Packaging system for thermally controlling the temperature of electronic equipment

A passive method and packaging system for thermally controlling the temperature of electronic equipment within a housing are disclosed. The packaging system provides for two cooling steps, each of which uses a different passive cooling technology. Cooling by changing the phase of Phase Change Material, assists in controlling internal ambient temperatures by limiting the effects of short term localized thermal phenomena which would otherwise further increase internal temperatures. Cooling is also provided for by switching on a second passive cooling means, preferably a heat pipe, as the internal temperature of the housing increases above a predetermined temperature and to switch it off as the internal temperature drops below the predetermined temperature. Turning the second passive cooling means on and off provides respectively, a low or high thermal resistance path between the interior and exterior of the housing. The second passive cooling means allows for cooling of the housing above the predetermined temperature as well as heating of the housing below the predetermined temperature by preserving heat generated by the equipment. The two cooling steps in combination, co-operate to limit both maximum and minimum temperatures within the housing and of components. The combination of cooling steps also allows control over the temperature delta between the maximum and minimum temperatures during internal temperature swings.

FIELD OF THE INVENTION 
This invention relates to a passive method and packaging system for 
thermally controlling the temperature of electronic equipment. 
BACKGROUND OF THE INVENTION 
Electronic equipment which is to be located outdoors and placed within a 
housing structure to protect the equipment from mother nature, generally 
requires some method of regulating or controlling the internal temperature 
of the housing and the temperature of the electronic components. It is 
well known that the reliability of electronic components decreases 
significantly if they are subjected to high temperature extremes or to 
large temperature swings, especially if these swings or cycles are 
frequent. These temperature cycles can be due for example to electronic 
loading (e.g. peak telephone traffic times) or seasonal day-night 
temperature differences. 
Conventional convection methods of temperature regulation within equipment 
housings have relied on maintaining internal ambient air temperatures 
within a predetermined range and to provide an airflow over the equipment 
to assist in the removal of component heat by an air medium. Component 
manufacturers have specific temperature ranges within which they guarantee 
their components will function reliably. A semi conductor component 
junction temperature range stated by a manufacturer will typically be in 
the range of -15.degree. C. to +85.degree. C. Generally speaking suppliers 
of electronic equipment will stipulate that their equipment (incorporating 
semi conductor components) if operated within a commercial operating zone 
(ambient temperature internal to a housing for example) of typically 
0.degree. C. to +50.degree. C. will operate reliably. In other words 
operating their equipment in this commercial operating zone will ensure 
that component junction temperatures (-15.degree. C. to +85.degree. C.) 
set by component manufacturers will not be exceeded. 
Service providers often provide air-conditioning units and or heating 
elements which are then thermostatically controlled. While this form of 
convection cooling (or heating) tends to be satisfactory for equipment 
housings which are either indoors or outdoors and in close proximity to 
electrical power, it is not an acceptable method for remote outdoor 
housing applications. Typical data communications cabling is not of 
sufficient size (gauge) to handle the power required by air-conditioning 
units or heating elements and hence at great expense, separate power 
cables must therefore be run out to the remote location. The ongoing year 
after year energy costs required to power such air-conditioning units 
and/or heating elements can also be significant. More importantly, it is 
generally accepted that if any degree of system reliability is to be 
realized, backup power systems (batteries/generators etc.) along with 
backup air-conditioning units and heating elements must be provided. These 
costs, all of which can be very significant, ultimately cut into profit 
margins of the equipment owners. In addition any breakdown in the heating 
and cooling equipment may result in disastrous and expensive breakdowns in 
the electronic equipment. 
Many service providers in some developing countries when installing 
equipment even indoors do not wish to or can not afford to provide this 
costly form of temperature control system yet still require the highest 
degree of reliability possible from their equipment. 
SUMMARY OF THE INVENTION 
The invention seeks to provide a passive method and packaging system for 
thermally controlling the temperature of electronic equipment which 
minimizes the shortcomings above. 
According to one aspect of the invention there is provided a method of 
cooling electronic equipment comprising the steps of providing a 
substantially sealed and insulated housing enclosing the electronic 
equipment; cooling the equipment using first passive cooling means 
comprised of phase change material disposed within the housing, by 
changing the phase of the phase change material as temperature within the 
housing increases above a first predetermined temperature thereby limiting 
further increases in temperature internal of the housing; cooling the 
equipment by switching on a second passive cooling means as temperature 
within the housing increases above a second predetermined temperature to 
provide a low thermal resistance path between the interior of the housing 
and the exterior of the housing; and switching off the second passive 
cooling means as temperature within the housing decreases below the second 
predetermined temperature so as to provide a high thermal resistance path 
between the interior of the housing and the exterior of the housing 
thereby preserving heat generated by the equipment. 
