CA1102010A

CA1102010A - Magnetic heat sink for semiconductor ship

Info

Publication number: CA1102010A
Application number: CA305,488A
Authority: CA
Inventors: Kenneth S. Sachar
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1977-08-12
Filing date: 1978-06-14
Publication date: 1981-05-26
Also published as: IT7826104A0; EP0000856A1; EP0000856B1; DE2861442D1; JPS5635026B2; JPS5432074A; IT1112288B

Abstract

MAGNETIC HEAT SINK FOR SEMICONDUCTOR CHIP

ABSTRACT OF THE DISCLOSURE
Heat generated in a multicontact semiconductor chip can be conducted away by the use of a magnetic fluid held in place by the field from a magnet which in turn conducts the heat transmitted through the fluid to a heat sink to which the magnet is bonded for thermal transfer.

Description

- 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.

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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.

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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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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 i~VZ~
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 i~32~:~;63 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.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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.

2. The heat transfer apparatus of claim 1 wherein said magnetic heat sink is made up of a magnet bonded to a finned heat transfer member.

3. The heat transfer apparatus of claim 2 wherein said magnetic heat sink has a shape producing a circular mag-netic flux from a peripheral ring to a centralized region.

4. 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.

5. 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 sink and a fluid composed of magnetically responsive particles suspended in a vehicle forming contact between said magnet and the opposite side of said chip.

6. The apparatus of claim 5 wherein said magnet pro-vides a shaped magnetic field from a peripheral region to a centralized region.