Apparatus and method for air-cooling an electronic assembly

An improved cooling apparatus and related method are disclosed for air-cooling an array of electronic components mounted on a substrate. The cooling apparatus directs high-speed jets of cooling air directly at special heat sinks bonded to the particular electronic components that generate the most heat (e.g., microprocessors), with the air thereafter being directed to flow tangentially across the remaining components, to air-cool those components, as well. This configuration enables the requisite component cooling to be achieved with substantially reduced air flow rates and with heat sinks of substantially reduced size, as compared to prior cooling apparatus.

BACKGROUND OF THE INVENTION 
This invention relates generally to apparatus for cooling electronic 
assemblies and, more particularly, to cooling apparatus configured to 
direct air to flow across the electronic assemblies. 
Apparatus of this particular kind are used in a wide variety of electronic 
equipment, especially equipment incorporating microprocessors, which can 
generate significant amounts of heat. Typically, such equipment mounts the 
microprocessor and other heat-generating components on one or more printed 
circuit (PC) boards, with one or more heat sinks also mounted on the 
board. A fan or blower directs a tangential flow of air across the PC 
board and heat sinks, to cool the components by convection. Properly 
cooling each microprocessor typically requires a volumetric flow of about 
10 liters per second, at a velocity of about 2 meters per second. 
Heat dissipation is a more difficult problem to address when the electronic 
equipment incorporates numerous microprocessors or other components and 
modules generating large amounts of heat. Some high-end servers, for 
example, can house as many as 64 microprocessors, with associated memory 
devices and ASICs, dissipating up to 12 kilowatts. 
In such cases, cooling apparatus of the kind described briefly above are 
necessarily very large and complex. In addition, the tangential, 
unidirectional nature of the air flow causes the multiple components to be 
cooled in series. Consequently, the downstream components are cooled by 
preheated air and thus are cooled by lesser amounts than are upstream 
components. This drawback is alleviated somewhat by using a high air flow 
rate and by using heat sinks having large surface area. Heat sinks 
measuring 125 mm by 125 mm by 40 mm are commonly used. The large bulk 
volume flow also can require the use of several blowers and large exhaust 
ducting. The resulting size and complexity have detracted significantly 
from such electronic equipment's commercial viability. 
It should, therefore, be appreciated that there is a need for an improved 
cooling system for high-performance electronic assemblies of the kind 
incorporating multiple microprocessors or other heat-generating components 
and modules, which is substantially smaller in size and less complex, yet 
which is reliable and effective in dissipating excess heat. The present 
invention fulfills this need and provides further related advantages. 
SUMMARY OF THE INVENTION 
The present invention is embodied in an improved cooling apparatus, and 
related method, for air-cooling an electronic assembly of a kind that 
mounts two distinct groups of electronic devices on a substrate, e.g., a 
printed circuit board. Each electronic device in the first group is 
configured to generate an amount of heat greater than a first 
predetermined threshold, and each electronic device in the second group is 
configured to generate an amount of heat less than a second predetermined 
threshold. An air source is included, for directing a separate jet of 
cooling air into the vicinity of each electronic device of the first 
group, to cool such devices with non-preheated air, after which the air is 
directed to flow past the second group of devices, to cool those devices, 
as well. The electronic devices thereby are cooled with enhanced 
efficiency, which leads to reduced size, complexity and cost. 
More particularly, the cooling apparatus further includes a separate heat 
sink appropriately secured to each of the electronic devices in the first 
group of devices, with each heat sink incorporating a central recess, a 
lateral periphery, and a plurality of air paths connecting the central 
recess with the lateral periphery. In addition, the air source is 
configured to direct a separate air jet into the central recess of each of 
the plurality of heat sinks, whereupon the air exits therefrom via the 
plurality of air paths, to cool each of the heat sinks and thereby the 
associated electronic devices. 
In more detailed features of the invention, the air source includes a 
plurality of ducts, each configured to direct an air jet to a separate one 
of the plurality of heat sinks. The air source further includes first and 
second blowers, each configured to supply pressurized air to a plenum, for 
delivery via the plurality of ducts to the heat sinks. The first and 
second blowers each can be operated at variable speeds, and a controller 
controllably supplies electrical power to the blowers, such that if both 
blowers are operating they are made to operate at relatively low speeds, 
whereas if only one blower is operating it is made to operate at a 
relatively high speed. 
