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Liquid Cooled Cold Plates for Industrial High-Power Electronic
Devices—Thermal Design and Manufacturing Considerations
Satish G. Kandlikara; Clifford N. Hayner IIb
a
Mechanical Engineering Department, Rochester Institute of Technology, Rochester, New York, USA b
Thermal Solutions Consultant, Rochester, New York, USA
To cite this Article Kandlikar, Satish G. and Hayner II, Clifford N.(2009) 'Liquid Cooled Cold Plates for Industrial High-
Power Electronic Devices—Thermal Design and Manufacturing Considerations', Heat Transfer Engineering, 30: 12, 918
— 930
To link to this Article: DOI: 10.1080/01457630902837343
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Heat Transfer Engineering, 30(12):918–930, 2009
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DOI: 10.1080/01457630902837343
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Liquid Cooled Cold Plates for
Industrial High-Power Electronic
Devices—Thermal Design and
Manufacturing Considerations
SATISH G. KANDLIKAR1 and CLIFFORD N. HAYNER II2
1
2
Mechanical Engineering Department, Rochester Institute of Technology, Rochester, New York, USA
Thermal Solutions Consultant, Rochester, New York, USA
Electronics cooling research has been largely focused on high heat flux removal from computer chips in the recent years.
However, the equally important field of high-power electronic devices has been experiencing a major paradigm shift from
air cooling to liquid cooling over the last decade. For example, multiple 250-W insulated-gate bipolar transistors used in
a power drive for a 7000-HP motor used in pumping or in locomotive traction devices would not be sufficiently cooled
with air-cooling techniques. Another example is a “hockey puck” SCR of 63 mm diameter used to drive an electric motor
that could dissipate over 1500 W and is difficult to cool with air because of the shape of the device. Other devices include
radio-frequency generators, industrial battery chargers, printing press thermal and humidity control equipment, traction
devices, mining devices, crude oil extraction equipment, magnetic resonance imaging, and railroad engines. This article
classifies the cold plates into four types: formed tube cold plate, deep drilled cold plate, machined channel cold plate, and
pocketed folded-fin cold plate. The article further discusses selection of cold plate type and channel configuration, and some
of the relevant manufacturing issues. It is recommended that the thermal designer be involved in the early stages during the
electrical design and layout of the devices.
INTRODUCTION
Background of Cooling Technologies for High-Power
Devices
Electronic devices are at the heart of almost all major industrial and military equipment. Some of these are power
drives, insulated-gate bipolar transistor (IGBT) controllers,
radio-frequency (RF) generators, magnetic resonance imaging (MRI) machines, traction devices for locomotives, battery
chargers, UPS (uninterrupted power systems), DC-AC converters, AC-DC inverters, and army tanks (using transmission fluid
already at a high temperature). The high-power, high-heat-flux
demands on the cooling system cannot be met with air cooling,
and advanced liquid cooling solutions are necessary.
The authors are grateful to Dr. William Grande of Ohmcraft, Honeoye Falls,
NY, for his contribution in the electronic device characterization.
Address correspondence to Professor Satish G. Kandlikar, Mechanical Engineering Department, Rochester Institute of Technology, Rochester, NY 14623,
USA. E-mail: sgkeme@rit.edu
There has been a dramatic shift in cooling high-power devices
in the industry during the past decade. Air cooling has sufficed
for many lower power electronic devices. Although it is quite
difficult to make a distinction based on total power dissipation,
it seems that beyond a range of about 1500 W dissipation, there
are many physical and design constraints that may dictate a shift
toward liquid as the preferred medium.
With air cooling, a heat spreader is critical in carrying
heat from the device–heat sink interface to the air-cooled
surfaces. The role of the heat spreader becomes less important at higher heat fluxes as the thermal resistance for lateral
heat conduction (and the resulting temperature drop) in the
heat spreader becomes unacceptably large in comparison to
the available temperature difference between the base (design
condition) and the inlet coolant temperature. For example, the
temperature drop across a 1 mm thick copper plate is approximately 0.25◦ C at a heat flux of 10 W/cm2 , while it increases
to 2.5◦ C at 100 W/cm2 and 12.5◦ C at 500 W/cm2 . Although
heat spreaders are quite effective at lower heat fluxes, their effectiveness reduces considerably at higher heat fluxes. This is
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S. G. KANDLIKAR AND C. N. HAYNER II
one of the main reasons why the air cooling option becomes
unattractive, and eventually not viable for very high heat flux
conditions.
The need for liquid cooling has resulted in a major paradigm
shift for many electrical system designers and manufacturers.
Introducing a liquid, which is often not a dielectric, in the vicinity of electronic components was unthinkable until not too long
ago. Often electrical or electronic engineers would design an
electrically efficient layout, often without allowing an increase
in cabinet size, larger fan, or enough power for a larger fan. This
brought the realization that liquid cooling offers a better, and
often the only feasible, solution. The compactness of the cold
plates and the supply and return lines, lower power consumption, and reduction in noise levels are some of the attractive
features.
The development of multichip module (MCM) conduction
cooling at IBM by Oktay and Kammerer [1] and Chu et al. [2]
provided major impetus to liquid cooling. Kishimoto and Ohsaki
[3] presented a liquid-cooled cold plate design for VLSI chips,
which employed twenty-nine 800 µm × 400 µm rectangular
minichannels on an 85 mm × 105 mm alumina substrate package. Each is designed to accommodate sixteen 25 W chips with
a total heat load of 400 W and a volumetric heat dissipation rate
of 10 kW/L, comparable to the immersion cooling techniques.
