GB1588477A

GB1588477A - Prefabricated composite metallic heat-transmitting plate unit

Info

Publication number: GB1588477A
Application number: GB22253/78A
Authority: GB
Original assignee: Semi-Alloys Inc
Current assignee: Semi-Alloys Inc
Priority date: 1977-06-29
Filing date: 1978-05-25
Publication date: 1981-04-23
Also published as: DE2826252A1; CA1083263A; NL7806751A; IT1105422B; IT7849996A0; FR2396263B1; FR2396263A1; JPS5412569A

Description

(54) A PREFABRICATED COMPOSITE METALLIC HEAT-TRANSMITTING PLATE UNIT (71) We, SEMI-ALLOYS, INC., of 888 South Columbus Avenue, Mount Vernon, New York 10550, United States of America; a corporation organized and existing under the laws of the State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a new and improved member for transmitting heat between a heat-absorbing medium and a body subject, during normal operation, to wide temperature excursions and having a predetermined temperature coefficient of expansion.

In the transfer of heat between a heat sink and another body, it is often desirable or even necessary in certain applications to employ an intermediary metallic member having a temperature coefficient of expansion approximately equal to that of the body but a relatively high heat-transfer characteristic. For example, in the semiconductor industry the general practice has been to make the semiconductor devices from single-crystal silicon. It is characteristic of these devices that heat is generated in the silicon die when they are put into use by the electrical current flowing through them. If such heat is not conducted away from the die, its temperature will rise to an intolerable level and the device will not operate properly or will undergo complete failure.

In order to conduct the heat away from the silicon die, it has been customarily soldered to a metal mount which acts as a heat sink. Because the temperature of both the silicon die and the metal mount increase and they are rigidly soldered to each other, the temperature coefficients of expansion of the single-crystal silicon die and the metal heat sink mount must be compatible - that is, they must be very close to each other over the temperature range which they experience. If this condition is not achieved, the brittle silicon die will undergo strain and either fracture to cause device failure or severely modify the operating characteristic of the electrical circuit in the silicon die to such a degree as to make it useless.

Metal alloys which reasonably match the temperature coefficient of expansion of the single-crystal silicon over the temperature range of 20"C to 4000C to the required degree are KOVAR (Registered Trade Mark), a nickel-cobalt-iron alloy, and ALLOY 42, a primarily nickel-iron alloy. Unfortunately, both alloys have very poor coefficients of heat transfer and can only be used for silicon die mounting where the power dissipation in the silicon is comparatively low. If the power dissipation characteristics required of the electronic circuitry incorporated into the silicon die are high, for example power amplifiers, power diodes, and rectifiers, then 'KOVAR' and ALLOY 42 cannot be used because the temperature rise of the silicon die will be too high and the semiconductor device will fail.

One metal that does satisfy the heat-transfer and the temperature coefficient requirement for single-crystal silicon is molybdenum. However, molybdenum is relatively high in cost, is very difficult to fabricate mechanically, and is difficult to electroplate.

Other approaches to the problem described above have been proposed: providing a metal supporting plate for a semiconductor device consisting of a sintered porous plate of a metal having an appropriate thermal coefficient of expansion in which the pores are filled with a metal having a higher coefficient of heat transfer; a plate of a material having a high heat-transfer coefficient through which are imbedded tungsten fibres; or an assembly in which the semiconductor device is supported from a copper header in which is imbedded a grid or mesh of a material having an appropriate thermal coefficient of expansion. All of the foregoing proposals for solving the problem would be extremely expensive as contrasted to Applicant's simple plate unit.

It is an object of the invention, therefore, to provide a new composite metallic heat-transmitting plate unit which overcomes the disadvantages of the above-described methods of transmitting heat between a body having a predetermined temperature coefficient of expansion and a heat-absorbing medium, such as a heat sink.

It is a further object of the invention to provide a novel composite metallic plate unit for transmitting heat between a heat-absorbing medium and a body which has a temperature coefficient of expansion approximately equal to that of the body and a satisfactory heat-transfer coefficient.

In accordance with the invention, there is provided a prefabricated composite metallic plate unit for transmitting heat between a heat-absorbing medium having a planar surface such as a heat sink and a body having a planar surface and subject during normal operation to wide temperature excursions and having a predetermined temperature coefficient of expansion which comprises a high-tensile-strength metallic plate member having a temperature coefficient of expansion substantially the same as that of the body, one surface of said member being adapted to be disposed in contact with the planar surface of the body and the other surface being adapted to be disposed in contact with the planar surface of said medium, a plurality of holes extending through such member, and a relatively soft metallic material filling such holes having a heat-transfer coefficient of at least 0.3 cal./cm2/cm/sec./ "C.

The present invention will be further illustrated, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view, partially cut away, of a heat-transmitting member embodying the invention; while Figure 2 is a perspective view showing the application of the heat-transmitting member of Figure 1 to a power header for mounting a power-type semiconductor device.

