Download Air and Conduction Cooling for 3U COTS Cards

Air and Conduction Cooling for 3U COTS Cards: An Overview
Thermal knowledge and innovation continue to improve cooling limits for aircooled and conduction-cooled cards, and this benefits 3U cards greatly due to their
shorter width and lower power.
Ivan Straznicky, Curtiss-Wright Controls Embedded Computing
There are many trends at the die level and circuit card level that can drive the
decision of which cooling method to use for 3U COTS cards in rugged applications.
One of these trends is the increased use of multicore processors on 3U cards. Placing
more processor cores on a die increases power dissipation. However, as more cores
are placed on a die, the size of the die increases, which actually decreases the power
density in terms of W/cm2 (Figure 1). This is a good thing from a thermal standpoint
because it reduces the spreading resistance down the heat removal path.
Unfortunately, increasing device power dissipation, combined with the long term
trend of decreasing junction temperatures (i.e., from 125 C to 105 C, or 100 or less),
tends to override the small benefit obtained from the decrease in power density.
One of the main causes of increased power dissipation for processors is the trend
toward increasingly smaller transistor geometries, which results in large increases
in static (or leakage) power. Some modern processors use new types of transistor
materials that reduce the amount of static power. For example, Intel uses hafnium
dioxide on its 45nm processors. These new transistor materials reduce the amount
of gate oxide tunneling and sub-threshold leakage current, two of the dominant
forms of leakage current.
At the board level, component miniaturization and the use of more highly integrated
devices is increasing the functional density on 3U cards, which is directly related to
heat density. As the amount of processing power per square inch per 3U card has
increased it has driven up heat density. Another factor driving up allowable power
dissipations on 3U cards is the support for higher voltages in the new VPX (VITA 46)
standards. VITA 46 defines support for 12V and even 48V, compared to the
standard, traditional 3.3 and 5V supported by VME. VPX also supplements the
traditional 0.8” pitch of VME with 0.85” and 1.0” pitches. This increase in pitch
enables the use of more, hotter devices on the rear side of the circuit card,
increasing the power dissipation per unit area and volume. The outcome is an
almost exponential increase of power at the circuit card level. Susceptibility to this
trend depends on the card’s functionality. The higher power cards are typically DSP
cards that have multiple multicore processors onboard for number crunching. In
comparison to DSP cards, general purpose processor and I/O cards typically follow
more of a flat curve in terms of power.
While direct air and conduction cooling have been able to keep up with these power
increases to date, it has been a challenge. The amount of thermal design, analysis,
and testing that is required on a rugged military COTS 3U card is many times what it
was five years ago. This increased work is basically the result of the increase in
power dissipation.
Direct forced air cooling
Direct forced air cooling is typically the starting point in terms of cooling
approaches for military COTS cards simply because most software and system
development begins in a laboratory environment with air-cooled cards in a
benchtop rack. Consequently, these cards are usually commercial temperature-rated
and not rugged.
Cooling begins at the device die and on most modern, high power devices, the die is
exposed. This is because most commercial cooling approaches employ an air-cooled
heatsink on top of the die, which provides the shortest and lowest resistance heat
removal path to the air. While forced air cooling takes advantage of this
arrangement, it may not always be desirable to have a large piece of metal on the
die. For this reason a heat spreader is sometimes placed between the heatsink and
the die to spread the heat and to provide some protection for the die. Unfortunately,
for today’s higher power devices, standard off-the-shelf aluminum heatsinks, with a
few fins per inch, may be insufficient. To address these hotter devices the heatsink
will likely need to be optimized. System designers can use computation fluid
dynamics (CFD) tools to design the heatsink they require for air-cooling. There are
subroutines available within CFD tools to design the heatsink’s optimum number of
fins per inch, thickness of fins, optimum gap, and height, etc. Another trend in aircooling today is the availability of higher flow and pressure fans and blowers, which
provide increased airflow for such high pressure drop heat sinks.
These new approaches coupled with advances in air cooling have resulted in
significantly improved cooling numbers. For example, today, a 150W 6U card can be
sufficiently cooled with a 71 deg C air inlet. Just five years ago, many designers
would have been hard pressed to believe this was achievable. For 3U cards, air
cooling is even more attractive because there are many fewer hot components on
the card that need to be cooled by the same air stream (by virtue of the 3U
geometry, i.e. 100mm in the airflow direction vs. 233mm).
There are, however, drawbacks for air-cooling. Air-cooled cards are typically not
very stiff compared to conduction-cooled cards. Unlike conduction-cooled cards,
they lack a metal stiffener plate on top to increase the stiffness. When air-cooled
cards are subjected to significant vibration, they can displace quite a bit and end up
experiencing fatigue problems more quickly than on stiffened cards. With that said,
this is less of a problem for air-cooled 3U cards than for 6U cards, because they have
a much shorter span between guide rails (Figure 2).
Another drawback for air cooling stems from the use of non-sealed chassis to enable
air to be blown directly over the electronics. Because the chassis isn’t sealed, the
cards and electronics may be exposed to contaminants, such as salt fog/sand/dust,
etc., in the ambient environment. Filtering the air will remove some of these
contaminants, but will come at the cost of an increase in pressure drop due to the
filter.
