JPH07161891A - Manufacture of lead frame for semiconductor device - Google Patents

Abstract

PURPOSE:To prevent the generation of whiskery burrs by the coincidence of a groove with a plating border, at the time of forming the groove in the periphery of a die pad, by making press-working time frictional force larger. CONSTITUTION:Coarse surfaces 28 are formed by coarsening the side surfaces of a punch 21 for groove formation by electric discharge. To form a V groove 29 using this punch 21, first of all, a silver plating boarder is arranged under the punch 21 with the die pad surface of a lead frame base material 10 upside. Next the punch 21 is caused to descend to perform press working. At this time, silver plating fragments scraped off are scattered and segmented by the progress of the punch 21 before becoming whiskery burrs, and become silver grains 25, and are buried in the nickel plating 2 on the bevels of a V groove, since the side surfaces of the punch are coarce, and a V groove 29 having punch transfer parts 24 where the punch surfaces are transferred is formed. Even if the silver grains 25 buried in float, it does not exert any bad influence on the characteristic since their magnitude is the order of mum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a lead frame for a semiconductor device, in which a groove for improving moisture resistance is arranged around a die pad, and more particularly to a method suitable for a lead frame for a power transistor. .

[0002]

2. Description of the Related Art Lead frames for semiconductor devices, for example,
In the case where a large heat dissipation is required such as a lead frame for a power transistor, a deformed section strip having the same section but different plate thickness is used. A large number of the modified cross-section strips are pre-plated before the lead frame is formed, and the ratio to the type plated after the lead frame is reversed.

FIG. 4 shows a flow of forming a lead frame by precision press working based on a pre-plated profiled strip. Nickel plating 2 is applied to the entire surface of the modified cross-section strip 1, and then silver stripe plating 3 is applied to form a lead frame base material 10 ((a), (b)). After that, using a press die including a punch 4, a stripper 5, a lower die 6, and the like, press molding is performed into a lead frame shape (c), washed (e), and shipped as a product (f).

FIG. 5 shows an example of a general lead frame base material for a power transistor and a lead frame completed by precision pressing. Lead frame base material 10 plated with nickel 2 and silver stripe 3
Is pressed to form two rows of lead frames 8 and 9 facing each other above and below the strip ((a) and (b)).
An enlarged view of one of the lead frames is shown in (c). Three leads 1 from the outer frame 12 having the regulation hole 11
3 is extended, a die pad 14 is formed at the center of the leading end of the lead, and a screw hole 15 for attachment is provided on the upper portion of the die pad 14. Wire bonding pads 16 are respectively formed on the remaining lead ends on both sides,
The three leads 13 are commonly connected by a tie bar 17.

As shown in the drawing, the die pad 14 for mounting the chip is usually provided with a square groove 1 around its periphery.
8 is formed. The shape of the groove 18 is arbitrary. The square-shaped groove 18 prevents the spread of the bonding material such as solder used when bonding the chips, prevents the intrusion of moisture from the outside after completion, and further improves the adhesion with the resin mold. Is formed to

Particularly above the die pad 14 adjacent to the screw hole 15 located at the boundary with the resin mold, the same groove is formed because it is necessary to further improve the moisture resistance and the adhesion with the resin mold. However, it is often the case that the upper side 18a of the square-shaped groove 18 is used as a substitute, or the groove is set separately (for example, Japanese Patent Publication No. 61-3074).
No. 6, Japanese Utility Model Publication No. 62-9735, etc.).

FIG. 6A shows the arrangement of the grooves 18 formed in the die pad 14. In this example, the setting groove to be formed above the die pad 14 is replaced by the upper side 18a of the square groove 18. is doing. If you substitute like this,
The larger the groove width and groove depth, the better. As shown in FIG. 6B, the cross-sectional shape of the square groove 18 is generally V-shaped or U-shaped. However, regarding the arrangement of the groove 18, it is necessary that the groove 18 is embedded in the mold because of the relationship with the boundary of the resin mold.

Since it is necessary to apply the silver plating 3 to the die pad 14, the silver plating boundary 3a and the upper side 1 of the square groove 18 are formed.
8a (or the setting groove) will be concentrated on the boundary portion of the resin mold. As a result, the upper side 1 of the square groove 18
8a is located just near the boundary 3a of the silver plating 3.

Therefore, when the square groove 18 is formed, as shown in FIG.
As shown in Fig. 3, the punch 2 when the silver plating boundary 3a is formed in the groove is formed.
When it is peeled off to 0 (peeled off), a beard 21 having a certain length is produced. Here, the whisker is a phenomenon in which the plating from the sagging portion of the material edge portion to the sheared surface is peeled off during press working to form a mustache.

The generation of the whiskers 21 is not preferable, but the generation is inevitable by the conventional groove forming method. Therefore, as a countermeasure, sit down to the occurrence of whiskers, reduce the size of the square groove so that plating is not applied to the groove, omit the upper side of the groove, or silver-plated area The only way to do this is to expand the area to expose the silver plating boundary outside the groove.

