JPH04249352A - Leadframe for semiconductor device - Google Patents

Abstract

PURPOSE:To provide a leadframe, for use in a semiconductor, coated with a nickel plating, which possesses a superior solderability without a flux. CONSTITUTION:A leadframe, for use in a semiconductor device, is composed of a substrate, which is made up of copper, a copper alloy or an iron alloy, coated with a nickel-cobalt alloy plating. The substrate thus plated is further coated with a nickel-iron-phosphorus alloy plating, which is composed of, in percent by weight, 1-10 iron, 2-10 phosphorus, and the balance nickel. Since an oxide layer, produced by the nickel-cobalt alloy plating, is not as hard as that produced by a pure nickel plating, the nickel-cobalt alloy plating is improved in solderability. However, the nickel-cobalt alloy plating tends to be impaired in colorfastness when used solely, and suffers the progressive growth of an oxide layer, thereby being deteriorated in solderability. To prevent this, the surface of the nickel-cobalt alloy plating is further coated with a thin film of the amorphous nickel-iron-phosphorus alloy plating which is superior in anticorrosion and not liable to the growth of the oxide film.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to lead frames for semiconductor devices, particularly lead frames for transistors or ICs.

0002

[Prior Art] Lead frames for transistors and ICs have conventionally been made by applying nickel plating, etc., to a long strip of copper alloy or iron alloy, and then punching it into the desired shape through a pressing process. . In this step, the plating position is controlled so that the element mounting area, the wire bonding area, and the soldering area during mounting become the plating area.

In a lead frame, a transistor or an IC chip is soldered to an element mounting portion. Further, in the actual device after assembly, the outer lead pins are attached to the board and soldered. As described above, a lead frame is required to have excellent solderability on both the element mounting portion and the outer lead portion, and nickel plating is performed as one method that satisfies both.

[0004] Watt baths are commonly used for nickel plating, and brighteners and stress relievers are added.

[0005] The soldering reaction is a reaction between the base metal and tin in the solder to form an intermetallic compound. Nickel plating generally tends to form a corrosion-resistant oxide film, and when an oxide film is formed on the surface of the nickel base metal, this reaction is significantly suppressed, resulting in poor solderability. Therefore, nickel plating can be said to be a plating film that usually has very poor solder properties.

[0006] When soldering nickel-plated electronic components, which have poor solderability, conventionally, the oxide film on the surface of the nickel plating was removed using flux, and then soldering was immediately performed. Since flux often contains active ions such as chlorine, it is necessary to wash the electronic components with an organic solvent such as fluorocarbon or trichlorethylene after using the flux to dissolve and remove the flux. In recent years, due to global environmental issues and safety standards for handling chemicals, the use of the cleaning solvents mentioned above has begun to be rapidly regulated, and as a result, it has become desirable to improve the efficiency of the post-cleaning process for electronic components. There is an urgent need to develop a nickel plating process that provides good solderability without the use of flux.

[0007] Conventionally, Japanese Patent Publication No. 333/1983 has been published regarding the improvement of the solderability, especially the soldering strength, of lead frames in which a substrate made of copper or copper alloy is plated with nickel.
In order to improve the solderability of a lead frame plated with bright nickel, Patent No. 12 discloses that a ring-containing nickel layer is provided on the bright nickel plating layer to prevent natural oxidation of the surface of the nickel layer due to the reducing effect of the phosphorus contained therein. A method has been proposed to improve the adhesive strength of solder by preventing this and improving wetting with the solder. In addition, special public service 61-224
In the method for improving solderability in which a phosphorous-containing nickel layer is provided on a bright nickel plating layer, the adhesive strength between an IC chip and a lead frame soldered on an automatic assembly machine varies, making it unsatisfactory. Therefore, in order to further improve solderability on an automatic assembly machine, a nickel plating layer containing 0.3 to 20% by weight of cobalt was provided as the first plating layer, and a nickel plating layer containing 0.3 to 20% by weight of cobalt was added on top of the nickel plating layer. A method has been proposed in which a nickel plating layer containing 6 to 20% by weight of cobalt and phosphorus is provided to improve the adhesive strength of soldering electronic components such as IC chips to lead frames using automatic assembly machines.

