JPH11163233A - Aluminum heat sink and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide both an adhesive insulating film and a part which allows wire bonding by coating a surface with an anode oxide film except for a conductive connection part where a base material of aluminum or aluminum alloy is exposed. SOLUTION: Related to an aluminum heat sink 1, a protruding streak part 2 is formed at a part corresponding to wire bonding position, and the protruding streak part 2 is cut at its wire bonding position to remove an anode oxide film from an aluminum substrate. Related to a method for removing the anode oxide film, cutting with a diamond bite is most excellent, where a bite is attached to a spindle which cuts to a flat surface at a high speed of 20,000 r,p.m. while moving, with a circular streak remaining on the surface of an aluminum material. Wire bonding is possible over the entire cutting surface by a diamond bite, and both an insulating film of good adhesion to a sealing resin and a part which allows wire bonding are provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiator plate made of aluminum or an aluminum alloy, and a method of manufacturing the radiator plate, among heat radiators used for lowering thermal resistance when packaging a semiconductor chip.

[0002]

2. Description of the Related Art A semiconductor chip is packaged by various methods corresponding to its use, the electrical characteristics of a device, or the degree of heat generation. For example, in the case of a plastic package molded by transfer molding, use a single-layer lead frame, mount the chip on the die pad, make electrical connection from the chip to each lead by wire bonding, and then use epoxy resin. The process is completed by sealing with a sealing agent, and performing outer plating and bending and trimming of the outer leads. In the case of low power consumption and microcomputers and memories that do not have special requirements for electrical characteristics, this plastic package has features that sufficient reliability can be obtained and that it is inexpensive. However, a ceramic package has been used for a device which consumes a large amount of power and has strict requirements on electrical characteristics as compared with the plastic package as described above.

Conventionally, various heat radiators have been attached to this ceramic package to improve the heat dissipation. However, the ceramic package has a disadvantage of being heavy in weight and has a high manufacturing cost. Many improvements have also been made to plastic packages with low thermal resistance and excellent electrical characteristics, and they have come into practical use. That is, there is a multi-layer structure in which a heat spreader or a ground plane is attached to a lead frame using a polyimide tape with a double-sided adhesive. A so-called drop-in type package in which a metal heat spreader is inserted at the time of resin molding is inexpensive and widely used. The radiator plate made of aluminum or aluminum alloy used for such a plastic package has an anodic oxide film on its surface so as to increase the adhesion to the resin and maintain its durability even in a usage method where a part is exposed outside the package. Is usually applied. The anodic oxide film has a property of enhancing heat radiation by radiation, but is often colored black in order to further improve heat radiation.

On the other hand, in a copper heat sink, a heat radiator may be grounded in order to improve characteristics as a semiconductor device. At this time, gold or aluminum bonding wires are bonded to the heat radiator using ultrasonic waves to achieve conduction. In the case of a heat sink made of aluminum or an aluminum alloy provided with an insulating anodic oxide film, bonding cannot be performed directly on the anodic oxide film, so that the anodic oxide film has to be cut off to form a bonding surface. Further, there is also a problem that an insufficient finish of the bonding surface adversely affects the bonding.

[0005] It is possible to bond a gold wire or an aluminum wire to an aluminum material. However, generally, the bonding strength is not stable, and a certain bonding strength cannot be obtained or the bonding cannot be performed. The present inventors have found that the cause is in the surface state of the aluminum material, and in order to enable the wire bonding for grounding to be provided with an anodized film, a heat sink in which a part of the anodized film is removed First, Patent Application 8-15
No. 1795.

[0006] In order to improve the characteristics of the semiconductor device, it has been clarified that a gold or aluminum bonding wire can be bonded to the radiator using ultrasonic waves when the radiator is grounded.
Even on the aluminum surface from which the anodized film has been removed,
The bonding strength was not stable, and in some cases, was lower than the bonding strength required for practical use, and a phenomenon of causing defects occurred.

[0007]

The object of the present invention is to form a bondable region on an aluminum or aluminum alloy radiator having an anodic oxide film formed on its surface. This stabilizes the bonding strength.

