Unitary slotted heat sink for semiconductor packages

A unitary heat sink for a semiconductor package having a plurality of cooling fin elements, each element having upwardly extending openings that divide the base into a plurality of leg portions, the heat sink having a plurality of flat base portions individually bonded to a surface of the package, each of the leg portions of an individual fin element being integral with different but adjacent flat base portions, in operation the heat sink preventing a build-up of stresses at the bonded interface of the base portions and package due to differential coefficient of expansions of the heat sink and package.

TECHNICAL FIELD 
This invention relates generally to heat transfer mechanisms and more 
particularly to an improved heat transfer mechanism for removing the heat 
generated in an electronic circuit module assembly. 
The high circuit densities in modern integrated circuit semiconductor 
devices require that the heat generated by their operation be efficiently 
removed in order to maintain the temperature of the devices within limits 
that will keep the operating parameters of the devices within 
predetermined ranges, and also prevent destruction of the device by 
overheating. The problems associated with heat removal are increased when 
the device is connected to the supporting substrate with solder terminals 
that electrically connect the device to appropriate terminals on the 
substrate as compared to a back bonded device where the support substrate 
acts as a heatsink. On such solder bonded devices, the heat transfer that 
can be accomplished through the solder bonds is limited, as compared to 
back bonded devices. Ordinarily the semiconductor devices are contained in 
an enclosure and the devices are mounted in or in contact with a heat 
sink. The heat sink can be cooled with liquid or air. However, when 
cooling requirements can be met, it is normally less expensive to 
dissipate the heat with a flow of air, which can be chilled, if desired. 
As the size of the substrate supporting the operating device increases, the 
more significant differences in coefficients of expansion of the materials 
of the elements of the semiconductor package become. During use the 
temperature of the package is inherently cycled. Thus when the support 
substrate is made of ceramic, the lid or enclosure over the devices is 
also preferably made of ceramic with a corresponding coefficient of 
expansion. However, ceramic does not conduct heat as well as metal. 
Therefore, cooling fins formed of ceramic are relatively inefficient when 
compared to metal and may not be suitable to meet the requirements. 
Individual metal fins can be bonded to the ceramic to overcome the 
coefficient of expansion difference problems. However, the finned lid is 
fragile and relatively expensive because the tedious and time-consuming 
operation of individually bonding the fins. A large unitary fin assembly 
can be bonded to the lid surface. However, since the coefficient of 
expansion of metal and ceramic are quite dissimilar either the fins and 
lids will separate, and/or the assembly would bow when heated. At the very 
least, stresses are generated which can be detrimental to the reliability 
over the life of the package. 
PRIOR ART 
U.S. Pat. No. 4,277,816 discloses a cooling system for a semiconductor 
package where a plurality of individual slotted hollow metal tubes are 
mounted by brazing or soldering, on the lid that encloses semiconductor 
devices. A blower and a baffled chamber are provided to provide 
impingement cooling of the packages i.e., air is directed axially into the 
tube mounted on the lid. 
SUMMARY OF THE PRESENT INVENTION 
It is a principal object of the present invention to provide a heat 
transfer mechanism for a large scale integrated circuit module that will 
provide efficient heat removal and which is compatible with ceramic 
materials. 
A more specific object of the invention is to provide an effective and 
efficient metal heat sink for mounting on ceramic lids that can be 
thermally cycled without imposing destructive or harmful stresses on the 
lid. 
