JPH0661392A - Semiconductor device lead frame - Google Patents

Abstract

PURPOSE:To prevent the rear side of an element mount exposed to the outside from discoloring at assembly. CONSTITUTION:A Ni plating film 15 is provided at least to an element mounting pad 6, a wire bonding part, and a rear side 10 of the element mounting pad 6 of a transistor lead frame. Furthermore, an Ni-P alloy plating film 16 is provided to a part where the glazing Ni plating film 15 is provided excluding the rear side 10 of the element mounting pad 6 to form a two-layered plating film. By this setup, only the glazing Ni plating film 15 is provided to the rear side 10 of the element mounting pad exposed to the outside after assembly, and the Ni-P alloy plating film 16 is not provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device lead frame, and more particularly, to a semiconductor device lead frame having an external exposed surface which is exposed to the outside after assembly and packaging.

[0002]

2. Description of the Related Art Generally, a process of assembling a transistor is performed as follows. First, a silicon element (silicon chip) of a transistor is placed on a base body called a lead frame, which is mainly made of a copper alloy, and solder, silver paste or Au is used.
After the element is bonded to the base by using —Si eutectic or the like, the element and the lead of the base are wire-bonded with Au or Al wire. Next, in many cases, resin sealing is performed to protect the element and the wire. After that, the outer leads are cut and the outer leads are solder-plated to complete the product.

By the way, when the silicon element is joined by silver paste or Au-Si eutectic, Ag plating is provided on the element mounting portion of the lead frame. However, when the element is joined by soldering, it is glossy. Two-layer plating in which a Ni—P alloy plating is provided to a thickness of about 0.2 μm in addition to a Ni plating has come to be used (for example, Japanese Patent Publication No. 60-33).
No. 312). This is because Ni-P has excellent solderability.

In the wire bonding portion of the lead frame, especially in the power transistor, the Al wire is used as the connecting wire although the cost is high in terms of reliability. Unlike the Au wire, the Al wire cannot be ball-bonded, and therefore ultrasonic wave wedge bonding is used for the first and second times. In ultrasonic bonding, if the surface of the substrate is hard and smooth, the applied ultrasonic waves will not be absorbed,
Bonding strength is increased. In this respect, the Ni-P alloy is hard even immediately after plating (Vickers hardness Hv 400 or more), 300
It becomes harder when subjected to heat treatment at ~ 400 ° C (H
v800), more N than when only bright Ni plating is provided
When the i-P alloy plating is thinly provided on the i-P alloy plating, the wire bonding strength is improved.

For the above reason, in the lead frame for power transistor in which elements are joined by soldering, at least the element mounting portion and the wire bonding portion of the lead are N excellent in solderability and bondability.
It is common to provide iP / bright Ni bilayer plating.
The reason for forming two layers is that Ni-based plating needs to be at least 1 μm in order to suppress the diffusion of copper from the underlying copper alloy material, but if this is all Ni-P alloy plating, the Ni-P alloy is very Since it is hard, cracks occur in the plating layer during bending or forming by pressing, resulting in plating defects. For this reason, after providing a relatively soft bright Ni plating, Ni-P alloy plating is applied to about 0.2 μm in order to improve the surface characteristics. This minimizes the occurrence of cracks.

Ni-P / gloss Ni2 for power transistors, which is excellent in solderability and bondability.
The layer-plated lead frame is manufactured as follows. First, the surface of the copper alloy strip is degreased and cleaned by pickling, and then bright Ni plating is provided to a thickness of about 3 μm. The plating solution is mainly a Watts bath to which a brightening agent is added. At this time, the plating is required at least on the element mounting portion and the wire bonding portion, but it is not necessary to provide it on the other portion, for example, the back surface of the element mounting portion or the outer lead portion, so the plating is omitted. There is also. However, when the back surface of the element mounting portion is exposed to the outside after assembly, plating is often provided for the reason described below. When the plating is omitted, a so-called partial strike plating method is used in which the unnecessary plating portion is covered with a masking tape or the like and only the necessary portion is plated in a strip shape.

[0007] Ni-P alloy plating is further provided on the portion where the bright Ni plating is provided by about 0.2 μm, and cleaning,
After drying, the plating process is completed. Next, a lead frame in which two-layer plating is provided on at least the element mounting portion and the wire bonding portion of the lead is formed by pressing.