Providing two cooling steps, each using a different passive cooling 
technology enables passive cooling of remote electronic equipment 
housings. Cooling by changing the phase of Phase Change Material, assists 
in controlling internal ambient temperatures by effectively limiting or 
regulating the effects of short term localized thermal phenomena which 
would otherwise further increase internal temperatures. Cooling by 
switching on and off a second passive cooling means above and below a 
predetermined temperature to provide respectively a low or high thermal 
resistance path between the interior and exterior of the housing. 
Switching on and off the second passive cooling means as described enables 
cooling of the housing above the predetermined temperature as well as 
heating of the housing below the predetermined temperature by preserving 
heat generated by the equipment when the second passive cooling means is 
in an off state. The two cooling steps in combination, co-operate to limit 
both maximum and minimum temperatures within the housing. Maximum and 
minimum temperatures within the housing and of components can thus be kept 
to within accepted manufacturer's specifications but as well the delta 
between the maximum and minimum temperatures during temperature swings can 
be controlled or reduced to further increase reliability. 
Advantageously, in many climates the changing the phase of the phase change 
material occurs at a first predetermined temperature which is higher than 
the second predetermined temperature of the second passive cooling means. 
Advantageously, changing the phase of the phase change material occurs in 
the range of between 30-80.degree. C. and wherein switching on and off of 
the second passive cooling means occurs at the second predetermined 
temperature of about 0.degree. C. 
Preferably, changing the phase of the phase change material occurs in the 
range of between 35-60.degree. C. 
According to another aspect of the invention there is provided an outdoor 
packaging system for housing electronic equipment comprising a 
substantially sealed and insulated housing for housing the equipment; 
first passive cooling means comprising phase change material disposed 
internal of the housing for absorbing heat above a first predetermined 
temperature thereby limiting further increases in temperature internal of 
the housing due the effects of short lived thermal phenomena; and second 
passive cooling means comprising; heat sink means disposed externally of 
the housing; passive switch means for switching on the second passive 
cooling means above a second predetermined temperature to provide a low 
thermal resistance path between the interior of the housing and the heat 
sink to effect cooling of the equipment and for switching off the second 
passive cooling means below the second predetermined temperature to 
provide a high thermal resistance between the interior of the housing and 
the heat sink to thereby preserve heat generated by the equipment and 
effect heating of the housing. 
The combination of technologies provides passive control over cooling 
required as a result of localized short term thermal spikes as well as 
continuous equipment power dissipation and external solar heating. This 
arrangement also benefits from having the ability to effectively turn off 
further cooling during periods of cold weather, thus preserving internally 
generated heat. 
Advantageously, the passive switch means includes a heat pipe to 
efficiently conduct heat almost isothermally from inside of the housing to 
the heat sink means. 
Preferably the passive switch means includes a Variable Conductance Heat 
Pipe, which is best suited to transferring heat at near constant 
temperature even where variable thermal loads and environments are 
encountered. 
Conveniently a heat distributing structure is disposed throughout the PCM 
to more evenly subject the whole of the PCM to local ambient temperatures 
and preferably the heat distributing structure comprises a metal honey 
comb structure or a metal filamentery material randomly disposed through 
out the PCM. 
Advantageously, the electronic equipment comprises electrical components 
thermally and electrically conductively attached to a substrate, the 
substrate being thermally conductively attached to a highly thermally 
conductive member to form a module, the module being removably thermally 
conductively attached to the second passive cooling means. 
Preferably the packaging system further comprises a highly thermally 
conductive heat spreading means, thermally conductively attached to the 
second passive cooling means and wherein each module is removably 
thermally conductively attached to the heat spreading means whereby 
individual module heat is combined and efficiently conducted to the second 
passive cooling means.

DETAILED DESCRIPTION 
Electronic equipment which incorporate semi conductor components must 
provide ways of controlling the temperature the components in order to 
maintain component junction temperatures within the manufacturers 
specified range of typically -15.degree. C. to +85.degree. C. Another 
reliability requirement is to keep the components as well as substrate 
surfaces at a minimum of 7-10.degree. C. above the external ambient 
temperature thereby minimizing humidity related problems from occuring. 