[0008]

[Problems to be Solved by the Invention] However, Japanese Patent Publication No. 61-22458 does not mention omitting the use of flux in the process, and there is still no mention of the development of a plating process that does not require the use of flux. It seems that the goal has not yet been achieved.

In particular, the above-mentioned prior art does not mention at all a lead frame made of a substrate made of iron alloy.

[0010] Therefore, it is desired to develop a nickel plating process that can be applied to substrates made of copper, copper alloys, or iron alloys and has good solderability without the use of flux.

An object of the present invention is to provide a lead frame for a semiconductor device, which is provided with nickel plating that has excellent solderability even without flux, for electronic component materials made of copper, copper alloy, or iron alloy, especially lead frames. It is about providing.

0012

[Means for Solving the Problems] The above-mentioned object of the present invention is to provide nickel-cobalt alloy plating on a copper, copper alloy or iron alloy substrate, and further to provide a nickel-cobalt alloy plating on which the proportion of iron in the alloy plating is iron 1. This can be solved by providing a nickel-iron-phosphorus alloy plating containing ~10% by weight, 2-10% by weight of phosphorus, and the balance being nickel.

[0013] Also, in the nickel plating method, the first
A particularly suitable lead frame can be obtained when the alloy component ratio of the nickel-cobalt alloy plating layer is 1 to 10% by weight of cobalt and the balance is nickel.

[0014] Nickel-iron-phosphorus alloy plating alone exhibits extremely excellent solderability. However, because it is extremely hard and brittle, it cannot be plated thickly on electronic parts that require bending. Therefore, after applying nickel-cobalt alloy plating to approximately 3 μm as the base plating, nickel-iron
Plating is performed in a two-layer structure with a thickness of about 0.2 μm of phosphorus alloy plating. With this plating method, the solder wettability is significantly improved compared to bright nickel plating alone or nickel-cobalt alloy plating alone, and the plating cracking phenomenon that accompanies bending does not occur.

Since nickel-cobalt alloy plating does not form an oxide film as strong as pure nickel plating, solderability is improved. However, nickel-cobalt alloy plating alone has poor discoloration resistance and gradually forms an oxide film, which impairs solderability.・Thin iron/phosphorus alloy plating is applied.

The thickness of the underlying nickel-cobalt alloy plating layer is preferably about 1 to 5 μm, and the thickness of the nickel-iron-phosphorus alloy plating layer is preferably in the range of 0.05 to 0.2 μm. If the thickness of the nickel-iron-phosphorus alloy plating layer is greater than 0.2 μm, cracking may occur in the plating when the electronic component is bent.

[0017] Even if the ratio of alloy components of the underlying nickel-cobalt alloy plating is high, the solder wettability is good;
Although not particularly limited, since it is expensive, it is preferable that the content of cobalt be 1 to 10% by weight, with the balance being nickel. In addition, the proportions of each alloy component in the nickel/iron/phosphorus alloy plating are iron: 1 to 10% by weight, phosphorus: 2 to 10% by weight,
Nickel accounts for the remainder. If the iron content is more than 10% by weight, the plated surface will be slightly discolored and the corrosion resistance of the nickel/iron/phosphorus alloy plating will decrease, so the iron content should be 1.
It may be up to 0% by weight. It is preferable that the underlying nickel-cobalt alloy plating be bright plating, and a brightener and stress relaxation agent are added to the plating solution.

[0018]

EXAMPLES For a better understanding of the present invention, specific examples are shown below, but it goes without saying that the present invention is not limited to these examples.

The test method for the solderability of the nickel-plated lead frames and the evaluation criteria for the soldering condition prepared in the following Examples and Comparative Examples are as follows.

