JPH0510826B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for punching lead frames for ICs (integrated circuits) and the like.

[Conventional technology and its problems] IC lead frames are generally produced by passing a cold-rolled steel strip to a predetermined thickness through a slitting machine and cutting it into multiple metal ribbons of a predetermined width. The metal ribbon is punched out using a press to form a predetermined shape and size. However, significant residual stress is generated at the side edges of the metal ribbon during the cutting process using the slitting machine, and when this is punched as is, some of the residual stress is partially released and the stress distribution changes. Due to the loss of balance, it deforms abnormally, making it impossible to obtain the shape accuracy required for a lead frame. The occurrence of strain due to the influence of such residual stress will be explained in more detail with reference to FIG. 4. FIG. The strain distribution in the longitudinal direction of the ribbon 1 is shown in FIG. That is, the side edges 2, 2 are elongated by cutting (hereinafter, elongation and tension are shown as +, and shrinkage and compression are shown as -).
This is a state where distortion is occurring. The distribution of residual stress generated based on this strain is shown in Figure c. That is, compressive residual stress is generated at the side edges of the metal ribbon 1, and to balance this, tensile residual stress is generated from the center. Now, when the metal ribbon 1 with residual stress is photo-etched into the strip shape shown in FIG. The sides 3 and 3, including the sides 3 and 3, expand more toward the outside because the compressive residual stress that had been increasing toward the outside is released, and as a result, the sides 3 and 3 bend significantly inward, causing residual stress. I understand. As can be seen from such a residual stress determination test, the metal ribbon that has been cut to a predetermined width has a very large residual stress, and when the ribbon is put into a press and stamped to form the lead frame 5, the pin spacing 6 becomes smaller. The twisted pin tip 4 is deformed in such a way that it lifts up and becomes loose, making it unusable as a lead frame 5. As a countermeasure against this problem, there is a conventional method of annealing the metal ribbon at a low temperature, but this method involves work-hardening the ribbon through cold rolling, which gives the material great strength, but it is said that the annealing softens the material. It was never the preferred method due to its drawbacks.

[Means for solving the problem] Therefore, the lead frame punching method according to the present invention involves cutting a strip material to form a metal ribbon of a predetermined width, and punching the metal ribbon. In the process of molding lead frames for integrated circuits, etc., only the corners of the side edges of the metal ribbon are processed and plastically deformed in the process before punching the metal ribbon. .

[Function] As shown in the cross-sectional view of the metal ribbon 1 in FIG.
A is pressurized by a press die, and the side edges 2, 2 are stretched outward as shown by solid lines in a of the same figure, and the width of the ribbon is slightly expanded. The elongation in the length direction is corrected to a small value in this widened portion, and the strain distribution is also averaged as shown in FIG. As a result, the distribution of residual stress is also modified to be gentle as shown in FIG. In other words, the compressive residual stress at the side edges becomes smaller and the gradient of the stress distribution becomes smaller, so that even in the judgment test by photoetching shown in Figure d, the sides 3, 3 including the side edges 2, 2 are as shown in Figure 4. It can be seen that the amount of deformation is smaller than that shown in Figure 2, and the distribution of residual stress is gentler.

[Example] A 42% nickel steel strip with a thickness of 0.25 mm and a width of 400 mm was cut using a slitting machine to obtain a metal ribbon with a width of 40 mm. Then, a pressing piece is installed on the material entry side of the punching die of the transfer press machine, and the corners of both side edges of the metal ribbon are pressurized by the pressing piece, plastically deformed into the shape shown in Fig. 2, and then This was then punched using the punching die to obtain a 24-pin lead frame. The variations in positional displacement of the tip of each pin of the lead frame punched in this way are (+0.06 mm, -0.03 mm) in the vertical direction and (+0.03 mm) in the horizontal direction.
0.04 mm, -0.03 mm) with extremely high accuracy. For comparison, a 24-pin lead frame of the same shape was punched from a metal ribbon whose corners had not been plastically deformed, and the vertical variation was (+0.31 mm, -0.24 mm), and the horizontal variation was ( +0.11mm, -0.03mm).

Note that since the side edges of the metal ribbon are cut after being assembled to the IC as a lead frame, changes in shape due to pressurization of this portion do not cause any practical problems.

In addition, bending in the plate thickness direction occurs during cutting, which also causes harmful distortion, but this bending is generally removed by installing a leveler on the entry side of the punching die of the press to correct this. can.

[Effects of the Invention] By processing and plastically deforming only the corners of the side edges of the metal ribbon in the pre-punching process, the residual stress in those parts can be alleviated and the product accuracy of the lead frame can be improved. Moreover, since it is a simple processing step of simply applying pressure, it is not costly, and it is extremely beneficial since there is no risk of reducing material hardness as in the case of conventional low-temperature annealing.

[Brief explanation of the drawing]

FIG. 1 shows a cross section of a metal ribbon according to the present invention.
Diagrams showing strain distribution, residual stress distribution, and residual stress determination test results; FIG. 2 is a cross-sectional view of the side edge of a metal ribbon showing an embodiment of the present invention; FIG. Figure 4 is a perspective view showing an example of a 12-pin lead frame, and Figure 4 shows the cross section of the as-cut metal ribbon, strain distribution, residual stress distribution, and residual stress determination test results for comparison with Figure 1. FIG. 1... Metal ribbon, 2, 2... Side edge, 2a,
2a...corner.

Claims (1)

[Claims] 1 In the process of cutting a strip material to form a metal ribbon of a predetermined width and punching the metal ribbon to form a lead frame for an integrated circuit, etc., in the process before punching the metal ribbon, A lead frame punching method characterized in that only the corners of the side edges of a metal ribbon are pressurized to cause plastic deformation.