JPH01201421A - Lead frame material and its production - Google Patents

Abstract

PURPOSE:To manufacture a lead frame material causing no deformation to a wire bonding part resulting from blanking by applying compressive stress in the longitudinal direction of the surface layer part of an alloy material and also applying tensile stress in the longitudinal direction of the central part contiguous to the surface layer part, respectively, at the time of manufacturing lead frame. CONSTITUTION:A lead frame material consisting of a cold-rolled sheet-like Ni alloy is subjected to cold tension and cold repeated bending simultaneously by means of a tension leveler, by which compressive stress is established in the longitudinal direction of the surface layer part, and simultaneously, tensile stress is applied in the longitudinal direction of the central part contiguous to the surface layer part. At this time, the maximum difference in sheet thickness-direction distribution between the applied longitudinal-direction stresses are regulated to >=2kg/mm<2>. By this method, the lead frame material causing no deformation, such as difference in level, to the wire bonding part at the time of blanking can be obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a lead frame molding material for integrated circuit elements, and more particularly to a lead frame material with improved processing accuracy during punching of the lead frame. .

[Conventional technology]

Lead frames for integrated circuit devices are manufactured by slitting a strip of 42% Ni alloy or the like into a predetermined width, followed by punching and bending. FIG. 8 shows the form of the lead frame.

However, according to the above manufacturing method, compressive stress due to shear force remains in the vicinity of the cut surface of the slit-processed strip, for example. If the strip plate in such a stressed state is subjected to the next step of punching, the lead frame will be significantly deformed due to the influence of the residual stress, and during bonding work etc. in which lead wires are connected to the wire bonding part 1, This poses a major obstacle to automatic positioning of the wire bonding part 1.

Conventionally, in order to prevent deformation of the wire bonding portion 1, a method has been proposed to reduce residual stress occurring in the strip. For example, a method of reducing residual stress by annealing, and a method of applying uniform rolling strain in the width direction of the strip by skin pass rolling to cancel the strain existing only on the cut surface and reduce the residual stress (Japanese Patent Laid-Open No. 59 -189016, 62-158502), etc.

[Problem that the invention seeks to solve]

However, according to studies conducted by the present inventors, it has been found that even if the residual stress is reduced, deformation of the wire bonding portion cannot necessarily be prevented.

In other words, even if the residual stress in the strip is reduced, it is directly affected by the strain caused by the subsequent punching process.
Deformations such as steps still occur in the wire bonding portion.

The present invention has been made in view of the above problems, and
The object of the present invention is to provide a lead frame material that prevents deformation such as a step difference in a lead frame wire bonding part even by punching, and a method for manufacturing the lead frame material.

[Means for solving problems]

As a result of various studies to prevent deformation caused by punching, the inventors of the present invention found that it is effective to prevent deformation caused by punching by allowing a specific stress to remain in the strip, rather than by reducing residual stress. I found out.

That is, the present invention is characterized in that a compressive stress in the longitudinal direction is applied to one surface layer part, and a tensile stress in the longitudinal direction is applied to a central part continuous to the surface layer part, and preferably the applied longitudinal stress is applied in the thickness direction. Maximum difference in distribution is 2 kg/m
This is a lead frame material with excellent punching workability.Here, the maximum difference in longitudinal stress distribution in the plate thickness direction is 2 kg/■2 or more.
This is because if it is less than 1/m'', it is not very effective in improving punching properties.

Further, the lead frame material in a stressed state as in the present invention can be obtained by cold stretching and repeated bending at the same time, for example, by applying a tension leveler.

〔Example〕

The present invention will be explained below using examples.

After the cold rolling process, a 42% Ni alloy thin plate having a thickness of 0.254 mm and a width of 70 m was obtained by performing a predetermined slitting process. After applying a tension leveler to this strip, we investigated the distribution of residual stress in the longitudinal direction in the thickness direction.

The procedure for determining the longitudinal residual stress distribution in the plate thickness direction will be explained.

The method used in this example was based on the obtained material to be measured, 4
Etching the 2% Ni alloy thin plate sequentially from the surface,
The residual stress in the portion removed by etching is sequentially calculated from the deflection due to stress release at that time.

This will be explained in detail below.

1. The moment generated in the part left after etching can be calculated using the following formula.

Here, M is the moment of the part left after etching, E is Young's modulus, R is the radius of curvature of deflection, X is the position in the thickness direction of the plate, t is the thickness of the part left after etching, and the subscript n is the nth etching. This shows the image after etching.

2. The moment acting on the neutral plane of the portion removed by the (n-1)th etching and remaining in the nth etching is expressed by the following equation.

Note that in the i-th etching, Mr, = -M immediately.

holds true.

Here, Mri represents the moment of the portion removed by the i-th etching, and σri represents the stress existing in the portion removed by the i-th etching.

3. Here, the following relationship holds from equations (1) and (2).

Therefore, the moment Mrn of the portion removed the nth time is determined.

4. Stress σr existing in the part removed the nth time
The following relationship exists between n and moment Mrn.

Here, σrn' represents an apparent residual stress that is not corrected for expansion and contraction after etching.