Further, the packages were assembled in a stack, allowing a
compact MCM cooling design employing liquid cooling with
significantly higher cooling rates than with other liquid cooling
designs. These authors also presented an analytical scheme to
provide uniform flow distribution in the channels. These concepts form the basis of today’s cold plate designs for cooling
high-power devices.
Currently the cold plates are used extensively to provide cooling platforms for electronic devices. Different cooling strategies
with single-phase liquid, air, jet impingement, pool boiling, and
flow boiling have been employed. An excellent survey of these
techniques has been recently published by Anandan and Ramalingam [4]. Sparrow et al. [5] looked into the thermal design
of the coolant passages and some enhancement techniques for
periodic heat sources. Some of the earlier reviews by Sathe
et al. [6], Incropera [7], Yeh and Chu [8], and Bar-Cohen and
Kraus [9] provide basic design equations and show the progression in literature toward using liquid cooling for high heat
flux removal. Kandlikar [10] pointed out the effectiveness of
small-diameter passages in meeting the high-heat-flux removal
challenges. Zuo et al. [11] provide an overview of the cooling technologies at seven different levels from the chip to the
cooling system. Schmidt [12] pointed out the need of moving
toward minichannel heat exchanger embedded heat sinks for
cooling high-flux devices.
Chu [13] presented an in-depth review of the electronics
cooling challenges faced with increasing power density. The reliability and redundancy requirements will be critical in designing the liquid cooled systems for reliable operation. Williams
and Roux [14] investigated the effect of different channel
inserts, including various copper fins and graphite foam, on
heat transfer engineering
919
the thermal performance of the heat sink. The authors employed computational fluid dynamics (CFD) analysis and presented a rank order of various techniques. Their results show
that using smaller channel sizes yields better thermal performance. Such optimization is necessary along with a pressure
drop and overall system level analysis in deciding the channel
configuration.
The available literature on the cold plate design is primarily focused on relatively low power dissipation. Although there
are a few publications that deal with CFD modeling and some
material developments available in literature (IGBT: Romero
and Martinez [15], Romero et al. [16], Rodriguez and Fusaro
[17], Lee, [18], Lasers-Liu et al. [19]; cooling and packaging of high-power diode lasers: Loosen [20]; cooling of irradiated targets by conductive cooling in nuclear applications:
Talbert et al. [21]), a comprehensive coverage is lacking. Although some of the recent publications deal with advanced
cooling systems using three-dimensional (3-D) cooling of devices [22, 23], porous plate for high-flux cooling [24], microcapillary pumped loop systems [25], microjets [26], diamond
substrate windows [27], liquid metal cooling [28], doubleside cooling of high-power IGBTs [29], and fin inserts and
other techniques in cooling hockey-puck type semiconductor
devices [30], a majority of high-power devices utilize liquidcooled cold plates. Some recent papers focus on cooling configurations and local heat transfer analysis for high-power devices such as diode-pumped lasers [31] and solid-state lasers
[32].
Iyenger and Bar-Cohen [33] present an excellent outline on
this topic for air-cooled heat exchangers used in electronic
cooling considering the manufacturing issues. They considered a number of fin designs and their manufacturing technologies, and performed optimization to identify the maximum heat transfer capability within a given design domain.
They also presented useful information on different types
of fins and their manufacturing techniques for air cooling
applications.
Although some novel cooling systems are being introduced
to handle the very high heat flux systems, liquid cooling is by far
the most commonly employed system for cold plates. Among
the advanced systems being investigated are spray cooling, jet
impingement cooling, and advanced single- and two-phase microchannel cooling [34–41].
This article primarily deals with cold plates for high-power
devices, covering some of the challenges faced by liquid cooling, thermal management solutions, and manufacturing aspects,
which are often ignored in the literature but are of great importance in practical implementation of these thermal design
solutions.
Thermal Characteristics of Electronic Devices
In this section, a brief introduction to the thermal characteristics and related issues of some of the electronic devices is
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S. G. KANDLIKAR AND C. N. HAYNER II
presented [20, 42]. The discussion is to help in understanding
some of the major electronic performance issues as they relate
to the operation of the cooling systems, and is by no means a
comprehensive summary.
The IGBT and the silicon-controlled rectifier (SCR) are both
minority carrier devices whose ultimate high-temperature limit
of operation is governed by intrinsic carrier generation. In other
words, at any given temperature there is a steady creation of
thermally activated electrons and holes. When the density of
these thermally generated carriers becomes comparable to the
engineered doping levels, the desired characteristics of the semiconductor layers break down. The limit of this phenomenon is
the intrinsic temperature, Ti , at which the intrinsic carrier concentration equals the doping level of the most lightly doped
layer. At this point, rectification of the p − n junction ceases and
the device cannot function. In many silicon devices the intrinsic
temperature is about 280◦ C. However, since all of the fundamental physical parameters of semiconductors are functions of
temperature to a greater or lesser degree, the upper temperature range of practical devices must be limited to well below
the intrinsic temperature to guarantee specified performance.
A common maximum temperature spec for silicon devices is
125◦ C, except for thyristors (including SCRs), which should be
kept cooler because of the large switching pulses that are used
in triggering.
In the case of IGBTs, the leakage current leads to heating
in the device. The leakage current increases dramatically with
temperature. A significant portion of total power is dissipated internally within an IGBT—10% of total load is not an uncommon
occurrence.