[0008]

By the way, when the back surface of the element mounting portion is resin-sealed to release heat from the element,
Many have a structure exposed to the outside. Therefore, if the copper alloy material is exposed, the copper may be discolored due to the heat history of the assembly process and the atmosphere during use, and the commercial value may be impaired. Therefore, even on the back surface of the element mounting portion exposed to the outside, in the same process as the front surface, Ni-
A P / bright Ni two-layer plating is provided to improve heat discoloration resistance and humidity discoloration resistance.

However, as a characteristic of the Ni-P alloy plating, it is superior to copper in heat resistance and discoloration resistance, but when it is heated in the air, nickel phosphate is produced and the color changes to yellow. Further, in an acid aqueous solution (PH3 or less), the color changes to black in a short time. It is presumed that this is because Ni is dissolved by the acid, and the phosphorus compound of Ni remains on the surface to turn black. The yellow discoloration is caused by Ni-P / bright Ni 2 layer plating lead frame placed in a furnace in a reducing gas atmosphere and heated in the element attaching process, when the furnace shield is poor and air enters, especially in the element mounting part. It selectively occurs on the back surface side, that is, on the heat block contact portion. In addition, in the step of providing molten solder on the outer lead part after resin sealing and cutting and molding are completed, after dipping in flux to remove the oxide film of the lead, put it in the molten solder bath and then wash the flux residue. However, since the flux is usually a strong acid, the Ni-P plating part that is exposed to the outside by the residual acid when cleaning is insufficient or when the frames are in close contact with each other and the acid remains between them. That is, black discoloration occurs on the back surface of the element mounting portion.

As described above, the Ni-P alloy plating is necessary for the element mounting portion and the wire bonding portion of the lead frame, but for the back surface of the element mounting portion where Ni-P is exposed to the outside, the assembly process is performed. If it is incomplete, discoloration may occur, which is one of the factors that significantly reduce the commercial value.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a novel lead frame for a semiconductor device which prevents discoloration of the rear surface of the element mounting portion exposed to the outside.

[0012]

According to the present invention, a Ni-P / bright Ni two-layer plated lead frame is assembled into a Ni-P.
Focusing on the phenomenon that the alloy-plated surface is discolored, N
The i-P alloy plating is based on the knowledge that only the functional surface and only the bright Ni plating excellent in discoloration resistance should be used for the surface exposed to the outside.

That is, according to the present invention, in a semiconductor device lead frame made of copper, a copper alloy, or an iron alloy, at least the element mounting portion, the wire bonding portion, and the back surface of the element mounting portion exposed to the outside after the assembly package are glossy. Ni plating is provided, and at least a portion of the element mounting portion excluding the back surface of the element mounting portion, which is provided with bright Ni plating, is further provided with Ni-P alloy plating, and the rear surface of the element mounting portion that is exposed to the outside after assembly and packaging is bright Ni. Only plating is provided,
The Ni-P alloy plating is not provided.

Thus, at least the element mounting portion and the wire bonding portion are Ni-P / bright Ni 2 layer plating,
By providing only the gloss Ni on the back surface of the element mounting portion, it is possible to improve the element attaching property and the wire bonding property, and at the same time, obtain the surface excellent in discoloration resistance that is not easily discolored during assembly.

[0015]

[Function] The bright Ni plating is less discolored due to the presence of an acid or heating in air than the Ni-P alloy plating. Therefore, in the lead frame manufacturing process, after at least the element mounting portion and its back surface and the wire bonding portion are provided with bright Ni plating, the back surface of the element mounting portion is covered with a masking tape or a belt mask, and then at least the element mounting portion and Ni-P alloy plating is provided on the wire bonding part,
It may be washed and dried. Bright Ni plating thickness is 3μm
Although the degree is good, the plating thickness is not limited to this.

