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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 6

2 2


etallic glues can serve as excel-

lent conductors for heat dissi-

pation and electrical current in

electronic devices and also as leak-resis-

tant seals for vacuum environments. The

potential market for these applications is

extensive and growing rapidly.


It is common practice to join two

solids together using a third substance

for gluing or soldering.



ly refers to the joining process that is

made in ambient conditions—at room

temperature, in air, and without pres-

sure, or with a small amount of mechan-

ical pressure


. Sealing an envelope

with polymer glue is a good example.

Despite this process being easy and

inexpensive, it often produces prop-

erties that make it unsuitable for use

in high-tech environments. For exam-

ple, polymer glue—unlike metallic sol-

der—is permeable to air and moisture,

degrades fast in ambient temperature

or environment, has low mechanical

strength, does not effectively conduct

electricity or heat, and does not retain

its function at high temperatures



In contrast,


usually refers

to the joining process that uses added

molten metal at increased tempera-

tures, generally much higher than room



. Similarly,




also involve high-temperature

melting, where brazing refers to joining

through added molten metal at even

higher temperatures than soldering,

and welding involves melting or fusing

the members to be joined, often un-

der an inert environment


. The joining

from such high temperature processes,

as compared to polymer glue, is me-

chanically strong, effectively conducts

electricity and heat, and degrades slow-

ly (if at all) in ambient environments.

Further, its leak resistance to air and

moisture goes from good to better with

time due to oxidation



Metallic gluing

refers to the pro-

cess of joining two solids with metal as

the connecting party, which operates

at room temperature, in air, and under

low pressure. Metallic glues feature

the combined advantages of the ambi-

ent condition of gluing and the supe-

rior properties of the joint from high-

temperature soldering (or welding and

brazing), making them beneficial to

many advanced technologies.

As an example, consider desktop

and laptop computers. The core of com-

puting is the central processing unit

(CPU), and connecting the CPU to exter-

nal components for heat dissipation or

electrical conduction is necessary. The

process of making the connection, if it

requires high temperature, can damage

the CPU by exceeding the thermal bud-



. For heat dissipation


, an ideal

connection conducts heat efficiently,

which makes metals with high thermal

conductivity desirable. However, if sol-

der is used, the temperatures necessary

to create a good bond can damage the

CPU. Also, solder bonds can be relative-

ly thick, resulting in reduced heat trans-

fer. Further, the thermal conductivity of

most solders is low, conducting roughly

5%-20% as effectively as a pure metal

such as copper



Thermal grease is often used as an

interface material, filling the space be-

tween the heat sink and CPU. However,

the thermal conductivity of this grease

is only a fraction that of copper—a mere



. This low conductivity limits

the amount of heat that can be dissi-

pated from the CPU and is a significant

barrier to further miniaturization and

reliability of devices such as tablets

and computers. Thermal greases also

suffer from problems such as pump

out, where grease is forced out of the

interface during thermal cycling, and

dry out


. Figure 1a shows the config-

uration of a CPU with a heat sink in a

laptop computer, for simplicity. Desk-

top computers often contain an addi-

tional protective and heat transferring

plate between the CPU and heat sink

with two separate interfaces requiring




Advancements in nanoscience are making it possible to metallically

glue two solids together at room temperature, in air, and under a

small amount of mechanical pressure.

Stephen Stagon and Alex Knapp, University of North Florida, Jacksonville

Paul Elliott and Hanchen Huang,* Northeastern University, Boston

*Member of ASM International