In other more detailed features of the invention, each heat sink has a 
generally cylindrical shape, with a first circular end secured to the 
electronic device being cooled and with a second, opposite circular end 
facing outwardly. The central recess is formed centrally in each heat 
sink's second circular end, and the plurality of air paths formed in each 
heat sink extend radially from the central recess to the outer periphery. 
In one embodiment, these air paths are arranged like radial spokes, 
whereas in an alternative embodiment, the air paths are arranged in a 
spiral pattern. The air paths preferably are evenly spaced around the 
circumference of each heat sink.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference now to the drawings, and particularly to FIG. 1, there is 
shown a cooling apparatus configured to cool an array of electronic 
components mounted on a printed circuit (PC) board 11. The electronic 
components include a number of microprocessors, several of which are 
depicted at 13a, 13b and 13c, which generate significant amounts of heat, 
e.g., 100 watts each. The electronic components further include additional 
devices 15, such as memory devices and ASICs, which generate lesser 
amounts of heat but nevertheless still require supplemental cooling. The 
cooling apparatus functions to efficiently cool all of these electronic 
components sufficiently to achieve optimal performance. 
Special slotted heat sinks 17a, 17b and 17c are bonded to the respective 
microprocessors 13a, 13b and 13c (e.g., by soldering, epoxy, or thermal 
compound). Alternatively, the heat sinks can be mechanically clamped to 
the microprocessors. In addition, a pair of centrifugal blowers 19a and 
19b are provided, for supplying pressurized air to a plenum 21, which in 
turn, supplies that pressurized air via ducts 23a, 23b and 23c directly to 
the sites of the respective heat sinks 17a, 17b and 17c. Specifically, 
each duct supplies a jet of non-preheated cooling air, having a speed in 
the range of 8 to 10 meters per second. Not shown in the drawings, 
additional heat sinks can be mounted on the PC board 11, each receiving a 
jet of cooling air from a separate duct connected to the plenum. 
The jets of cooling air delivered to the heat sinks 17a, 17b and 17c, after 
moving past the heat sinks, then form a tangential flow across the other 
components mounted on the PC board 11, for discharge via an exhaust port 
25. It thus will be appreciated that the particular components that are 
most in need of supplemental cooling (e.g., the microprocessors 13a, 13b 
and 13c) are cooled by high-speed jets of non-preheated air, while the 
remaining components, which require less supplemental cooling, are cooled 
by a tangential flow of air that has been preheated. Not only does this 
approach increase cooling efficiency, but it also enables the heat sinks 
to be reduced in size as compared to what would otherwise be required. 
FIG. 2 depicts one suitable configuration for the heat sinks 17a, 17b and 
17c. The depicted heat sink has a generally cylindrical shape, with a 
cylindrical recess 27 formed in the center of each heat sink's exposed, 
upper circular surface, and with a number of narrow slots 29 (e.g., 36 
slots) formed between that recess and the heat sink's outer cylindrical 
surface. These slots are arranged at uniform circumferential intervals, 
and they extend from the upper circular surface nearly all of the way to 
the lower circular surface. 
Each heat sink 17a, 17b or 17c can be formed from round metal stock, and 
its cylindrical recess 27 and plurality of narrow slots 29 can be formed 
by machining. The heat sinks preferably are formed of aluminum or, 
alternatively, a composite of aluminum and silicon carbide, available from 
Alcoa. These materials have a relatively low specific gravity, but high 
specific heat, and they are suitable for use with solder, epoxy, and/or 
thermal compound bonding. Optionally, a circular disc 31, of similar 
material, can be interposed between the heat sink and the underlying 
electronic component. 
In use, and as shown in FIGS. 1 and 4, the ducts 23a, 23b and 23c are 
arranged to direct cooling air directly into the cylindrical recesses 27 
of the respective heat sinks 17a, 17b and 17c. This air then passes 
radially outwardly through the plurality of narrow slots 29. The 
cylindrical recess and slots, together, provide substantial surface area 
from which heat generated by the underlying microprocessors 13a, 13b and 
13c can be transferred. 