The foregoing and other objects and advantages are accomplished with a heat 
sink assembly for mounting on a ceramic lid of a semiconductor package 
which heat sink is comprised of a plurality of elongated cooling fin 
elements bonded to a lid wherein the heat sink assembly is provided with 
plurality slots in the lower side that separate the lower end of each fin 
element into a plurality of leg portions, said slots terminating short of 
the upper end, and a plurality of flat quadralateral base portions bonded 
to the surface of the lid where each quadralateral base portion is joined 
to the leg portions of one of each of the adjacent cooling fin elements, 
the flat quadralateral base portions joining said plurality of said hollow 
cooling fin elements and forming a unitary assembly which will relieve 
stresses generated by differences in coefficients of expansion of the 
material of the heat sink assembly and the lid during thermal cycling.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, an air cooling system for high density integrated 
circuit modules for utilizing the heatsink of my invention is 
schematically illustrated. Large scale integrated circuit chips (not 
shown) are packaged in modules 10. The modules are in turn supported by 
printed circuit board 11. The modules 10 each have a heat conductive 
covering surface and attached to this covering surface is a unitary 
slotted heatsink element 12. An air plenum 15 is spaced a suitable 
distance from the top surface of the integrated circuit module. Associated 
with the air plenum chamber 15 is an air moving device 16. Internal to the 
air plenum 15 in the surface 17 facing the integrated circuit board 
assembly of modules are a plurality of openings 18. Under each opening 18 
is preferably a module 10 having the unitary heatsink of my invention 12 
mounted thereon. In the base of the assembly beneath the air plenum 15 and 
the circuit board 11 is a slit 19 which permits the exhausted air to be 
exited from the air cooling system. 
Referring now to FIGS. 2-4 of the drawings, there is illustrated a 
preferred specific embodiment of the unitary heatsink embodiment 12 of 
invention. Heatsink 12 is adapted to be bonded directly to the surface of 
a heat generating body, more preferably the lid of a semiconductor 
package, wherein heat must be dissipated. The heatsink structure permits 
bonding directly to materials having a dissimilar coefficient of expansion 
since the bonding surface is broken up into a plurality of small surfaces 
that are free to move or flex to the degree necessary for adjusting to the 
contraction of the surface to which it is bonded. 
Heatsink 12 has a plurality of upstanding cooling fin elements 20 that are 
hollow. The fin elements, as viewed in FIGS. 2 and 3 from the top surface 
of the assembly 12, are formed by the intersection of parallel slots 44 
with parallel slots 46. The slots 44 and 46 extend downwardly from the top 
surface but stop short of the height of the assembly. The combination of 
slots 44 and 46 shape the exterior side surfaces of cooling fin elements 
20. Each fin element 20 has four upstanding leg portions 22 that are 
joined at the top of the fin element 20. The leg portions 22 are capable 
of flexing to accommodate for expansion and contraction of the surface 
that the heatsink 12 is bonded to, as will be explained and illustrated in 
more detail. The leg portions 22 are preferably formed by the intersection 
of parallel slots 26 and parallel slots on the bottom side of assembly 12 
that separates the lower end of each fin element into four parts. Parallel 
slots 26 and 28 also shape the base portions 30, as seen in FIG. 4, that 
will be bonded to the surface of the package lid. An opening 24 is 
provided in the top portion of each cooling fin when impingement cooling, 
in the combination in FIG. 1, is used for dissipating heat. The opening 24 
extends downwardly to at least the slots 26 and 28 providing a passage 
through the fin element 20 with openings in the lower portion to allow 
cooling medium to flow downwardly and outwardly. This flow of fluid, 
normally air, provides effective heat dissipation. Turbulence is caused by 
air flowing outwardly about the four leg portions 22 after entering top 
opening 24, which dispels the boundary air layer that normally would 
impede heat transfer. 
As shown more clearly in FIG. 4, heatsink 12 has a plurality of square 
quadralateral shaped flat base portions 30 which, in use, are bonded to 
the body to be cooled. In the preferred embodiment, the base portions 30 
are square, but could be rectangular or round as well. The base portions 
30 are attached to the cooling fin elements 20 but in a very special 
manner. Each of the leg portions 22 of a cooling fin element 20 are each 
attached to different but adjacent base portions 30. This relationship is 
most clearly shown in FIG. 2. Stud 20A has four leg portions on the lower 
end i.e., 22A, 22B, 22C and 22D. Leg 22A is mounted on the base 30A; leg 
22B is mounted on base 30B; leg 22C is mounted on base 30C; and leg 22D is 
mounted on base 30D. Each base portion 30 is thus attached to four 
different fin elements. The adjacent corners of four base portions 30 thus 
support a single fin element. Thus, any relative movement of the 
individual base portions 30, due to heating or cooling effects, is 
absorbed by flexing of the leg portions 22 of the fin elements. The 
heatsink structure is unitary because the base portions 30 are all tied 
together by the upper portions of the cooling fin elements 20. In addition 
the large area of base portions 30 provide effective thermal transfer of 
heat to the heat fin elements 20. In addition a hot spot, caused as for 
example by an active semiconductor device, beneath a base portion 30 
results in transfer of heat to four separate fin elements which reduces 
thermal resistance and also promote a more uniform temperature profile in 
the heat generating body. 