For power applications, generally the cooler a device can be
maintained during operation the better it will perform. Higher
operating temperatures can conspire with power transients, triggering signals, noise, and localized heating effects to produce
failures that would not occur at lower temperatures. Long-term
materials interactions, particularly at device interfaces, are a major concern for lifetime and are often accelerated exponentially
with temperature. A rule of thumb for silicon devices is that
failure rates often double for every 10–15◦ C rise in operating
temperature beyond 50◦ C [5].
GENERAL DESIGN ISSUES
The most effective way to deal with heat removal is to
consider the thermal requirements at the design stage of a
new or upgraded product. A thermal engineer as part of the
design group will help create the least costly mechanical
and electrical design. More often than not the electrical designs will be completed, and then the thermal designers work
to meet the thermal requirements under original and added
constraints resulting from decisions made by electrical engineers. This leads to more expensive cooling designs and more
operational compromises, often resulting in reduced product
performance.
heat transfer engineering
LIQUID TYPES—COOLANT ISSUES
The type of liquid, fluid and system pressure, fluid flow, inlet
temperature, cold plate weight, type of material, and the allowable or desired pressure drop are major factors in the cooling
system design. Plain water is the optimum cooling choice and
will be used only in controlled environments, laboratory conditions, or requested solutions. Tap water may contain active ions
or other impurities, which will attack the inside of aluminum
flow channels. Given time, those aluminum channels will corrode, causing a leak path and ultimately equipment failure. That
is why copper in tube or channel form is the preferred solution
with water and other liquids.
More often an ethylene glycol–water solution of a given
percentage is specified, since it lowers the freezing point and
raises the boiling point. Corrosion inhibitors must be used if any
aluminum is in the flow path, such as piping, tubes, manifolds,
tanks, fittings, or cold plates. Many other fluids are available and
each has its own specific heat, viscosity, and handling characteristics. Such fluids are polyolefin, gasoline, kerosene, mineral oil,
transmission fluid, JP-5, seawater, etc. Matching or optimizing
the available fluid to the target temperature of the heat sink is
the challenge.
Fluorinert is difficult to utilize since it will evaporate through
microscopic crevices, making proper containment a difficult
must. Its lower thermal conductivity and heat capacity compared
to water also make it unattractive as a single-phase coolant.
Distilled water or DI water is a challenge to cool with. A
suitable corrosion inhibitor must be incorporated into the system
so as not to dissolve metal from the cold plate or soldered
connections. DI water without an inhibitor will attack any stress
points (such as tube bends) and cause a leakage path with dire
results.
COLD PLATE CLASSIFICATION
The substrate and the fluid flow channels can be arranged in
several different configurations depending on the device size and
power dissipation requirement. These cold plate configurations
are classified into four major types as described next.
Formed Tube Cold Plate (FTCP). The coolant tubes are attached to the cold plate substrate by soldering or using a thermal
epoxy. In this design, shown in Figure 1, copper plate is generally
used, although aluminum is sometimes employed in lowpower
applications. This is one of the simplest cold plate designs, but
its performance is rather poor, limiting its use in the low-power
applications.
Deep Drilled Cold Plate (DDCP). As the power dissipation
increases, the contact resistance of the plate and the tube wall
become unacceptably high. In this design, shown in Figure 2,
deep holes are drilled in the plane of the substrate plate, generally
made of copper. These holes are then configured with end caps
(or plugs) to create coolant flow paths through the substrate. The
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S. G. KANDLIKAR AND C. N. HAYNER II
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Figure 1 FTCP—formed tube cold plate with copper plate soldered to the
cold plate substrate.
placement of the electronic devices often influences the coolant
passage layout. It is not uncommon to implement two or more
parallel paths for the coolant flow to meet the pressure drop,
coolant distribution, or temperature rise considerations.
Machined Channel Cold Plates (MCCP). As the heat flux
and power increase, it becomes necessary to improve the thermal
performance of the channels. In this design, shown in Figure
3, channels are machine-cut into the base plate and a cover
is soldered in place to form the flow passages. Depending on
the thermal performance desired, these channels can be several
millimeters wide or as small as 200 µm wide microchannels
for extremely high heat flux applications (over 100 W/cm2 ).
Cross-rib patterns, shown in Figure 3, or other enhancement
features may be incorporated in the channels, depending on the
performance requirements.
Pocketed Folded-Fin Cold Plates (PFCP). The local heat
transfer coefficient, as well as the surface area in the coolant
passages, can be enhanced by implementing fins in the coolant
passages. In this design, shown in Figure 4, recessed pockets
are machined to accept various folded fin inserts, which are
soldered inside the passages. Similar to MCCP, a cover plate is
soldered in place to form the enhanced flow channels.
Pocketed fins of various designs are available from various
manufacturers. Figures 5a and 5b show some of the designs
(courtesy Robinson Fin Machine Co., USA). Other designs include straight fins with square edges, straight fins with rounded
edges, herringbone, ruffled, lazy ruffled, lanced, offset, lanced
and offset, perforated, and triangular.
In all the designs just described, appropriate provisions are
made for coolant inlet and outlet locations. Depending on the
Figure 2 DDCP—deep drilled cold plate with single-pass or multi-pass
coolant passages drilled in the copper cold plate.
heat transfer engineering
921
Figure 3 MCCP—machined channel cold plate with channel passages machined in the cold plate to match the device location and thermal dissipation
requirements.
heat flux, total heat removed, and available pressure drop, a
specific design may be selected. The cost is an important factor, as it will vary significantly between these design choices.
Further discussion on these aspects is presented later in the
article.
