Note: Descriptions are shown in the official language in which they were submitted.
8 BACKGROUND OF THE INVENTION
- 9 Ai the capabillty to lncorporate more and more devices ln large
scale semlconductor chlps advances, however tiny the power dissipa-
11 tion ln an indlvldual devlce, the dlfflculty of gettlng the heat out
12 of the chlp becomes compounded by several conflicting considerations.
13 For example, as the devlce~ become smaller, the contact area devoted
14 to signal input and output becomes totally lnadequate to also be relied
upon to conduct the heat generated within the chlp. Further, because
16 of the llmited physlcal strength of the small connections rigid mechanical
J 17 contact for thermal transfer, as a general rule, cannot be employed because
lB a mismatch in thermal expanslon coefficients can result in an unacceptably
19 large stress.
'~ .
SUMMARY OF THE INVENTION
21 Through the use of a magnet thermally bonded to a large area
22 heat sink, the heat 8enerated in the chip can be conducted through
23 a magnetic fluid and (held in place and shaped by the flux emanating
24 from the magnet) covering a large area on the back of the chip. The
fluid permits both conduction and convection heat transfer and allows
26 for repair and changes to the chip.
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More particularly, there is provided:
Heat transfer apparatus for conducting heat from a
semiconductor chip to a heat sink comprising in combina-
tion:
a fluid composed of magnetically responsive particles
suspended in a vehicle positioned in contact with a semi-
conductor chip and in contact with a magnetic heat sink.
. . .
There is also provided:
In semiconductor integrated circuit apparatus of the
type wherein signals to and from a semiconductor chip are
entered and leave the chip from one side, the improvement
comprising:
means for dissipating heat from the back of said chip
via a fluid composed of magnetically responsive particles
suspended in a vehicle, said fluid cooperating with a
shaped magnetic field emanating from a magnet bonded to a
heat sink to thereby conduct heat from said chip to said
heat sink.
: _ _.. _ .... ... ..... . . .. ,.. , . .. ...... . .. . . _ ., .. .. . " .. . .
There is further pro~ided: ...
. Semiconductor integrated circuit apparatus comprising
in combination:
a substrate;
a semiconductor integrated circuit chip;
a plurality of interconnections between electrical
points on said substrate and electrical points on a first
side of said chip;
a heat sink, a magnet thermally bonded to said heat
sir.k and a fluid composed of magnetically responsive
particles suspended in a vehicle forming contact between
said magnet and the opposite side of said chip.
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1 DESCRIPTION OF THE DRAWINGS
2 FIG. 1 is a sketch of the magnetic heat sink of the invention, and
3 FIG. 2 is a cross sectional view of FIG. 1 showlng the details
4 of the magnetic flux of the magnet.
DETAILED DESCRIPTION OF THE INVENTION
6 Where semiconductor chips are mounted on a substrate which
7 generally carries the wiring to communicate with the other parts of the
8 apparatus of which the chip i8 a part, there will be a plurality of
9 electrical current carrying pads on the chip. These pads for signal
speed and for area conservation are generally so small that the heat
11 generated by the individual active elements in the chip cannot be
12 dissipated through them. In addition to area and signal response con-
13 siderations, thermal expansion coefficient mismatches may be encountered
14 that limit the ability to increase the physical size of the pads.
Through this invention a magnetic fluid i8 applied to the back
16 of the chip. The fluid is shaped and its thermal path is governed by
17 the flux from a magnet thermally bonded to a large heat sink. This allows
18 the different elements to move with respect to each other for coefficient
19 of expansion problems. At the same time a maximum of thermal transfer
is achieved.
21 Referring now to FIG. 1 a substrate 1 is provided which may be
22 one or a plurality of insulating layers containing wiring and having signal
23 pads 2 to which connections 3 to a corresponding pad on chip 4 are made.
24 The connections illustrated involve the solder reflow technique wherein
surface tension of a solder quantity limited in area by the size of the
26 pad operates to lift the chip above the substrate and at the same time
27 provide an electrical connection. Another chip connection technique,
28 well known in the art, involves wire bonding from a lead frame. This
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1 technique was not illustra~ed but it will bè clear that the principle
2 involved in both generallzed techniques or any other light involves
3 fairly small connections whose physical size is governed by use of
4 available area, signal responsiveness, and coefficient of expansion
matching such th~t making the individual connections large enough to carry
6 the heat generated in the chip, becomes less and less practical.
7 In accordance with the invention, on the back of the chip 4, a
8 magnetic fluid 5 is applied. This fluid is one of a number available in
9 the art and conslsts of a vehicle in which magnetic particles are suspended.
The fluid has a viscosity such that it will not creep over the edge of the
11 chip and has a particle sugpension such that a magnetic fluid will change
12 the shape of the fluid quantity. One commercially available fluid is
13 manufactured by a corporation known as Ferro Fluidics Corporation. The
14 fluid must remain stable through the operating temperature of the chip.
The magnet 6, which is preferably a permanent magnet for simplicity, in
16 turn is thermally bonded, generally by direct contact to a heat sink 7
17 which is shown as having fins 8, although ie will be apparent to one
18 skilled in the art that any heat sinking approach including direct contact
19 with further fluids, is acceptable within the purview of the invention.
Referring next to FIG. 2 a cross sectional area of FIG. 1 showing
21 the details of the magnet 6 and the magnetic fluid 5 are provided. In
22 FIG. 2 the chip 3 is agaln shown using the solder reflow type connections 2
23 to the substrate 1. On the back of the chip 3 is shown the magnetic fluid
24 5. The magnet 6 has a shape such that the lines of flux go as is illus-
trated from an outside region 9 to a centralized region 10, such that the
26 flux causes the magnetic fluid 5 to assume a shape permitting a larger
27 fluid quantity than viscosity would ordinarily permit and in turn facilita-
28 ting increased convection and conduction of the heat from the back of the
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1 chips to the magnet 6 which in turn is bonded thermally to the heat sink 7
2 which ln turn transfers the heat through the fins 8 to the surrounding
3 environment.
4 The magnet 6 has a shape through its perlphery 9 and its central
portion 10 to accomplish the following functions. It shapes the magnetic
6 fluid 5 to permit more fluid than viscosity will permit if needed, and it7 holds the fluid in place during vibrations and thermal expansion. The
8 shape of the fluid is such that heat transfer by both conduction and con-9 vection is enhanced. The non-uniform magnetic field generally draws the
fluid to the region of higher magnetic field strengths which is generally
11 the center of the chip. If the quality of fluid is property ad~usted, the12 fluid will seal the region between the back of the chip and the magnet so13 that good thermal contact is produced between the chip and the heat sink,14 and the fluid connection will be independent of orientation of the structure.
Since no rigid contact exists the chip and the magnet can readily be brought
16 together and separated allowing assembly and repair to be accomplished con-
17 veniently.
18 What has been described i8 the use of a magnet and a magnetic
19 fluid to enhance both conduction and convection of heat from the back of
a chip to a heat sink while at the same time maintaining a non-rigid but
21 thermally efficient contact.
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