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• Stability/lifetime: Consistent
performance during manufacture,
transportation, storage, initial
application (print/dispense/placement/
reflow, etc.), and processing. Will the
TIM give a consistent performance
in the deposition tool over the stated
product data sheet lifetime?
• Reliability: Once in place as a TIM
in a finished device or module, key
performance evaluation parameters
are drop shock, thermal cycling and
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power cycling. Figure 8: Indalloy 51E jetted on bare silicon.
Reliability must be assessed based on
the application level to be discussed. A
primary example is the TIM1 in an HPC
package. The selection of a reflowed soft
solder such as 100% indium in a relatively
thick preform may be required both for
ease of handling and, more importantly,
for the purpose of addressing coefficient
of thermal expansion (CTE) mismatches
within the package from environmental
thermal changes, and also from power
cycling. The thick sTIM preform,
therefore, offers an additional degree
of freedom in controlling the stresses
inherent in the use of a heavy copper
lid attached to an organic carrier as the
substrate, with a relatively rigid edge- Figure 9: Classification of gallium-based TIMs (GBTs).
seal adhesive bonding the two together. A these lower viscosity liquid metals is their
thicker preform may be selected for this fluidity, and the relatively poor wetting
purpose, although the greater thickness of these first-generation materials onto
may be contrary to thermal design needs, silicon and copper. This means that, in
as the CTE mismatch and allowance for practical application, all other electronic
substrate bowing are highest in criticality components around the logic die being
for package overall reliability. This is cooled must be protected from “leakage,”
a specific example of where a reflowed that is, against metal droplets shorting
solder, as a TIM1, is selected for reasons out contacts on adjacent devices, such as
that are not found in a TIM2 application. decoupling capacitors.
The rheology and metallurgy of
The promise of liquid metals hybrid-liquid metal (HLM) paste-like
Indium metal and its alloys used as sTIMs materials can be designed to withstand
have proven their worth for many years thermomechanical stresses of power
for high-end logic devices [11]. However, and thermal cycling caused by the
the evolving needs of heterogeneously- gross CTE mismatches between silicon
integrated HPC modules, and the larger (~4ppm/K) and other module materials
bondline thicknesses typically used for (typically >16ppm/K for TIM solution
sTIMs (typically 150-300µm) have begun to metals and substrate materials). These types Figure 10: Second-generation stable GBT paste.
necessitate TIMs that can give low bondline of high-viscosity fluids, therefore, seem around 20 to around 75µm) with zero
thicknesses (BLTs) on the order of 10-75µm. ideally suited for TIM 0, and, potentially, leakage. No leakage ensures there is no loss
This new approach, which is now TIM 1 applications as replacements for gels of TIM at the die edge, and also removes the
gaining a great deal of interest in the HPC and phase change materials. The promise need for electrical protection of surrounding
market involves metallic alloys based on of the second-generation of gallium-based components. Moreover, the paste must be
gallium. Figure 8 shows a first-generation TIMs (GBT), and especially HLM pastes, applied without damaging thinned die,
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liquid metal material, Indalloy 51E is the next evolution of the liquid metal such as those in HBM stacks, while still
(eutectic GaInSn), that has been jetted onto concept (Figure 9). meeting the criteria described earlier. Any
a bare silicon die in a high-volume HPC HLM pastes are designed for minimal use of gallium-based paste must include
application. However, a major drawback of bondline thicknesses (anywhere from consideration of the presence of other

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