One more drawback associated with air cooling that is worth noting is that because
air is compressible and has low heat capacity and low thermal conductivity, it is not
a very good coolant. Because of these properties, an increase in air flow rate over a
card can result in an asymptotic curve with respect to cooling vs. airflow. At the
same time, the pressure drop is increasing steadily so the cooling limit is reached
relatively quickly with air flow cooling.
Conduction cooling
Conduction cooling is a popular choice and used quite widely for deployed 3U
systems because it is inherently more rugged (Figure 3). The “backbone” of
conduction cooling is the stiffening and thermal frame, typically made of aluminum.
Today’s increased power dissipation levels are leading to increased use of copper as
well.
Because of copper’s high density it is used only where needed, for heat spreading for
example. Composite materials are also showing some promise, not necessarily as a
replacement for the thermal frames, but localized in areas of the thermal frames.
However, most composites are orthotropic with regards to thermal conductivity,
with the in-plane thermal conductivity being quite high. For example the claimed
thermal conductivity for pyrolytic graphite is very good - about 1500 watt per meter
per degree Kelvin (W/m°K) in either in-plane direction. Compared to aluminum,
which is 180 W/m°K, pyrolytic graphite should provide performance almost an
order of magnitude better. However, the through thickness thermal conductivity for
pyrolytic graphite is only around 20 W/m°K, and needs to be taken into account. If
large amounts of heat are being moved across large planes or long distances a
composite can be a good solution. But trying to move heat through the thickness is
more difficult. There are several research groups working on developing materials
that have isotropic conductivity better than copper (which is 400 W/m°K), have the
density of aluminum or lighter, and can be produced at reasonable cost. However,
because they are highly engineered, cost often ends up being an issue.
Another option for increasing the thermal conductivity of thermal frames on 3U
cards is to use phase change devices such as heat pipes or vapor chambers. CurtissWright has undertaken substantial research and development with heat pipes and
has successfully implemented them in rugged products. Heat pipes are very effective
at moving heat with a very low temperature drop. A drawback of heat pipes though,
is that that they are orientation dependent, which makes it critical to understand
how they behave under the effects of body forces such as acceleration, vibration and
gravity. The various performance limitations of heat pipes, such as capillary,
entrainment and condenser limits, also must be understood to implement them
properly. When properly implemented, heat pipes can handle fairly high power
densities and power dissipations. A heat pipe implemented in the axial direction
provides effective conductivity in the thousands of W/m°K, which is an order of
magnitude higher than the metals used for cooling.
Advances in conduction-cooled cards now make it possible to cool in excess of
170W at an 85 C card edge temperature, the upper limit of what is seen with
military COTS circuit cards. A standard chassis with air-cooled sidewalls, though, is
not capable of cooling even one of these cards. Air cooling through the side of a
chassis wall, with conduction cards slotted between rails, is just not capable of
cooling the level of power density at the circuit card edge. In such a case, other
approaches, such as a liquid-cooled chassis, in which liquid is flowed through the
sidewalls, must be employed.
Thermal resistances in the heat path of a conduction card have improved greatly, for
example thermal interface materials, over the last decade. Ten years ago the best
thermal interface material you could find was in the order of 1-3 W/m°K. Now there
are several in the order of 10-15 W/m°K, which is basically an order of magnitude
improvement. However you do have to be careful with some of these materials.
When tested, some that have been advertised as having a certain thermal
conductivity actually exhibited only 1/10th of that value. Had that material been
designed into products without testing, thermal failures may have resulted. Because
these materials have been designed for use in commercial applications they also
need to be tested for rugged properties such as long-term thermal cycling, exposure
to humidity, etc., to ensure that they can be used in rugged applications.
The perceived “Achilles Heel” of conduction cooling is the thermal contact resistance
between the conduction card edge and the chassis rail that results from metal-tometal contact. These metal surfaces are not truly flat and smooth at the microscopic
level. Metal peaks, called asperities, contact each other and create low resistance
paths through which much of the heat flows. The heat flowing across this junction is
a surprisingly complex phenomenon. The thermal contact resistance for a given pair
of surfaces depends on a complicated combination of variables including contact
area, contact pressure, surface flatness, surface roughness, and hardness.
Typical values of thermal contact resistance used for VME or VPX cards are in the
range of 0.3-0.5 °C per Watt. So for a 100W card, with 50W going to either card edge,
the 50W is multiplied by the .5 °C per Watt resulting in a 25 °C temperature
difference across that interface. This is huge in terms of the temperature budget
between the coolant and the electronics. For example, the temperature budget for
cooling a part that has 100 °C maximum junction temperature with 55 °C ambient
air is only 45 °C. By investigating and optimizing the contributing factors to thermal
contact resistance, much lower values in the range of 0.1-0.2 °C/W can be achieved.
Note that these values are at sea level and they will change (increase) at altitude.
With these numbers and our example of 100W card, a temperature difference of
only 5 to 10 degrees can be obtained. This is still significant and must be accounted
for in system level thermal analyses, but 5-10 °C is certainly more appealing to
system integrators than 25 °C. These improvements should be sufficient for 3U
cards for the foreseeable future, making the 3U form factor a good choice for small
form factor (SFF) electronics.
Curtiss-Wright Controls Embedded Computing, Leesburg, VA. (703) 737-3660.
www.cwcembedded.com