When a masking tape is attached to the back surface of the element mounting portion to shield it after providing the bright Ni plating, the tape does not adhere if the back surface is wet. However, if the water is evaporated to remove water, heat is also applied to the Ni-P alloy plating layer on the surface, and the oxide film is formed by drying,
It is better not to heat and dry it because the solder wettability deteriorates. In this case, after providing the bright Ni plating, the tape can be attached by draining only the back surface with an air knife, and the oxidation of the Ni-P alloy plating on the surface hardly poses a problem.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A lead frame for a transistor according to an embodiment of the present invention and a manufacturing process of a power transistor using the same will be described below. A coil of a deformed copper alloy strip made of 12Sn-OFC which is an alloy of Sn and oxygen-free copper (OFC) was prepared. As shown in FIG. 2, this modified copper alloy strip S has a thick plate portion in the central portion and thin plate portions on both sides, and has a cross-sectional shape having different thickness portions in the same cross section. The strip width is about 50 mm and the weight is about 200 kg.
This was installed in a mass production device for automatic striped strip manufacturing.

First, when the entire surface is subjected to cathodic electrolytic degreasing and then press-molded, a functional surface plating portion 2 (element mounting portion, a portion corresponding to a wire bonding portion) and a back surface plating portion 4 of the element mounting portion (element mounting The heat-adhesive masking tapes 1 and 3 were attached to the other portions 21 and 23 (portions to be outer leads) except for the portion corresponding to the back surface of the placing portion (FIGS. 3 and 4). Then, once again, through the steps of cathodic electrolytic degreasing and pickling, the parts 2 and 4 to which plating is to be applied, to which the masking tapes 1 and 3 are not attached, are cleaned.

Next, bright Ni plating 1 is applied to the portions 2 and 4.
5 was provided by an electrolysis method with a thickness of about 3 μm, and after the collection and washing steps, high-pressure air was blown to the side of the strip S that contacts the back surface of the element mounting portion with an air knife to remove the attached water and paste From the top of the masking tape 3
The masking tape 11 was laminated and attached so that the exposed portion of the plated surface was covered (FIG. 5). The masking tape 11 is attached only to one side of the strip S, and the surface on the element mounting portion side and the wire bonding portion side is not attached.

After that, Ni-P electrolytic plating was performed. Since the backside plating portion 4 of the element mounting portion is all tape-masked, the Ni-P alloy plating 16 was provided on the surface of the element mounting portion and the wire-bonding portion, which was bright Ni-plated. The thickness of Ni-P alloy plating is 0.2
μm. And the front and back masking tapes 1, 3, 1
1 was removed, and after one step of multi-step water washing and drying,
A copper alloy strip having only i-P / bright Ni two-layer stripe plating 13 and the other surface having a bright Ni stripe plating 15 was prepared (FIG. 6). This copper alloy strip was formed into a desired transistor lead frame shape 12 shown by a dotted line by a press machine.

After the transistor element 7 is joined to the element mounting portion 6 by soldering, the transistor element 7 and the wire bonding portion 17 are wire-bonded with the Al wire 8 and then resin-sealed with the plastic resin 5. After that, a transistor was manufactured by cutting the outer leads 9 (FIGS. 1A to 1C). In this case, the back surface 10 of the element mounting portion of the transistor lead frame is not covered with the mold resin 5 and is exposed to the outside.
As described above, this is to radiate the heat generated from the element 7.

For comparison, both the front and back surfaces were made of Ni-
A transistor was assembled in the same manner as in the above-mentioned embodiment even with a conventional lead frame manufactured from a stripe plating strip provided with P / bright Ni two-layer plating.

Next, the transistors respectively assembled using the lead frame of this example and the lead frame of the comparative example were left to stand in an aqueous solution of 40 ° C. adjusted to pH 3.0 with hydrochloric acid and exposed to the outside. Back of the element mounting part 10
The discoloration situation of was observed. Although it is the reason for the immersion in the hydrochloric acid aqueous solution of pH 3.0, the outer lead 9 of the transistor is usually provided with molten solder. Therefore, in order to remove the oxide film of the lead 9, the lead 9 is immersed in a flux composed of hydrochloric acid, zinc chloride, organic acid, etc.
Immersed in a molten solder bath. Next, a cleaning process is performed to remove the residual flux, but it is empirically recognized that if the cleaning is insufficient, the plating surface is eroded by the acid in the flux. Therefore, a hydrochloric acid aqueous solution having a pH of 3.0 simulating the residual flux components is prepared and the assembled transistor is immersed.