OVERVIEW OF DESIGN APPROACH
As high-power electronic devices dissipate more power and
face new constraints due to space and weight limitations, liquid cooling seems to offer a superior alternative for systems
traditionally equipped with air cooling. Cooling of high-power
electronic devices poses some unique challenges that are somewhat different from those in IC chip cooling. The combination
of high heat flux and high power requirements necessitates efficient thermal and fluid management in the heat sink. Some of
the overriding design considerations are:
•
Temperature and heat dissipation requirements of each individual device (simultaneous peak load of relevant components
in a group).
• Pressure drop and flow rate requirements for the cooling fluid.
In designing a cold plate, the designer works with a given set
of inlet fluid temperature, mass flow rate, and pressure drop limits, as well as individual device power dissipations and junction
temperature requirements, and placement of devices relative to
each other. The design of individual devices and the thermal
Figure 4 PFCP—pocketed folded-fin cold plate with folded fins inserts providing enhanced heat transfer passages for coolant; a machined channel cold
plate shown at the outlet.
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S. G. KANDLIKAR AND C. N. HAYNER II
Figure 6 Coolant flow arrangements on a cold plate: (a) series, and (b) parallel arrangement.
temperatures Tf,in and Tf,out :
Figure 5 Straight (a) and lazy ruffled (b) copper fin designs employed in
pocketed folded-fin cold plates. Courtesy Robinson Fins, USA.
ṁ = ṁ(Q, Tf,in , Tf,out )
The local channel wall temperature under a device with the local
coolant temperature Tf , local heat flux q , and local heat transfer
coefficient h is given by:
Tw = Tw (Tf , q , h)
resistance at the contacting interface with the cold plate determine the allowable resistance for heat transfer to the fluid.
These issues are covered in detail in literature, while the primary focus of this article is on the thermal performance of the
cold plate through the selection of cold plate type and channel
layout. The coolant exit temperature depends on its mass flow
rate for a given total heat load. It is also limited by the thermal performance requirement of the last device near the coolant
exit, since it has a lower available temperature difference for
heat dissipation. Devices that can sustain a higher temperature
should therefore placed toward the exit end. Placement of these
devices is also dictated by their functional criticality, as a lower
junction temperature will result in a lower failure rate.
Consider the schematic arrangement shown in Figure 6a
where all devices are placed on a cold plate with the coolant
serving them in a series configuration. The basic heat transfer/fluid flow relations take the following form:
Total coolant flow rate ṁ may be expressed as a function of
the total load Q on a cold plate, and coolant inlet and outlet
heat transfer engineering
(1)
(2)
Since the coolant outlet temperature is the highest near the
exit, it is important to check if the maximum allowable device
temperatures near the exit are exceeded. The local channel wall
temperature under a device located near the channel exit is given
by:
Tw,exit = Tw ,exit (Tf ,out , qexit , hexit )
(3)
The total pressure drop with an equivalent fluid flow resistance
of Rf,eq in the coolant passages from the inlet to exit is given by:
p = p(ṁ, Rf,eq )
(4)
Figure 6b illustrates a parallel arrangement that provides the
lowest coolant temperature at the entrance for each device. Since
the fluid flow rate in each parallel channel is reduced, this may
adversely affect the heat transfer coefficients in the channels if
the flow is in the turbulent region. When using microchannels
(<200 µm) or minichannels (<3 mm), the flow is generally in
the laminar region, and the heat transfer coefficient penalty may
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S. G. KANDLIKAR AND C. N. HAYNER II
not be significant. For enhanced channels, one needs to know
the performance characteristics of the channels or fin inserts if
pocketed fins are employed. A combination of series and parallel
configurations may also be employed in arriving at a satisfactory
design arrangement.
The problem then becomes one of optimizing the heat transfer under the given pressure drop constraint to achieve the desired base temperature. The main variables available to a cold
plate designer in this optimization process are the channel shape
and size.
Additional considerations are introduced in the overall design of the fluid flow loop. The issues encountered here are the
pressure drop in the cooling channels, flow maldistribution in
parallel channels, fluid routing across different cooling zones,
and overall fluid flow pathways to meet the thermal constraint
of each device. Since each heat sink design is unique, some of
the issues raised here are discussed in relation to a few specific
cooling system designs.
Locating the regions of different thermal loading becomes
critical to plan uniform cooling (relative to the device requirements) and achieve satisfactory target temperatures. In cases
where heat fluxes of the devices are uniform and the temperature limits are similar, placement of the devices along the flow
path is not critical. However, if there are devices that have different heat fluxes and different temperature requirements, their
placement becomes quite important. Devices that can tolerate
higher temperatures should be placed near the exit end, whereas
devices that have a lower temperature limit should be placed
near the entrance region of the coolant channels.
In an actual design, the heat flux and total heat load have
different significance. In order to maintain the design junction
temperatures, a higher heat flux in a given footprint would require a lower thermal resistance of the conduction path in the
heat sink and a larger hA (heat transfer coefficient times heat
transfer surface area) in the coolant channels. This has a direct
implication on the channel size and number of channels covering the footprint of a device in an electronic component. The
total heat load, on the other hand, dictates the mass flow rate
and the coolant temperature rise. Thus, for the case where heat
load is large and heat flux is high as well, the coolant channels
need to be smaller, with a shorter pitch between adjacent parallel channels. Additional factors that are available to a designer
are the aspect ratio of the channels (deeper channels providing
larger heat transfer surface area) and enhanced channels, with
internal fins, such as microfins, offset strip fins, or other folded
fin configurations.