(Soldering test method) Lead frame 1
Place the piece in a clip and use an automatic lifting device to solder it with eutectic solder (62% tin, 38% lead) without using flux.
) was immersed in a molten solder bath for 10 seconds and then pulled out. Increase the temperature of the solder bath from 240℃ to 320℃ in 20℃ increments.
The solder wetting state of each sample at each solder bath temperature was visually observed. It can be said that the lower the temperature at which the plating film gets wet with the solder, the better the solderability of the plating film.

[0021] Note that the lead frame is always subjected to thermal history during assembly. Therefore, in order to simulate and evaluate the solderability after the thermal history of the assembly process, a part of the lead frame after each plating was heated at 100°C for 30 minutes or at 150°C for 30 minutes.
The samples were heat-treated for 0 minutes, and a solderability test was conducted on the samples before and after heating.

The immersion test was conducted three or more times per condition.

(Evaluation criteria for soldering condition) ○: The entire surface is uniformly wetted.

Δ: Almost wet, but the underlying plating surface is slightly exposed and a dewetting phenomenon is observed.

×: The solder is repelled and the underlying surface is clearly exposed.

(Example 1) After cleaning a lead frame made of copper alloy by degreasing and pickling, the entire surface was cleaned (Table 1).
) Nickel-cobalt alloy plating was applied to a thickness of approximately 3 μm at a current density of 4 A/dm 2 using a plating solution having the composition shown in FIG. On top of this, nickel/iron/phosphorus alloy plating was applied at a current density of 4A/dm2 using a plating solution with the composition shown in Table 2.
The thickness was approximately 0.2 μm. For nickel/iron/phosphorus alloy plating, phosphorous acid (H3 PO
The concentration of 3) was varied in the range of 0 to 20 g/l so that the amount of phosphorus precipitated in the alloy was 0 to 15 g/l.

(Comparative Example-1) Using a plating solution with the composition shown in Table 3, a lead frame with only bright nickel plating of approximately 3 μm and bright plating was provided, and then nickel/iron/phosphorus alloy plating ( A lead frame with a phosphorus content (8% by weight) of about 0.2 μm was produced. In addition, (Example-1) and (Comparative Example-1)
The components of the plating alloy were determined by volumetric analysis.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031] Also, (Example-1) and (Comparative Example-1)
The evaluation results of solderability are shown in (Table 4).

[0032]

[Table 4]

As a result, the nickel-iron-phosphorus alloy/nickel-cobalt alloy two-layer plating lead frame of the present invention has a phosphorus content in the nickel-iron-phosphorus alloy plating of 2 to 2.
It was found that excellent solderability was exhibited without flux even after heating in a range of 10% by weight.

(Example-2) Similarly to (Example-1), after cleaning the surface of the copper alloy lead frame, a nickel-cobalt alloy plating of about 3 μm was applied to the entire surface, and then a nickel-iron-phosphorus alloy was applied. Plating was provided to a thickness of about 0.2 μm. At this time, the concentration of phosphorous acid in the nickel-iron-phosphorus alloy plating solution was 10 g/l, and iron sulfate (
The amount of FeSO4 7H2 O) added was varied in the range of 0 to 20 g/l.

According to the above soldering test method and evaluation criteria,
The results showing the soldering state are shown in (Table 5).

[0036]

[Table 5]

As a result, the iron content in the plating film was reduced from 1 to
It was found that excellent solderability was exhibited within the range of 10% by weight.

[0038]

As described above, since the lead frame of the present invention does not require the use of flux during soldering, there is no fear of contamination due to residual flux, and the reliability of the semiconductor is significantly improved.

Furthermore, since there is no need for cleaning after semiconductor assembly, there is a great economical effect.

Claims (1)

[Claims] Claim 1: A nickel-cobalt alloy plating is provided on a copper, copper alloy or iron alloy substrate, and the proportion of iron in the alloy plating is 1 to 10% by weight of iron and 2 to 1% by weight of phosphorus.
A lead frame for a semiconductor device, characterized in that it is coated with a nickel-iron-phosphorous alloy plating of 0% by weight and the balance being nickel.