heat pipe reliability in high power applications

Power Electronics Technology August 2004 www.powerelectronics.com40

Heat Pipe Reliability inHigh-Power Applications

A well-designed, carefully built and appropri-ately applied heat pipe can achieve an operat-ing life in excess of 15 years. With short-term,high- temperature screening and numericalanalysis, heat pipe life can be predicted.

By CCCCCh rh rh rh rh ristististististopher A.opher A.opher A.opher A.opher A. S S S S Souleouleouleouleoule, Engineering Director, Thermshield,LLC, Gilford, N.H.

Over the past decade, the use of heatpipes in electronic cooling applicationshas increased dramatically, primarily innotebook computers. In fact, virtuallyevery notebook computer manufac-

tured today uses at least one heat pipe assembly. Typicallyused to carry less than 25 W of power, these parts are lowin cost and highly reliable.

Use of heat pipes in high-power (>150 W) cooling ap-plications has been limited to custom applications re-quiring either low thermal resistance or having a severelyrestricted enclosure area. The cost of these larger diam-eter heat pipes was high due to a limited number of

manufacturers and handmade assembly times.Enter now the latest generation of IGBT and other semi-

conductor power modules. These modules offer high-power outputs and even more challenging power densi-tiescooling of the modules at full rated output power isvirtually impossible. As in modern microprocessors, theremoval of waste energy in the form of heat has arguablybecome the most challenging engineering task of the me-chanical design effort.

These heat loads and flux densities are so high that inmany cases conventional air-cooled aluminum extrusionand even bonded fin heatsinks will not provide sufficientcooling. Using forced air-cooling, they cannot remove heatfast enough to keep the module from exceeding its maxi-mum recommended junction temperatures. The introduc-tion of a solid copper heat spreader (copper has 2X theconductivity of extruded aluminum) into the base of anextrusion also will not suffice.

Historically, some high-power systems have used heatpipes to enhance base-plate heat spreading in differentmodes as the solution to keep systems using air-coolingand keep them away from liquid cooling (Fig. 1). A heatpipe has an apparent conductivity many times greater thancopper and relies on the latent heat of vaporization of aworking liquid inside a heat pipe to operate.

Reliability and longevity of these closed-loop coolersand the systems they are used in now become a large issue.What will happen if the heat pipe stops working? WhatMTBF can be expected from these large diameter heatmovers? Is there any way to ensure they keep working upto and beyond the expected life of the system?

Heat Pipe OperationBasically a heat pipe is a partially evacuated, closed ves-

sel that recirculates a small amount of working fluid, whichthrough the addition of heat, changes from liquid to gas.

Fig. 1. The extreme heat loads and flux densities experienced by thelatest generation of semiconductor power modules demand the use ofheat pipes to enhance base-plate heat spreading. When heat pipes aremounted in the Z-axis of the heatsink, perpendicular to the base-mounting surface (as shown above), they can boost fin efficiencies bynearly 100%.

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HEAT PIPE RELIABILITY

Condensing that gas back to a liquid and releasing the ab-sorbed heat requires additional cooling surface or othermeans of heat removal.

In operation, a heat pipe absorbs significant amountsof heat in the evaporator section as it reaches a set tem-perature. The working fluid and the partial pressure in-side the pipe set this temperature. Heat of vaporization ofthe liquid allows for high quantities of heat to be absorbedat a given temperature. This is similar to liquid water at100C vs. steam at 100C. The additional heat absorptionis required to change phase.

Heated gas moves to the cold end of the heat pipe atnearly the speed of sound and under nearly isothermalconditions. At the condenser section the gas cools slightly,releasing the heat gain, reverting back to a liquid. This en-tire cycle usually happens with lessthan a 5C differential from oneend of the pipe to another.

Operation takes place at virtu-ally the same temperature anddoes not depend on where theheat enters or leaves the heat pipe.Depending on pipe diameter, thisprocess can move hundreds of watts a distance of manyinches, offering an apparent thermal conductivity of thou-sands of W/mK (Fig. 2).

