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Metals as TIMs
The choice of metals as TIMs is a simple
one: they typically exhibit high bulk thermal
conductivity and low interfacial resistances.
Depending on the application, different form
factors and alloys are available, enabling
a wide range of allowable application
temperatures, even for cryogenic quantum
computing applications [7]. The strong
correlation (Figure 7) between electrical
and thermal conductivity is because
the electron and phonon transmission
mechanisms in metals are very similar.
Across the whole thermal management
materials market, metallic TIMs may be
relatively less well understood, as their
application has been generally restricted
to some rather narrowly-defined market
segments for specific types of devices.
Examples are the decades-long use of flat
indium metal foil for flange-mount power
devices for military/aerospace and similar Figure 7: Conductivities of metallic elements.
system applications as a TIM2 (traditionally Indium Corporation’s LV2K flux coating) or to reduce the thermal contact resistance
referred to as a “shim” by power device processed with a separately-applied liquid for TIM2 applications. Patterned indium-
engineers). In the semiconductor test market, flux. These fluxes are typically organic acid- silver preforms clad with a barrier metal to
indium foil is used for complex test heads based, or a gaseous flux (typically formic avoid sticking have been demonstrated to be
and must potentially survive thousands of acid) may be used to reduce the amount of successful for burn-in and CLASS testing.
contact and release cycles [8]. metal oxides at solderable surfaces. There is When a preform becomes large enough,
Solder TIMs (sTIMs) are preforms (metal an increasing trend towards the use of lower typically >1x1”, it is referred to as a foil, and
shapes), typically of either pure indium metal, temperatures to minimize overall package whole system foils in use today may be as
or an indium-silver alloy, which are applied as and module warpage. large as 20cm x 20cm and are used as sTIMs
a TIM1 to the die top surface, then reflowed, Vacuum is extremely effective at for an array of device types with dedicated
providing a reliable thermal and mechanical drastically reducing voids when the solder thermal solutions for each interface. The
joint to the lid. Solder TIMs have been used TIM is above its liquidus. Voiding is caused large area reduces assembly steps and
for two decades as a known-good solution in by small gas bubbles trapped within a solder promotes some degree of heat spreading.
high-end server-die applications. The sTIM joint. These bubbles create areas of very Paste-like metal powder-based and
reflow process has traditionally used a liquid high thermal resistance, and are minimized fusible systems in polymeric carrier vehicles
flux to remove surface metal oxides, but using specific reflow profiles, solderable have proven promising for achieving
now, formic acid is increasingly being used, low-oxide surface finishes and thicknesses, near-preform-like results, with excellent
because it leaves no organic residues [9]. and different types of flux. The presence flexibility in application. The powder within
These lidded soldering applications naturally of voids needs to be predictably controlled, the paste creates thermally-conductive
necessitate the use of solderable surfaces; especially if subsequent thermal excursions channels between the die and the heat sink
the solder forms intermetallics with the are anticipated for the package (or, for during fusion or compression [10].
metallized surface. Typical surfaces include example, during subsequent reflow profiles
Au/Ni as the die backside metallization used in manufacturing and assembly of Stability and reliability
(BSM) or nickel on the copper lid. the device as a ball grid array [BGA]). Further considerations for TIM
Metal thermal solutions, including Preform thickness and XY dimensions evaluation and selection for system design
indium-based alloys, are used because they create challenges for processing and include system reliability goals over time
are commercially available in forms suitable handling, especially if the preform is made and temperature, desired (or, in many
for high-volume manufacturing. For a of a soft alloy. As discussed earlier, a thin instances, known-good) application
TIM1 application, the most readily available TIM bondline is advantageous for thermal processes available and suitable for the
solution is a preform, which is a specifically- performance, but such thinning may create assembly factory, application unit volumes
shaped and toleranced solder alloy piece a preform that is difficult to handle with (units/hour manufacturing capability),
with controlled thickness, surface condition standard placement and packaging solutions. and related physical characteristics of the
and other parameters. The preform creates Metal TIM solutions may also be created system operating environment.
the desired die-to-lid standoff and is with bondline accommodation treatments As a materials supplier, we typically
typically delivered to the assembly floor including cladding and a collapsible divide our thoughts on a material’s
in tape and reel, or similar packaging. pattern. These systems may be processed as utility into two categories: stability and
Preforms may be supplied pre-fluxed with described for standard preforms, but often reliability, as noted below.
a well-controlled dry flux-coating (such as are used in a solid state, and pressure is used

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