For example condensation causes moisture buildup on surfaces which for 
electronic equipment may cause electrolytic/galvanic corrosion. As well, 
ion ladened moisture on and between signal tracks may also reduce 
reliability by causing leakage from one track to the other. 
Daily temperature swings will vary from one geographical location to 
another and for illustrative purposes only, one example of a worst case 
positive and worst case negative outdoor ambient temperature extreme 
(T.sub.EXT) is shown in FIG. 6. An insulated outdoor equipment housing as 
is known in the art when subjected to the worst case positive or negative 
temperature extreme will undergo a delayed internal temperature swing 
(T.sub.INT) and will generally peak at a temperature which is higher than 
the external ambient maximum as well as reach a minimum temperature which 
will be above the external ambient minimum. Differences between respective 
maximums and respective minimums is due largely to the heat produced by 
the actual equipment which is internal to the housing. As can be seen from 
FIG. 6, anytime an outdoor housing without some form of cooling, is 
subjected to an external ambient (T.sub.EXT) temperature peak (e.g. 
+50.degree. C.) the temperature internal (T.sub.INT) to the housing can 
very often reach a peak of around +90.degree. C. (shown in chain dot) 
which is well above the desired maximum internal ambient temperature of 
+50.degree. C. Similarly, anytime an outdoor housing is subjected to an 
external ambient (T.sub.EXT) temperature minimum of -55.degree. C., the 
temperature internal to the housing can very often reach a minimum of 
around -30.degree. C. (also shown in chain dot) which is lower than the 
desired minimum internal ambient temperature of -10.degree. C. As was 
stated earlier, operating electronic equipment at these extremes will 
shorten component life span and generally the reliability of the 
equipment. It is also known that cycling operating temperatures even if 
the minimums and maximums are within the rated range, reduces the life 
span and reliability of equipment. The greater and more frequent the 
temperature swing the less reliable the equipment may be. In many 
applications such as communications for example this is not satisfactory. 
Ideally, to increase reliability it is desirable that the equipment in its 
ambient surroundings, be only subjected to temperatures within the 
commercial operating zone and in particular, the delta between maximum and 
minimum temperatures (.DELTA.T.sub.3) during any temperature cycle. 
Equipment enclosure 10 of the embodiment of FIG. 1 comprises a 
substantially sealed and insulated housing 12 which utilizes a combination 
of PCM and heat pipe technologies to provide efficient passive conductive 
cooling of electronic equipment. The housing 12 of the embodiment of FIG. 
1 includes a diffusion passage for water vapor and a conventional access 
door (both of which are not shown). The diffusion passage ensures that the 
absolute humidity levels inside and outside of the housing are the same 
which helps to reduce condensation related problems. Components 14 are 
thermally and electrically conductively attached using known techniques to 
a substrate 16 (printed circuit board) to form a circuit pack 18. The 
substrate 16 of each circuit pack 18 is in turn thermally conductively 
attached to a facing surface of a major planar arm 22 of an `L` shaped 
support member 20 having high thermal conductive characteristics such as 
aluminum for example. The `L` shaped support member 20 can be better 
visualized with reference to FIG. 3 in which it is shown in perspective 
view. The major planar arm 22 of each `L` shaped support member 20 is of 
comparable surface area to the substrate 16 of a respective circuit pack 
18. A circuit pack 18, connected to a respective support member 20, form 
an electronic module 30. Electronic modules 30 in turn are thermally 
conductively attached in the following manner, to a highly thermally 
conductive heat spreading plate 32 which is disposed directly above the 
electronic modules 30. One surface of the small planar arm 24 of each `L` 
shaped support member 20 is thermally conductively attached using known 
techniques (thermal epoxies etc.) directly to a facing planar surface of 
the large rectangular heat spreading plate 32. A convection heat sink 40 
forms at least a portion of the housing's 12 exterior wall directly above 
and spaced apart from the heat spreading plate 32. Fins 42 of the heat 
sink 40 are open to the ambient environment external to the housing 12. 