Heat pipes offer many advantages in their use and op-eration. First, a heat pipe by itself does not remove or dis-

sipate heat. It is only a conduit through which heat can bemoved from one point to another with a low thermal re-sistance. To make it operational, a heat pipe must have asmuch cooling surface area in form of fins as the equiva-lent-size air-cooled heatsink without a heat pipe. It must

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Heat pipe diameter - mm (inch)

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Fig. 2. Heat pipe capacity as a function of diameter.

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To make it operational, a heat pipe must have as muchcooling surface area in form of fins as the equivalent-size air-cooled heatsink without a heat pipe.



also have a high conductivity thermalcontact to the heat source to bringheat into the heat pipe.

Heat pipes are orientation-sensi-tive in relationship to gravity. Heatpipes will carry large amounts of heatwhen they operate in a heat-down,cooling-up attitude. This orientationallows for the rapid return of cooledliquid to the evaporator. Heat pipesalso will operate with little loss of con-ductivity in a horizontal attitude.However, operation in a heat-up/cool-

ing-down orientation must be care-fully engineered. Depending on thestyle of wick or liquid return capillary,most heat pipes will lose some effi-ciency.

Z-axis Heat RemovalMost high-power heat pipe appli-

cations have used heat pipes or vaporchambers (a flat heat pipe) to helpheat spreading under the base of apower module. Many times a series ofround heat pipes are embedded in a

heatsink base to help average the tem-perature of the aluminum mountingplate.

Although this is a positive step, itstill leaves conventional air-cooled finsor extended cooling surfaces operat-ing at fin efficiencies often as low as50% to 60%. Heat pipes used in theZ-axis, perpendicular to the base-mounting surface, can offer fin effi-ciencies approaching 100%.

Fig. 3 shows a comparison of finefficiencies and overall thermal per-formance for three types ofheatsinksa high ratio extrusion, abonded-fin heatsink, and a heatsinkwith a heat-pipe assembly mountedin the Z-axis.

For an extrusion with a fin area of1X, the fin efficiency is 70% to 90%,and the overall thermal performanceof the heatsink is 100%. For a bondedfin heatsink of the same size, a fin areaof 1.5X is achieved, which produces afin efficiency of 60% to 80% and anoverall thermal performance of 150%.In the case of the heatsink with heat-pipe assembly, a fin efficiency of 3X ispossible. This heatsink achieves a finefficiency as high as 90% to 95% andan overall thermal performancegreater than 200%.

Heat pipes with the ability to moveheat with near-zero temperature riseare employed as conduits to eliminatethis fin efficiency problem. Z-axiscooler design uses large diameter heat

(b) (C)(a)

Fig. 3. A comparison of fin efficiency for a highratio extrusion heatsink (a), a bonded-finheatsink (b), and a heatsink with heat pipeassembly embedded in the Z-axis (c). Overallthermal performance varies from 100% for theextrusion to 150% for the bonded-fin heatsinkto greater than 200% for the heatsink withheat pipe assembly.


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pipes mounted through a base plateheat spreader, perpendicular to theheatsink mounting surface. Heatspreaders integral to the heat pipes arepositioned in the base plate to makemaximum contact to the high heatflux sites (die positions) under thepower module.

The heat pipes move heat awayfrom the base plate and use a series ofthin, copper fins attached to the pipesto dissipate this heat into a forced airstream. Due to the effects of the heatpipe, the copper fin furthest from thebase plate will have virtually the samefin efficiency as the closest fin. Thisallows significant increases in fincount and cooling surface over an ex-trusion type heat sink. In many casesZ-axis coolers can increase cooling ofa power module (IGBT or similar de-vice) by up to 100%.

Heat Pipe ReliabilityOver the past 40 years of heat pipe

design and manufacture, reliabilityand consistency of performance havealways been issues. Do heat pipes leakover long periods of time? Can theycontinue to operate at their limits foryears? What are their limits?