[0008]

According to the present invention, there is provided a heat sink made of aluminum or an aluminum alloy used for a semiconductor device, the surface of which is covered with an anodic oxide film except for a conductive connection portion where a base made of aluminum or an aluminum alloy is exposed. This is a radiator plate made of aluminum or aluminum alloy. The exposed conductive connection portion is a heat sink made of aluminum or an aluminum alloy formed by cutting using a cutting tool. The maximum roughness of the streak formed by the cutting process is 10 μm or less, and the period of the streak is 40% of the tip diameter of the wire bonding capillary or 2.2 times the diameter of the metal wire bonded by the wire bonding. The heat radiating plate is made of aluminum or an aluminum alloy that is smaller than at least one of the numerical values.

In a method of manufacturing a heat sink made of aluminum or aluminum alloy used for a semiconductor device,
This is a method for manufacturing a heat radiator made of aluminum or aluminum alloy by cutting aluminum or aluminum alloy covered with an anodized film using a diamond cutting tool.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a method for forming a bonding surface for wire bonding by removing an anodized film from the surface of a heat radiator made of aluminum or an aluminum alloy having an anodized film formed thereon. Indicates that a large wire bonding surface is formed.

FIG. 1 is a view for explaining an example of the aluminum heat radiator of the present invention. The aluminum radiator plate 1 has a ridge 2 at a portion corresponding to a wire bonding position.
The anodic oxide film is removed from the aluminum substrate by cutting the wire bonding position of the ridge. FIG. 2 is a diagram illustrating a cross section of FIG. 1 taken along line AA. When the anodic oxide film 3 is removed, the convex portion 2 corresponding to the wire bonding position is also cut together with the anodic oxide film until the convex portion is substantially at the same height as the base surface of the aluminum heat sink. I do.

There are various methods for removing the anodic oxide film. Table 1 shows the results of measuring the bonding strength using a gold wire after removing the anodic oxide film from the surface of the aluminum material by various methods.

[0013]

Table 1 Anodized film removal method Bonding evaluation result (g) A Alkaline etching Maximum strength: 10.1 Minimum strength: 6.8 B Chemical polishing Maximum strength: 11.6 Minimum strength: 7.4 C After chemical polishing Removal of oxide film Maximum strength: 8.7 Minimum strength: 5.7 D Cutting with carbide tool Maximum strength: 11.5 Minimum strength: 6.2 E Cutting with diamond tool Maximum strength: 11.4 Minimum strength: 8.6

In Table 1, each treatment was performed under the following conditions. (A) Alkali etching An aqueous sodium hydroxide solution (50 g / l) was heated to 50 ° C., and the aluminum material was immersed for 60 seconds to dissolve the surface. Subsequent to the water washing, the substrate was immersed in nitric acid having a concentration of 30% by weight to remove smut, which is a residue from aluminum dissolved together with alkali remaining on the surface. (B) Chemical polishing 30 ml of nitric acid was added to 1000 ml of a chemical polishing liquid (Lasabrite manufactured by Rasa Kasei), and the aluminum material was immersed in a chemical polishing bath heated to 95 ° C. or higher. went. Processing time is about 90
Seconds. After the treatment, it was sufficiently washed with running water. (C) Removal of oxide film after chemical polishing The aluminum material after the above chemical polishing is replaced with phosphoric acid (35 m
1 / l) and a bath of chromic acid (20 g / l) heated to 95 ° C. or higher for 3 minutes to dissolve and remove the surface oxide film. (D) Cutting with Carbide Tool Using a tool bit made of tool steel coated with titanium nitride, the surface of the aluminum material was milled at a rotation speed of 6000 rpm using machine oil as lubricating oil. (E) Cutting by Diamond Tool Using a cutting tool using sintered diamond, the aluminum surface was milled at 18,000 rpm using kerosene as a lubricating oil.

The evaluation of the bonding strength of the bonding performed after each treatment was performed using a 30 μm gold wire and a load of 80 μm.
g, an ultrasonic output of 100 W, a pressurization time of 15 msec, and a temperature of 250 ° C., 10 tests were performed, and the maximum and minimum bonding strengths performed 10 times were shown.

Among these removal methods, the cutting method using a diamond tool is the most excellent. However, even when cutting using a diamond tool, the bonding strength is not stable and becomes lower than the practical bonding strength. Occasionally a phenomenon occurred. When we examined the cause of joint failure when cutting with a diamond bite by observing the joints, it was estimated that the occurrence of joint failure was related to the cutting streak. In order to clarify the condition, cutting was performed using a pointed tool bit having a tip angle of 150 ° while changing the feed conditions, and the bonding strength at each wire bonding position was measured.

