US3449506A - Aluminum rectifier base having copper insert - Google Patents

Description

June 10, 969 H. wElNsTElN ET Al- 3,449,506,

ALUMINUM RECTIFIER Iii/XSE` HAVING QOIIER INSERT Filed May 11, 1967 United States Patent Ol 3,449,506 ALUMINUM REC'IIFIER BASE HAVING COPPER INSERT Harold Weinstein, Van Nuys, and Ernst Ortner, Los Angeles, Calif., assignors to International Rectifier Corporation, El Segundo, Calif., a corporation of California Filed May 11, 1967, Ser. No. 637,785 Int. Cl. H05k 5/04 U.S. Cl. 174-52 5 Claims ABSTRACT 0F THE DISCLOSURE An aluminum rectifier base for supporting a semiconductor wafer is formed of aluminum, and has a copper insert extending into and slightly above the central surface of the base. The outer periphery of the insert and the internal periphery of an opening in the stud which receives the insert have cooperating gear-shaped convolutions which mesh with one another, and an annular groove extends around the interior of the opening in the insert. A heading operation forces the gear-shaped convolutions into intimate thermal and electrical contact with the insert material, and insert material flows into the groove to lock the insert in position. The insert may be cylindrical or conical in shape. The at bottom surfaces of the insert and opening can be brazed.

This invention relates to semiconductor device bases, and more particularly relatesto a base or stud for semiconductor devices which has a majorportion thereof composed of aluminum, and a central wafer-receiving insert of copper, which is bonded to an opening Within the base and extends above the upper surface of the stud.

There are many advantages in using aluminum as the material for a base which is to carry a semiconductor wafer. These advantages include the increased strength of aluminum, as compared to the commonly used copper, a weight reduction of the stud, and reduction in cost. 'I'he disadvantages of aluminum are that it has poor solder ability characteristics for soldering to the wafer, and a poorer match than copper to the thermal coefficient of expansion of the molybdenum expansion plates which commonly carry the semiconductor wafer.

In accordance with the present invention, the advantages of both aluminum and copper are fully exploited in a novel combined stud, having a major body portion of aluminum, which has an insert extending into the upper surface thereof of copper. The copper then serves as the pedestal for receiving the semiconductor wafer, thereby taking advantage of its superior solder-ability characteristics for soldering to a wafer and its closer coeflicient of thermal expansion characteristics to the` molybdenum plates secured to the opposite surfaces of the semiconductor wafer. The remainder of the stud, which is aluminum, imparts to the overall stud a significant weight reduction, cost reduction and increased strength.

In order to create a good bond between the copper insert and the main aluminum body, the outer periphery of the insert and the interior of the opening in the base which receives the insert have an increased circumferential length by causing these surfaces to have a gear-type shape. In addition, an annular groove is formed in th-e interior opening of the aluminum base body, whereby a cold-heading operation can be used forcing the insert into the stud opening, with material flow from the insert and into this groove locking the insert and stud body securely to one another. The same heading operation causes intimate contact between the registering gear shapes of the insert and opening.

3,449,506 Patented June 10, 1969 ICC By positioning the insert in the center of the base region, it has been found that there will be balanced thermal expansion between the central copper insert and the exterior or surrounding aluminum material even though these two bodies have different coeflicients of expansion. Thus, there will be balanced expansion of both the insert and the aluminum base, tending to reduce forces created due to differential thermal expansion and contraction which might tend to loosen the insert and the base body.

An improved interface contact between the insert and the aluminum body can be obtained by brazing the insert into the body as well as by cold-heading. Where brazing is used, a suitable brazing compound, such as palladium, can coat the surfaces which are to be connected to one another, and the temperature thereafter is suitably raised. Alternatively, a brazing compound, such as palladium, can be connected only to the flat engaging interfaces of the stud and insert, and in combination with the coldheading operation for intimately bonding the peripheral regions of the insert and base, the flat engaging surfaces can be bonded by the brazing compound.

Accordingly, a primary object of this invention is to provide an improved base for carrying semiconductor wafers which has the combined desirable properties of aluminum and copper.

Another object of this invention is to provide a novel base for semiconductor devices which has a copper insert region for mounting a semiconductor wafer, and an exterior and main aluminum body portion which receives the insert.

Another object of this invention is to provide a novel base structure :for carrying semiconductor wafers which can be easily soldered to a semiconductor Wafer, has increased strength and decreased weight and cost.

Yet another object of this invention is to provide a novel arrangement for a copper insert in an aluminum base body in which the insert and body have balanced expansion and contraction during thermal cycling.

These and other objects of this invention will become apparent from the following description when taken in connection with the drawings in which:

FIGURE 1 is a top view of the aluminum base body ofy the present invention.

FIGURE 2 is a cross-sectional view of FIGURE 1 taken across the lines 2-2 in FIGURE 1.

FIGURE 3 is a top View of a cylindrical copper insert which is to be bonded in the opening of the aluminum base of FIGURES l and 2.

FIGURE 4 is a side plan view of the gear-shaped cylindrical Vcopper insert of FIGURE 3.

FIGURE 5 is a cross-sectional view showing the insert of FIGURES 3 and 4 bonded into the interior of the aluminum base of FIGURES 1 and 2.

FIGURE 6 is similar to FIGURE 5, but shows a conically-shaped insert contained in a conically-shaped opening in the aluminum base body.