Referring now to Figure 1 of the drawing, there is represented a prefabricated composite metallic heat-transmitting plate unit comprising a high-tensile-strength metallic member 10 having a temperature coefficient of expansion substantially the same as that of the body with which it is associated. As illustrated, the member 10 has opposed substantially parallel planar surfaces, one adapted to be disposed in contact with the planar surface of the body and the other disposed to be in contact with a heat sink. Formed in the metallic member 10 are a plurality of uniformly distributed holes 11 which occupy 45% or less of the total areas of the surfaces of the member 10 and 45% or less of the volume of the member 10. Each of the holes 11 is filled with a metallic material having a heat-transfer coefficient of at least 0.3 cal./cm2/cm/sec./ C and is formed of a soft metal preferably of the group comprising silver, copper, aluminum, and alloys of any of these metals.

In Figure 2 there is illustrated a power header for mounting a power-type semiconductor device utilizing the heat-transmitting member of the invention. Such header comprises a stud 20 having a unitary enlarged cylindrical head 21 and of a material having a relatively high heat-transfer coefficient, such as copper. The stud 20 is threaded as shown for screwing into any conventional heat sink. A disc-shaped member 22 is a heat-transmitting member fabricated in the same manner as the plate member 10 of Figure 1. It is preferably then gold plated and brazed to the head 21 of stud 20. A semiconductor device 23 of the power type, such as a power amplifier or a rectifier, is then soldered to the member 22, usually using a solder such as a gold-silicon or a gold-tin eutectic alloy.

The power mounting device thus described is usually hermetically sealed by a cup-shaped cover soldered to the periphery of the head 21, the conductive leads from the semiconductor device 23 extending through the cover. Such sealing cover and the leads from the semiconductor device 23 are not shown in the drawing since they form no part of the present invention.

Thus, by the use of the heat-transmitting member 22 having a temperature coefficient of expansion approximating that of the semiconductor device 23 and a relatively high heat-transfer coefficient, a substantial amount of heat developed in the semiconductor device 23 is transferred through the member 22 to the power header 20, 21 while the member 22 and the semiconductor device 23 expand and contract compatibly, avoiding disruptive stress on the semiconductor device 23.

In selecting the materials for the metallic member 10 and the materials for filling the holes 11, the following characteristics are significant: Temperature Coefficients of Expansion The temperature coefficients of expansion of pertinent materials over the temperature range of 20"C to 400"C in cm/cm/ C are approximately as follows: Semiconductor silicon ......... ................... 4.8 x 10-6 'KOVAR' ........................................ 4.9 x 10-6 ALLOY 42 ...................................... ..... 5.2 x 10-6 Silver ....................... ......................... . 19.7 x 10-6 Copper ........................................ 16.4 x 10-6 Composite member 10: ALLOY 42 and 30% copper by volume ..... 7.0 x 10-6 'KOVAR' and 30% copper .. ............... 6.5 x 10-6 Thus, while copper and silver have high heat-transfer coefficients as discussed below, their temperature coefficients of expansion differ so widely from those of silicon that, if attached to the silicon die to absorb heat therefrom, the adverse effects on the silicon die described above would result. On the other hand, members of 'KOVAR' or ALLOY 42, with copper-filled holes described above, have temperature coefficients of expansion closely matching those of silicon, so that the use of the composite member 22 avoids the adverse effects on the silicon die 23 as its temperature varies over a wide range.

Heat-transfer coefficients The heat-transfer coefficients of pertinent materials over the temperature range 20 C to 400 C in cal./cm2/cm/sec./ C are as follows: 'KOVAR' . ....... . 0.04 ALLOY 42 .... . ....... ......... 0.035 Silver ....... . 0.99 Copper . ....... . 0.90 Composite: ALLOY 42 and 30% copper . . 0.25 'KOVAR' and 30% copper .. . 0.24 Thus, while 'KOVAR' and ALLOY 42 have temperature coefficients of expansion closely matching those of silicon, their heat-transfer coefficients are only about 4% of those of silver and copper and quite inadequate to dissipate heat from the silicon die 23 over an extended temperature range if used alone. However, such coefficient of the composite member 22 of ALLOY 42 and 30% copper and of 'KOVAR' and 30% copper have heat-transfer coefficients six to seven times those of 'KOVAR' and ALLOY 42 alone and adequate for all usual applications.

The heat-transfer member 10 may be formed by any of several well known methods. For example, after holes 11 have been drilled in the member 10, they may be filled with copper or silver by electrolytic deposition. Alternatively, the member 10 may be formed into conventional expanded metal by cutting parallel short slits in the member and pulling it to form the slits into openings which are then filled with silver, copper, or the like. In this form of the invention, it is preferable to laminate two sheets to form the member 10 such that the direction of slits in the two sheets are approximately perpendicular to each other.