Thermal transistors in the form of a heat pipes 50 (only one being visible 
in FIG. 1) are disposed in the region between the heat sink 40 and the 
heat spreading plate 32 and are thermally conductively attached to an 
exteriorly facing surface of the heat spreading plate 32 and to the 
inwardly facing surface of the heat sink 40. Phase change material is 
encased within a highly thermally conductive framework to form circuit 
pack PCM modules 60. Each module 60 is sealed to the extent that it is 
able to contain the PCM during all stable phase states and transitions. 
PCM modules 60 are thermally conductively attached to the rear surface of 
the major planar arm 22, directly opposite respective circuit packs 18. 
Wall PCM modules 70 having different dimensions to the circuit pack PCM 
modules 60, are thermally conductively attached to an interior surface of 
each of the side walls 15 of the housing 12. 
In operation, two cooling technologies are used in combination to provide 
efficient passive cooling of the components 14 within enclosure 10. FIG. 6 
illustrates graphically an example temperature swing .DELTA.T.sub.3 
internal (T.sub.INT) of the housing resulting from a temperature swing 
external (T.sub.EXT) of the housing. Shown in chain dot are the maximum 
and minimum peak ambient internal temperatures that the housing would be 
subjected to without any form of cooling in place. A housing in accordance 
with the invention can clip or limit the maximum peak internal temperature 
at the PCM threshold temperature (T.sub.PCM) and as well limit the minimum 
peak internal temperature at the Heat Pipe threshold temperature 
(T.sub.HP). Raising the Heat Pipe threshold temperature (T.sub.HP) to a 
temperature above the -10.degree. C. lower limit set by the commercial 
operating zone further reduces .DELTA.T.sub.3 therefore further increasing 
reliability. Heat pipe technology is used to efficiently conduct heat 
almost isothermally from inside of the housing 12 to a convection heat 
sink 40, external of the housing whenever the ambient internal temperature 
is above a predetermined first threshold temperature (T.sub.HP) of for 
example 0.degree. C. Phase Change Materials in the form of circuit pack 
PCM modules 60, 70, 80 further assist in controlling internal ambient 
temperature by effectively limiting or regulating the effects of short 
term localized thermal phenomena which might further increase internal 
temperatures above a second predetermined PCM threshold temperature 
(T.sub.PCM). For reasons of minimizing costs and enclosure size, heat 
pipes are designed, generally for average daily thermal conditions and not 
short term peak thermal phenomena. Short lived localized thermal loads or 
peaks (e.g. adjacent circuit packs) due to for example to increased 
equipment loads can be effectively absorbed by the PCM modules 60 through 
the latent heat of fusion, above a second predetermined temperature 
(T.sub.PCM) for example in the range from 30-60.degree. C. to thus limit 
or prevent any further increase in internal temperature. Wall PCM modules 
70 similarly may prevent any further increase in internal temperature as a 
result of external solar temperature peaks. During periods of colder 
weather the internal minimum ambient temperature is regulated by 
effectively turning off the heat pipe 50 to thus provide a high thermal 
resistance between the interior of the housing 12 and the heat sink 40 to 
thereby preserve heat generated by the components 14 and effect heating of 
the housing 12. 
In more detail, power in the form of heat dissipation from components 14, 
is thermally absorbed by the major arm 22 of respective `L` shaped support 
members 20 and through conduction is carried via each major arm 22 to the 
heat spreading plate 32. Each `L` shaped support member 20 operates in 
this manner to absorb and conduct heat away from respective circuit packs 
18 and thermally carry it through conduction to the heat spreading plate 
32. When the heat spreading plate 32 reaches a predetermined temperature 
(T.sub.HP, signifying the need for cooling), the thermal transistor or in 
the embodiment the heat pipe 50 effectively turns on and conducts the heat 
energy from the heat spreading plate 32 out of the housing to heat sink 40 
where fins 42 then vent this excess heat to the external ambient air. 