As a result, there are drawbacks in that a large chip cannot be mounted and the moisture resistance measure is inferior. Further, the beard burrs have a problem that they have a bad effect such as a short circuit of a wiring when a predetermined length is formed and a short circuit of withstand voltage due to mixing during resin molding.

[0012]

As described above, the conventional technique has the following problems because the beard of silver plating is generated when the groove is formed by the punch.

(1) It is necessary to reduce the die pad area to keep a distance from the resin mold boundary, and as a result, it is difficult to mount a large chip.

(2) There is a case where the beard is removed with a brush or the like to surpass the urgent situation, but the workability is poor, and there is a risk that other parts may be damaged or deformed.

(3) The groove cannot be deep and large. As a result, the spread of solder or the like at the time of chip bonding, the penetration of moisture from the outside, and the adhesiveness with the resin mold could not be sufficiently improved.

(4) When the silver plating boundary is exposed outside the groove, moisture resistance and silver migration may be sacrificed, which is not preferable.

(5) In some cases, it was necessary to reduce the silver stripe plating width to put the silver plating region inside the groove (the die pad side), but this would hinder the chip mount.

(6) In order to suppress the occurrence of whiskers,
It is necessary to set the dimension of the plating boundary with extremely high precision, which increases the manufacturing cost.

(7) If the burrs cannot be removed, the characteristics of the finished semiconductor product may be deteriorated and the reliability may be impaired.

An object of the present invention is to effectively prevent the occurrence of burrs caused by the fact that the groove and the plating boundary are coincident with each other when forming the groove provided in the die pad portion by increasing the frictional force at the time of press working. It is an object of the present invention to provide a method for manufacturing a lead frame for a semiconductor device, which can achieve

[0021]

A method of manufacturing a lead frame for a semiconductor device according to the present invention comprises a step of forming a groove for improving moisture resistance in a plated die pad portion of a lead frame base material, When roughening the surface of the punch for groove processing and press-molding the groove using this punch,
The plating separated by the press molding is divided by friction generated by roughening the punch surface.

In this case, the surface of the punch for grooving is roughened by electric discharge machining, or a rough grindstone is used to roughen the surface of the punch with a nondirectional scratch or a directional streak. It is preferable.

[0023]

When a punch having a roughened surface is used as the punch for grooving, friction occurs between the roughened punch surface and the lead frame substrate. Therefore, when a groove is formed in the boundary portion of the plating applied to the surface of the base material, the plating peeled off by press molding is divided by friction, so that the burrs do not occur.

Even if the burrs are generated, the burrs are divided and embedded in the surface of the base material as strips with the progress of the roughened punch.

The plating thus peeled off does not cause burrs and is finely divided, so that it does not adversely affect the product.

When the plating applied to the base material is silver, it is preferable that the punch surface for forming the plating groove is roughened in the order of several μm or more. The shape of the groove is arbitrary regardless of whether it is V-shaped or U-shaped.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the method for manufacturing a lead frame for a semiconductor device of the present invention is applied to a lead frame for a power transistor will be described below with reference to the drawings.

FIG. 1 is an explanatory view of a first embodiment in which a V-groove 29 is formed on the surface of the lead frame base material 10 by using a V-groove punch 21 having a roughened contact surface. In the processing of the V-groove processing punch 21, first, the punch material is formed into a punch shape in the same manner as in the conventional case to manufacture the V-groove processing punch 21. After that, the punch side surface that actually contributes to the groove processing is roughened by electric discharge machining to form the rough surface 28. Roughening is performed on the order of μm. The lead frame base material 10 to be grooved is formed by applying nickel plating 2 on the entire surface of the modified cross-section strip 1 and then silver stripe plating 3.

A groove 29 is formed as follows using the punch 21 having a roughened side surface. here,
The position of the silver plating boundary provided on the surface of the base material 10 coincides with the expected V groove.

First, the surface of the die pad of the lead frame base material 10 for power transistors is faced up, and the silver plating boundary is placed under the punch 21. Next, the punch 21 is lowered to press-mold the surface of the base material 10. At this time, if the side surface of the punch is smooth, the silver plating 3 is peeled off from the sagging portion of the base material edge portion to the shear plane, and a whisker is generated.

However, in this embodiment, since the side surface of the punch is roughened, the peeled silver is divided and dispersed into the silver grain 25 before it becomes a whisker due to the progress of the punch 21,
A V-groove 29 having a punch-transferred portion 24 on the punch surface, which is embedded in the nickel plating 2 on the slope of the V-groove which is being formed, is formed. Even if the embedded silver particles 25 are lifted up, the size thereof is in the order of μm, and does not adversely affect the characteristics of the lead frame or the power transistor.

As described above, even if the groove planned on the upper portion of the die pad of the power transistor lead frame and the silver plating boundary coincide with each other, it is possible to effectively prevent the occurrence of whiskers at the time of forming the groove. Further, even if the silver plating boundary is coincident with the surface of the die pad, no burrs are generated, so that a groove can be formed in an extremely narrow portion on the die pad.

