulated to be electrical
insulators which is a desirable property when bonding or
potting most types of electronic components. In those
cases where thermal and electrical conductivity is required,
there are specially formulated adhesives that conduct both
heat and electric currents.
Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | [email protected]
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From Insulator To Conductor
One obvious barrier to using any adhesives or potting
compounds to help dissipate heat is that polymeric
materials in their natural state are actually good thermal
insulators. Unfilled epoxy, for example, has thermal
conductivity of just 0.14 W/mK. Compare that value to
a world-class conductor like aluminum, which comes in
around 200 W/mK.
With the addition of various metallic, ceramic or even
nanotech fillers, formulators can improve on an adhesive’s
baseline thermal conductivity values by a factor of 10 or
more. The resulting products, while never approaching the
conductivity of metals, do conduct enough heat to become
a valuable part of a comprehensive thermal management
system. And remember that thermal adhesives or potting
compounds do eliminate another troublesome insulator—
the thermally insulating air gaps that would otherwise exist
between heat transfer surfaces.
Most of Master Bond’s thermal adhesives and potting
compounds, of which there are dozens of individual grades,
have conductivity values that fall between 1.5 to 3 W/
mK. That conductivity range covers the vast majority of
commercial electronics bonding and potting applications
(see sidebar). In special cases, however, we are developing
epoxies with thermal conductivity values 4 W/mK and
above without significantly compromising the mechanical
performance of the adhesive.
And that emphasis on mechanical performance is important
with thermal adhesives because the same inorganic fillers
that impart conductivity tend to reduce bond strength.
Compare an unfilled and filled version of the same epoxy,
for instance, and the unfilled will have higher bond strength
every time. The reason why is simple: The more highly
filled products have proportionally less epoxy available for
bonding. The same reasoning applies to silicone products
as well.
Fortunately, this trade-off between strength and
conductivity is not an issue in the majority of electronics
applications. Even when filled with thermally conductive
additives, epoxies and silicones still offer more than
enough bond strength to withstand the minimal forces
seen by most power dissipating components. For example,
potted components may have to withstand some level of
stress and strain during the manufacturing process and in
use, but they don’t often experience the high forces that
characterize a true structural adhesive bond.
There are some applications in which the adhesive plays
both a structural and thermal management role. In these
cases, engineers need to keep the strength-conductivity
trade-off in mind—and potentially take steps to design
around it.
For further information on this article, for answers to any
adhesives applications questions, or for information on
any Master Bond products, please contact our technical
experts at Tel: +1 (201) 343-8983
Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | [email protected]
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Popular Thermal Adhesive Grades
Mix
Ratio
by
weight
Mixed
Viscosity
RT, cps
Supreme
11AOHT
1/1
thick
paste
Supreme
10ANHT
no
mixing
flowable
paste
Master Bond
Grade
EP21TDC-2AO
EP30AN-1
1/3
10/1
paste
5,000-6,000
Service
Temp
Range, °F
Thermal
Cond,
BTU•in/
W/m-K
Features
30-40
24-36 hrs @ 75°F
1-2 hrs @ 200°F
-100°F to
+400°F
1.44
Two Component, Room Temperature Curing,
Thermally Conductive, Electrically Isolating Epoxy
For Bonding & Sealing Featuring High Temperature
Resistance Along With High Shear & Peel Strength
N-A
1 hr @ 250°F300°F
4K to
400°F
2.883.60
One Component, Heat Curing Epoxy Adhesive
Featuring both High Shear and High Peel Strength
for Optimal Bonding Performance & Exceptionally
High Thermal Conductivity
>90
48-72 hrs @ 75°F
3-4 hrs @ 150°F
-300°F to
+250°F
1.30
Two Component Highly Flexiblized Thermally
Conductive Epoxy Resin Compound For High
Performance Bonding, Sealing, Coating, And
Encapsulation
30-40
24-48 hrs @ 75°F
1-2 hrs @ 200°F
-60°F to
+250°F
3.1
Two Component, Room Temperature Curing, Low