Referring first to FIGURES 1 and 2 there is illustrated a base body 10` which is of aluminum, and has a hexagonal heard portion 11, with a threaded stud 12a extending from the bottom of head portion 11. Note that the threaded stud 12m can be eliminated, with the base body still being constructed in accordance with the invention. A gearshaped opening 13 is formed in the upper surface of head portion 11 by any suitable machining technique, and an annular groove 14 is thereafter machined around the interior of opening 13 at a central axial position therein. Typically, the head portion 11 of base 10 may lit within a circle having a diameter of about 2 inches; the height of head portion 11 could typically be 1/2 inch; the depth of opening 13 could typically be of an inch; and the maximum diameter of opening 13 could be about 11/2 inches.

A copper insert 15, shown in FIGURES 3 and 4', is then prepared, which has a gear-shaped outer periphery which conforms generally to the gear-shaped interior of opening 13. Preferably, the teeth of insert would have about 0.020 inch clearance within opening 13 so that the insert can be easily loaded into the opening. The height of insert 15 is greater than the depth of opening 13 and could, for example, be about 1/2 inch. The insert 15 is then loaded into opening 13 and, by a cold-heading operation, a strong impact force is applied to the Itop of insert .15, causing the insert to be crushed downwardly, with its outer periphery extending into intimate bonding relation with the interior surface of opening 13. In addition, material from insert 15 will flow into groove 14 which can have a depth of about Ms inch below the troughs of the gear-shaped interior of opening 13. The ilow of material into groove 14 insures a `good mechanical lock between insert 15 and head portion 11. As an alternate method the copper insert can be loaded in the opening of an aluminum slug, before the base is formed. The A1 base with the copper insert can be formed this way in one single cold-heading operation. y

The upper surface of insert 15 is then lapped at after the coldheading operation to insure a fiat receiving surface for such semiconductor wafers. For this purpose, it is preferred to have the copper insert material continuing to extend above the surface of head portion 11 after the cold-heading operation. A silicon wafer, preassembled in the usual manner with molybdenum expansion plates may then be easily soldered to the top surface of insert 15, as illustrated in dotted lines in FIGURE 5.

In order to obtain a good bond between the opposing at surfaces forming the bottom of opening 13 and the bottom of insert 15, the bottom of openingr13 may be coated with a thin palladium layer 16, as shown in FIG- URE 5. Brazing can then be accomplished between bottom surfaces of opening 13 and insert 15, thereby insuring good electrical andy thermal connection between them. Alternative to the use of palladium, it will be apparent that the bottom of opening 13 can be loaded with any desired electrically and thermally conductive compound. l

While FIGURES 1 to 5 show opening 13 and insert `15 of cylindrical configuration, they may both be conical in shape, as illustrated in FIGURE 6. In FIGURE 6, components similar to those of FIGURES 1 to 5, receive similar identifying numerals where it is seen that the opening 13 in FIGURE 6 has a conically undulating outer periphery. Similarly, the insert 15 has a cooperating conical shape where, again, a cold-heading operation intimately bonds the exterior gear-shaped surface of conical section 15 to the cooperating convolutions of conical opening 13, with further locking obtained by flow of material into groove 14.

It will also be apparent that palladium or another brazing material can be contained between the bottom at surfaces of aluminum portion 11 and insert y15, and

even the gear-shaped outer peripheries of these members.

By using a conical insert, there are certain advantages over the cylindrical insert of FIGURES 1 to 5 in that the flat surfacetosurface area between the bottom of opening 13 and the bottom of insert 15 is reduced, and the mechanical strength of the stud body 11 is increased in this region.

Although this invention has been described with respect to its preferred embodiments, it should be understood that many variations and modifications will now be obvious to those skilled in the art, and it is preferred, therefore, that the scope of the invention be limited not by the specific disclosure herein, but only by the appended claims.

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

1. A mounting base for a semiconductive device comprising an aluminum body having a generally at upper surface and a'copper insert for said aluminum body; said aluminum body having a central opening extending thereing from said flat upper surface; said copper insert secured in said central opening and having an upwardly extending portion extending above said flat upper surface; said upwardly extending portion of said insert having a flat wafer-receiving surface generally parallel to said at upper surface; the peripheral wall of said insert mechanically and electrically bonded to the surrounding wall of said opening; said opening in said aluminum body having a central annular groove therein; said insert having an annular bead extending therefrom and intosaid groove.

2. The base of claim 1 wherein said insert has a flat bottom and said opening has a flat bottom of area coextensive with and in engagement with the area of said flat bottom of said insert; and wherein the periphery of said opening and the periphery of said insert have cof operating gear shapes for increasing the area of engagement of said aluminum body and copper insert.

3. The base'of claim 1 which includes a brazing compound at thebottom of said opening for brazing the bottom surface of said insert to the bottom surface of said opening.

4. The base of claim 1 wherein said insert and opening have cooperating conical shapes tapering inwardly toward their said bottom surfaces. 5. The base of claim 1 wherein said aluminum body has a threaded extension extending from the bottom surface thereof. I

References Cited UNITED STATES PATENTS 3,361,868 1/1968 Bachman 174-52 LEWIS H. MYERS, Primary Examiner.

D. A. TONE, Assistant Examiner.

U.S. Cl. X.R. 317--234

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