Integrated circuit heat transfer element and method

An integrated circuit heat transfer element (6,30) is made by selecting thermally conductive fibers having aspect ratios of length to diameter of more than 1, selecting a resin and combining the fibers and the resin to create a formable resin/fiber compound. The resin/fiber compound is formed into a composite material in part by applying pressure to the formable resin/fiber compound, which aligns the fibers, and when cured creates a thermally anisotropic composite material to maximize heat conduction along the aligned fibers. The thermally anisotropic composite material has a coefficient of thermal expansion (CTE) of less than about 10.times.10.sup.-6 cm/cm/.degree. C. The composite material has a thermal conductivity in the direction of the carbon fibers of at least 50 W/m.degree. K. The IC device is preferably secured to the heat transfer element using a thermally conductive adhesive.

BACKGROUND OF THE INVENTION 
Many integrated circuit (IC) devices, such as microprocessors, generate 
enough heat so that heat dissipation is a concern. In many applications, 
heat transfer elements of the type called heat slugs and heat spreaders 
are attached directly to the backside of the integrated circuit device to 
spread and remove heat away from the device. Conventional heat slugs and 
heat spreaders are typically made of copper, copper/molybdenum alloys or 
copper/tungsten alloys. 
One of the problems with using copper is its high (17.times.10.sup.-6 
cm/cm/.degree. C.) coefficient of thermal expansion (CTE) versus the much 
lower CTE (5-6.times.10.sup.-6 cm/cm/.degree. C.) for silicon and the IC 
circuit device. This CTE mismatch requires that elastomeric silicone 
adhesives and uncured thermal greases be used to attach the heat transfer 
element to the IC device. Doing so prevents the IC subassembly from being 
destroyed by thermal stresses when undergoing thermal cycles. However, the 
use of elastomeric silicone adhesives and uncured thermal greases creates 
reliability problems due to inherently poor bond strength. In addition, 
silicone-based products are poor thermal conductors and restrict heat 
transfer away from the IC device. 
To alleviate the problems caused by the CTE mismatch, copper/tungsten and 
copper/molybdenum alloys can be used. The CTE match for these alloys is 
much closer to that of silicon and ceramic substrates, 
6.7-9.0.times.10.sup.-6 cm/cm/.degree. C. versus 5-6.times.10.sup.-6 
cm/cm/.degree. C. The closer CTE match permits a more intimate bond, which 
allows greater thermal conduction. However, these alloys are quite 
expensive, thus substantially increasing the cost to the ultimate 
purchaser. 
Another problem with conventional, copper-based, heat transfer elements is 
that they are heavy. While for standalone units this may not be a problem, 
it is a substantial concern when dealing with computers and other 
electronic devices for which weight is an important consideration because, 
for example, they are carried about by the user. 
Some electronic devices are low powered and therefore have very low heat 
outputs. These devices very often have lids molded in place using a 
polymeric molding compound, typically an epoxy filled with spherical 
quartz. Although cost-effective, the low thermal conductivity of these 
molding compounds means that they do not transfer heat very well and thus 
are totally unsuited for IC devices which generate substantial amounts of 
heat, such as microprocessors. Rather, lids for high-powered 
microprocessors are typically made from aluminum or an aluminum/ceramic 
composite material. The ceramic materials are added to decrease the CTE of 
the aluminum to a level closer to that of silicon. The lids are typically 
bonded in place using a hermetic solder, silicon adhesive or other 
polymeric adhesive. 
One of the problems with the conventional aluminum lids is that the CTE is 
24.times.10.sup.-6 cm/cm/.degree. C. requiring elastomeric silicon 
adhesives and uncured thermal greases to be used between the lid and the 
IC device. This creates the same type of problems as discussed above with 
reference to heat slugs and heat spreaders. With aluminum/ceramic 
composite lids the thermal conductivity is relatively low compared to pure 
aluminum but the manufacturing processes used to make the hardware are 
more difficult than with aluminum-based hardware. 
SUMMARY OF THE INVENTION 
The present invention is directed to an integrated circuit heat transfer 
element and method having a similar CTE as the IC device to permit the use 
of high strength, reliable, high thermal conductivity adhesives to secure 
the heat transfer element to the IC device. The heat transfer element is 
made of a thermally anisotropic composite material including thermally 
conductive fibers oriented generally perpendicular to the IC device for 
enhanced heat transfer from the IC device. In addition, the invention 
permits heat transfer elements to be made much lighter than equivalent 
metallic heat transfer elements. 
