Thermally conductive liquid materials for electronics packaging
eliminating this resistance to heat flow. material, filled with thermally conductive
Thermal conductivity is not the only fillers is naturally a function of the thermal
variable affecting thermal performance— conductivity of the filler and the polymer.
otherwise sheets of copper would do In addition the increasing proportion
nicely. Conformability and wetting of surfaces of filler increases thermal conductivity
5-7
.
is critical to success. The rheology of the Finally, the aspect ratio of the particles is
TIM, i.e. the deformation behavior of key to determining the thermal conductiv-
the material under stress is critical. The ity of thermally conductive polymers.
interface between surfaces has gaps on two As seen in Figure 3, heat travels in
different length scales. The first is small- pathways that are a combination of series
scale roughness (Figure 1)—typically O (1 and parallel steps through polymer (slow
Equation 1a
µm)—from which air is eliminated by flow heat transfer) and filler (fast heat transfer).
and wetting by the interface material. As the filler fraction increases, so does
The second is related to larger gaps due thermal conductivity, however it does not
either to the non-planarity of surfaces and increase indefinitely.
poor co-planarity—as seen in Figure 2. Ther- This is because filler particles are hard
Equation 1b
mal conductivity plays a more important and therefore have a limiting volume
Here
role here but so does the rheology of the fraction due to packing constraints. In
k
c
is composite thermal conductivity
TIM. The thermal interface material needs addition there are significant bottlenecks
k
p
is polymer thermal conductivity
to be able to conform to the surfaces, with to heat transfer from particle to particle
k
f
is filler thermal conductivity
a low external stress to produce deforma- since heat (or phonons) have to traverse
φ is filler content
tion without straining the electronic the small polymeric region between par-
A is a function of particle geometry
components. ticles (which themselves have very limited
This is an area where liquid materi- particle-particle contact). This is shown in
als excel, when compared to pre-formed Figure 4.
Aspect Ratio Value of A
gaskets, since the component stress during One model that has been widely used
1.0 0.68 smooth spheres
assembly is orders of magnitude lower. is the Nielsen model
7
, which reproduces
0.44 rough spheres
The key properties of liquid thermal the basic behavior of filled polymer com-
6-8 0.44 plates
interface materials, which affect the final posites. See Equations 1a and 1b.
performance, are thermal conductivity, Table 2 depicts the typical value of 18 0.32 plates/rods
rheology, yield stress and size of the par- A for various particle geometries—in
23 0.26 rods
ticulates added to the material. many ways this value is very similar to a
maximum packing fraction in rheological
27 0.18 rods
Thermal conductivity models.
Table 2. Typical value of A for various particle
The thermal conductivity of a polymeric
geometries.
Rheology and Wetting Have Major Impact Bulk Thermal Conductivity and Rheology Have Major Impact
Module cover/heat sink
Larger gap due to
mechanical constraints
or poor coplanarity
~O (1 uni03BCm)
Interface material
Component surface
~O (10-1000) uni03BCm
Figure 1. A typical thermal interface. Figure 2. Interfaces with larger gaps.
Filler
Poor particle-particle
"JS
_

8N,
contact is a severe
Polymer
Thermal Conductivity
bottleneck





t
'JMMFST
_

8N,





t
1PMZNFST
_

8N,
High f_iller loading





t
$PNQPTJUF
_

8N,
increases viscosity
much faster than
thermal conductivity
Figure 3. Heat transport in filled dispersion. Figure 4. Heat transfer limitation in dispersions.
www.globalsmt.net Global SMT & Packaging – December 2008 – 19
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