JPH03159266A - Semiconductor device for lead frame - Google Patents

Abstract

PURPOSE:To obtain a new lead frame possessed of large mechanical strength and excellent direct bonding properties by a method wherein pure copper or copper alloy excellent in direct bonding properties is used as a base material for the lead frame, and an alloy layer of the base material and different metal is formed on the surface of the base material excluding at least the bonding region of an inner lead. CONSTITUTION:Metal such as zinc, tin, or the like is comparatively easily diffused into pure copper or copper alloy which services as a base material to form an alloy layer high strength with the base material. The alloy layer concerned can be formed either before or after a tab 12, inner leads 4, outer leads 6, and the like are carved, but usually it is previously formed on a lead frame in a plate material phase, and then a carving work is carried out. Therefore, an alloy layer is not formed on the side faces of the lead frame, so that the lead frame has a sandwich structure, and the used different metal is controlled in type, diffusion concentration, diffusion depth, and the like, whereby every part of the lead frame can be made to have a required strength. By this setup, a lead frame excellent in both mechanical strength and direct bonding properties can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to a lead frame used for mounting semiconductor chips such as ICs (integrated circuits) and LSIs (large scale integrated circuits).

[Prior Art] In general, when assembling a semiconductor device, tabs (semiconductor chip mounting plate) and inner leads (conducting wires for internal wiring) are prepared in advance by pressing or etching.

A so-called lead frame is used, which has the shape of outer leads (conductor wires for external wiring) etched into it.

To assemble a semiconductor device, after placing the semiconductor chip on the central tab, each electrode of the chip is connected to the inner end of the corresponding inner lead using a gold or copper wire (wire bonding). .

Next, the semiconductor chip and its peripheral parts (including the inner ends of the inner leads and wire bonding parts) are integrated with a synthetic resin mold, and then that part is separated from the frame of the lead frame along with the 7 outer leads to form the desired flat type. Or obtain an in-line type semiconductor device.

Typically, lead frames are made of high-strength copper alloy materials in which dissimilar metals such as iron, nickel, tin, and silicon are added to copper, or these dissimilar metals are diffused into the surface area of pure copper or a copper alloy base metal. It is made of a surface-strengthening material (for example, see Japanese Patent Laid-Open No. 105259/1983) on which an alloy layer is formed.

This type of material has sufficient mechanical strength as a lead frame, as well as excellent properties in many aspects such as pressability, bendability, solder wettability, plating performance, and resin adhesion. Recently, when these materials are used as they are, the added or diffused dissimilar metals form an oxide film.
It has been found that this oxide film significantly impairs the adhesion (direct bondability) between the inner lead and the bonding wire.

However, this problem can technically be solved by gold plating or silver plating the inner lead. However, as a recent new trend, the demand to completely omit the plating process is rapidly increasing, so the use of high-strength copper alloy materials and surface-strengthening materials is likely to be restricted in the future.

On the other hand, pure copper-based materials containing almost no additive metals, such as C1
Precipitation-type alloy materials such as 51 (Cu-0,1% Zr) are superior in that they do not generate oxide films that inhibit direct bonding properties, but copper alloy materials with high mechanical strength as lead frames are preferred. It has the essential drawback that it is considerably inferior to other surface-reinforced materials.

[Problems to be Solved by the Invention] As mentioned above, conventional lead frame materials have problems in strength and bonding that are incompatible with each other, and there is a strong desire for an excellent lead frame that solves this contradiction. It is rare.

Therefore, an object of the present invention is to provide a novel lead frame that has both high mechanical strength and good direct bondability.

[Means for solving the problem] The above problem is solved by using pure copper or a copper alloy with excellent direct bonding properties as a base material, and forming a base metal and a dissimilar metal on the surface of the base material excluding at least the bonding area of the inner lead. This can be solved by forming an alloy layer of The base metal must not contain more than 1% of additional elements, because if the added elements exceed 1%, the direct bonding properties will rapidly deteriorate.

The alloy layer is formed by coating the surface of the base material with a dissimilar metal using an appropriate method, then heating the whole to a high temperature to diffuse the dissimilar metal into the base material. The method of coating dissimilar metals is to apply a paint-type metal penetrant containing the powder (Japanese Unexamined Patent Publication No. 6
2-54086), a plating method, a vapor deposition method, a thermal spraying method, an ion implantation method, etc. can be used.

No alloy layer should exist in the bonding area. Therefore, after forming an alloy layer on the entire surface of the base material, a mask is applied to areas other than the bonding area, and unnecessary alloy layers are selectively removed by chemical etching, etc. On the other hand, it is necessary to mask the bonding area and selectively diffuse the dissimilar metal to form an alloy layer.

Dissimilar metals that form an alloy layer with the base metal include:
Although it is possible to use various metals other than copper, it is particularly desirable to use one or more metals selected from zinc, tin, silver, aluminum, nickel, and titanium.

The thickness of the alloy layer is determined by the balance between the required strength and conductivity, but it is desirable that the alloy coverage on one side of the shell is in the range of 3 to 20% of the total plate thickness. If it is less than 20%, hardly any effect of increasing strength can be expected, while if it is more than 20%, the conductivity will be significantly reduced and the difference between the area where the alloy layer is formed and the area where it is not formed (or the area where the alloy layer is removed) will be reduced. If the plate thickness difference becomes too large, problems such as an increase in manufacturing time and effort will occur.

