Thermally conductive liquid materials for electronics packaging
D
Thermal
90
conductivity
Contact resistance
Viscosity
>D
90
φ
Filler loading
max
10-20% rods
20-40% plates
> 60% spherical
Figure 7. Bondline limitation from particle size. Figure 8. Optimizing properties for applications.
of the final application. In this case the Application performance Typical gasketing materials may have
rheology of the material is tailored in order Issues relate to thermal performance a modulus ranging between 105-107 Pa
to suit the manufacturing requirement. have been related in detail in many whereas the liquid dispersions have a yield
One may have a highly thixotropic material other texts
3,4
]. Here we will highlight those stress significantly below 103 Pa. Therefore
that is dispensed in place and does not features of thermally conductive liquids the assembly stress needed for achieving
flow away or one that is self leveling, i.e. that distinguish them from gaskets. First, similar bondlines is at least 2 orders of
low viscosity even at low shear. the liquids are available in various formats magnitude less for liquids than it is for
If the bondline is not set by external depending upon suitability in an applica- typical gaskets. Looking at it in another
mechanical constraints, then the final tion. These include the following general way, for a given assembly stress (dictated
interface thickness is determined by one categories: by the component or board mechanical
of the following factors: (1) External force Thermal Greases: These are relatively strength)
on the interface and the yield stress of lower viscosity polymer liquids that have
the material or (2) largest particles in the been loaded with thermally conductive par-
particulate dispersion. ticulates. These are thermoplastic, i.e. they
Particulate dispersions can behave like will deform continuously under external
plastics that need a minimum stress to de- stresses without limit.
Equation 5
form. This plastic behavior can be modeled Thermal Pastes or Gels: These are
by, among others, the Herschel-Bulkely relatively higher viscosity materials with Where τ
0
is the yield stress.
model
18
(Equation 5). higher molecular weight linear or branched
The Herschel-Bulkley model reduces to polymer matrix. This gives them a certain
the familiar Bingham plastic model when rheological stability and some elastic prop-
n = 1. erties. Ultimately these are thermoplastic
As an example τ
0
< 50 Pa is easy like greases.
to pour like milk and are self leveling; Curable/Reactive Liquids: These may
Equation 6
between 50-120 Pa the dispersions are be either adhesives or form-in-place gaskets
like thick but pourable like milk shake; (gap fillers) and may be one component
Here θ is thermal impedance, T
1
and T
2

between 150-250 Pa they are quite thixo- (1-k) or two component (2-k). These are
are temperatures measured at two differ-
tropic and a liquid bead will hold its shape thermoset materials that crosslink into a
ent locations and P is the power output
when dispensed; higher yield stress materi- network that do not deform until the in-
from the electronic device or package.
als can create problem if proper pumping ternal stress in the material resists further
equipment is not chosen. deformation.
If the external stress is sufficient to The following properties distinguish
deform the liquid, the size of the particles thermally conductive liquids from gaskets:
their concentration (or loading) and their Thermal impedance: Thermal impedance
Equation 7
modulus determines the final bondline. between two points in any electronics as-
Typically the ceramic or metal particles sembly refers generally to a measured tem-
Where θ
C
is the contact impedance,
used in thermally conductive liquids are perature difference and power dissipation.
which is a function of the surface rough-
high in modulus and crush strength. For a general overview of thermal man-
ness, wetting and flow between sur-
Therefore one cannot get below the thick- agement, Equation 6 shows θ as thermal
faces. l is the bondline thickness, which
ness of typically D
90
of the particle size impedance. T1 and T2 are temperatures
is a function of the application geometry,
distribution—90% of all particles are below measured at two different locations, and
material rheology (yield stress, viscosity),
this particle size. In some case, depending P is the power output from the electronic
particle size of dispersion, AND external
upon the loading and assembly process, device or package. This thermal impedance
stress. K is the thermal conductivity of
the particles may not flow past each other is can be roughly modeled as shown in
the dispersion and A is some effective
and get ‘locked.’ In this case one could get Equation 7.
area for heat transfer between surfaces.
stacking of particles 3-4 deep.
22 – Global SMT & Packaging – December 2008 www.globalsmt.net
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