Page 28 - Chip Scale Review_May June_2021-digital

0.1°C/W range, heat sinks can achieve 350W TDP, the size and
case and junction temperatures (Tc and weight of the heat sink
Tj, respectively) well inside a typical apparatus can become
target range of 75°C - 100°C. For many quite substantial. So,
of today’s 50mm x 50mm to 70mm a compromise must
x 70mm packages, this approach can be st r uck bet ween
still be effective (Figure 5). Because r e li a b i li t y a n d
these heat sinks are inexpensive, easy ergonomics.
to manufacture, reliable, and simple to
use, they are an attractive choice for the Liquid-cooled
test engineer. Additionally, there are no solutions
moving parts, no settings to adjust, and for compact
no fan noise – all features that make them performance
popular with end-users. Compared to solid
With today’s die scaling capabilities at metal or heat pipe-based
the 7nm and 5nm nodes, we are seeing approaches, leveraging Figure 5: A heatsink design for 200W TDP that is integrated into a socket lid
devices with dramatically higher TDP in the much higher specific and stays within the device packaging outline.
similar-sized packages. Holding package heat capacity (C p ) of
size constant, and with TDP increasing liquids can dramatically
to over 250W, the surface volume of the i m p r o v e t h e r m a l
heat sink must expand significantly to d i s s i p a t i on w h e n
provide the required cooling. To create combined with laminar
this greater surface volume, the heat flow across the heat
sink designer has two choices: “out” source. The decision
or “up.” As described earlier, when the to implement liquid
system board allows the heat sink to cooling is rife with use-
extend well beyond the periphery of the model considerations.
DUT socket in the x and y dimensions, Ma ny eng i neer i ng
additional cooling can be achieved with teams avoid liquid
a traditional design. However, when the at all costs for two
thermal solution must stay inside the x/y reasons. First, liquids in
boundary of the DUT socket, significant electronic applications
improvements in z-axis heat movement h a v e h i s t o r i c a l l y
are required. increased the risk of
Heat pipes can enable the z-axis damage by shorting.
heat movement described above very Today, this risk is
effectively (Figure 6). Using vapor mitigated significantly
phase change and capillary action, heat thanks to the increased
pipes can move heat away from the case availability of lower-
much more efficiently than solid metal, cost nonconductive
with ϴ (thermal resistance) values below f luids. Still, a bias
0.05°C/W. When combined with a heat against using liquids
sink, a well-designed heat pipe solution in electronics setups
integrated into a socket lid can maintain remains. Second, liquid Figure 6: Simulations of heat pipes used to move heat in the z-axis to be
T c and T j in the target range for upwards solutions require setup dissipated in a large fin array above the DUT.
of 600W TDP [4]. At Smiths Interconnect, and control by the user.
we perform extensive thermal simulations While a test team may create a setup that As shown in Figures 7 and 8, a non-
to help our users determine if this type of works extremely well when handled by test chilled liquid flow combined with a heat
design will achieve their targets. A major professionals in a development lab, that sink – even at a moderate 0.75g/min flow
advantage of a heat pipe plus heat sink same setup may not work so well when rate – can deliver solid power dissipation
solution is that it is extremely reliable shipped to a characterization engineer for a 500W device. By chilling the liquid
and highly portable across a user base. focused only on getting the needed to below ambient temperature (such as
For this reason, many engineering teams measurements. For these reasons, static, 15°C or 10°C) the effectiveness of liquid-
prefer to provide this type of design to air-cooled solutions are often preferred as based cooling is even more dramatic. It is
their internal and external customers. more resistant to “pilot error” compared to expected that chilled liquid solutions will
With no liquid to manage, chiller settings liquid-cooled solutions. need to be implemented for the majority
to handle, etc., these setups can be When TDP requirements exceed 600W of >800W TDP designs. However, when
delivered to teams around the globe with (particularly for smaller package sizes using a chilled liquid, condensation may
minimal risk of user error. However, it is where the thermal density will be higher), become a consideration at the interface
important to note that for designs above liquid cooling can become a necessity. point. To address this problem, socket
vendors must provide carefully designed air

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