w w w . M a s t e r B o n d . c o mt e l + 1 . 2 0 1 . 3 4 3 . 8 9 8 3f a x + 1 . 2 0 1 . 3 4 3 . 2 1 3 2

W h i t e P a p e r @ M a s t e r B o n d . c o m1 5 4 H o b a r t S t r e e t

H a c k e n s a c k , N J 0 7 6 0 1 U S A

Thermally ConductiveAdhesives Keep

Things CoolThermally conductive adhesives

and potting compounds are an importantweapon in the war against heat raging within

today’s powerful electronic devices

T E C H S P O T L I G H T

1Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | whitepaper@masterbond.com

Thermally Conductive Adhesives Keep Things CoolThermally conductive adhesives and potting compounds are an important weapon in the war against heat raging within today’s powerful electronic devices.

T E C H S P O T L I G H T

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,

1

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.

eXtra BONd LiNe CONsideratiONs

2Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | whitepaper@masterbond.com

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.

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.

Right: Master Bond’s line-up of thermal interface materials

includes one- and two-component epoxies as well as

silicones and solvent-based compounds.

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.

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.

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.

3Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | whitepaper@masterbond.com

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

4Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | whitepaper@masterbond.com

POPuLar thermaL adhesive Grades

Master Bond Grade

Mix Ratio

by weight

Mixed ViscosityRT, cps

Set-Up Time-

Minutes, RT

Cure ScheduleTemp/Time, °F

Service Temp

Range, °F

Thermal Cond,

BTU•in/W/m-K

Features

supreme 11aOht

1/1thick

paste30-40

24-36 hrs @ 75°F

1-2 hrs @ 200°F

-100°F to

+400°F1.44

Two Component, Room Temperature Curing,

Thermally Conductive, Electrically Isolating Epoxy

For Bonding & Sealing Featuring High Temperature

Resistance Along With High Shear & Peel Strength

supreme 10aNht

no mixing

flowable paste

N-A1 hr @ 250°F-

300°F

4K to

400°F

2.88-

3.60

One Component, Heat Curing Epoxy Adhesive

Featuring both High Shear and High Peel Strength

for Optimal Bonding Performance & Exceptionally

High Thermal Conductivity

eP21tdC-2aO 1/3 paste >9048-72 hrs @ 75°F

3-4 hrs @ 150°F

-300°F to

+250°F1.30

Two Component Highly Flexiblized Thermally

Conductive Epoxy Resin Compound For High

Performance Bonding, Sealing, Coating, And

Encapsulation

eP30aN-1 10/1 5,000-6,000 30-4024-48 hrs @ 75°F

1-2 hrs @ 200°F

-60°F to

+250°F3.1

Two Component, Room Temperature Curing, Low

Viscosity Epoxy for Potting, Sealing, Coating and

Bonding Featuring Exceptionally High Thermally

Conductivity and Excellent Electrical Insulation

Properties

eP37-3 FLFaO

1/118,000-

22,00075-90

4-5 days @ 75°F

3-5 hrs @ 200°F

4K to

275°F

1.30-

1.44

Two Component, Low Viscosity, Room Temperature

Curing, Thermally Conductive, Electrically Isolating

Epoxy For Potting, Bonding, Sealing & Coating

Featuring Flexibility. Meets NASA Low Outgassing

Specification

eP112FLaN-1 100/8025,000-

35,000cps4-6 hrs

2-3 hrs @ 220°F-

250°F

4-6 hrs @ 300-

320°F

-60°F to

+500°F2.75

Two Component Flexibilized Low Viscosity Heat

Curing Epoxy Resin System High Performance

Thermal Conductive & Electrical Insulation for

Potting & Encapsulation Applications

eP36aOno

mixing

Liquid @

180°FN-A

30 min @ 180°F

2-2.5 hrs @ 300°F

-80°F to

+500°F

1.30-

1.44

One Component, Thermally Conductive, High

Temperature Resistant Flexibilized Epoxy System

For Electrical Potting and Encapsulation, Coatings

and Bonding

ms 705tCno

mixingpaste N-A

24 hrs or

longer @ 70°F

-75°F to

+400°F2.1

One Component, Thermally Conductive,

Electrically Isolating, Room Temperature Curing,

Non-Corrosive Silicone for High Performance

Bonding, Sealing & Coating

ms 151aO 100/05

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°F1.5

Two Component, Low Viscosity, Thermally

Conductive, Silicone Compound for High

Performance Casting, Potting and Encapsulation

eP21tCht-1 100/60light

paste45

18-24 hrs @ 75°F

1-2 hrs @ 200°F

4K to

400°F1.44

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

eP42ht-2aO 100/40

Part A:

45,000-

55,000

Part B:

100-1000

60-902-3 days @ 75°F

2-3 hrs @ 200°F

-100°F to

+400°F1.45

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

FL901aOno

mixingfilm N-A

1 hr @ 250°F

30-40 mins @

300°F

-100°F to

+400°F1.44

One Component Thermally Conductive,

Electrically Insulative Epoxy Adhesive Film

Featuring Convenient Handling, Good Storage

Stability & Fast Cure

X5tCno

mixingflowable

pasteN-A 5-30 min @ 75°F

-75°F to

+250°F1.61

Thermally Conductive, Elastomer Based, Room

Temperature Curing, One-Component Adhesive

for High Performance Bonding and Sealing

5Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | whitepaper@masterbond.com

THERMALLY CONDUCTIVE ADHESIVE FUNDAMENTALS

LIQUID

Here's a look at the features to keep in mind when working with thermal adhesives in their liquid and film forms:

• 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

For a complete list of thermally conductive bonding and potting applications, visit

http://www.masterbond.com/lp/tabs/tp_pp_thermcond.html

• Die attach hybrid/MCMs

• Encapsulate transformers, sensors, coils and micro circuitry

• Laminate silicone wafters

• Manage heat in aerospace and NASA- approved 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

FILM

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

HEATSINK

ELECTRONIC COMPONENT

THERMAL INTERFACE

ELECTRONIC COMPONENT

THERMAL INTERFACE