In cutting with a diamond tool,
A single crystal of diamond or a combination of fine grains is used. Generally, a diamond cutting tool is mounted on a rotating shaft and is moved while rotating at a high speed of 20,000 revolutions per minute, thereby cutting a flat surface. Therefore,
The shape of the tip of the diamond bite is left in a circle as a streak on the surface of the aluminum material.

FIG. 3 is an enlarged view for explaining a streak when cutting with a diamond cutting tool. The striations 4 are formed in a circular shape, but when observed with a microscope or the like, they appear to be formed in a substantially linear shape. The bonding wire 5 is pressed and bonded by a bonding capillary. Indentations 6 are formed on the surface of the aluminum material by a bonding capillary, and a wire joint 7 of the bonding wire 6 is formed.
Is formed.

FIG. 4 is an enlarged view for explaining an example of the shape of the cross section of the streak. Since the diamond bit has a V-shaped tip, a V-shaped continuous cross-sectional shape as shown in FIG. 4 is formed. In this example, the feed pitch in the horizontal direction is 50 μm, and the height of the unevenness in the vertical direction is 10 μm.
m. The pitch of the formed marks is determined by the moving distance of a tool such as a diamond tool, that is, the feed length, and at the same time, the height of the surface irregularities formed by the marks is determined.

The diameter of the bonding portion by bonding varies depending on the wire diameter of the bonding wire, the shape of the bonding capillary, and the bonding conditions. The quality of the bonding is determined by the relationship between the diameter of the bonding portion and the cross-sectional shape of the striation. That is, for example, if the pitch of the streaks is sufficiently larger than the diameter of the joint, bonding in the recesses of the streaks is performed well. However, in this case,
Since the height of the protruding portion of the streak is high, bonding extending to the protruding portion is likely to be defective. In these phenomena, the pitch of the streaks affected the bondability in both the ball bonding method and the wedge bonding method.

On the other hand, if the feed of the cutting tool is made sufficiently smaller than the diameter of the joint, no joining failure occurs, but there is a problem that the time required for cutting becomes longer and the manufacturing time becomes longer. Therefore, the present invention provides a cutting method using a diamond tool having excellent properties, by reducing the projection formed by the streak accompanying the cutting to 8 μm or less, thereby enabling wire bonding on the entire surface of the cutting surface by the diamond tool. It has been found that it is possible to manufacture an aluminum alloy heat sink having an insulating film having good adhesion to the sealing resin and a portion capable of wire bonding.

The maximum roughness of the streak formed by the cutting process is 10 μm or less, and the period of the streak is 40% of the tip diameter of the capillary of the wire bonding or the diameter of the metal wire bonded by the wire bonding. It is preferable that the value is smaller than at least one of the numerical values.

If the maximum roughness of the streak formed by the cutting process is larger than 10 μm, the load is not uniformly applied to the bonding portion if the bonding portion formed by bonding is located on the convex portion of the streak, resulting in poor connection. It is not preferred because it is easy If the period of the streak is larger than one half of the capillary diameter in wire bonding, the joint may be formed only at the convex portion or the concave portion of a single streak. May occur. Preferably, the joint is formed over a plurality of projections or depressions.

The heat radiating plate of the present invention has high bonding reliability of wire bonding, and makes it possible to manufacture a plastic package having both excellent thermal and electrical characteristics as compared with a conventional package.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the embodiments of the present invention. Example 1 An Al-2% Fe alloy having a thickness of 1.5 mm was cut into 24 mm squares, and stepped by press working to produce a heat dissipation plate. On the back surface, an annular protrusion having a height of about 0.02 mm was provided as shown in FIG. After this aluminum material was attached to an anodizing jig, anodizing treatment was performed in a 16% by weight sulfuric acid bath using a lead electrode as a counter electrode. The electrolysis conditions were as follows: bath temperature 15 ° C., current density 1.5 A / dm ² , energizing time 30
In about a minute, a coating of about 15 μm was obtained. Sealing treatment was performed to ensure corrosion resistance. Sealing treatment is nickel acetate 5g /
l, 6 g / l of boric acid were dissolved in water, and acetic acid and sodium hydroxide were added to adjust the pH to a range of 5 to 6; .

