High Heat Flux

Space Constrained

Harsh Environment

Application NoteHeat Sink Design Using Two

Phase Devices

Contact Informationwww.celsiainc.com

gmeyer@celsiainc.com

Heat Sink Design Using Liquid Two-Phase Devices: Heat Pipes & Vapor Chambers

At some point in a thermal system design project it

may become apparent that the basic methods of

increasing thermal efficiency - solid base, fin & fan -

just aren’t sufficient. Reasons include:

• Keep out zones prohibit a larger heat sink (thicker

base, added fin area, etc).

• Enclosure size and/or airflow can’t be increased.

• Transitioning to a solid copper heat sink, in whole

or in part, adds too much weight and in some

cases too much cost.

• Component power /density necessitates heat be

moved to a remote location more than 40-50mm

away from the heat source.

Regardless of the reason, most thermal engineers are

going to need a two-phase cooling solution using

either heat pipes or vapor chambers on numerous

projects with which they are involved. But, which one

is likely the best choice? This application note provides

a topline overview of structural differences and

thermal design considerations between these very

similar yet somewhat unique two-phase devices.

It probably goes without saying, but the operating

principles of all two-phase devices are identical. A wick

structure (sintered powder, mesh screens, and/or

grooves) are applied to the inside walls of an enclosure

(tube or planar shape). Liquid (usually water) is added

to the device and vacuum sealed at which point the

wick distributes the liquid throughout the device. As

heat is applied to one area, the liquid turns to vapor

and moves to an area of lower pressure where it cools

and returns to liquid form whereupon it moves back to

the heat source by virtue of capillary action. In this

sense, heat pipes and vapor chambers are the same

thing.

For simplicity’s sake, let’s focus on the most common

type of two-phase device: an all copper vessel, using a

sintered copper wick structure with water as the

working fluid.

Figure 1 – Inner Workings of a Two-Phase Device

Application Note

Two-Phase Device Thermal Resistance

The most commonly asked question regarding the

design of a heat pipe / vapor chamber cooling solution

is - what is the effective thermal conductivity (W/m-K)

of the device? 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. A third resistance, the

condensation resistance, is much smaller than the other

two. In the vast majority of applications, the

evaporation resistance is the dominate one.

Therefore, these devices are somewhat length

independent. This means that a two-phase device with

a transport distance of 75mm will have almost the same

Tsource -Tsink as one with a 150mm 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 with standard designs and 500-

1,000W/cm2 with custom modifications.

The vapor transport resistance is expressed in similar

terms, but refers to the cross sectional area of the

vapor space. Keep in mind, changes in temperature or

working fluid will change these values. The values

presented are typical values for a water-based vapor

chamber operating at electronics cooling temperatures.

This resistance is 0.01 C/W/ cm2.

The next chart shows common vapor chamber cross

sections of 2.0mm to 3.5mm thicknesses and widths

from 20mm to 80mm. The cross sections are calculated

and the terms expressed in simple C/W for each size.

Chart 1 – Typical Two-Phase Evaporator Resistance

Chart 2 – Vapor Chamber Typical Vapor Resistance

Structural Differences & Thermal Design Considerations

Heat Pipes

For decades, heat pipes have been the default two-

phase device of choice for thermal engineers due

largely to the cost difference relative to vapor

chambers. They were used both for heat transport, for

which they still have an advantage, and for heat

spreading, typically using multiple pipes in close

proximity to one another. For lower power applications,

perhaps requiring only a single, small heat pipe, or

those where heat must be effectively transported, heat

pipes still maintain dominance due to their low cost and

design flexibility.

Figure 2 – Notebook Heat Pipe Application (Wikipedia)

Greater total power and power densities eventually led

to multiple heat pipes being used to solve the thermal

challenge. Both images below are heat sinks for a small

form factor, high performance desktop PC. The one on

the left uses a copper base plate in between the heat

source and the heat pipes, as is common with heat pipe

applications (indirect contact).