Design Issues for IGBTs
The manufacturers of IGBT devices have incorporated a
number of features to improve the heat transfer from the devices to the base plate. Some of these include incorporating a
higher thermal conductivity ceramic, such as aluminum nitride,
between the power chip and the cold plate. Sometimes the base
heat transfer engineering
923
Figure 7 Different IGBT units, counterclockwise from lower right: two-bolt,
four-bolt, four-bolt, and eight-bolt units. Courtesy Powerex Corp.
plate itself is used as the heat sink for direct heat removal with
built-in coolant passages. Use of internal fins in the passages,
microchannels, or a graphite foam mix further improves their
thermal performance.
IGBTs come in different mounting configurations. As the
number of mounting holes increases from two to eight, as shown
in Figure 7, the location and number of transistors and diodes
increase, as does the heat generated.
Two-Bolt Devices
A single cooling path under the center of this device may
suffice if the load is small or a higher temperature is allowed,
around 75 to 85◦ C. Since the two bolt holes are most often
centered on the longest ends, positioning a single path along
the length of the IGBT may not be possible (since the bolts
themselves will interfere). Further, a single path running perpendicular to the long axis may not provide cooling to the target
temperature, and several path turns may be needed to obtain the
necessary heat removal rate.
The next choice is multiple paths in a uniform direction or
counterflow direction to reach the target temperature. Creating
multiple channels under the device is accomplished by machining coolant flow paths or by placing folded fins in a cavity.
There are small copper extrusions that are employed for small
heat loads, but are more expensive and not suited for large loads,
and hence not treated in this article.
Four-, Six-, and Eight-Bolt Devices
As the power dissipation increases, the size of IGBTs increases and the number of bolts used also goes up. Four-, six-,
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S. G. KANDLIKAR AND C. N. HAYNER II
and eight-bolt IGBTs share the same concerns listed earlier. The
difference is now that the power to dissipate may range up to
7000 W or higher. With the largest IGBTs measuring some 100
to 150 mm, placing the cooling channels at the correct locations
is paramount. Each IGBT manufacturer uses a grid pattern to
locate the transistors and diodes. Generating this pattern will
facilitate the subsequent design of the cooling channels.
The thermal resistance of the cold plate can be estimated using the standard heat transfer calculation methodology widely
available in heat transfer literature. However, there is little information available for modern-day transistors that are embedded
within silicone, potting epoxies, and plastic moldings. A more
realistic method is to determine the desired cold plate temperature for a particular task and then to specify the maximum
allowable cold plate temperature. For any cold plate there will
be a hot spot generally located under the center of the device,
or slightly off center in the direction of the fluid flow (due to
fluid temperature rise resulting from heat addition). Since this
hot spot is the likely location of failure of the mounted device,
the design goal is to lower the hot spot temperature to the specified limit. If an average cold plate temperature is used in the
design, the device may function but may result in a reduction in
expected product life.
In order to minimize the hot spot, or hot spots if multiple
devices are to be mounted, the following several questions must
be answered.
7. If the inlet and outlet are shown, or desired to be on the same
edge, are there any other piping arrangements that would
enhance the cooling performance of the cold plate? Usually
the same edge location costs more since space is needed for
a return path by milling, deep drilling, or external piping.
8. Is there a group of devices that need series cooling, or may
they be arranged for the liquid to flow under each individual device through a manifold? For the series path, the last
device’s hot spot is the one that becomes the target for temperature control as described earlier. For the parallel paths, a
larger cold plate is necessary for inlet and outlet manifolds,
and may result in higher average temperatures for all devices
since the liquid flowing per device is less.
9. Can the devices be rotated 90 degrees to allow more surface
area to contact the cooling paths directly? Often the electrical
design is inflexible, but an early discussion may allow this
change.
10. Can any of the devices be moved so the hottest ones are
shifted to the front, or conversely can the fluid enter from a
different side where the hottest devices are located? Often
the layout is fixed, but a small shift in location to allow for
deep drilled paths is often possible.
11. Is the customer committed to an aluminum product or will
a copper solution be accepted? Better performing thermal
properties of copper are often needed for the higher heat
loads presented.
12. Is it possible to include any design-specific items that are
uncovered in discussions with the initiating designer? Milled
slots, cavities, or large holes introduce thermal resistances
in the heat flow paths and must be carefully accounted for.
1. Are the outside dimensions of the cold plate known? This
may determine not only the mounting and space for drilling
holes or for milling paths and manifolds, but also the type
of liquid cooling that can be applied.
2. Where are the device bolt holes and solid connections to the
cold plate located? The depth of these holes may require a
thicker base and therefore may cost more. With multiple devices (up to 12), with six or eight bolt holes each, a multitude
of potential leak paths will exist.
3. Is pumping more fluid through the cold plate an option? This
is often not possible since the planned pump has only minimum power allocated to it, and a more detailed optimization
at the system level (entire system served by the pump) may
be needed.
4. Is the target temperature realistic, or an aspired result that
may not be achievable? A discussion with the submitting
engineer may provide a changed answer. Once the target is
known, a first-pass solution can be made.
5. Can the fluid type be changed to provide better cooling?
Again this is often dictated by the marketing factors or by a
customer’s system requirements.
6. If only a single inlet and outlet are requested, are multiple
inlets or outlets possible? This could allow the coldest fluid
to be directed toward the predicted hot spot(s).