Utilizing a thermal transistor enables the housing 12 to contain or 
preserve internally generated heat from the components 14 for example 
during extreme cold periods such as in the winter or during a cold night 
portion of a daily temperature cycle. Preserving internally generated heat 
by turning off the conduction of heat to the heat sink 40 below a 
predetermined temperature provides two advantages. Firstly, it effectively 
limits any further downward trend of the internal temperature below a 
predetermined engineered value (e.g. 10.degree. C.). Benefits gained by 
doing this result from increasing the temperature differential 
(.DELTA.T.sub.2 FIG. 6) between ambient temperatures inside and outside 
the housing (humidity/condensation concerns) and of further reducing the 
temperature delta .DELTA.T.sub.1 between internal cycle minimums and 
maximums. As was mentioned earlier, cycling operating temperatures even if 
the minimums and maximums are within the rated range, reduces the life 
span and reliability of equipment. As well, providing a means to 
effectively remove or switch off the low thermal resistance path provided 
by the heat pipe when it is essentially on, may under certain atmospheric 
conditions prevent moisture within the air surrounding the circuit packs 
18 from condensing onto component 14 and circuit pack substrate 16 
surfaces. For example under certain conditions a night sky can represent a 
radiant temperature in the range of -50.degree. C. even though the ambient 
temperature external to the housing may only be in the range of 
-10.degree. C. In a housing for example having a heat sink facing the 
night sky and which is directly thermally coupled to the electronic 
equipment (i.e. no thermal transistor or switch in series with the heat 
sink), there will be a substantially small but constant thermal resistance 
between the night sky and the equipment of typically around 10.degree. C. 
Under these conditions, the air around the electronic equipment may reach 
temperatures as low as -40.degree. C. which is significantly lower than 
the external ambient temperature (of -10.degree. C.) and hence moisture 
from the air surrounding the equipment will inevitably condense onto 
component and substrate surfaces (reducing the reliability of the 
equipment). However, in a housing according to the invention the thermal 
transistor (the heat pipe 50 in the embodiment) turns off under the same 
conditions and effectively then provides a very high thermal resistance in 
series with the heat sink or between the night sky and the equipment, thus 
reducing condensation concerns by preventing the interior ambient 
temperature from dropping below the exterior ambient temperature. 
It should be apparent from FIG. 6 that there is a predictable maximum 
duration of time (for a given geographical area) where the temperature 
internal to the housing may exceed the rated maximum or minimum 
temperatures of the equipment due to external temperature fluctuations in 
ambient surroundings. These short time duration's are usually only a few 
hours for many applications. Short lived internal thermal increases may 
also for example be due to increased equipment electrical loads. In the 
telecommunications industry for example there are predictable periods 
within each day of the year where telephone equipment usage increases 
dramatically (e.g. over the lunch hour, Christmas Day, etc.) which 
increases the load on respective equipment and hence power dissipation of 
the equipment. These short lived thermal increases can raise the internal 
ambient and component temperatures above the rated values. 
Phase change materials (PCM) exhibit characteristics which when used 
internally to an equipment housing will help to effectively cool the 
equipment by limiting and regulating the temperature within the housing to 
a predetermined value. It is known that Phase Change Materials through the 
latent heat of fusion are able to absorb significantly more heat during 
the change of state phase than in their stable states, i.e. either before 
or after the change of state phase. During the change of phase state, PCM 
absorbs heat energy with little or no increase in the temperature of the 
material itself. PCM can therefore be used to passively cool and regulate 
the temperature within a housing. The degree of cooling achievable is 
dependent upon the amount of PCM used, the type used and the placement of 
the PCM within the housing. As well, different types of PCM's change phase 
at different predetermined threshold temperatures and hence the point at 
which cooling takes place can be engineered by choosing the appropriate 
material; for example, Paraffin Waxes change phase in the range of 
43-56.degree. C., Disodium Phosphate Dodecahydrate changes phase around 
36.degree. C., and Neopentyl Glycol changes phase around 43.degree. C. In 
this specification phase change materials (PCM) include not only those 
materials which go from a liquid phase to a solid phase a back to a liquid 
phase such as Paraffin Waxes do, but also includes those materials such as 
Neopentyl Glycol which go from a first solid phase to a second solid 
phase; with the difference between first and second solid phases being 
that each solid phase exhibits a different crystalline structure. By 
definition these materials are phase change materials which change from 
one solid phase having one crystalline structure to a second solid phase 
having a different crystalline structure. 
For a predictable short term thermal spike such as high noon sun loading or 
peak equipment load times, the amount of PCM disposed within the housing 
can be engineered to effectively limit and regulate internal temperatures 
over the complete duration of the thermal spike. Where larger amounts of 
PCM are to be used for peak thermal spikes lasting a few hours or more it 
has been found that as the PCM (e.g. paraffin wax) changes phase it often 
does not do so uniformly and hence the desired temperature regulating 
characteristics during the phase change may not be realized. As well some 
of the available thermal absorption capacity of the PCM will not be 
realized if all of the material is not permitted to undergo the phase 
change (i.e. some of it does not melt). 