The concentration of different metals in the alloy layer, especially the surface concentration, is determined by the range of solid solution of the different metals in the base metal, in other words, the range of solid solution of the different metals in the base metal. It is desirable that the range is such that the composition of the α phase is maintained.

[Function] Metals such as zinc, tin, silver, aluminum, nickel, and titanium diffuse relatively easily into the base metal of pure copper or copper alloy, forming a high-strength alloy layer with the base metal. do. Therefore, a lead frame having this type of alloy layer on its surface has a strength comparable to that of a lead frame made entirely of a high-strength copper alloy.

The alloy layer can be formed either before or after forming the tabs, inner leads, outer leads, etc. Ideally, it would be desirable to form the alloy layer after engraving, but such a procedure is not suitable for mass production. For this reason, 1. Usually, an alloy layer is formed in advance at the plate stage, and then engraving is performed. Therefore, there is no alloy layer on the side surfaces of each part of the lead frame, and the lead frame has a sandwich structure in which the alloy layer exists only on the front and back sides.

By controlling the type, diffusion concentration, diffusion depth, etc. of the dissimilar metals used, it is possible to provide each part of the lead frame with the required strength.

[Example] Five types of copper alloys with different compositions were melted in a conventional manner, and then subjected to hot working, cold working, and annealing to obtain an intermediate plate material with a thickness of 0.5 mm. Zinc is applied to the entire surface of this intermediate plate material.
After applying a paint-type metal penetrant containing aluminum or tin powder, heating was performed at a temperature higher than the melting point of each metal to diffuse each metal into the intermediate plate material to form a surface alloy layer. Further, the intermediate plate material on which the surface alloy layer was formed was cold rolled to obtain a final plate material having a thickness of 0.25 cm and a width of 25 cm. on the other hand.

For comparison, another final plate having the same thickness and width as the final plate was obtained by cold-rolling an intermediate plate with a thickness of 0.5 cm as it was (without forming a surface alloy layer).

Table 1 shows the results of comparing the tensile strength of each of the two final plate materials obtained in this way.As is clear from the table, the final plate material with one surface alloy layer formed has a surface alloy layer The tensile strength of both sheets is improved compared to the final plate material that was not formed.

Next, the final plate material with the surface alloy layer formed thereon was chemically etched to produce a lead frame as shown in FIG. In the figure, 1 is a semiconductor chip shown for reference, 2 is a rectangular tab for mounting the chip, 3 is a tab lead for supporting the tab 2 at its four corners,
4 is a plurality of inner leads that protrude so as to face the periphery of the tab 2 (only two inner leads are shown in the figure), and 5 is a frame for supporting the outer ends of both leads 3.4. 6 are outer leads carved corresponding to the tab leads and the inner leads, and 7 are sprocket holes provided at regular intervals along both side edges of the frame portion 5.

In order to confirm the wire bonding performance, the shaded area 8 including the bonding part of the inner lead 4 was
The core material was exposed by etching to a depth of . Then, wire bonding was performed on the tips (inner ends) of the inner leads 4 and the crimping rates were compared for the sample in which the core material was exposed in this way and the sample in which one surface alloy layer was left intact.

The results are shown in Table 2. The wire bonding was performed using an ultrasonic thermocompression bonder under the conditions of 300° C., a load of 75 g, and a time of 16 hours for 1 hour. The wire used was 30.1111 gold wire.

As is clear from Table 2, the sample with the surface alloy layer left as it is shows only an extremely low crimp rate, but the sample with the surface alloy layer removed to expose the core material has a large amount of added elements. Except for sample 10 (Cu-1,3%Sn), a good crimp rate that can withstand normal use is shown. In addition, sample 1
In the case of 0, the compression bonding rate is somewhat better with the surface alloy layer removed, but this level of compression bonding rate is still insufficient for normal use. Therefore, the amount of the additive element relative to the base metal is limited to at most 1%.

[Effects of the Invention] By using the lead frame according to the present invention, direct bonding properties can be improved while maintaining high strength of the outer lead portion. Therefore, it is possible to perform direct wiring while responding to thinner lead frames and increased number of pins.

[Brief explanation of the drawing]

FIG. 1 is an external view showing an embodiment of a lead frame according to the present invention. <Explanation of symbols> 1...Semiconductor chip, 2...Tab, 3...Tab lead. 4... Inner lead, 5... Frame portion, 6... Outer lead, 7... Sprocket hole, 8... Core material exposed portion. In the above example, sample 6 in which aluminum was diffused In the case of , the solder wettability was abnormally reduced, so we attempted to improve it by further applying copper plating on top of the alloy layer to create a three-layer structure. In actual use, such a structure may be used.

Claims (1)

[Claims] 1. The base material is pure copper or a copper alloy with excellent direct bondability, and an alloy layer of the base metal and a different metal is formed on the surface of the base material excluding at least the bonding area of the inner lead. A lead frame 2 for a semiconductor device characterized in that the lead frame 3 according to claim 1 is characterized in that the amount of the additive element to the base metal is at most 1% or less, the dissimilar metal is zinc, The lead frame 4 according to claim 1 or 2, characterized in that the lead frame 4 is made of at least one metal selected from tin, silver, aluminum, nickel, and titanium, and the thickness of the alloy layer is equal to or less than the total thickness of the lead frame 4. The lead frame 5 according to any one of claims 1 to 3, characterized in that the alloy coverage on one side is in the range of 3 to 20% with respect to the plate thickness, and further on the alloy layer. The lead frame according to any one of claims 1 to 4, characterized in that it has a three-layer structure by being plated with copper.