FIG. 3 depicts an alternative configuration for the heat sinks 17a, 17b and 
17c. The depicted heat sink 13' has a cylindrical shape and a cylindrical 
recess 33 like that of the heat sink configuration of FIG. 2, but it 
differs from that configuration in that it incorporates narrow walls 35 
that cooperate to define a plurality of spiral slots. Periodic gaps in the 
walls cooperate to define concentric channels 37 that aid in establishing 
the desired air flow pattern. This configuration for the slots provides 
substantial surface area for transferring heat to the cooling air moving 
through them. Suitable heat sinks having this configuration can be 
obtained from a Japanese company called Alpha. They are formed of aluminum 
and are produced by forging. 
It will be appreciated that heat sinks having other configurations also can 
be used. For example, the heat sinks can each have a generally rectangular 
shape, with parallel channels through which the jets of air can pass. The 
air can be directed to flow tangentially through the channels, if vertical 
space above the heat sink is limited. Alternatively, the heat sinks can 
each have the same radial and spiral slot configuration as the heat sinks 
of FIGS. 2 and 3, except that their cylindrical side walls can be modified 
to have one or more flat sides. 
With reference again to FIG. 1, the two centrifugal blowers 19a and 19b 
that deliver cooling air to the plenum 21 are operable at variable speeds. 
The blowers are sized such that, in normal operation, each is operated at 
just one half of its maximum speed. The connection of the two blowers to a 
single plenum ensures that if either blower fails, the remaining one can 
be operated at its full capacity without adversely affecting the 
apparatus' cooling capability. This control is achieved by a controller 
38. 
The plenum 21 is depicted in greater detail in FIG. 5. It is formed from a 
plastic sheet that is formed into a wall 39 having a generally oval shape, 
after suitable preheating. End caps 41a and 41b close off the plenum's 
interior. Side-by-side inlet ports 43a and 43b are provided in the end cap 
41a, for connection to the respective blowers 19a and 19b. In addition, an 
array of outlet ports 45a, 45b, etc. are formed in the oval wall, for 
individual connection to the plurality of ducts 23a, 23b, etc., 
respectively. A flapper valve 47 is pivotally secured to the inward facing 
side of the end cap 41a, between the two inlet ports 43a and 43b, to 
ensure uninterrupted operation when one of the blowers is shut down. 
The blowers 19a and 19b are each sized to provide a nominal volume flow 
rate of about three liters per second per location. At this flow rate, 
twenty separate heat sinks 17 can be accommodated by supplying air at a 
volume flow rate of about 60 liters per second, or about 125 cubic feet 
per minute. This flow rate is approximately one third that of a similarly 
sized electronic assembly cooled by a conventional cooling apparatus. 
Suitable centrifugal blowers can be obtained from Ametek, Inc., of Kent, 
Ohio, at pressures up to about 30 inches of water. Multiple blowers can 
provide the requisite air flow rate. 
One drawback to the jetted air-cooling apparatus described above is that it 
can generate significant amounts of noise, particularly from the air being 
drawn into the blowers 19a and 19b. This noise can be alleviated by 
encapsulating the blowers with foam-plastic barrier material (not shown) 
and by placing this same material around the plenum 21 and its outlet 
ports 45a, 45b, etc. One suitable barrier material is a PBM material 
available from a company called Soundcoat, of Irvine, Calif. The noise 
also can be alleviated by configuring the inlet ducts for the blowers to 
have a serpentine pattern. 
It should be appreciated from the foregoing description that the present 
invention provides an improved cooling apparatus for an array of 
electronic components mounted onto a PC board. The cooling apparatus 
directs high-speed jets of cooling air directly at special heat sinks 
bonded to the particular electronic components that generate the most 
heat, with the air thereafter being directed to flow tangentially across 
the remaining components, to cool those components, as well. This 
configuration enables the requisite component cooling to be achieved with 
substantially reduced air flow rates and with heat sinks of substantially 
reduced size, as compared to prior apparatus. This, in turn, enables a 
substantially higher packing density, and lower cost, to be achieved. 
Although the invention has been described in detail with reference only to 
the presently preferred embodiment, those of ordinary skill in the art 
will appreciate that various modifications can be made without departing 
from the invention. Accordingly, the invention is defined only by the 
following claims.