CFD modeling plays an important role in the thermal design
of cold plates. When the thermal designer has a good handle
regarding the issues just listed, it is recommended that a first solution pass be made with a CFD program [e.g., 18]. The target
temperature requested is compared with the predicted hottest
spot, along with allowable pressure drop and the cooling fluid
inlet temperature at a given flow. If any of the required parameters are not met, solutions can be sought by altering many
parameters. These include possible cooler inlet temperature,
higher fluid flow rate, material change, mixing cold plate materials, more aggressive fin configurations, rerouting fluids, or
requesting a target temperature change. More often than not,
a solution can be arrived at through in-depth discussions with
the customer (or electrical engineers). The customer will hopefully recognize reality and make allowances to continue toward
a buildable product. For example, the IGBT may not put out 700
A continuously, but would have reliable deliverable 600 A over
long hours of usage.
Any time a double-sided load is presented, a number of additional issues arise. Are the heat loads to be dissipated and
their locations the same for both sides? The exact locations
of the loads are critical since the straighter the tube runs,
the less costly is the construction. Are there different target
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Advanced Cooling System Design Strategies
heat transfer engineering
vol. 30 no. 12 2009
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S. G. KANDLIKAR AND C. N. HAYNER II
temperatures per side? Is there a range of target temperatures on
each side based on different devices? Are there duplicate, parallel, or counter flow paths? Are there single or multiple inlet
and outlets? Where are the inlet and outlet—opposite ends, at
90 degrees, or on the same end or side? Each required parameter
dictates the direction of a thermal solution.
Will the cold plate be placed inside of a cabinet or frame for
support, or will the cold plate be a support structure for multiple
devices? The answer will affect the weight of a part by allowing
material for mounting-bolt holes along the sides or corners of
the cold plate. If used to support multiple IGBTs, stiffness in
the cold plate may be important and the plate may require more
mass, to allow connected copper electrical buss work to stay
positioned.
When a cold plate is to be used in any portable project—
person-carried, airborne, or vehicle—total weight becomes a
critical feature. Therefore the first design scenarios should start
with an all aluminum construction. Often aluminum will not
achieve the target junction temperatures so a mixed metal solution may be required. If the devices are mounted on a copper
machined fin set, an o-ring gasket may seal the liquid flow path,
preventing dissimilar metals from touching, thereby keeping the
weight to a minimum.
Manufacturing Issues
Manufacturing issues are often overlooked by the thermal
designer, but play a critical role in practical implementation of
the design. Some of the issues related to manufacturing considerations are presented in this section. This section is intended to
make the thermal designer aware of some of the manufacturing
issues, not to present a comprehensive summary.
When a tube is bent or folded to make a parallel return path,
the bend radius must allow for full flow and not be crimped.
Tube bending mandrels are often needed to create the least
deformation of the tube at the return bend. With a medium wall
thickness of about 0.9 mm (0.035”) a 9.5 mm (3/8”) OD tube can
be formed into a return path with 25+ mm (1+”) centerline-tocenterline dimensions. Limiting deformation is most imperative
when the top surface of a cold plate is fly cut to achieve an overall
flatness for device mounting. If too much metal from a tube bend
radius is removed or if it was crimped, a thin tube section and
probable leak location may occur. Once made, all cold plates
must have 100% testing to assure leak-proof functionality.
Drilling small holes for thermocouples in the cold plate under
the predicted hot spot is very costly, since they will often need
to be several centimeters deep. Therefore, surface mounting
thermocouples on isothermal lines next to the device will be
able to check the hot-spot temperature (predicted from CFD
analysis if conducted during design stage), although these types
of measurements are subject to a number of measurement errors
(e.g., contact resistance, altering local coefficients). Thermal
imaging can also determine the temperatures along the edge
of the device if the view is clear, and thereby predict if the
heat transfer engineering
925
target temperature has been achieved. The best testing is most
often done in the customer’s laboratory, where use test or field
conditions can be duplicated.
Manufacturing Cost Considerations
When a part must have a machined liquid flow path for obtaining the correct target temperature, several methods of construction must be compared. The cost of a machined path must
be compared with building a pocketed set of fins. The machine
setup time, with machine speeds and feeds, requires a specific
amount of time to create the appropriate paths. This milling
machine has an hourly cost to arrive at a specific part cost. One
must assume that there will be some percentage of scrap created,
added to the cost. There will be set up time and machining time
necessary for pocketing a set of folded fins. This is added to the
cost for the required fin set, scrap expected, for a comparison
part cost.
For every Critical to Function or Critical for Design designation on such items as hole locations, hole depths, bosses, milled
paths, pocket depths, edge features, or drawing notes, there will
be a corresponding cost associated with creating or providing
those features. Most likely there will be a need for a fixture that
tightly positions the part, and an extra programming charge to
carefully drive the milling machine. In each case just described
there will be a cover that needs to be soldered to the base, with
some additional cost.
For the pocketed folded fin set, care must be taken during
manufacture to insure that all the flow paths are maintained
and that no solder runs down into the channels and blocks the
flow paths. This is less of a problem for the machined path
solution. There is often no clear answer for which direction is
preferable since the production quantity affects all machining
costs. Setup time for running the entire job at one time amortized
over the total number of parts. Parts that have a series of machine
runs require multiple machine set ups amortized on smaller
quantities, and therefore are more costly.
Standard machine tools can be purchased for nominal prices
and are used for creating the machined flow paths, pockets, and
tube attachments. If, however, a modified or nonstandard tool
size is required or specified, the machine tool costs could go
up by a factor of six and require a larger number of these tools
purchased to allow for any breakage. Sometimes a wider flow
path can be created by running a smaller tool in two passes to
create the correct width. However, this affects the cost by adding
machine time.
