IBM researchers presented an innovative approach for improving the cooling of
computer chips, an increasingly urgent need given the large amount of heat
released by today’s more powerful processors and the additional energy required
for removing that heat. The technique, called "high thermal conductivity
interface technology," allows a twofold improvement in heat removal over current
methods. This paves the way for continued development of creative electronic
products through the use of more powerful chips without complex and costly
systems simply to cool them.

The approach used by IBM addresses the connection point between the hot chip
and the various cooling components used today to draw the heat away, including
heat sinks. Special particle-filled viscous pastes are typically applied to this
interface to guarantee that chips can expand and contract owing to the thermal
cycling. First lab results are impressive. The team has demonstrated cooling
power densities of up to 370 Watts per square centimeter with water as coolant.
This is more than six times beyond the current limits of air-cooling techniques
at about 75 Watts per square centimeter. Yet, the system uses much less energy
for pumping than other cooling systems do.

At the BroadGroup Power and Cooling Summit here today, IBM (NYSE: IBM)
researchers presented an innovative approach for improving the cooling of
computer chips, an increasingly urgent need given the large amount of heat
released by today’s more powerful processors and the additional energy required
for removing that heat.

The technique, called "high thermal conductivity interface technology," allows a
twofold improvement in heat removal over current methods. This paves the way for
continued development of creative electronic products through the use of more
powerful chips without complex and costly systems simply to cool them.

As chip performance continues to progress according to Moore’s Law, efficient
chip cooling has become one of the most vexing problems for designers of
electronic products. The IBM technique outlined today is one of several being
explored by scientists from the IBM Zurich Research Laboratory to address the
issue.

"Electronic products are capable of amazing things, largely because of the more
powerful chips at their heart," said Bruno Michel, manager of the Advanced
Thermal Packaging research group at IBM’s Zurich lab. "We want to help
electronics makers keep the innovations coming. Our chip-cooling technology is
just one tool at our disposal to help them do that."

The approach used by IBM addresses the connection point between the hot chip and
the various cooling components used today to draw the heat away, including heat
sinks. Special particle-filled viscous pastes are typically applied to this
interface to guarantee that chips can expand and contract owing to the thermal
cycling. This paste is kept as thin as possible in order to transport heat from
chip to the cooling components efficiently. Yet, squeezing these pastes too thin
between the cooling components and chip would damage or even crack the chip if
the conventional technologies are used.

Using sophisticated micro-technology, the IBM researchers developed a chip cap
with a network of tree-like branched channels on its surface. The pattern is
designed such that when pressure is applied, the paste spreads much more evenly
and the pressure remains uniform across the chip, allowing the right uniformity
to be obtained with nearly two times less pressure, and a ten times better heat
transport through the interface.

This unique and extremely powerful design for chip cooling is borrowed from
biology. Systems of hierarchical channels can be found manifold in nature, e.g.
tree leaves, roots, or the human circulatory system. They can serve very large
volumes with little energy, which is crucial in all organisms larger than a few
millimeters. Ancient water irrigation systems also used the same approach.

The demonstrated prototype is part of a large effort within IBM’s Research and
Development organizations to improve cooling performance of next and future
generations of computer systems.

The cooling bottleneck results from the demand for ever more powerful computer
chips and becomes one of the most severe constraints of overall chip
performance. Today’s high-performance chips already generate a power density of
100 Watts per square centimeter — one order of magnitude more than that of a
typical hotplate. Tomorrow’s chips may attain even higher power densities, which
would create surface temperatures close to that of the sun when not cooled
(approx. 6000 degrees C). Current cooling technologies, mainly based on forced
air convection (fans) blowing across heat sinks with densely spaced fins, have
essentially reached their limits with the current generation of electronic
products. To make matters worse, energy needed to cool computer systems is
rapidly approaching the power used for calculations, thus almost doubling the
overall power budget.

"Cooling is a holistic challenge from the individual transistor to the
datacenter. Powerful techniques, brought as close as possible to the chip right
where the cooling is needed, will be crucial for tackling the power and cooling
issues," states Michel.

Looking beyond the limits of air-cooling systems, Zurich researchers are taking
their concept of branched channel design even further and are developing a novel
and promising approach for water-cooling. Called direct jet impingement, it
squirts water onto the back of the chip and sucks it off again in a perfectly
closed system using an array of up to 50,000 tiny nozzles and a complicated
tree-like branched return architecture.

By developing a perfectly closed system, there is also no fear of coolant
getting into the electronics on the chips. What’s more, the IBM team was able to
enhance the cooling capabilities of the system by devising ways to apply it
directly to the back of the chip and thereby avoiding the resistive thermal
interfaces in between the cooling system and the silicon.

First lab results are impressive. The team has demonstrated cooling power
densities of up to 370 Watts per square centimeter with water as coolant. This
is more than six times beyond the current limits of air-cooling techniques at
about 75 Watts per square centimeter. Yet, the system uses much less energy for
pumping than other cooling systems do.

Schematic HCTI 72dpi IBM Next Gen Chip Cooling Technologies

The image shows a cross-sectional schematic of the cooling
architecture using the high thermal conductivity interface. A highly viscous
paste is brought between the chip cap and the hot chip in order to reduce the
thermal resistance. Thanks to its tree-like branched channels, the architecture
allows the paste to spread very homogenously and attains a thickness of less
than 10 micrometers. With this technique, two times less pressure is needed to
apply the paste and a twofold increase in cooling performance can be achieved.

Schematic JAC text 72dpi IBM Next Gen Chip Cooling Technologies

The image shows a cross-sectional schematic of the jet
impingement cooling system that eliminates the thermal interface. Here, the hot
chip is directly cooled by a multitude of small streams of water. The technique
employs a distributed return architecture with alternating inlets and outlets to
squirt small amounts of water onto the chip and suck them off again, The 50,000
channels are 30-50 micrometers wide and made with microtechnology (MEMS). With
the JAC a cooling performance of up to 370 W/cm2 was demonstrated with water as
coolant.

Image1 HCTI 72dpi IBM Next Gen Chip Cooling Technologies

Close-up of the micrometer-sized tree-like hierarchical
channel design.

Image2 HCTI 72dpi IBM Next Gen Chip Cooling Technologies

The image shows the high thermal conductivity interface after
the paste has been applied. The pattern of paste arises from the hierarchical
channel design of the interface that controls and optimizes the spread of the
paste.

Image3 JAC 72dpi IBM Next Gen Chip Cooling Technologies

Three-dimensional representation of one inlet and outlet tree,
similar to the human vascular system.

Image4 JAC 72dpi IBM Next Gen Chip Cooling Technologies

A scanning electron microscope image showing a cross section
through the jets and four hierarchical layers of manifolds (jets). Blue arrows
indicate the water flow.

Image5 JAC 72dpi IBM Next Gen Chip Cooling Technologies

Image of a jet impingement cooling module (in the foreground)
and chip (in the background) before assembly. The module has been placed on a
finned copper air heat sink weighing several kilograms. Such a sink would be
required to obtain a sufficient cooling performance for the next generation of
computer chips.