Research done by the applicant suggests that as larger amounts or masses of 
PCM change state, interface regions may be created between that portion of 
the material which is still in the solid state and that portion which is 
in the liquid state. It is believed that the outer melted layers form and 
effective insulation region between the still solid layers and the heat 
source; or stated in other words, the thermal resistance of the melted 
layer increases in effect and as a result causes the PCM to not change 
state uniformly. This non-uniform change of state may result in a slower 
and uneven rate at which the PCM can absorb heat energy and hence a loss 
in heat regulation may be realized. Preferably the PCM may be placed 
within a heat distributing structure constructed of any material having 
good thermal conducting properties. An example of such a structure is 
illustrated in FIG. 7. The honey-comb structure 84, fabricated from 
aluminum is encased within a framework 82 to contain the PCM and form a 
PCM module 80. The honey-comb heat distributing structure 84 more evenly 
subjects the whole of the contained PCM to local ambient temperatures 
which allows the PCM to change state at an even rate. Filamentary 
structures 85 as shown in FIG. 8, resembling coarse steel wool but made of 
aluminum, have also provided very good results with regard to distributing 
heat evenly throughout the PCM. 
Heat pipe technology in combination with Phase Change Materials can control 
temperature levels more efficiently within an equipment enclosure. Heat 
Pipes require no external power, have no moving parts, are sealed units, 
can be designed to operate over a very wide temperature range, and can be 
constructed to take on virtually any desired shape (tubes, flat plates 
etc.). Heat Pipes can transfer high heat loads while incurring only very 
small temperature drops (typically 1-2.degree. C.) across the heat pipe. 
In their most basic form (FIG. 5) a heat pipe 100 comprises an evaporator 
110, a vapor transport medium 120, a condenser 130, and a liquid return 
medium 140. In operation the evaporator 110 portion of the Heat Pipe 100 
is subjected to an undesired heat load where at a predetermined 
temperature, the liquid internal to the evaporator 110 vaporizes. The hot 
vapor is then allowed to flow along the vapor transport medium 120 towards 
the condenser 130 where it then condenses. While condensing, heat 
contained within the vapor is rejected and the liquid is returned back to 
the evaporator 110 by capillary forces within the liquid return medium 140 
so that the process may be repeated. Some form of heat sink is disposed in 
thermal contact with the condenser 130 portion of the heat pipe 100 so as 
to conduct the undesired heat out of the housing to the external ambient 
air. Heat pipes can be designed to effectively turn off at a predetermined 
temperature by choosing a liquid which will essentially freeze at that 
temperature. Once the liquid freezes the evaporate/condense cycle ceases 
and the heat pipe effectively presents a very high thermal resistance 
between the heat sink and the inside of the housing. 
The Heat Pipe configuration just described generally is referred to as an 
Isothermalizer and can have multiple evaporators and condensers and may 
use either gravity or a wick as the liquid return medium. A preferred heat 
pipe 50 shown in FIG. 1 and more clearly in FIG. 4, referred to as a 
Variable Conductance Heat Pipe (VCHP) includes a reservoir 135 of 
non-condensable gas where the gas expands or contracts depending on 
temperature and hence pressure of the vapor from the Evaporator. Generally 
in a VCHP, vapor temperature in the Evaporator increases as a result of an 
increased thermal load which in turn causes the vapor pressure within the 
Evaporator to increase. As a result of this increase in pressure the 
non-condensable gas in the reservoir compresses, thus exposing more active 
condenser surface to the hot vapor. A Variable Conductance Heat Pipe is 
best suited to transferring heat at near constant temperature in 
applications where variable thermal loads and environments are 
encountered. 
In the embodiment of FIGS. 1, 4 heat pipe 50 is used to switch on and off 
the cooling provided by one of the passive cooling devices, namely the 
heat sink 40. The heat pipe turns on the cooling provided by the heat sink 
40 by providing a low thermal resistance between the heat spreading plate 
32 and the heat sink 40 when the temperature of the heat spreading plate 
32 is elevated above a predetermined threshold value (T.sub.HP). 
Conversely, the heat pipe 50 effectively turns off the cooling provided by 
the heat sink 40 by providing a high thermal resistance between the heat 
spreading plate 32 and the heat sink 40 when the temperature of the heat 
spreading plate drops below the predetermined threshold value (T.sub.HP). 