To understand heat pipe longevityand potential failure, it is necessary tounderstand the manufacturing stepsand design for reliability. Virtually100% of prematurely failed heat pipescome from:

Improper cleaning/oxidation ofthe interior.

Improper filling or charging. Poor sealing or potential leakage

over time. Incompatible materials. Overtemperature during assem-

bly.Failures also can be seen as the re-

sult of designers not understandingthe limitations of heat pipes in appli-cation and long-term use. In terms oflongevity, how a heat pipe is appliedis just as important as how it is as-sembled. In short these failure modesare:

Dry-out (high heat loads/heatfluxes).

Improper orientation to gravity.

at the end of life, due to wear-out. Af-ter passing the first few hours of op-eration, a heat pipe will normally op-erate for many tens of thousands ofhours before failure occurs.

In many real-life, high duty-cycleapplications, large diameter heat pipeshave been in operation for more than20 years without failure. These appli-cations include steel-wheel locomo-tive and traction drive, electrically

Sealing/crimp problems. Flex failure due to shaping. Catastrophic failure due to too

high temperature at assembly or inoperation (Fig. 4).

Predicting Operational LifeHeat pipes are similar to semicon-

ductor electronics in that they dem-onstrate higher failure rates at start-up, due to initial infant mortality, and




powered people movers, as well as wind power generators,high-horsepower ac motor drives and region buses.

One predictor of potential heat pipe life is the use ofshort-term, high-temperature testing to induce failure andmathematically predict wear-out. The use of controlledtemperature chambers to accelerate the heat pipe destruc-tion can be used along with predictive models to demon-strate this long-term reliability. As per a recently publishedMTBF model*, the following life model can offer a reason-ably predictive model. Operating life at 60C can be pre-dicted by operation time. This example uses 36 hours asan indicator.MTBF hours of operation at 60C = Test time at 180C 2(DT/10)

MBTF hours of operation at 60C = 36 hours 2(180-60)/10

36 hours at 180C is equivalent life of 147,456 hoursor ~ 17 years at 60C

This type of accelerated life has its roots in the exten-sive work in predicting the life and reliability of semicon-ductor electronics. Based on weeding out infant mortal-ity in populations of electronic components, the same prin-ciples of temperature-induced stress hold true for heatpipes.

Use of burn-in or stress-screening techniques similarto those used for PC boards can be used on heat pipes.

Short-term, high-temperature screening of full productionlots can help to eliminate the infant mortality problemsseen in the first few hours of use. This can increase lot re-liability and virtually eliminate field failures.

The use of and need for heat pipes in high-power ap-plications exists today and is growing as the semiconduc-tor power outputs increase. The use of heat pipes or otherhigh conductivity materials is a growing necessity in state-of-the-art electronic cooling designs. This is true, in part,because of:

Increased heat outputs from power modules isdriving high-technology cooling solutions.

The markets avoidance of water or other coolingliquids due to cost and maintenance concerns.

Innovative use of heat pipes to remove heat in the Z-axis direction from a power module will increase heatsinkefficiency and provide an increased level of air-cooling.

Heat pipe reliability, like air mover reliability, is keyto system reliability.

Service life of a well-designed, carefully built andappropriately applied heat pipe will be in excess of 15 years.

Based on short-term high-temperature screening, heatpipe life can be predicted using numerical analysis similarto semiconductor life. PETech

References1. Hinged Heat Pipes for Cooling Notebook PCs, 13th IEEESemi-Therm Symposium 1997, San Jose, Calif.2. Vukovic, Mirjana and Wakins, John. Reliability Charac-terization of Copper/Water Heat Pipes Below 0C, NortelNetworks, Canada3. Faghiri, Amir. Heat Pipe Science and Technology,Copyright 1995, Taylor and Francis, Washington, D.C.

Fig. 4. Testing heat pipes at the upper limits of temperature screeningseven hours at 300Creveals the worst-case examples ofcatastrophic heat pipe failure.


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* Heat pipe reliability test method from 13th IEEE SemiTherm Symposium.

heat pipe reliability in high power applications

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