Power Electronics

Technologyupdate
Matt Connors
Application Engineering Manager
Thermacore, Inc.
Vapor Chambers Improve Cooling
of Power Semiconductors
Using vapor chambers can be an efficient way to
manage heat in today’s small die, high-power semiconductor devices where effective cooling helps
ensure long component life and reliability.
A
s the heat density increases at the die level
of more powerful semiconductor devices,
there becomes a greater need for new and
innovative cooling solutions. Standard
Aluminum extrusions and bonded fin
heat sinks no longer have the capacity to
sufficiently cool the semiconductor dies.
These heat sinks are limited by the amount of thermal
spreading in the base of the heat sink. As shown in Fig. 1,
with increased flux, hot spots are apparent in the base of a
standard aluminum heat sink. The fins on the extremities
of the part are cool and are not providing any additional
cooling. The location above the heat source has the concentrated heat load, causing localized high temperature
areas in the sink and ultimately increased junction temperatures.
By inserting a vapor chamber into the base of the heat
sink, fins now heat up evenly ultimately making the heat
sink more efficient (Fig. 2). Also, the hot spot over the
die is now evenly spread out, significantly decreasing the
junction temperature.
In implementing a vapor chamber, the gains from adding additional surface area by increasing the footprint of
the heat sink are much more realized. In a standard heat
sink as shown in Fig. 1, if the size of the heat sink were to
Fig. 1. Heat sinks are limited by the amount of thermal spreading in their base,
so hot spots can occur in a standard aluminum heat sink.
18 Power Electronics Technology | August 2010
increase, there would be no real improvements in overall
resistance of the heat sink since the fins that were added
would be ineffective (shown in blue in Fig. 1). The vapor
chamber allows you to expand the heat sink in width
or length nearly without limit and still greatly improved
thermal resistance.
simulating thermal performance
In order to show this spreading phenomenon, a CFD
(computation fluid dynamics) simulation was created for
a typical power semiconductor IGBT and its cooling sink
(Fig. 3). For this example, a 9 X 4 X 2 in. low-profile
aluminum heat sink with a standard extruded pitch (7 FPI
@ .070 in. thick fin) was used to cool an IGBT component
(Standard Die locations and thermal spreading in package,
Total Power = 2400 watts). As shown in Fig. 1, the high
temperature gradients are evident in the cross-section
view. The fins toward the outside edges are quite cool
compared to the hot spot directly over the electrical component. The thermal resistance of the heat sink with 50
CFM of airflow is 0.054 °C/Watt, which is the temperature rise from ambient to the hottest spot on the surface
of the heat sink over the total power dissipated.
The challenge now becomes the ability to spread that
heat efficiently through the base of the heat sink without
changing its existing geometry. Meeting this challenge
allows the designer to stay within the same form factor or
original package size without a long and costly redesign
of the component enclosure. In order to reduce this high
thermal resistance, the heat sink metallic base needs to
be replaced with a “super” conducting material. In this
Fig. 2. Inserting a vapor chamber into the base of the heat sink causes its fins to
heat up evenly, making it more efficient. Hot spots over the semiconductor die
are now evenly spread out, decreasing its junction temperature.
www.powerelectronics.com
Fig. 3. CFD (computation fluid dynamics) simulation for a typical IGBT and its
all-aluminum standard extrusion heat sink.
example, a vapor chamber can be used as the medium to
spread heat in the base more efficiently.
Vapor Chamber Benefits
Vapor chambers are essentially flat or planar heat pipes
that use the principles of evaporation and condensation to produce a very high conductivity thermal plane.
www.powerelectronics.com Vapor chambers, like traditional cylindrical heat pipes,
are evacuated vessels with an internal wick and a working
fluid. The wick helps transport the working fluid back
to the evaporator surface without the use of any moving
parts. Once the fluid evaporates, it travels to the cooler
section of the chamber, condenses in the wick and the
cycle continues.
