Electronics Protection - Winter 2014 - (Page 16)
A Better Alternative to Heat Pipes: Integrating Vapor
Chambers into Heat Sinks
George Meyer, CEO
Often in high power density or low profile heat sink applications, the spreading resistances in the base of the heat sink limits
the performance of the design. Once it is determined that normal
heat sink materials are either insufficient or too bulky to meet the
design objectives, the obvious next step is to look at two phase
spreading devices, such as heat pipes or vapor chambers.
Either technology is often an improvement in these applications. The use of vapor chambers offers two distinct advantages
over heat pipes, direct contact to the heat source and uniform
spreading in all directions. Integrating heat sinks and vapor chambers is simpler than most people think and this integration often
allows for further improvements in performance.
Integrating Vapor Chambers into Heat Sinks
One typical design incorporates three basic parts: the vapor
chamber, an aluminum frame for
mechanical attachments and a
fin pack, which is often made of
aluminum. These three parts are
soldered into one assembly.
An alternative to this design is
to simply add the vapor chambers to the base of an extruded
heat sink. In Figure 1, several
standard-size vapor chambers
are shown imbedded into the
base of a large heat sink to provide an isothermal base.
The heat sink in Figure 2,
a cooling solution for highbrightness light emitting diodes
(HBLEDs), shows how a vapor
chamber can be integrated into a
fin stack directly.
Vapor Chamber Thermal Resistance
The most commonly asked question relating to the design of
a vapor chamber cooling solution is what is the effective thermal
conductivity (W/m-K) of the vapor chamber? Because two phase
devices do not exhibit a linear heat transfer behavior, this number
is application specific. There are two main resistances within all
two phase heat transfer devices: the evaporator resistance and the
vapor transport resistance. The third resistance, the condensation
resistance, is much smaller than the other two. In the vast majority
of applications, the evaporation resistance is the dominate resistance; therefore, making these devices somewhat length independent. This means that a vapor chamber with a transport distance
of 75 mm will have almost the same Tsource -Tsink as one with a
150 mm transport distance. This, in effect, doubles the effective
thermal conductivity for the longer devices.
Evaporator resistance is expressed in units of degree C/W/ cm2.
At lower power levels, 5 to 10 W/ cm2, this resistance is on the
order of 0.1 C/W/ cm2. As power densities increase, the evaporator
resistance decreases until a performance limit is reached. This limit
can extend to 200 W/cm2 and higher, depending on the vapor
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chamber design. Figure 3 shows the evaporator resistance for one
particular vapor chamber design.
The vapor transport resistance is expressed in similar terms,
but refers to the cross sectional area of the vapor space. Keep
in mind, changes
or working fluid
will change these
values. The values
typical values for a
at electronics cooling temperatures.
This resistance is
0.01 C/W/ cm2.
Figure 4 shows
chamber cross sections of 2.0 mm to
3.5 mm thicknesses
and widths from 20
mm to 80 mm. The
cross sections are
calculated and the
in simple C/W for
each size. 
The thermal models in Figure 5 compare a copper-based 1U
heat sink with a vapor chamber-based 1U heat sink. In this type of
application, where the heat is being spread uniformly more than it
is being transported a long distance, the typical effective thermal
conductivities are on the
order of 1,000 to 1,500
W/m-K. In a small form
factor such as a 1U heat
sink where the transport
length is short the effects
of the vapor chambers is
an improvement of 3°C to
4°C or about a 10 percent
improvement over a copper base. This improvement is often critical in high ambient applications or where the gain is used to lower
fan speeds for noise considerations.
In summary, vapor chambers are easily integrated into thermal
solutions and can offer thermal performance improvements on
the order of 10 percent to 30 percent over copper and heat pipe
based solutions and can often be lighter in weight than equivalent
extruded or copper based heat sink. These improvements allow
for designers to design for higher ambient or lower noise due to
low required fan speeds.
References - 1. Garner, S.D., "Heat Pipes for Electronics Cooling
Applications", Electronics Cooling, Vol. 2, No. 3, 1996.
For more information visit www.celsiainc.com.
Table of Contents for the Digital Edition of Electronics Protection - Winter 2014
EMI Compliance: Choosing the Right Shielding and Gasketing
Thermal-Fluid Modeling for Flat Thin Heat Pipes/Vapor Chambers
Increase Rack Cooling Efficiency and Solve Heat-Related Problems
Seven Essential Cabinet Design Considerations for Protecting 19 Inch Electronics
A Better Alternative to Heat Pipes: Integrating Vapor Chambers Into Heat Sinks
Common IP Testing Failures and How to Avoid Them
Electronics Protection - Winter 2014