There are several sets of fixtures needed for any machining
of production parts. First, the material must be held for precise
replication. Second, fixtures for side features (if any) are also
necessary. All features such as holes, cavities, bosses, slots or
trenches, and dowel pin holes require exactness of location.
Some holes may be necessary for T-bolts to hold the fixture to the
milling machine base. The fixturing will consist of locating pins,
blocks, multiple clamps, and other necessary machining. There
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S. G. KANDLIKAR AND C. N. HAYNER II
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may be fixtures created to hold various components together for
final assembly. They will include locating pins, blocks, multiple
clamps, supports, or braces to allow uniform heating and prevent
warping when heated.
Flatness of parts is a critical issue as well. Generally a flatness
ratio of 0.001”/” (inches per inch length, or mm/mm length) is
an acceptable standard for machined cold plates. This is acceptable for most electrical devices, since there will be an interface
material of some thickness, between the cold plate and the device. Usually about 100 to 150 µm (4 to 6 mils) thick, these
materials allow for close contact between surfaces. For a tighter
requirement, .0005”/”, there will be a corresponding cost increase. As the requirement gets closer to dead flat, more precise
manufacturing processes are needed. Blanchard grinding is the
final most expensive step.
Folded Fin Sets
If machined path fins are not able to achieve the target temperatures due to the large number of fins (surface area) required,
the alternative is folded fin sets. These will be set into a pocket
or cavity with a manifold needed to avoid maldistribution. The
type of folded fin sets are straight fin, herringbone fin, wavy
fin, square fin, lanced fin, offset fin, triangle fin, perforated fin,
ruffled fin, and rounded fin. The thickness of the desired fins dictates the fin choices. All of these types are available with short
fin heights, short flow lengths, and very thin material. Since removing significant amounts of heat requires rather large surface
areas and greater thickness, fin type selections are limited.
For example, if 1 cm tall fins of 0.6 mm (0.25”) thickness
are specified at 8 fins/cm (20 fins per inch) and are 380 mm
(15”) long, the manufacture of these fins may not be possible,
even if the CFD program created this answer. Therefore another
solution needs to be found to achieve the target temperature.
This is where compromises get made, such as higher pressure,
shorter fins, or a different fin set. An entirely new design may
be needed to meet the required target temperature.
The most common choice is straight folded fins of a given
height, pitch, thickness, length, and cutoff or overall width. If
these fins are to be pocketed between the base and cover, an
addition of two fin thickness to the height will reduce pressure
drop across the fin inlet and outlet.
A second common choice is a ruffled or lazy ruffled fin set.
Again the height, pitch, material thickness, length, and cutoff
are specified. The same pocketing applies. However, these fin
sets have a smaller manufacturing tolerance, which makes cold
plate construction easier. However, it is recognized that this
fin will create a higher pressure drop. The additional surface
area gained by the ruffling makes them better for higher heat
transfer duties. Often a compromise with the thermal engineer
will enable ruffled fins to be used with the attendant higher
pressure requirement.
All of the items just listed may become part of the cost for
a particular solution. The optimum thermal solution is based
heat transfer engineering
on the heat to be dissipated, number of devices to be cooled,
their location on the cold plate, material, fluid type, fluid flow
rate, inlet temperature, and pressure drop, weight, and plate
temperature targets. That is why no two cooling solutions are
quite alike.
SAMPLE CASE STUDIES
A few sample case studies are presented here to show the
selection of the cold plate type and placement of the channels
in different applications.
Power Drives
Conventional three-phase motors powering fluctuating loads
operate at a given voltage and amperage. Whenever they operate
at lower or reduced loads they become inefficient by consuming
more energy than necessary for the load presented. The application of IGBTs to motor drives offers a more efficient operating
method. Circuit control of the IGBTs will only draw as much
amperage or energy at a given voltage as is required for the
load presented. Even so, the IGBT is not 100% efficient, so
any excess heat generated to create the step wave form must be
removed for steady operation.
In Figure 8 there are four 8-bolt 4800-W IGBT devices
mounted on a cold plate. This is one of three cold plates used in
the system. There are also three 400 W devices centered in the
middle of the cold plate. This design required cooling to 75◦ C
on the surface of the second device in series. This could only
be accomplished by mounting ruffled folded fin sets pocketed
in the base and by creating manifold like flow paths in and out
of each fin set. Fluid flow was also channeled below the center
devices to remove 400 W each, above a soldered cover.
In a clamshell design, two small copper plates are machined
with diagonal grooves, as shown in Figure 3, allowing for an
internal manifold spreading and collecting the fluid. The cooling
fluid flows up and down repeatedly as it crosses the cold plate,
so this design could easily have a double-sided load of IGBTs.
Leak testing is required to ensure long-term performance.
Figure 9 depicts another power drive cooling channel configuration. Ten parallel paths are milled in the base for 10 flat tubes
to be pressed in and fly cut to the required flatness. The total
dissipated heat load was 8900 W. The external inlet manifold
Figure 8
IGBTs.
Cooling channel configuration for a cold plate serving four 8-bolt
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S. G. KANDLIKAR AND C. N. HAYNER II
927
RF Generators
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Figure 9 Flat tube manifold pressed into aluminum base plate.
provided the coolest fluid to each IGBT. The dies were located
so that the hottest spots could be targeted with the flat tubes.
SCRs are also widely employed in high-power electronic
systems. They are very sensitive to high temperature, as shown
in Figure 10. SCR efficiency drops drastically beyond 125◦ C.
When cooling SCRs, the design must provide adequate cooling
to limit the hot spot temperatures, which usually occur near the
center of the device. Most electrical designs using SCRs will
have multiple devices in banks or racks. Therefore the lowest
temperature coolant must be supplied to each device present for
uniform performance.
