JPS6216549B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor device manufactured using a lead frame partially plated with a precious metal, and particularly to a method for partially plating a lead frame for a semiconductor device without masking. , relates to a plating method for partially applying silver plating, gold plating, etc.

Lead frames for semiconductor devices are generally made of relatively inexpensive materials such as iron, copper, or their alloys, and are formed into a predetermined size and shape by pressing or etching.
In order to improve the bonding properties of the thin metal wire of the electrode lead, a plating film of an expensive noble metal such as silver or gold is formed with a predetermined thickness.

Conventionally, this plating film of silver, gold, etc. has been applied to cover the entire surface of a lead frame substrate formed by lead molding, which inevitably increases the price of the lead frame.

Currently, as a method of drastically reducing the price of lead frames, a partial plating method is being adopted that minimizes the amount of plating required for silver, gold, etc. In other words, the adhesive part of the element in the lead frame,
This is a partial plating method that reduces the amount of silver or gold required by forming a plating film of silver or gold only at functionally necessary locations, such as the bonding portions of lead wires.

Conventionally, one method of partial plating is to cover parts of the lead frame that do not require plating with liquid resin or resin tape, and then immerse the entire lead frame in a plating bath to perform plating. After that, there is a method to remove the masking. Such a partial plating method is often performed by continuous plating of a coiled lead frame. As a batch type partial plating method, there is a method in which a part of the lead frame is mechanically covered with rubber or the like, and a plating liquid is brought into contact with the uncovered portion by squirting or the like.

Although these plating methods certainly save on the cost of plating materials, they have the disadvantage of increasing the cost of masking materials and equipment. In addition to these drawbacks, if the lead frame material is mainly iron, it is necessary to apply copper or nickel plating to the entire surface before silver plating to prevent rust. If a partial plating method such as that described above is performed after the plating, a large amount of cost will be required in terms of man-hours as well.

The present invention enables lead frames made of iron as a main component to be used without causing defects such as rust.
Another object of the present invention is to provide a method for performing partial silver plating, partial gold plating, etc. at low cost without masking.

Next, an example of partial silver plating will be described in detail with reference to the drawings.

FIG. 1 is an example of a lead frame commonly used in semiconductor devices. The ends of the three leads 11, 12, 13 are connected to a belt-like band 14.
The other end is connected to the frame plate 10. Reference numeral 7 indicates the area necessary for mounting the device and bonding the thin wire, and indicates the area where noble metal plating such as silver or gold is required, for example. Reference numeral 8 denotes a part to be soldered to a printed circuit board or the like as a semiconductor product, and is generally an area where preliminary soldering is applied.

Next, an embodiment of a plating method in which silver plating is applied to a predetermined thickness only in the area 7 in FIG. 1 will be described with reference to FIGS. 2 and 3. Here, the lead frames in FIGS. 2 and 3 are A-
It is shown in section A'.

Figure 2 shows a lead frame material 1 whose main component is iron having the structure shown in Figure 1, which has been previously coated with copper plating 2 to a thickness of 0.5μ or more, and then a strike silver plating 3 is applied in a plating bath. This shows that the process is carried out up to the surface 4 of the surface.

FIG. 3 shows that after the strike silver plating, main silver plating 5 is applied to the surface 6 of the plating bath. Here, it is desirable that there be a distance of several mm or more between the surfaces 4 and 6.

The reason for this will be explained below.

That is, at the surface 4 of the strike silver plating bath, the plating liquid creeps up around the lead wires due to surface tension. Initially, even if silver plating is applied to this part, the supply of silver ions is poor, so the surface is easily corroded, and copper and iron are particularly susceptible to corrosion. Therefore, it is not preferable to immerse the surface 4 in a plating bath for a long period of time, such as performing main silver plating at the position of the surface 4. In order to minimize corrosion, the immersion time should be limited to the time required for strike silver plating. It is necessary to keep it. Here, the thickness of the copper plating layer 2 is 0.5
μ or more is required, and a dense surface is preferable.
For this purpose, it is appropriate to adopt the so-called PR method.

Next, when performing main silver plating, if the lead frame is pulled up several mm, for example, about 5 to 10 mm, the strike silver layer 3 will be at the position of the main silver plating liquid surface 6. Yes, the silver surface is stable because it is less susceptible to corrosion due to plating solution creeping up. In order to avoid corrosion, the thickness of the strike silver layer 3 needs to be 0.2μ or more.

The lead frame plated in this way is
Bonding of elements and thin wires is possible on the surface of the silver plating layer 5, and since the surface area other than the silver plating layer 5 has a copper plating layer 2 and a strike silver plating layer 3, it can be covered with molten solder. Therefore, there is no inferiority in characteristics to the case where a silver plating layer is provided on the entire surface of the lead frame. In addition, since masking is not required, plating can be performed without increasing the number of processing steps, making it possible to significantly reduce the cost of silver plating materials.

[Brief explanation of the drawing]

Figure 1 is a plan view showing a general lead frame shape, Figure 2 is a sectional view of the lead frame during strike silver plating, and Figure 3 is a sectional view of the lead frame during main silver plating. show. 1... Lead frame material, 2... Silver plating layer, 3... Strike silver plating layer, 5... Real silver plating layer, 10... Frame board, 11, 12, 13... Lead.

Claims (1)

[Claims] 1. In a partial precious metal plating method in which a predetermined part is immersed in a plating bath and electrolytic plating is performed, and other parts are exposed outside the plating bath liquid level and a plating layer is not provided, the requirements for the lead frame are as follows: When applying strike precious metal plating to the area and then applying main precious metal plating to the areas where the strike precious metal plating is required, the strike precious metal plating is immersed in a plating bath slightly deeper than the areas where the strike precious metal plating is required. plating, and the main precious metal plating is performed by immersing the lead frame in the plating bath to a shallower depth than in the strike precious metal plating, so that the required part has a predetermined thickness. A partial plating method for a lead frame for a semiconductor device, which is characterized by plating precious metal with steps as shown in FIG.