The heat transfer element is made by selecting thermally conductive fibers, 
typically carbonaceous fibers, at least 30% having aspect ratios of length 
to diameter of more than 1, selecting a resin and combining the fibers and 
the resin to create a formable resin/fiber compound. The resin/fiber 
compound is formed into a composite material in part by applying pressure 
to the resin/fiber compound to create a thermally anisotropic composite 
material in which the fibers are generally aligned in a chosen direction 
to maximize heat conduction along the chosen direction. 
The thermally anisotropic composite material preferably has a coefficient 
of thermal expansion (CTE) of less than about 10.times.10.sup.-6 
cm/cm/.degree. C., more preferably has a CTE within about 60% of the CTE 
of the IC device, and most preferably has about the same CTE as the IC 
device, that is about 5-6.times.10.sup.-6 cm/cm/.degree. C. The fibers are 
preferably graphite fibers but could be other carbonaceous fibers or 
fibers coated with thermally conductive materials such as nickel, silver, 
gold, diamond or ceramics. As used herein, carbonaceous fibers include 
fibers which start out being carbon, or substantially carbon, as well as 
fibers which are made of carbon precursors and can be transformed into 
carbon during an oxygen-free heating step. In the latter case, the resin 
can also be a carbon precursor resin so that after the oxygen-free heating 
step, the composite material is a carbon/carbon composite for further 
enhanced thermal conductivity. 
The composite material has its thermally conductive fibers aligned by 
applying pressure to the resin/fiber compound while still in its formable 
state. This can be achieved by procedures such as cavity molding under 
pressure and by extrusion. The heat transfer elements can be pressed 
singly in a compression molding process or in a multiple cavity tool using 
a transfer molding process. The molds would typically be heated to above 
about 175.degree. C. (350.degree. F.) and molded at pressures of about 1 
to 2.times.10.sup.-8 dynes/cm.sup.2 (1,500-3,000 psi). 
The resin/fiber compound is preferably extruded at temperatures of 
190-200.degree. C. and pressures of 1500-3000 psi. The heat transfer 
elements can be created from the extruded resin-fiber compound by cutting 
slices of the extruded resin/fiber compound and then machining surfaces as 
needed. 
The resultant composite material preferably has a thermal conductivity in 
the direction of the thermally conductive fibers of at least 50 
W/m.degree. K and a density of less than about 3 gm/cc, and more 
preferably less than about 2.3 gm/cc. 
Different types of integrated circuit assemblies are made with the heat 
transfer element adjacent to the IC device and with the thermally 
conductive fibers oriented generally perpendicular to the IC device for 
enhanced thermal conductivity away from the IC device. The heat transfer 
element can take a number of forms, including a heat slug or heat spreader 
when, for example, the IC assembly is part of a wire bonded IC assembly 
design. Alternatively, the heat transfer element can be formed as a 
thermally conductive chip lid in, for example, a ball grid array design of 
an IC assembly. The heat transfer element can also be made as an integral 
part of a heat sink used with either of the wire bonded and ball grid 
array designs. 
The IC device is preferably secured to the heat transfer element using a 
non-elastomeric, thermally conductive adhesive. The heat transfer element 
can be plated with a metal if desired.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD 
FIG. 1 illustrates a typical IC assembly 2 of the wire bonded design. IC 
assembly 2 includes a heat sink 4 to which a heat slug or heat spreader 6 
is mounted. An IC device 8, such as a microprocessor, is positioned on top 
of heat transfer element 6. Wire bonds 10 extend from IC device 8 to a 
copper lead frame 12. A ceramic substrate 14 overlies heat sink 4 in areas 
other than heat transfer element 6. 
The above-described IC assembly 2 is generally conventional. With 
conventional IC assembly 2, heat transfer element 6 is typically copper 
requiring an elastomeric silicon adhesive or an uncured thermal grease to 
be used between IC device 8 and heat transfer element 6. When an 
elastomeric adhesive is used, the bond between IC device 8 and heat 
transfer element 6 is not very strong; in addition, the elastomeric 
adhesive is a fairly good thermal insulator, thus reducing the heat 
transfer from IC device 8, through heat transfer element 6 and into heat 
sink 4. When thermal greases are used, which enhances heat transfer 
between IC device 8 and heat transfer element 6, bonding between IC device 
8 and heat transfer element 6 is substantially nonexistent. 
According to the present invention, heat transfer element 6 is a thermally 
anisotropic composite material in which thermally conductive fibers, 
typically graphite fibers, are aligned generally along a chosen direction 
16. Direction 16 is generally perpendicular to IC device 8 so to maximize 
heat conduction along the chosen direction and thus away from the IC 
device. 
