Online proceedings - EDA Publishing Association

F
F
A
B
Fig. 1: Metal micro-textured thermal interface material concept
conductivity imposed by multiple point-to-point contacts in
conventional TIMs is eliminated. In addition, the possibility
of void formation is significantly reduced since these microtextured
contact points are fixed in an array. Air voids,
which may become entrapped in the organic phase, will not
affect metal contact density. The result is an array of
conformable, yet continuous solid metal features of high
effective thermal conductivity that are in intimate contact
with the mating surfaces due to the plastic deformation of
raised features. Furthermore, by employing pure metals
such as copper, silver or aluminium, the features of the
MMT-TIM are both good thermal conductors and relatively
compliant.
A preliminary investigation characterized the performance
of an array of hollow silver cones (measuring approximately
2 mm tall by 1 mm diameter, on 2mm pitch) which exhibited
promising results with effective thermal conductivities up to
5 W/m·K. Furthermore, these MMT-TIMs demonstrated
significant compliance (up to 85% strain) with a relatively
constant application pressure. More importantly, the
effective thermal conductivity remained relatively constant
over a large deformation range [6]. Kempers et al. [6] have
also developed a numerical model that characterizes the
mechanical and thermal response of a given MMT-TIM
geometry during its compressive deformation.
The present study investigates the performance of several
different MMT-TIM feature geometries and presents some
experimental results that demonstrate the significance of
certain geometrical factors on both the mechanical and
thermal response of the MMT-TIM. Ultimately, this will
lead to a design tool that can be used to develop optimum
MMT-TIM geometries for a given set of criteria.
A
B
7-9 October 2009, Leuven, Belgium
contribute to a low overall uncertainty and a robust error
analysis provides uncertainties for all measured and
calculated quantities. Details regarding the design and
uncertainty analysis of this apparatus are provided in [7].
The temperatures at the meter-bar contact surfaces, T a and
T b , and the heat flux, Q, for each meter-bar were obtained by
performing least squares regression of the axial temperature
distribution to a straight line and computing the resulting y-
intercept and slope at the contact surfaces. As a result, the
uncertainty of T a , T b and Q depend on both the thermal and
spatial uncertainties of each thermistor. Details regarding
the uncertainty propagation through the least squares
regression are presented in [7].
The heat transfer rate through each meter-bar is then
computed by
Q m k A
mb mb
(1)
where m mb is the temperature gradient through each meterbar,
k mb is the thermal conductivity of each meter-bar, and A
is the cross-sectional area of the meter-bar.
The apparent thermal resistance of the TIM is then
calculated as
mb
=
R =
( T T )
a −
Q
(2)
where Q is the mean heat transfer rate through the meterbars.
The effective thermal conductivity of the TIM can then
be calculated using
QL L
keff = =
A( Ta
− Tb
) AR
(3)
where L is the thickness of the specimen bond line. The
decrease in sample height as they are compressed was
computed by subtracting the bondline thickness from the
initial height. Pressure is computed using the apparent
contact area of the apparatus (1600 mm 2 ).
III. MMT-TIM GEOMETRIES
The prototype MMT-TIMs investigated in the present
study were fabricated by first creating a 3D model of the
desired surface-negative using a conventional CAD package
(in this case ProEngineer). This form or mandrel was then
built in wax directly using a high-resolution wax 3D printer.
Silver was electroplated onto the wax mandrel to a desired
thickness and the wax subsequently removed. The resulting
b
II. EXPERIMENTAL APPARATUS
The design of the experimental apparatus was based upon
a popular implementation of ASTM D5470 where wellcharacterized
meter-bars are used to extrapolate surface
temperatures and measure heat flux through the sample
under test. Measurements of thermal resistance, effective
thermal conductivity, and electrical resistance can be made
simultaneously as functions of pressure and sample
thickness. This apparatus is unique in that it takes advantage
of small, well-calibrated thermistors for precise temperature
measurements (±0.001 K). Careful implementation of
instrumentation to measure thickness and force also
Fig. 2: Cross-section of hollow conical MMT-TIM geometry
©EDA Publishing/THERMINIC 2009 211
ISBN: 978-2-35500-010-2