As processor heat increased in the subsequent

generation of this product, the company encountered

thermal issues, but did not want to radically redesign

the thermal solution as can be seen on the image to the

right. Here a vapor chamber replaced the copper base

plate, spreading heat more evenly across the heat

source and transferring it more effectively to the heat

pipes. This is a great example of how both types of two-

phase devices can be used together.

A potential alternative to this problem might have been

to implement what some call ‘direct contact’ heat pipes.

But, this solution has its drawbacks as well. As seen in

Figure 5, this design option uses slightly flattened and

machined heat pipes cradled in an aluminum mounting

bracket to make direct contact with the heat source.

While eliminating the base plate and additional TIM

layer – decreasing thermal resistance – it doesn’t

spread the heat as effectively as a vapor chamber

solution.

Traditional (2-Piece) Vapor Chamber

Most manufacturers of vapor chambers use a

traditional two piece design. While studies and practical

application shows that the performance of heat sinks

using vapor chambers can be enhanced by 20-30% over

their heat pipe counterparts, a two piece design has

cost implications of roughly the same magnitude versus

a multiple heat pipe configuration. Nonetheless, vapor

chamber usage has grown with the increasing power

and power densities of today’s devices.

Because they do an incredible job of spreading heat,

allow for low profile heat sinks, can be made into

virtually any shape, embossed, and make direct contact

with heat source, these devices are used in a wide

variety of higher power applications.

Figure 3 – Heat Pipes with Copper Base Plate (ixbtlabs.com)

Figure 4 – Vapor Chamber Replaced Copper Plate

Figure 5 – Direct Contact Heat Pipes (silverstonetek)

Figure 6 – Two-Piece Vapor Chamber Cooling ASIC Array

As mentioned earlier, the increased cost of this design

sometimes limits its incorporation into thermal

solutions. Another potential drawback is that there’s

little design flexibility in the z-direction. Making a U-

shape for instance, while conceivably possible, would

be impractical from a manufacturability/cost

perspective.

Hybrid (1-Piece) Vapor Chamber

Available from a growing number of manufacturers,

one-piece vapor chambers are a cost reduced version of

their two-piece counterparts, yet maintain the thermal

performance characteristics while adding some unique

capabilities (e.g. U-shape bending).

Like heat pipes, a one-piece product begins its life as a

single copper tube, hence the 1-piece moniker. Like

traditional two-piece designs, one piece vapor

chambers make direct contact with the heat source,

have a multi-directional heat flow, and can support

clamping forces of up to 90 PSI. But they’re less

expensive to produce because they require less tooling,

don’t use individual support posts, and don’t have to be

welded on all four sides. Below are a few examples of

one-piece vapor chambers.

The thing to remember about two phase devices is that

heat pipes favor moving heat over spreading it, while

the reverse is true of vapor chambers. For sure, there

are numerous thermal challenges where either could be

used with good results so it’s important to do a

thorough review process of both designs for settling on

one.

Two-Phase Thermal Review Process

A typical, although not ideal, thermal design scenario is

one in which several key variables are already defined.

These include enclosure shape / size, component layout

(with associated keep out zones) and airflow, as well as

the total power, power density, and size of the heat

source(s).

Step #1 – Start by looking at fin location

Given these constraints, the focus is on the heat sink

itself and one should start by understanding if the

condenser is remote or local to the heat source.

• If remote by more than 40-50mm begin the

investigation with heat pipes. Design flexibility is

high allowing bending and flattening to conform to

almost any shape in all three dimensions.

• If local and copper alternatives have been ruled

out then a vapor chamber solution that spreads

the heat is the best starting point provided the

perimeter ratio of the heat sink to the heat source

is greater than 30:1.

Figure 7 – Possible Shapes of 1-Piece Vapor Chamber

Figure 8 – 1-Piece Vapor Chamber Heat Sink Assembly

Step #2: Run an Excel model to determine heat exchanger performance

Based on this input a simple excel model should be run

to determine the performance of the heat exchanger.