Vapor chambers can have a number of different shell
materials and working fluid combinations. The selection of these materials depends mainly on the operating
temperature of the cooling system. The most common
combination in the electronics cooling field is copper and
water due to its operating temperature of about 10°C to
250°C, but other liquids and materials can be used for
extreme temperature ranges.
Bulk conductivities for vapor chambers have been
measured at over 30 times the conductivity of copper,
and over 10 times the conductivity of pyrolytic graphite
and diamond in the same flat plane configuration. In
addition, vapor chambers can be bonded into an existing
extrusion or used as the base itself, in which case fins
can be soldered directly to it. Vapor chamber sizes can
August 2010 | Power Electronics Technology
19
HEATSINKperformance
shown here with an embedded vapor chamber and all of
the parameters held constant exhibits a resistance of only
0.044 °C/watt. Vapor chambers also allow easier implementation of a folded fin or stamped fin. These fins can
be directly soldered or glued onto the top of the chamber.
The fin densities can be dramatically improved by using
these types of fins due to the ability to increase pitch.
Since the heat flux is so low exiting the vapor chamber,
the additional resistance going through this fin-attach
interface is inconsequential.
HIGHER METAL HEAT SINK PERFORMANCE
Fig. 4. With a vapor chamber integrated into the heat sink base the heat spreads
evenly across the entire heat sink, causing a drop in thermal resistance .
range from as small as 1 X 1 in. to as large as 13 X 30 in.
Standard thicknesses range from 3 to 9 mm so they can be
easily inserted into existing bases.
In today’s electronics cooling market, vapor chambers are used in various applications. The military uses
these high-conductivity heat sinks in cooling radar TWTs
(traveling wave tubes), IGBTs (Insulated-gate bipolar
transistors), and other high-flux electronics. The medical
industry uses them to warm blood uniformly. A multitude
of heat sinks in mid- to high-range computer servers use
vapor chamber technology to manage the heat from highflux, high-computing-power CPUs that define the speed
and performance of the system.
To illustrate the thermal performance improvement
that a vapor chamber can provide to an all-metallic heat
sink, let’s examine the same heat sink described earlier,
but with a vapor chamber integrated into the heat sink
base. As shown in Fig. 4, the heat is spread much more
evenly across the entire heat sink, causing a drop in
overall thermal resistance of 19 percent. The heat sink
As illustrated, the enhanced performance of the vapor
chamber improves the thermal performance of an allmetallic heat sink significantly. The improved thermal
performance allows the electronic component designer
to easily manage component frequency speed and power
increases within the existing architecture, and at the same
time, allow for much more computing/transmitting power
for new designs in a more compact space. For example,
if an IGBT component in a given system is reaching its
maximum junction temperature at 1500 Watts, the vapor
chamber can potentially increase the dissipated power to
1800 Watts without changing junction temperature. This
is a great advantage for mechanical designers where changing the form factor of a given heat sink would prompt a
costly overhaul of the enclosure layout.
Fig. 5 shows a Therma-Base vapor chamber, which
enables lower device temperature and greater component
reliability. Like conventional heat sinks, vapor chambers
are versatile enough to be freeze/thaw resilient and able to
withstand military shock and vibration standards, but it’s
important to consider those variables in the specification.
Operating temperature, gravity orientation, and length
of power transport are all factors to account for when
tailoring a vapor chamber’s internal structure for a given
application.
While conventional heat sinks may be suitable for use
in low power, large heat source applications, it is applications where the performance of the system is limited by
thermal/mechanical constraints (heat flux, overall power,
space, mass) that the vapor chamber offers the ability
to obtain the next level of speed and power in the same
space.
SUMMARY
Fig. 4. With a vapor chamber integrated into the heat sink base the heat spreads
evenly across the entire heat sink, causing a drop in overall thermal resistance .
20 Power Electronics Technology | August 2010
The Therma-Base vapor chamber has an enhanced
capability to accept higher heat fluxes than a traditional
aluminum or copper surface. Its smaller size improves
system packaging and provides quieter operation through
less air flow. Able to operate in any orientation, the
Therma-Base passes shock and vibration testing, and thermal cycling (freeze/thaw).”
www.powerelectronics.com