Metal Thermal Interface Materials in Power Devices
Jordan Ross, Bob Jarrett, Indium Corporation 10/1/07
As today’s technology devices continue to get smaller and more powerful, the
need for high performance thermal interface materials is becoming more critical.
Metal thermal interface materials are the ideal solution to fill this need. The high
thermal conductivity of metals and alloys determines how they can be used in
thermal solutions. Metals dominate heat sink, spreader, and heat pipe
applications due to their high conductivity, as well as their ease and flexibility in
fabrication. In critical heat flow situations, metals are frequently used as the
thermal interface material (TIM) in the thermal solution.
Table I-Thermal Conductivity of Selected Materials
Material
Silver
Ag
Copper
Cu
Gold
Au
Aluminum
Al
Indium
In
Tin
Sn
Gallium
Ga
Lead
Pb
Bismuth
Bi
Phase Change
Materials
Thermal Grease
Ag - Filled Die Attach
Molding Compounds
BT Epoxy
FR - 4
Electrical
Thermal
Conductivity Conductivity
(% AICS
@85°C
@1.72
(W/m·K)
μohm·cm)
429
108.6
396
100
318
73.4
240
65.9
86
24.0
73
15.6
41
12.3
35
7.9
8.0
1.3
3-8
.75 - 6
1.3 - 5
0.6 - 0.7
0.19
0.11
Metal TIMs must make intimate contact with the working surfaces to take
advantage of the superior thermal conductivity. To achieve this bond, metals are
used in four different forms—solder, liquid metals, phase change metals, and
compressible metals.
Each of these metal TIMs achieves intimate contact with the mating surfaces
through different assembly methods. The solder joint requires a reflow cycle to
melt the metal well above the operating temperature to form a bond that is solid
when the device is in use. Liquid metals are liquid during application and stay
liquid when the device is in service. Phase change metals are solid during
installation and melt when the device is in service to wet to the contact surfaces.
Compressible metals are mechanically soft materials that can be plastically
deformed when in service to make intimate contact with the mating surface. In
the IGBT (insulated-gate bipolar transistor) and power amplifier industry, solders
and compressible metals TIMs are more commonly used.
In this article, we define a metal as a thermal interface material if it is used
primarily to conduct heat. In such applications, metals do not necessarily need to
conduct electricity, except perhaps for grounding. Die-attach and solder shim are
terms used in the industry to describe the metal TIMs in these specific
applications.
The die-attach application of a metal TIM involves soldering the die to a substrate
or heat spreader which provides a heat-conducting path and mechanical
attachment. Electrical connections are made to the opposite side of the die using
wire bonding technology. The first level TIM connecting the individual substrates
onto the IGBT base is often in the form of a solder paste or solder preform with a
lower melting point than the die-attach material. With a solder shim the metal TIM
is clamped in place between two surfaces to improve the contact, as in the case
of the field attachment of an IGBT module.
The picture below is a graphical representation showing where metal interfaces
would go in an IGBT module.
In the most demanding applications, metal TIMs are used instead of polymers
due to their low thermal resistance and stable thermal breakdown. Compressible
metal TIMs have been used for years in many applications, especially power
amplifiers. Recent advancements in altering the surface of soft metals have
produced new metal thermal materials called Soft Metal Alloy TIMs, or SMATIMs. These SMA-TIMs have been designed to compress at lower pressures
and with more uniformity than standard foil. The relative performance of the
SMA-TIM versus high-performance thermal greases is shown in the plot below.
Pure indium, for example, is very soft - in fact it is 4 times softer than lead with a
compressive yield stress of only 2.14 MPa (310 psi). Therefore, you will find that
many of SMA-TIMs are indium-containing alloys.
SMA-TIMs rely on clamping force to breakdown the contact resistance. For
applications with high compressive loading (such as the screw-mounted IGBTs
or power amplifiers), conventional metal TIMs outperform polymer solutions. In
these devices, SMA-TIMs offer substantially lower thermal resistance than these
materials and extend the application down to pressures of ~0.3 MPa (40 psi).