Next, the anodized aluminum material is fixed to a jig, and the surface of the alumite is cut with a diamond tool, and the annular projection on the back surface has a height of 0.002 mm or less from the surrounding anodized film surface. I cut it down to become. An aluminum base appeared in that part. As shown in Table 2, the feed of diamond cutting tool is 88, 66,
48 and 38 μm. At this time, the height of the streak measured using a surface roughness meter (manufactured by Kosaka Laboratories) was as shown in Table 2.

A gold wire having a diameter of 30 μm was applied to the streak surface with a tip diameter of 17 μm.
Ultrasonic bonding was repeated 10 times with an 8 μm wire bonding capillary, and the bonded gold wire was pulled to measure the bonding strength. Table 2 shows the measurement results. Feed pitch 66 μm or less, ie, streak height 10 μ
m or less, all of the ten times of bonding exhibited stable adhesion with a tensile strength of 6 g or more. Furthermore, when the pitch of the streak was reduced and the streak height was lowered, the bonding strength increased. However, when the pitch is 48 μm or more, if the direction of the streak with respect to the bonding wire is horizontal, the strength may be insufficient depending on the bonding position of the bonding wire. Therefore, when the pitch is further reduced to 38 μm,
The bonding strength was stable regardless of the horizontal or vertical direction with respect to the bonding wire.

[0028]

[Table 2] No. Cutting conditions Streak height Bonding strength (unit g) 1 Feed pitch 88 μm 16 μm Maximum strength: 9.8 Minimum strength: 0.9 2 Feed pitch 66 μm 10 μm Maximum strength: 10.2 Minimum strength: 6 .23 feed pitch 48 μm 8 μm maximum strength: 10.1 minimum strength: 8.34 feed pitch 38 μm 7 μm maximum strength: 10.5 minimum strength: 8.1

The aluminum heat sink covered with an insulating anodic oxide film can be used as a ground plane by bonding, similarly to the conventional copper heat sink, and the bonding strength of the bonding can be secured stably.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of an aluminum heat radiator of the present invention.

FIG. 2 is a diagram illustrating a cross section of FIG. 1 taken along line AA.

FIG. 3 is an enlarged view for explaining striations when cutting with a diamond cutting tool.

FIG. 4 is an enlarged view for explaining a cross section of a streak.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Aluminum radiator, 2 ... Convex stripe, 3 ... Anodized film, 4 ... Trace, 5 ... Bonding wire, 6 ... Indentation by bonding capillary, 7 ... Wire joint

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoichi Miura 1-1-1, Ichigaya-Kagacho, Shinjuku-ku, Tokyo Inside Dai Nippon Printing Co., Ltd. (72) Inventor Shinichiro Hori 1-1-1, Ichigaya-cho, Shinjuku-ku, Tokyo No. 1 Inside Dai Nippon Printing Co., Ltd. (72) Inventor Masato Sasaki 1-1, Ichigaya Kaga-cho, Shinjuku-ku, Tokyo Inside Dai Nippon Printing Co., Ltd. (72) Inventor Yoshio Hirayama 1 Fuyodai, Mishima-shi, Shizuoka 4-12 Hirayama Technician Office at Chome 12 (72) Inventor Takayuki Hayade 610 Nagaji Gosho, Okaya City, Nagano Prefecture Sode Nagano Co., Ltd. Japan High Purity Chemical Co., Ltd.

Claims (4)

[Claims] 1. An aluminum or aluminum alloy radiator plate used for a semiconductor device, the surface of which is covered with an anodic oxide film except for a conductive connecting portion where a base made of aluminum or an aluminum alloy is exposed. Heat sink made of aluminum or aluminum alloy. 2. The heat radiating plate made of aluminum or aluminum alloy according to claim 1, wherein the exposed conductive connecting portion is formed by cutting using a cutting tool. 3. The maximum roughness of a streak formed by cutting is 10 μm or less, and the period of the streak is 40% of the tip diameter of a capillary of wire bonding or the diameter of a metal wire bonded by wire bonding. 3. The heat radiating plate made of aluminum or an aluminum alloy according to claim 2, wherein the heat radiating plate is smaller than at least one of 2.2 times. 4. A method of manufacturing a heat sink made of aluminum or aluminum alloy used for a semiconductor device, wherein aluminum or aluminum alloy covered with an anodized film is cut using a diamond cutting tool. And a method for manufacturing an aluminum alloy radiator.