Thermally conductive liquid materials for electronics packaging
thinner bondline can be achieved with Manufacturing considerations liquids or stenciling them (if large particle
liquids and hence much lower thermal 1-k or 2-k liquids can be applied in a vari- size does not allow easy screening). Figure
impedance. In addition, this significantly ety of ways to the surface(s). Much depends 10 is an example of liquid gap filler sten-
lower yield stress allows liquids to flow and upon factors like volume usage, packaging, ciled in various patterns.
wet out the surface better thereby reducing shelf life and application requirements. Abrasion: Typical thermally conduct-
the contact resistance, θ
C
, significantly over One advantage that accrues in using ing liquids contain abrasive ceramic
gaskets. liquids (besides performance as noted particles. In pumping these materials there
Mechanical performance: Curable liquids above) is the fact that the material usage is abrasion of moving parts that come
(especially adhesives) couple stresses is targeted and optimized leading to lower into contact with the liquid. As such the
between two surfaces much better due materials (and therefore operating) costs. manufacturing has to account for wear
to better wetting and flow. This means Leveling/thixtotropy: Depending upon and replacement of worn parts or invest in
that liquid adhesives can provide better the geometry and the time between up front fixed costs in abrasion resistant
adhesion strength when applied at lower dispensing and final assembly the liquids parts. Many equipment manufacturers of
pressures compared to laminating adhesive may need different rheology. For thick pumping and dispensing equipment have
gaskets or pressure sensitive adhesives. bondlines and longer open time between addressed these issues.
One property worth noting is elongation dispensing and assembly, there would be Filler separation/caking: As noted earlier,
to break or toughness. As the filler volume need for a material with high yield stress under high shear the particles may not be
fraction increases above a critical volume that will flow easily when stress—i.e. a able to flow past each other and start aggre-
fraction, the elongation to break of the thixotropic material. For complicated gating. In addition, if there are sharp cor-
final cured gasket goes down significantly— geometry where liquid has to flow and wet ner or fittings, the particles may separate
the higher the aspect ratio, the lower out varying topology there will be a need under centrifugal action and accumulate in
this critical fraction. It is typical to have for a self-leveling material—i.e. one with no dead spaces. Finally, if the fitting the not
elongation to break below 10% above this yield stress. designed well for the pressure needed for
loading. These properties are of deep inter- Assembly stress: While a liquid dis- pumping, the polymer fluid may actually
est for adhesive applications where power pensed material will apply less stress to an leak slowly and cause the local concentra-
or thermal cycling produces deformation assembly, there is nonetheless stress when tion of filler to build up until a cake forms.
of the interface due to CTE mismatch. pressure is applied to cause the material to All these can cause the clogging of tubing
Property optimization: In order to achieve flow. If the rheology of the material allows and fittings [19]—typically loose fittings or
better thermal conductivity the dispersions it to flow easily under stress then one sharp corners are the most implicated in
need to be concentrated with particles. option may be to dispense a ‘bead’ using poor pumping performance. Equipment
However this also leads to higher yield a thin nozzle. Typically a bead traces a manufacturers are now taking these factors
stress and potentially higher bondline, pattern on one surface that allows all active into account to produce robust pumping
which have an adverse impact on thermal areas to be covered once the assembly is systems.
performance (see Figure 8). made. Various patterns are evaluated, and Open/working time: This is applicable
In addition higher loading leads to visually inspected for efficient materials us- mainly to curing materials where the
poor contact resistance as well as poor wet age, by using a transparent Plexiglas cover rheology changes with time—i.e. viscosity
out and lowers toughness. All these also to simulate the other surface. increases. Open time refers to the time
impact thermal and mechanical perfor- In some applications, either due to window available after dispensing when
mance adversely. Therefore the design of higher viscosity or fragile components (like the final assembly needs to be completed.
thermally conductive liquids has to be tai- LTCC or thin ceramic boards) the material When the lag time between dispensing and
lored to the application—in other words the may be applied in a more uniform pattern assembly is large, there will be a need for
properties need to be optimized based on in order to reduce localized stress build long open time. Another related concept
performance trade offs as well as manufac- up. It may also be possible to automate is pot life, which refers to an adhesive that
turing and reliability considerations (more dispensing in a dot or line pattern as seen is pre-mixed before application. Sometimes
on that below). in Figure 9. adhesives or gap fillers may be pre-mixed
Other options include screening the before screening or stenciling. In this case
there is limited time before the liquid vis-
cosity increases and makes it unworkable—
this is the pot life.
Shelf life: Unfilled liquid polymer sys-
tems have storage or shelf life limitations
based on the lifetime of reactive species.
Over time reactivity is lost and the materi-
als do no cure to the same final properties.
Thermally loaded polymer systems also
have similar limitations. In addition, filler
settling imposes another limitation of
shelf life. Most ceramics are denser than
the polymer liquid they are dispersed in
and hence settle over time. The settling
rate of course depends upon the particle
size, concentration and density difference
Figure 10. Dispensed pattern. Figure 11. Stenciled liquid gap filler.
between the particles and the polymer
24 – Global SMT & Packaging – December 2008 www.globalsmt.net
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