When the expansion and contraction of the material to be measured due to etching is corrected to the equation (4) σrn', the following equation is obtained.

tn,. -1 (5) In this way, the residual stress σrn of the portion removed by the n-th etching is determined, and the stress distribution in the plate thickness direction can be obtained by sequentially performing etching.

The results of the above calculation method are shown in FIG.

In the figure, positive stress indicates tensile stress and negative stress indicates compressive stress, but when a tension leveler is applied, it can be seen that compressive stress remains on the surface layer of the strip, and tensile stress remains inside the strip.

The tension leveler conditions are roll diameter 16mm.
φ, roll pitch 25 nm, number of rolls 21, and elongation of the 42% Ni alloy thin plate using a tension leveler was 0.
It is 2z.

Next, the strip plate having the above stress distribution was punched into a 64-pin lead frame as shown in Fig. 9 (
(Figure 9 is a diagram in which most of the leads are omitted). Figure 2 shows the level difference in the wire bonding part after punching, and the level difference in the wire bonding part is -50 μm.
It can be seen that the range is from -160 μm. Note that the pin numbers in FIG. 2 correspond to the pin numbers in FIG. 9.

In addition, as a comparative example, the above 42
%Ni alloy thin plate was set to 0.05%, and the other conditions were the same as in FIG. 1, the stress distribution and the lead step difference after punching were investigated. The strip plate used was made of the same material and had the same dimensions as in the above embodiment.

Figure 3 shows the stress distribution, and Figure 4 shows the results of the lead level difference.

The maximum difference in stress distribution shown in Figure 4 is 1.5 kg/+
The lead height difference in this case is in the range of -250 μm to 100 μI, and it can be seen that the lead height difference is hardly improved with this level of stress distribution.

On the other hand, as a comparative example, the stress distribution and the lead step difference after punching were investigated when residual stress was removed by annealing without applying a tension leveler. The strip plate used was made of the same material and had the same dimensions as in the above example, stress relief was carried out at 700°C x 1.2w1n (furnace residence time), and punching was performed in the same manner as in the above example. .

Figure 5 shows the stress distribution, and Figure 6 shows the results of the lead height difference. Although the stress has been almost eliminated, the lead height difference is in the range of -300 μm to 100 μm, compared to the present invention. It is clear that the lead height difference is large.

The lead frame material of the present invention shown here and the conventional lead frame material obtained in the above comparative example were broken in a tensile test.

FIG. 7a is a diagram showing the morphology of the fractured surface of the material of the present invention,
FIG. 7 is a diagram showing the form of a fractured surface of a conventional material. In the material of the present invention, the edge of the fracture surface 2 was extremely sharp compared to the comparative example, and the fracture state was such that the surface layer was less deformed.

Furthermore, while the comparative example showed a dimple-like ductile fracture surface, the material of the present invention had almost no dimple-like ductile fracture surface. This is thought to be due to the loss of ductility of the lead frame material due to stress distribution, especially compressive stress in the surface layer.

It is believed that by imparting such a fracture characteristic with low ductility to the lead frame material, the occurrence of steps in the wire bonding portion due to deformation during punching was suppressed.

〔Effect of the invention〕

The lead frame material of the present invention has less influence of distortion due to lead frame punching process, and can provide a lead frame with fewer lead steps, and can be used for work that requires accuracy of lead parts such as mold adjustment during punching and wire bonding. However, it is possible to significantly improve efficiency.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the stress distribution in the thickness direction of the lead frame material of the present invention, Fig. 2 is a graph showing the relationship between the pin No. and the step after punching of the lead frame material of the present invention, and Fig. 3 is a graph showing the relationship between the pin No. after punching of the comparative example and the step, FIG. 4 is a graph showing the relationship between the pin No. after the punching of the comparative example and the step, and FIG. 5 is the graph of the conventional lead frame material. Figure 6 is a graph showing the relationship between the pin number and level difference after punching of conventional lead frame material, and Figure 7 is a graph showing the stress distribution in the thickness direction of the lead frame material when it is broken in a tensile test. FIG. 8 is a diagram showing one form of a lead frame, and FIG. 9 is a diagram showing a lead frame material according to the present invention. Figure 1 shows the punching form in the example 0 0.127
0.2 hiro@)l-(mm) Fig. 2 EOONN○. Figure 3 Figure 4 εON N. Figure 5 Figure 6 Ren 0n No. Figure 7a Figure 7 Figure 8 Figure 9

Claims (1)

[Scope of Claims] 1. A lead frame material with excellent punching workability, characterized in that a longitudinal compressive stress is applied to a surface layer part, and a longitudinal tensile stress is applied to a central part continuous to the surface layer part. 2. The lead frame material according to claim 1, wherein the maximum difference in the distribution of stress in the applied longitudinal direction in the plate thickness direction is 2 kg/mm^2 or more. 3. A method for manufacturing a lead frame material with excellent punching workability, which is characterized by cold stretching and repeated bending at the same time.