Viscosity Epoxy for Potting, Sealing, Coating and
Bonding Featuring Exceptionally High Thermally
Conductivity and Excellent Electrical Insulation
Properties
1.301.44
-60°F to
+500°F
2.75
Two Component Flexibilized Low Viscosity Heat
Curing Epoxy Resin System High Performance
Thermal Conductive & Electrical Insulation for
Potting & Encapsulation Applications
N-A
30 min @ 180°F
2-2.5 hrs @ 300°F
-80°F to
+500°F
1.301.44
One Component, Thermally Conductive, High
Temperature Resistant Flexibilized Epoxy System
For Electrical Potting and Encapsulation, Coatings
and Bonding
paste
N-A
24 hrs or
longer @ 70°F
-75°F to
+400°F
2.1
One Component, Thermally Conductive,
Electrically Isolating, Room Temperature Curing,
Non-Corrosive Silicone for High Performance
Bonding, Sealing & Coating
base resin:
17-24,000cps
curing agent:
50-150
3-4 hrs
24-48 hrs @ 73°F
4-6 hrs @ 65°C
2-3 hrs @ 100°C
-65°F to
+400°F
1.5
Two Component, Low Viscosity, Thermally
Conductive, Silicone Compound for High
Performance Casting, Potting and Encapsulation
18,00022,000
EP112FLAN-1
100/80
25,00035,000cps
EP36AO
no
mixing
Liquid @
180°F
MS 705TC
no
mixing
MS 151AO
100/05
100/60
light
paste
EP42HT-2AO
100/40
Part A:
45,00055,000
Part B:
100-1000
FL901AO
no
mixing
X5TC
no
mixing
EP21TCHT-1
Cure Schedule
Temp/Time, °F
Two Component, Low Viscosity, Room Temperature
Curing, Thermally Conductive, Electrically Isolating
Epoxy For Potting, Bonding, Sealing & Coating
Featuring Flexibility. Meets NASA Low Outgassing
Specification
1/1
EP37-3
FLFAO
Set-Up
TimeMinutes,
RT
75-90
4-5 days @ 75°F
3-5 hrs @ 200°F
4K to
275°F
4-6 hrs
2-3 hrs @ 220°F250°F
4-6 hrs @ 300320°F
1.44
Two Component Room Temperature Curing
Thermally Conductive, Electrically Insulating,
and Heat Resistant Epoxy Compound For High
Performance Bonding and Sealing. For Service Up
to 400°F
1.45
Two Component, Thermally Conductive Room
Temperature Curable, Heat Resistant Epoxy
Adhesive, Sealant, Coating & Casting System
Featuring Resistance to Medical Type
Sterilization. Ideal for Potting & Encapsulation
-100°F to
+400°F
1.44
One Component Thermally Conductive,
Electrically Insulative Epoxy Adhesive Film
Featuring Convenient Handling, Good Storage
Stability & Fast Cure
-75°F to
+250°F
1.61
Thermally Conductive, Elastomer Based, Room
Temperature Curing, One-Component Adhesive
for High Performance Bonding and Sealing
45
18-24 hrs @ 75°F
1-2 hrs @ 200°F
4K to
400°F
60-90
2-3 days @ 75°F
2-3 hrs @ 200°F
-100°F to
+400°F
film
N-A
1 hr @ 250°F
30-40 mins @
300°F
flowable
paste
N-A
5-30 min @ 75°F
Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | [email protected]
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THERMALLY CONDUCTIVE ADHESIVE FUNDAMENTALS
Here's a look at the features to keep in mind when working
with thermal adhesives in their liquid and film forms:
LIQUID
FILM
HEATSINK
THERMAL INTERFACE
ELECTRONIC COMPONENT
KEY FEATURES
• Flexible chemistry
• Wide range of physical properties available
• Application versatility for bonding, potting
and sealing
• Room temperature cure options
• Uniform bond line thickness for optimal
cooling
• Convenient handling, no mixing
• Speeds production
• New grades eliminate cryogenic storage needs
APPLICATIONS
• Assemble CSPs
• Assemble SECC format devices
and microprocessors
• Attach base plates
• Attach surface mounted devices (SMDs)
• Bond and pot power supplies, rectifiers
and voltage regulators
• Bond and pot RF components
• Bond flexible and rigid circuit boards
• Bond ground planes
• Bond high power LEDs to PCBs
• Bond integrated circuits to heat sinks
• Bond power supply components
• Bridge thermal gaps
• Create BGA-die heat spreader interface
• Die attach hybrid/MCMs
• Encapsulate transformers, sensors, coils and
micro circuitry
• Laminate silicone wafters
• Manage heat in aerospace and NASAapproved environments
• Mount heat sinks
• Perform in-line and stack-chip bonding
operations
• Repair electronic modules, printed circuits,
wave guides, flat cables & high frequency
shields
• Seal and pot sensors, connectors and pins
• Stake transistors, diodes, resistors, integrated
circuits and other thermal-sensitive
components to printed circuit boards
For a complete list of thermally conductive bonding and potting applications, visit
http://www.masterbond.com/lp/tabs/tp_pp_thermcond.html
Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | [email protected]
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