As a result, in the transistor of the comparative example, it was observed that the Ni-P alloy plated surface of the device mounting portion rear surface 10 which is an externally exposed portion was discolored black by immersion for about 20 minutes.
However, the transistor using the lead frame of this example did not show any discoloration even after 2 hours, and maintained the metallic luster.

Ten lead frames of this example and the comparative example were placed in a constant temperature bath at 350 ° C. for 10 minutes each, and then the discoloration of the plated surface was observed. As a result, in the lead frame of the comparative example, it was confirmed that the Ni—P alloy plated surface was subjected to air oxidation and slightly discolored. On the other hand, in the lead frame of the present example, the Ni-P alloy plated surface was slightly discolored in yellow, but the surface of the device mounting portion back surface 10 only having the bright Ni plating was not discolored and maintained the metallic luster. .

As described above, the Ni-P alloy plating is a plating film having excellent wire bonding and soldering properties, but discoloration may occur. In the lead frame for a transistor according to this example, Ni—P alloy plating was provided only on the functional surface, and the back surface of the element mounting portion, which is the externally exposed portion, was a glossy Ni plating surface rich in heat discoloration resistance and acid resistance. Is. From this, it was confirmed that yellow discoloration due to high temperature oxidation and black discoloration due to flux residue hardly occurred during assembly. It should be noted that the occurrence of discoloration does not mean that the functional characteristics are deteriorated, but the discoloration markedly reduces the commercial value, and therefore the occurrence thereof is a property that should be absolutely prevented.

[0027]

According to the present invention, the Ni-P alloy plating is provided only on the functional surface, and the back surface of the element mounting portion, which is the externally exposed surface, has a bright Ni plating surface rich in heat discoloration resistance and acid resistance. As a result, it is possible to effectively prevent the occurrence of discoloration defects on the externally exposed surface during assembly.

[Brief description of drawings]

FIG. 1 is a conceptual view of a functional surface of a transistor assembled using a lead frame according to the present embodiment, a conceptual view of a transistor viewed from the side, and an external view of the back surface of an element mounting portion.

FIG. 2 is a perspective view of a modified copper alloy strip used in this embodiment.

FIG. 3 is a perspective view of a modified copper alloy strip showing a masking tape position and a plated surface when a bright Ni plating is provided on the surface of the element mounting portion and the wire bonding portion according to the present embodiment.

FIG. 4 shows a gloss Ni on the back surface side of the element mounting portion according to the present embodiment.
The perspective view of the variant copper alloy strip which shows the masking tape position at the time of providing plating, and a plating surface.

FIG. 5 is a perspective view of a modified copper alloy strip showing the position of a masking tape that covers the back surface when Ni-P alloy plating is further provided only on the bright Ni plating on the functional surface according to the present embodiment.

FIG. 6 is Ni-P / gloss N on one side according to an embodiment of the present invention.
FIG. 3 is a perspective view of a copper alloy strip to be a lead frame for a transistor, in which only i2 layer stripe plating and the other surface is provided with bright Ni stripe plating.

[Explanation of symbols]

 1 Masking tape 2 Functional surface plating part 3 Masking tape 4 Backside plating part of element mounting part 5 Plastic resin 6 Element mounting part 7 Transistor element 8 Al wire (bonding wire) 9 Outer lead 10 Device mounting as externally exposed part Back surface of the part 11 Masking tape 12 Lead frame shape for transistor 13 Ni-P / bright Ni 2 layer plating part 15 Bright Ni plating 16 Ni-P alloy plating 21 Part to be the outer lead 23 Part to be the outer lead S Deformed copper alloy Article

Claims (1)

[Claims] 1. A lead frame for a semiconductor device made of copper, a copper alloy or an iron alloy, at least an element mounting portion,
A bright Ni plating is provided on the back surface of the wire bonding portion and the element mounting portion exposed to the outside after the assembly package, and at least the element mounting portion and the wire bonding portion excluding the back surface of the element mounting portion are further Ni-P alloy. A lead frame for a semiconductor device, wherein plating is provided, and the back surface of the element mounting portion is not provided with Ni-P alloy plating.