Other forms of thermal transistors or switches may be used to provide the 
switching on and off of cooling provided by the heat sink 40. Bi-metallic 
technology for example may be used to switch on and off cooling by using 
known expansion characteristics of dissimilar metals to provide an 
increase (switched on) or a decrease (switched off) in pressure between 
the heat spreading plate 32 and the heat sink 40. Thermostatic technology 
(such as is used in automobile thermostats) which utilizes the change in 
volume of a material through expansion properties of solids and liquids 
may also provide a passive mechanical thermal switch by increasing 
(switched on) or decreasing (switched off) the pressure between the heat 
spreading plate 32 and the heat sink 40. Although a convection heat sink 
40 is used in the embodiment the invention is not limited to convection 
heat sinks. 
In the event of short term increased thermal loads resulting from increased 
equipment loads, each `L` shaped support member 20 may not be capable of 
efficiently conducting this extra heat to the heat spreading plate 32 or 
as discussed earlier, the heat pipe 50 by design may not have the capacity 
to remove this extra heat or be able to respond quickly enough to prevent 
component temperatures from exceeding rated maximum temperatures. Circuit 
pack PCM modules 60 as illustrated in FIGS. 1, 2 are disposed adjacent the 
rear side of circuit packs 18 and are held in thermal contact with a back 
surface of the major planar arm 22. Increased equipment loads will 
invariably result in circuit packs 18 dissipating larger amounts of heat 
energy. This heat energy may be absorbed by the circuit pack PCM modules 
60 hence permitting localized cooling of each circuit pack 18 during short 
term thermal peaks during those time intervals of peak time equipment 
loads. 
Cooling effectiveness using PCM may be enhanced or optimized by the 
relative placement of the PCM internal to the housing 12. As may be seen 
in FIG. 1 locating PCM modules 60 generally in close proximity to each 
circuit pack 18 effectively permits localized cooling of each circuit pack 
18 during short term thermal peaks during those time intervals of peak 
time equipment loads. Cooling effectiveness may be further enhanced by 
having PCM modules 60 thermally conductively attached to respective major 
planar arms 22 which are in turn thermally conductively attached to 
respective substrates 16 of each circuit pack 18. Placement of PCM modules 
70 on interior walls 15 of the housing 12 generally will aid in the 
cooling of the internal ambient temperature should it reach the particular 
PCM threshold temperature. PCM modules for use adjacent circuit packs 18 
may contain different PCM from PCM modules positioned elsewhere in the 
housing (walls for example) to effectively provide staggered turn on 
thresholds. One or more walls 15 of the housing 12 may also be used 
depending on the degree of cooling required. Certain applications may 
require or benefit from PCM modules being disposed in direct thermal 
contact with the heat spreading plate 32, alongside the heat pipe 50. 
Other alternatives are for more than one type of PCM to be used within a 
single PCM module in order to act as temperature buffers at various times 
of the year. 
Equipment having shelving arrangements which utilize one or more 
conventional printed circuit board receiving stations may also benefit by 
having PCM disposed adjacent to respective receiving stations for cooling 
respective circuit boards when in use. 
In the embodiment described the phase change threshold temperature 
(T.sub.PCM) for the PCM was chosen to be in the range of 35-60.degree. C. 
which is higher than the turn on temperature (T.sub.HP) for the heat pipe 
of 0.degree. C. (i.e. the heat pipe effectively turns on before the PCM). 
In other applications or climatic conditions other than North American 
type climates or even for different applications (e.g. passive cooling of 
indoor equipment enclosures), the PCM temperature thresholds could be 
chosen to be the same as or lower than the heat pipe turn on temperature. 
PCM modules disposed on one or more exterior walls can be used to 
advantageously absorb and store solar heat energy during one phase 
transition and to later release heat energy during the a second phase 
transition. 
Developments in liquid metal technology have provided liquid metals such as 
Indalloy 117, Indalloy 136, and Indalloy 19 which may be used in PCM 
modules and which generally have higher densities for the same or similar 
latent heat of fusion of the more traditional phase change materials. The 
liquid metals listed above are alloys containing; Indalloy 117 (44.7% Bi, 
22.6% Pb, 19.1% In, 8.3% Sn, and 5.3% Cd), Indalloy 136 (49% Bi, 21% In, 
18% Pb, and 12% Sn), and Indalloy 19 (51% In, 32.5% Bi, 16.5 Sn).