Whether a metal is applied as solder or as a compressible TIM, the bulk thermal
resistance (a function of the conductivity alone) is very low. Both solder and liquid
metal TIMs wet to the surfaces resulting in minimum contact resistance and the
lowest possible thermal resistance. Compression interfaces rely on the plastic
deformation of TIMs to make intimate contact—so the thermal resistance is
somewhat higher than the 0.02-0.03 cm²-°C/W value seen with solder and liquid
alloy TIMs. However, compressible SMA-TIMs are applied without the reflow
operation or the containment barrier required for the solders or liquid metal TIMs.
Thermal Resistance
0.20
Thermal Resistance (cm2-oC/W)
Thermal Grease #1 (50 um)
0.18
Thermal Grease #2 (50 um)
0.16
75 um Indium Foil
0.14
75 um HEAT_SPRING
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Pressure (MPa)
The reliability of metal over polymers, especially in compressible applications, is
very clear as the useful life of the device increases to 5 or more years. In the
diagram below you can see a difference in the way various interface materials
perform during a bake test. In this example, the thermal test vehicle (TTV) was
baked for about 1600 hours at 90°C. The increase in thermal resistance when
the grease dries out is shown on the right side. The compressible interface does
not melt or separate from the surfaces. In addition, metal interfaces do not
experience pump-out or migration of material during power cycling. The
compliance of pure indium allows it to bridge the interface between materials of
widely different coefficients thermal expansion (CTE). This property also provides
durability for high-shock and high-stress environments.
As for using solder as an interface, there are very few interface materials that are
more durable than a well soldered metal. The creation of an inter-metallic bond
between the surfaces of the interface creates a very durable joint that can
withstand many years of high stress life.
It is because of the reliability of metal interfaces that they are now more
commonly used in IGBT and power amplifiers. The design life of power
amplifiers in base stations for RF extends over many years. During that life cycle,
they experience high temperatures and considerable thermal cycling. Therefore,
it is important that for these applications the alloys are engineered to fit these
unique requirements. In some applications, low temperature bismuth-containing
alloys are used in a step soldering process. Indium is added to increase
compliance in some metal systems. In contrast, silver is added to increase
mechanical strength. Because IGBT and power amplifiers have multiple layers,
or stack ups, metals can be used in different forms and compositions in different
layers for optimum performance. This process would begin with a high
temperature solder for die-attach, and end with a low temperature solder or a
compressible metal for the final interface.
The advent of compressible SMA-TIMs has allowed companies in the power
amplifier and IGBT markets to use these highly reliable TIMs in place of
polymers, which tend to be very sensitive to thicker bond lines and have a
shorter useful life. These metal materials can be engineered to melt at specific
temperatures to work either as a phase change material or to alter the melting
point so that the highly conductive material stays solid during high temperature
use.
Metal interfaces can be made in many different forms and are no longer limited to
just solder applications. In addition, in some applications, metal TIMs are totally
reworkable and recyclable. Metal TIMs are offered in a wide range of RoHS
compliant alloys and can be packaged in many forms, such as silicone- and
halide-free.
Metal thermal interfaces have been used for many years in the form of solder. In
recent years the need for better performing TIMs in such devices as power
amplifiers and IGBT modules have prompted suppliers to explore other types of
metal TIMs such as liquid metals, phase change metals, and SMA-TIMs. The
soft or compressible metal thermal interface material (SMA-TIM) is the most
easily adopted metal TIM because it does not need to be reflowed or contained
in a gasket like a solder or liquid metal. In long life devices, such as IGBT and
power amplifiers, metal TIMs prove to be a viable solution because of their ability
to stay thermally stable and not break down like polymers. Metal TIMs are very
thermally conductive, reliable, and in the case of compressible metals, easily
adopted.