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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
M
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.
TECHNOLOGICAL RELEVANCE
It is common practice to join two
solids together using a third substance
for gluing or soldering.
Gluing
usual-
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
[1]
. 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
[2,3]
.
In contrast,
soldering
usually refers
to the joining process that uses added
molten metal at increased tempera-
tures, generally much higher than room
temperature
[1]
. Similarly,
welding
and
brazing
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
[1]
. 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
[1]
.
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-
get
[4]
. For heat dissipation
[5]
, 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
[6,7]
.
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
1%-2%
[7,8]
. 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
[5]
. 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
METALLICGLUE FOR
AMBIENT ENVIRONMENTS
MAKINGSTRIDES
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