Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Characterization of Metal Micro-Textured
Thermal Interface Materials
Roger Kempers 1,2 , Anthony Robinson 2 & Alan Lyons 1,2,3
1 Alcatel-Lucent
Blanchardstown Industrial Park
Dublin 15, Ireland
2 Department of Mechanical &
Manufacturing Engineering
Trinity College Dublin,
Dublin 2, Ireland
3 Department of Chemistry
City University of New York,
College of Staten Island
New York, USA
Abstract- To address performance limitations of conventional
thermal interface materials (TIMs), a metal micro-textured
thermal interface material (MMT-TIM) has been developed
that consists of a thin metal foil with raised micro-scale features
that plastically deform under an applied pressure thereby
creating a continuous, thermally conductive, path between the
mating surfaces. Here, the influence of various geometrical
parameters on the mechanical and thermal performance of
hollow conical MMT-TIMs is investigated experimentally. The
results demonstrate the influence of feature size, shape, array
density and foil thickness. The results also serve to highlight
the underlying challenge of characterizing the thermal contact
resistance of MMT-TIMs. Future efforts for this project are
discussed including the validation of a numerical thermalmechanical
model and development of a relationship between
electrical and thermal contact resistance for MMT-TIMs that
would allow estimation of the thermal contact resistance using a
straightforward electrical measurement.
I. INTRODUCTION
The mitigation of thermal contact resistance is essential to
the performance of conduction-based electronic thermal
management solutions. Typically the most feasible strategy
to reduce thermal contact resistance is to insert a thermal
interface material (TIM) of higher thermal conductivity
between the mating surfaces to conform to the contacting
surface asperities and displace any micro and macroscopic
air voids, thereby providing a path of improved heat
conduction.
To work effectively, TIMs must physically conform to the
mating surfaces under reasonable assembly pressures and
exhibit low contact resistance with adequate bulk thermal
conductivity. The bond-line thickness values are kept to a
minimum to help reduce bulk thermal conductivity, however
the thickness must be sufficiently large to enable the TIM to
comply to surface irregularities and non-planarities. For
assembly of microprocessors, where surfaces are relatively
smooth and flat, TIMs are thin. However for other
demanding applications, such as the assembly of high
powered wireless amplifiers where surfaces can be rough
and undulating, relatively thick TIMs are required to ensure
good contact across the entire surface. Many different TIMs
are commercially available that attempt to meet these
requirements in different ways. These include a range of
adhesives, greases, elastomeric pads and various phasechange
materials [1].
The main weakness of many commercially available TIMs
is their relatively poor thermal performance. Often the TIM
consists of a low-conductivity organic phase, such as
silicone grease, interspersed with higher conductivity metal
(e.g. silver, copper) or ceramic particles (e.g. aluminium
oxide, zinc oxide or boron nitride) to boost the overall
effective thermal conductivity of the material. The end
result is a material whose effective thermal conductivity is
limited by multiple point-to-point contacts between adjacent
particles. Despite using extremely high conductivity filler
materials, such as silver (k ≈ 420 W/m·K), the effective
thermal conductivity of the best commercially available
TIMs is on the order of 5 to 10 W/m·K, which is
considerably lower than the thermal conductivities of typical
mating components. In addition, dispensing and flow of the
particle-matrix composite results in voids being trapped
within the bond.
Indeed in many high thermal energy dissipating systems,
the TIMs can account for up to 50% of the available thermal
budget of the package [2]. With the inevitable
implementation of high performance liquid cooling
strategies, this percentage will become even greater. If the
thermal management of an electronic device is inadequate,
unacceptable temperature levels may be reached which can
adversely affect device performance, reliability and lifespan
[2]. These thermal issues have spawned a global effort
towards the development of novel TIMs with complex
formulation and very high performance [3-5].
To address these issues, a novel TIM has been developed
called metal micro-textured thermal interface materials
(MMT-TIMs) [6]. These materials consist of an array of
small-scaled raised metal features on a thin metallic
substrate. When this structure is compressed between two
mating surfaces, the features plastically deform and conform
to the contacting bodies as illustrated in Fig 1. This
approach reverses the conventional TIM paradigm by
creating two interpenetrating continuous phases – one of
high-conductivity plastically deformable metal features and
a second of an optional organic compound which flows
around these features. The constraint on thermal
©EDA Publishing/THERMINIC 2009 210
ISBN: 978-2-35500-010-2