Electronics Protection - Summer 2017 - 6

Feature
Evonik continued from page 5

A more problematic sedimentation is the one that can occur
after jetting/application, in the beginning of the curing process.
Before the actual curing starts and the liquid adhesive begins
to gel and to turn into a solid, there is a drop in the viscosity
level because of the elevated temperature necessary for curing. That short period of time can be enough to cause a vertical
concentration gradient of the micro filler particles inside the gap
between chip and substrate. Figure 3 demonstrates this effect
on a macroscopic level. The upper sample (specimen a), which
contains micro particles only, shows sedimentation, clearly visFigure 3. Specimen with micro particles only (a) and with
ible because of its translucent upper half. The specimen bemicro and nano particles (b). Specimen a shows sedimenneath contains micro- and nanoparticles. Nanoparticles do not
tation, while NANOPOX in specimen b prevent a settlement and therefore looks much more homogeneous.
agglomerate due to repulsive forces between them, the same
effect also prevents sedimentation. Moreover nanoparticles can
even prevent micro particles from sedimentation (Figure 3, specimen b).
As stated previously, sedimentation can occur
during the curing process, especially in heat cured
systems. A vertical concentration gradient of filler
particles between chip and substrate (Figure 4) can
cause several negative effects, these can include:
* Thermal stress. In areas with less particle concentration cure shrinkage is more emphasized.
This might cause inner tensions.
* CTE in areas of low particle concentrations
can be higher. In turn the fatigue performance
Figure 4. SEM micrograph of an IC package cross section (light-gray area
might be decreased under temperature cycling. = solder bonds, black area = resin, medium-gray dots = micron scaled
filler particles). The underfill used contained micro particles only. The
* Reduced thermal conductivity. In the IC packvertical concentration gradient of the filler can clearly be seen. Especialage shown in figure 4 the area of reduced heat
ly in the upper area (enlarged section on the right) particles can hardly
conductivity is directly beneath the chip, where be found. This limits the heat removal of the whole IC package.
the heat is generated. This means the poorest
heat conductivity is where it is needed the most, i.e. where the heat needs to be transferred from the chip into
the resin.
Nanoparticles prevent sedimentation, after curing of the respective underfill, the dispersion quality is high and the
filler is homogeneously distributed (Figure 5). CTE and cure shrinkage are at a on a consistent level and the real heat
transfer of the IC package is at its optimum.
NANOPOX can improve the overall heat transfer - not only by filling the
interstitial spaces between the bulk material, allowing higher filler loadings
at constant viscosities - but also by reducing the resistance to heat transfer
at the interface chip/adhesive. In thin layer systems like underfills, the resistance to heat transfer at the interface chip/adhesive often becomes more
important than thermal conductivity of the adhesive itself. Micron scaled
particles cannot get very close to the chip/substrate surface. Therefore,
there will be a small layer of pure resin between them that limits the heat
transport. The thermal conductivity of pure EP resin is only ~0.35 W/mK. In
contrast, nanoparticles can get closer to the chip/substrate surface (Figure
6) and can therefore increase this first heat transfer and in turn increase
the overall heat removal in a given IC package.

Summer 2017 * www.ElectronicsProtectionMagazine.com

Figure 5. SEM micrograph of an IC package
cross section. The underfill used contained
60 wt.% micro and 10 wt.% nano scaled particles. No sedimentation occurred.

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Table of Contents for the Digital Edition of Electronics Protection - Summer 2017