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December 2005
library  >  Application Notes  >  General Articles

Alleviating Thermal Spreading Resistance in Telecommunication Systems


 

 

by geoff thyrum
thermacore, inc.


 

 

components are getting smaller and hotter

 

the electronic components used in telecommunications equipment (microprocesors, asics, rf amps, etc.) are dissipating increasing amounts of power while at the same time decreasing in size. higher device temperatures are directly associated with decreased reliability and increased failure rates. field repair costs could unexpectedly skyrocket unless new thermal management solutions are designed in today.

 

the heat sinks needed to cool the latest generation rf amps have become much larger than the devices that they cool. a hot spot develops directly over the device, which is caused by "thermal spreading resistance". the higher the power and the smaller the source, the greater the spreading resistance. when a heat source is placed at the edge or corner of a heat sink, the spreading resistance is even greater.


thermal spreading resistance increases local temperatures
for different heat source sizes and placements.


unfortunately, it's not as simple as just using a bigger heat sink. the amount of heat that can be transferred from an rf amp into the air depends on the surface area of the heat sink and the speed of air flowing over its surface. the brute force approach to overcoming spreading resistance has been to increase the size of the heat sink, increase the size and speed of the fan, or change the material from aluminum to something with higher conductivity such as copper. these steps all increase weight, noise, system complexity, and expense.

 

the most aggressive way to eliminate spreading resistance is to utilize a two-phase heat transfer process that is revolutionizing the heat sink industry. it is called the vapor chamber heat sink and it is the foundation for thermacore's new therma-base product line.

 

therma-base heat sink operation

 

a therma-base heat sink consists of a vapor chamber integrated with fins for natural or forced air convection cooling. the therma-base is a vacuum vessel with a wick structure lining the inside walls that is saturated with a working fluid. as heat is applied to the therma-base, the fluid at that location immediately vaporizes and the vapor rushes to fill the vacuum. wherever the vapor comes into contact with a cooler wall surface it will condense, releasing its latent heat of vaporization.

 

the condensed fluid returns to the heat source via capillary action, ready to be vaporized again and repeat the cycle. the capillary action of the wick enables the therma-base heat sink to work in any orientation with respect to gravity.


internal structure of a therma-base.

 

 

thermal performance features

 

the temperature drop associated with the vapor spreading is typically 3-5°c, providing an effective means of spreading the heat from a concentrated source to a large surface. as the size of the therma-base increases, the effective thermal conductivity increases dramatically. when the ambient temperature is below the freezing point of the working fluid, the therma-base stops operating as an effective heat transfer device. however, when the temperature rises above freezing again, the therma-base resumes normal operation.

 

typical applications


ambient temperature
-40 ºc < ta < 70 ºc
operating temperature
10 ºc < tvsp < 100 ºc
power/component
50 watts < q < 500 watts
heat flux
50 w/cm2 < q/a < 500 w/cm2
length or width of heat sources
0.1" < lc < 3"
length or width of vsp
2" < ls < 12"

 

due to the way the therma-base operates, the heat source can be placed anywhere on the contact surface of the therma-base with negligible affect on it's thermal resistance. in addition, there can be multiple heat sources dissipating the same or different amounts of power. the rate of fluid vaporization at each source will stabilize and the therma-base will be nearly isothermal.



temperature gradients of traditional heat sink (above) and therma-base heat sink (below).
the therma-base heat sink showed a 12°c reduction of the component temperature.


added benefit for multiple devices

 

when multiple devices are in series the first device preheats the air flowing over the second device, so the first device runs cooler and the second device runs hotter. this is usually exacerbated when both devices are aligned, so that the hottest air from the first device is flowing right over the second device. when the devices are cooled by two therma-base heat sinks, the air temperature is even across the entire width of each sink. when the devices are cooled by one therma-base heat sink, both devices are kept at the same intermediate temperature instead of one being hotter than the other.


when two devices are aligned in series, the hottest air flows
right over the downstream device.


when two devices are cooled by separate therma-base heat sinks,
the air temperature is even across the entire width of each sink.



when two devices are cooled by one therma-base heat sink,
both devices are kept at the same intermediate temperature
instead of one being hotter than the other.


working fluid and wick structure

 

of the working fluids available for use in a therma-base heat sink, the thermodynamic attributes of water make it an order of magnitude better than any other fluid for the majority of electronics cooling applications. its high latent heat of vaporization spreads more heat with less fluid flow. water's high surface tension, when presented to a wick with small pore size (such as sintered copper powder) generates a large capillary force that allows operation in any orientation.

 

its high thermal conductivity minimizes the dt associated with conduction through the wick. in addition to its thermodynamic properties, water is safe. the amount of water in a therma-base is only enough to saturate the wick structure.

 

if a unit were ever punctured air would leak in to the vacuum space but no water would leak out - the water being held in by the wick's capillary force. therma-baseheat sinks with a sintered powder wick have been freeze/thaw tested without any adverse thermal or mechanical effects.

 

the passive copper/water technology used in these therma-base heat sinks has been proven to be reliable in millions of heat pipes used in notebook computers every year and the copper/water materials combination has been life tested in thermacore heat pipes for over 17 years now.

 

mechanical features

 

custom mechanical and attachment features can be readily designed into a therma-base heat sink. the therma-base is captured in a plastic or aluminum frame, then assembled to the fin structure, allowing for a variety of mounting and alignment features that can be matched to a specific system assembly process. in addition, therma-base heat sinks can be manufactured with through-holes for attaching components.

 

attaching cooling devices to high flux heat sources can present a multitude of issues regarding interface resistance, electrical connectivity, ability to withstand shock and vibration, etc. one feature that has been developed is a precision flatness (0.001" per inch) surface finish. another feature is an internal post structure that can transfer 100-psi direct pressure contact to the device that needs to be cooled.

 

for the most critical applications, a therma-base can be incorporated into the device package. this delivers maximum control of the most critical thermal interface and provides the flexibility of using a variety of heat sinks.

 

manufacturing process

 

thermacore has developed high volume manufacturing capacity in anticipation of the emerging applications for therma-base heat sinks. the manufacturing process starts with copper stampings that provide precision flatness and, when applicable, multiple heat input surfaces at different heights to accommodate components of different thicknesses.

 

the plate material is copper - the best of only a few materials proven to be compatible with water at elevated temperatures over many years of life testing. a copper powder wick is sintered into place, ensuring an integral conduction path from the wall to the vapor/wick interface where vaporization occurs.

 

the stamped plates are sealed with a simple, clean welding process that introduces no brazes or solders. the therma-base is assembled with one of a variety of heat sink materials depending on the application, charged with working fluid and processed. the enhanced surface area of the heat sink can be extruded, stamped, or cut with a gang saw.

 

bonded fins and folded fins are also options. and since much of the volume of the therma-base is filled with vapor, therma-base heat sinks are lighter than alternative cooling solutions.


summary

 

electronic components such as rf amplifiers are dissipating increasing amounts of power, while at the same time decreasing in size. the associated heat sinks are becoming larger than the devices they cool and a thermal spreading resistance occurs. since the thermal resistance associated with vapor movement is small, the two-phase heat transfer process used in therma-base heat sinks provides an effective way to spread heat evenly across the base, especially for extremely high heat flux situations.

 

therma-base heat sinks keep devices cooler than traditional heat sinks, while being smaller, lighter, and requiring less air flow. a variety of mounting and alignment features are available with the high volume manufacturing process that is in place today. therma-base solutions should be considered when faced with the most challenging thermal management requirements.

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