WHAT WE CLAIM IS: 1. A prefabricated composite metallic plate unit for transmitting heat between a heat-absorbing medium having a planar surface and a body having a planar surface and subject during normal operation to wide temperature excursions and having a predetermined temperature coefficient of expansion, said plate unit comprising: a high-tensilestrength metallic plate member having a temperature coefficient of expansion substantially the same as that of the body, one surface of said member being adapted to be disposed in contact with the planar surface of the body and the other surface being adapted to be disposed in contact with the planar surface of said medium; a plurality of holes extending

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Semiconductor silicon ......... ................... 4.8 x 10-6 'KOVAR' ........................................ 4.9 x 10-6 ALLOY 42 ...................................... ..... 5.2 x 10-6 Silver ....................... ......................... . 19.7 x 10-6 Copper ........................................ 16.4 x 10-6 Composite member 10: ALLOY 42 and 30% copper by volume ..... 7.0 x 10-6 'KOVAR' and 30% copper .. ............... 6.5 x 10-6 Thus, while copper and silver have high heat-transfer coefficients as discussed below, their temperature coefficients of expansion differ so widely from those of silicon that, if attached to the silicon die to absorb heat therefrom, the adverse effects on the silicon die described above would result. On the other hand, members of 'KOVAR' or ALLOY 42, with copper-filled holes described above, have temperature coefficients of expansion closely matching those of silicon, so that the use of the composite member 22 avoids the adverse effects on the silicon die 23 as its temperature varies over a wide range. Heat-transfer coefficients The heat-transfer coefficients of pertinent materials over the temperature range 20 C to 400 C in cal./cm2/cm/sec./ C are as follows: 'KOVAR' . ....... . 0.04 ALLOY 42 .... . ....... ......... 0.035 Silver ....... . 0.99 Copper . ....... . 0.90 Composite: ALLOY 42 and 30% copper . . 0.25 'KOVAR' and 30% copper .. . 0.24 Thus, while 'KOVAR' and ALLOY 42 have temperature coefficients of expansion closely matching those of silicon, their heat-transfer coefficients are only about 4% of those of silver and copper and quite inadequate to dissipate heat from the silicon die 23 over an extended temperature range if used alone. However, such coefficient of the composite member 22 of ALLOY 42 and 30% copper and of 'KOVAR' and 30% copper have heat-transfer coefficients six to seven times those of 'KOVAR' and ALLOY 42 alone and adequate for all usual applications. The heat-transfer member 10 may be formed by any of several well known methods. For example, after holes 11 have been drilled in the member 10, they may be filled with copper or silver by electrolytic deposition. Alternatively, the member 10 may be formed into conventional expanded metal by cutting parallel short slits in the member and pulling it to form the slits into openings which are then filled with silver, copper, or the like. In this form of the invention, it is preferable to laminate two sheets to form the member 10 such that the direction of slits in the two sheets are approximately perpendicular to each other. WHAT WE CLAIM IS:

1. A prefabricated composite metallic plate unit for transmitting heat between a heat-absorbing medium having a planar surface and a body having a planar surface and subject during normal operation to wide temperature excursions and having a predetermined temperature coefficient of expansion, said plate unit comprising: a high-tensilestrength metallic plate member having a temperature coefficient of expansion substantially the same as that of the body, one surface of said member being adapted to be disposed in contact with the planar surface of the body and the other surface being adapted to be disposed in contact with the planar surface of said medium; a plurality of holes extending

through said member; and a relatively soft metallic material filling said holes having a heat-transfer coefficient of at least 0.3 cal./cm2/cm/sec./ C.

2. A heat-transmitting plate unit in accordance with claim 1, in which the metallic member has a tensile strength of at least 35 kg/mm2.

3. A heat-transmitting plate unit in accordance with claim 1 or 2, in which the material filling said holes is a soft metal of the group consisting of silver, copper, and aluminum, and alloys of any of these metals.

4. A heat-transmitting plate unit in accordance with claim 1, 2 or 3, in which said plurality of holes in said metallic member are uniformly distributed with respect to its planar surfaces and filled with said metallic material.

5. A heat-transmitting plate unit in accordance with claim 3, in which the aggregate area of all the holes at the planar surfaces of the metallic member is 45% or less of the total areas of such planar surfaces, after removal of the material to form the holes.

6. A heat-transmitting plate unit in accordance with claim 3 or 4, in which the total volume of said holes is 45% or less of the volume of said metallic member, after removal of the material to form the holes.

7. A heat-transmitting plate unit in accordance with any preceding claim, in which the body to be supported is a semiconductor body and the member is a nickel-iron alloy having a temperature coefficient of expansion of about 5 x 10-6 cm/cm/ C over a temperature range of from 20"C to 4000C.

8. A heat-transmitting plate unit in accordance with any one of claims 1 to 6, in which the body to be supported is a semiconductor body and the member is a nickel-cobalt-iron alloy having a temperature coefficient of expansion of about 5 x 10-6 cm/cm/ C over a temperature range of from 20"C to 400"C.

9. A heat-transmitting plate unit in accordance with claim 3, in which said relatively soft metallic material filling said holes is electrolytically deposited copper.

10. A prefabricated composite metallic heat-transmitting plate unit substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.