2 and 3 show a modification of the method of the present invention. FIG. 2 shows a V-groove punch 22 having a non-directional scratch 26 formed on the side surface of the V-groove punch 22 with a rough grindstone or the like. In addition, in FIG. 3, a streak 27 is provided on the side surface of the U-grooving punch 23 along the punch traveling direction by a rough grindstone or the like.

Also by these, as in the first embodiment of FIG. 1, the friction works effectively, so that the occurrence of whiskers can be effectively prevented.

As described above, in the present embodiment, among the lead frames for semiconductors, particularly in a lead frame for mounting a large-sized power chip, various ones are provided in the periphery of the chip bonding portion to improve the moisture resistance. A punch having a roughened surface is used when forming a groove having a shape and a depth. Therefore, even if the silver plating boundary coincides with the V groove, it is possible to prevent the occurrence of whiskers due to the separation of burrs when scraping off the plated portion on the surface of the pre-plated substrate. As a result, the following various effects occur.

(1) Since the silver plating boundary may be arranged in a narrow range between the upper surface of the die pad and the lower end of the screw hole, it is possible to secure a larger die pad area than before in the resin mold of the same size, and particularly power It is possible to allow an increase in the size of the chip required for the transistor.

(2) Regarding the machining of the punch itself, only the roughening by electric discharge machining or scratches or streaks made by a rough grindstone does not require particularly strict accuracy, and therefore the punching time and cost hardly increase. .

(3) Since deep and large grooves can be effectively arranged in a narrow area of the die pad, it is possible to improve functions required for the grooves, such as improvement in moisture resistance.

(4) Since the groove has a roughened shape,
Adhesion with resin is better than before, and moisture resistance is greatly improved.

(5) Since no burrs are generated even when the plating position moves to enter the V-groove planned area or the punch position moves to enter the plating area during stripe plating, there is no particular problem in characteristics. Increased design latitude.

Although the lead frame for the power transistor has been described in the above embodiment, the present invention can be applied to other lead frames for semiconductor devices as long as the groove is formed. Further, as described above, the above-mentioned groove is not limited to the square shape, and it can be naturally applied to a setting groove which is not a substitute.

[0042]

EFFECTS OF THE INVENTION (1) According to the invention described in claim 1, since the groove is press-formed by using a punch having a roughened surface, the peeled plating is divided by friction, It is possible to effectively prevent the occurrence of whiskers.

(2) According to the invention described in claim 2, the surface of the groove punch is roughened by electric discharge machining, so that the surface can be easily roughened.

(3) According to the invention described in claim 3, since the surface of the groove punch is roughened with a rough grindstone to give a non-directional scratch, roughening is further facilitated. And it can be cheap.

(4) According to the invention described in claim 4, since the line parallel to the traveling direction of the punch is provided, the roughening can be performed easily and at a low cost.

[Brief description of drawings]

FIG. 1 is a first embodiment in which the method for manufacturing a lead frame for a semiconductor device according to the present invention is applied to a lead frame for a power transistor, in which a V groove processing punch having a roughened contact surface is used as a base material. Explanatory drawing when forming a V groove on the surface.

FIG. 2 is an explanatory view when a V-groove is formed on a surface of a substrate by using a V-groove punch having a contact surface roughened by providing a scratch having no directionality according to a second embodiment.

FIG. 3 is a plan view of a U-grooving punch having a roughened contact surface with unidirectional streaks according to the third embodiment.
Explanatory drawing when a groove is formed.

FIG. 4 is a process drawing showing the flow of processing a general lead frame for a power transistor.

FIG. 5 is an explanatory diagram of a lead frame base material used for a power transistor lead frame, and an explanatory diagram showing an example of a power transistor lead frame shape.

6A and 6B are a layout explanatory view of a groove formed in a peripheral portion of a plated die pad of a conventional lead frame base material for a semiconductor device, and a sectional view showing a sectional shape of the groove.

FIG. 7 is an explanatory view when a groove is press-formed using a conventional punch, and an explanatory view showing a silver plating whisker occurrence condition when a silver plating boundary and a V groove coincide with each other.

[Explanation of symbols]

 1 Deformed section strip 2 Nickel plating 3 Silver plating 10 Lead frame base material 21 V groove processing punch 22 punch 23 punch 24 punch transfer part 25 silver grain 26 scratch 27 streak 28 rough surface 29 V groove 30 U groove

Claims (4)

[Claims] 1. A step of forming a groove for improving moisture resistance in a plated die pad portion of a lead frame base material, the surface of a groove processing punch is roughened in advance, and the punch is used to perform the above-mentioned process. A method of manufacturing a lead frame for a semiconductor device, characterized in that, when the groove is press-molded, the plating that is peeled off by the press-molding is divided by friction generated by roughening the surface of the punch. 2. The method for manufacturing a lead frame for a semiconductor device according to claim 1, wherein the surface of the groove punch is roughened by electric discharge machining. 3. The method for manufacturing a lead frame for a semiconductor device according to claim 1, wherein the surface of the groove punch is roughened by a rough grindstone to give a scratch having no directivity. 4. The lead frame for a semiconductor device according to claim 1, wherein the surface of the groove punch is roughened with a rough grindstone to form a line parallel to the direction of travel of the punch.