To create a heat transfer element 6 made according to the invention, 
graphite fibers having aspect ratios of length to diameter of more than 1 
are selected. The aspect ratio selected will usually vary from more than 1 
to about 200 depending on the degree of thermal anisotropy desired. Other 
thermally conductive fibers, including carbonaceous fibers made of carbon 
precursor materials and fibers coated with thermally conductive materials 
such as nickel, silver, gold, diamond or ceramics, can be used instead of 
graphite fibers. The graphite fibers are combined with a suitable resin to 
create a formable resin/fiber compound. Examples of suitable resins 
include epoxy, cyanate ester, phenolic, phenolic triazine cyanate esters, 
bismaleimide and polyimide. The formable resin/fiber compound is then 
formed (typically through the application of heat and pressure) to create 
the desired thermally anisotropic composite material. 
The alignment of the carbon fibers is achieved by applying pressure to the 
resin/fiber compound while still in its formable state which causes the 
fibers to align in a direction generally perpendicular to the direction of 
pressure during cure. Each transfer element 6 can be created in a 
compression molding process or a multiple cavity tool using a transfer 
molding process. The molds would typically be heated to above 350.degree. 
F. and the parts would be molded at pressures between about 1,500 and 
3,000 psi. Another method of manufacturing heat transfer elements would be 
to press or extrude long, constant-profile bars of the resin/fiber 
compound to create elongate bars of thermally anisotropic composite 
material. The bars could then be sliced to the proper thickness and, if 
necessary, further machined or worked to the proper shape or form. 
Heat transfer element 6 can be plated with copper, silver, gold or another 
thermally conductive metal, ceramic or polymer. This helps to ensure the 
heat transfer element remains sealed to ensure cleanliness in the 
application of the typically graphite fiber-based heat transfer element. 
Such plating or sealing may not always be required depending on the 
particular application. 
Heat sink 4 can also be manufactured using a thermally anisotropic 
composite material as discussed above with reference to heat transfer 
element 6. In some situations, heat transfer element 6 and heat sink 4 
could be a single, one-piece part. 
FIG. 2 illustrates an IC assembly 20 of a ball grid array design. Assembly 
20 is mounted on a printed wiring board 22 by solder balls 24. Assembly 20 
includes a ceramic substrate 26 to which an IC device 28 is mounted. Heat 
transfer element 30 is in the form of a chip lid and is conventionally 
made of, for example, aluminum. Chip lid 30 is conventionally secured to 
ceramic substrate 26 using a silicone adhesive to alleviate CTE mismatch 
problems. A silicone thermal grease, used for improved heat transfer, is 
used between heat-generating IC device 28 and chip lid 30. Heat sink 32 is 
secured to the upper surface 34 of heat transfer element 30. With 
conventional IC assemblies 20, heat sink 32 is typically aluminum. 
With the present invention, chip lid 30 is made of thermally anisotropic 
composite material having graphite fibers oriented generally perpendicular 
to heat transfer element 6, that is in direction of arrows 36, using the 
method discussed above. Further, heat sink 32 could be made using the same 
type of thermally anisotropic composite material. Also, as discussed with 
preference to FIG. 1, heat transfer element 30 and heat sink 32 could be 
made as a one-piece, unitary structure. 
A thermally anisotropic composite material from which heat transfer 
elements 6, 30 are made can also be made using carbon fiber/carbon matrix 
molded parts. The process used to make the parts would be identical to the 
steps discussed above. After the part is formed and/or then machined, the 
parts would be carbonized in an inert (for example, nitrogen) atmosphere 
furnace or vacuum furnace. During the carbonization process, which 
involves heating the parts to temperatures above 1,000.degree. F., 
typically above 1500.degree. F., the carbonaceous resin in the part will 
convert from its current state (carbon precursors or carbon and carbon 
precursors) into a carbon state. To reduce the voids in the parts and to 
ensure suitable structure or strength, a char yield of at least about 50% 
by weight (and more preferably at least about 60% by weight) is desired. 
That is, the mass of the resin (typically phenolic or phenolic triazine 
cyanate esters) before carbonization should be no more than twice the mass 
of carbon created by the carbonization of the resin. The carbon 
matrix/carbon fiber part can then be plated (e.g., using electroless 
plating techniques) to seal the surface and add a degree of extra 
toughness to the part. Using the carbon fiber/carbon matrix option creates 
a heat transfer element which has a lower CTE in the direction 
perpendicular to that of the fibers as opposed to the same part before the 
carbonization step. In addition, the thermal conductivity in the direction 
perpendicular to the direction of the fibers will increase due to the 
increase in the amount of more highly conductive carbon matrix versus the 
less highly conductive carbon precursor matrix. 
Modification and variation can be made to the disclosed embodiments and 
methods without departing from the subject of the invention as defined in 
the following claims. For example, it is preferred that all of the fibers 
have aspect ratios of more than 1; however, the invention is also intended 
to cover situations in which at least 30% have aspect ratios more than 1.