This tells us how much of the thermal budget - fin to air

and air temperature rise – are being used. This provides

information on how much is left for conduction and

interfaces. Since these two components, fin to air and

air temperature rise, are the largest resistances in the

system, the tail that wags the dog, a design review at

this point is normal to optimize the area for the fins and

the air flow and pressure drops.

• If the remaining thermal budget for conduction is

less than 10oC, a look at copper or heat pipes. For

small devices at low powers a single heat pipe is

often sufficient. With the total power we can

estimate the number of heat pipes required to

carry the heat. For example, a single 6mm heat

pipe can carry the power from a 45 watt device.

• If it is less than 5oC then a vapor chamber may be

required. For small devices at high powers typically

a vapor chamber is the best solution. Vapor

chambers, on the other hand, generally are not

run to their performance limits so they are sized to

cover as much of the base of the heat sink as

possible. Due to their flat format there is a direct

contact between the VC and the heat generating

component.

The image to the right is the summary page of such a

model for an LED application. In this example two 8mm

heat pipes were compared against a single 15mm wide

vapor chamber of similar cost. Each ‘case scenario’

represents the use of different length fins. As you can

see, the vapor chamber solution provides an additional

4-5 degrees Celsius of thermal headroom, but this delta

is often higher.

Figure 9 – Simple Excel Model Summary Page

Step #3 – Run a more sophisticated Excel model and CFD analysis to optimize the design

Available soon on Celsia’s website, but also downloadable from several other sources, a more comprehensive heat pipe

and vapor chamber modeling solution will aid in refining the two-phase design as it accounts for changes to wick

characteristics, vapor space, wall thickness, working fluid, case metal, and orientation.

Subsequently, CDF modeling is often used to determine the performance of variations to the full heat sink assembly.

However, sometimes the best use of time and money is simply to prototype and test a couple of thermal module

iterations.

Figure 10 – Advanced Excel Model Summary Page with CFD Picture

About Celsia Inc.

Established in 2001

Celsia’s client roster includes

Fortune 500 and mid-market

companies. In recent years, it

has shipped over 2 million

solutions to a global customer

base and has been granted

over 70 patents.

Fast Turnaround Times

Two to four week fulfillment

times for design modeling,

prototypes and production

orders. Existing designs are

quoted within days.

Quality First

Take the guess work out of

sourcing from Asia. Celsia’s

wholly owned Taiwan

operation is ISO 9001 certified

with QC procedures that are

among the tightest anywhere.

Solving DifficultThermal Challenges

Our design engineering,

manufacturing, and worldwide

distribution teams are

experienced at delivering

thermal solutions for the most

demanding environments.

Liquid Two-Phase Heat Sink Experts

Through its US headquarters and Taiwan design & production facility, Celsia Inc.

specializes in custom heat sinks using liquid two-phase devices.

High Heat Flux

Constantly Innovating

Harsh Environment Space Constrained

One Piece Vapor Chambers

Affordable, thin and bendable two-phase heat

spreaders designed to provide a cost comparable

yet thermally superior alternative to heat pipes. As

thin as 1mm with length and width dimensions as

large as of 300 x 100mm.

Two Piece Vapor Chambers

Limitless design options and impressive heat

spreading capability make these devices ideal for

one or more heat sources up to 500 W/cm2 and

those applications where the heat sink must

effectively work against gravity.

Purpose Build Heat Pipes

Design flexibility, unmatched power handling, and

the ease with which they can be integrated into

hybrid designs make Celsia heat pipes the most

versatile two-phase device on the market.

Solutions as Unique as our Customers

We tune product shape, wick characteristics, wall

thickness, working fluid, case metal, and fin

structure for each application in order to meet

tough thermal performance and packaging

requirements.Spacer

WickCase

Website: celsiainc.com

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