As is obvious from FIG. 4, the base portions that support the fin elements 
along the edges of the heatsink 12 are approximately one-half the size of 
the centrally located base portions 30. This is necessary because the 
outside row of base portions 30 are required to secure the leg portions of 
only two fin elements. 
The heatsink can be made of any suitable material having a thermal 
conductivity sufficient to meet the demands of the particular application. 
However, metals are preferred. Preferred metals are copper and aluminum. 
The heatsink can be bonded to the heat generating body by any suitable 
technique, as for example, by brazing or soldering. As previously 
mentioned, the support element, i.e. the lid of a semiconductor package, 
need not have a coefficient of expansion that matches the material of the 
heatsink. It is frequently desirable, particularly in larger semiconductor 
modules, to use a ceramic lid when the substrate is formed of ceramic. 
Material expansion mismatch between the material of the heat generating 
body is effectively reduced by reducing the size of the bonding interface, 
i.e. by breaking the area of the base portion of the heatsink into small 
individual areas that are free to move relative to one another. 
The unitary heatsink of my invention can be fabricated by the method which 
will now be described. It will be apparent that the relatively complex and 
intricate heatsink structure can be fabricated by performing a number of 
simple routine machining operations. In forming the heatsink a solid 
rectangular or square block of material having the desired size is 
selected. A first plurality of parallel spaced slots 26, as most clearly 
shown in FIG. 4, is machined in the bottom of the selected block. The 
depth of the slots is obviously less than the thickness of the block 
itself. A second plurality of parallel spaced slots 28 that are transverse 
to slots 26 is subsequently machined in the bottom of the block. This 
forms the surface configurations of base portions 30 that serves as the 
bonding interface. A third plurality of slots 44, positioned between slots 
26 which overlap in the block, are machined in the top opposite side of 
the block. Subsequently, a fourth plurality of parallel slots 46 are 
machined from the top of the block which are positioned between slots 28 
and are also in overlapping relation. These slots 44 and 46 form the 
exterior shape of the cooling fin elements 20. Holes 42 are drilled from 
the top of the block to open the fins for impingement cooling. 
As most clearly shown in FIG. 4 slots 44 and 46 from the top of the block 
shape the exterior surface of the fins 20. Slots 26 and 28 from the bottom 
of the block form the lower four leg portions of the fins 30 and also 
shape the lower leg portions 22 that support the fin. 
The heatsink 12 of the invention can be fabricated in any other suitable 
manner, as for example by casting. If the heatsink is cast, the slots 26, 
28, 44 and 46 can be slightly tapered to facilitate removal of the heat 
sink 12 from the mold. The depth of the slots and the height of the 
cooling fin elements can be varied to meet the requirements of the 
specific application. In general, the width of slots 26 and 28 constitutes 
approximately 1/3 to 1/2 of the total width of the fin elements 20. The 
relative dimensions of the slots 26 and 28 to the overall height of the 
cooling fin elements 20 determines the cooling effectiveness and capacity 
of the structure. Preferably the ratio of the height of the cooling 
element 20 to the width of slots 26 or 28 is the range of 5 to 50. 
While the invention has been illustrated and described with reference to 
preferred embodiments thereof, it is to be understood that the invention 
is not limited to the precise construction herein disclosed and the right 
is reserved to all changes and modifications coming within the scope of 
the invention as defined in the appended claims.