Conveyor Systems
In mining operations moving ore to a processing plant is
often done by 5000+ HP motors. Efficient motor drives that
follow this type of loading, unloading, and varying loads are
powered by SCRs. Cooling of these drives has been done with
self contained liquid systems coupled to an air cooled radiator,
since water is often scarce.
Figure 10 SCR voltage–temperature performance [20].
heat transfer engineering
Typically RF generators have a large number of devices
mounted on a cold plate. A deep drilled design shown in Figure 2
is appropriate for this case. Many small mounting holes are located very close to the 224 mm long paths. This cold plate can
remove 3600 W. Since the cold plate is only 12 mm (0.46”)
thick, many of these plates are stacked in a cabinet, maximizing the space available for the RF devices. Proper coolant flow
to each cold plate is accomplished through external manifolds.
Typically these cold plates are the mounting structure for the
electronics, so the weight of the cold plate is not a concern.
FLOW MALDISTRIBUTION
Flow maldistribution in the parallel fluid flow passages is a
major concern, as it may degrade the thermal performance of
the cold plate below the acceptable limit. Lu and Wang [43] and
Liu et al. [44] present a detailed analysis on the effect of inlet
and outlet locations on the flow maldistribution in a cold plate.
As an illustration, a CFD analysis of flow maldistribution was
carried out and is shown in Figure 11 for the flow arrangement
shown in Figure 9. There are in total 10 parallel channels. Each
channel is 9 mm × 33 mm and 241 mm long, and the header is
18 mm in diameter and 812 mm long. For the inlet and outlets
on the opposite sides, CFD simulation was carried out for a
total water flow rate of 0.085 kg/s. Figure 12 shows a similar
analysis for the case shown in Figure 9, but with the outlet on
the same side as the inlet. Both these figures show that the flow
is significantly skewed. Water exiting from the lower flow rate
channels will be at significantly higher temperatures, resulting
Figure 11 Normalized flow rates in individual channels for cold plate shown
in Figure 9 with inlet and outlet on the opposite sides.
vol. 30 no. 12 2009
928
S. G. KANDLIKAR AND C. N. HAYNER II
q Q
R
T
heat flux at the exit, W/m2
total heat dissipation, W
flow resistance in the cold plate
fluid temperature
Subscripts
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eq
exit
f
in
out
w
equivalent
at the exit section
fluid
channel inlet
channel outlet
wall
REFERENCES
Figure 12 Normalized flow rates in individual channels for cold plate in
Figure 9 with inlet and outlet on the same side.
in temperature overshoot of the devices mounted directly above
them.
Flow maldistribution has the potential of causing a catastrophic failure of a cold plate. It is highly recommended that
each design be analyzed using a CFD program [e.g., 18]. Since
flow maldistribution depends on the actual operating conditions
such as temperature and flow rate, such simulation under different operating scenarios is needed for a reliable design throughout
the operating range.
CONCLUSIONS
A brief overview of liquid cooled cold plates is presented.
Based on the current practice, a classification scheme is introduced to identify cold plate types. Liquid cooling of high-power
electronic devices requires unique solutions, a few of which
have been presented in this article. Each task has its own target
temperatures and design constraints, which provide challenges
for the thermal designers. Some of the thermal design issues,
manufacturing constraints, and cost considerations have been
presented. A decision tree of design choices is outlined in the
process for arriving at an optimum design. The inclusion of a
thermal engineer at the outset of a new or upgraded project will
result in the best design at the least cost.
NOMENCLATURE
h
ṁ
p
heat transfer coefficient, W/m2 K
coolant flow rate, kg/s
pressure drop in the cold plate
heat transfer engineering
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vol. 30 no. 12 2009
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S. G. KANDLIKAR AND C. N. HAYNER II
1997 and the Trustees Outstanding Scholarship Award in 2006. Currently he
is working on a DOE-sponsored project on fuel cell water management under
freezing conditions.
Satish Kandlikar is the Gleason Professor of Mechanical Engineering at Rochester Institute of Technology (RIT). He received his Ph.D. degree from the
Indian Institute of Technology in Bombay in 1975
and was a faculty member there before coming to
RIT in 1980. His current work focuses on the heat
transfer and fluid flow phenomena in microchannels
and minichannels. He is involved in advanced singlephase and two-phase heat exchangers incorporating
smooth, rough, and enhanced microchannels. He has
published over 180 journal and conference papers. He is a Fellow of the ASME,
associate editor of a number of journals including ASME Journal of Heat Transfer, and executive editor of Heat Exchanger Design Handbook, published by
Begell House. He received the RIT Eisenhart Outstanding Teaching Award in
Clifford Hayner earned his B.S. in mechanical
engineering from the University of Rochester and
joined the U.S. Navy. After the Navy, he worked
for Rochester Gas & Electric Corporation for over
30 years. He primarily helped in solving the energy
needs of major industrial customers. Throughout his
career at ERM Thermal Technologies Inc. (now Vette
Corp.) he has designed over 800 successful thermal solutions. He is now retired from the industry and
works as a thermal solutions consultant in Rochester,
New York. He was the president of the local chapter of Toastmasters International, president of the Irondequoit Chamber of Commerce, and is a member of
the Otetiana Council and on the Board of Directors of the Rochester Chapter of
ASME.
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Plate subject to Inlet Locations, Journal of Enhanced Heat Transfer, vol. 14, no. 1, pp. 65–76, 2007.
heat transfer engineering
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