JPH0471259A - Preparation of lead frame - Google Patents

Abstract

PURPOSE:To resolve a problem of slight dimensional change and to prevent the occurrence of various deformations such as strain, distortion, warp, and dislocation and so on of the tip of inner lead by removing the internal residual stress completely leaving the internal residual strain as it is. CONSTITUTION:When forming wire member C for lead frame, wire member B for lead frame is molded leaving part of the area to be removed unpunched so as to reduce the stress of the cross section where a stress exceeding an allowable range for an added tension. Next, a tension that the apparent internal stress falls in a range of 0.2-1.2kg/mm<2> is applied to wire member B in the longitudinal direction, the wire member B is then heated in a furnace with an atmospheric temperature of 400 deg.C-800 deg.C to remove the internal residual stress from wire member B leaving a resident internal residual strain as it is. Furthermore, after heat treatment, the area of the front part marked with slanting lines is punched out to mold wire member A for final lead frame.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a method for manufacturing a lead frame used in a semiconductor device, and more specifically, the present invention relates to a method for manufacturing a lead frame used in a semiconductor device. This invention relates to an improvement in the manufacturing technology of +J-frames that is effective in preventing extremely slight changes.

[Conventional technology]

When plastic working is applied to metal, non-uniform plastic strain, that is, local dimensional changes occur due to differences in the degree of working during this working process. Therefore, the deformation of that part and the surrounding part is elastically adjusted, and a balanced residual stress is generated within the metal.

In the manufacturing process of lead frames for semiconductor devices, similar residual stress and residual strain occur in areas removed when forming patterns such as inner leads, outer leads, and positioning reference holes, and in their surrounding areas.

Residual stress and strain are released after the lead frame is formed or during heating in post-processes such as semiconductor device assembly, causing lead frame distortion, twisting, warping, and positional displacement, which can have a significant impact on shape and dimensional accuracy. affect As a result, the quality and yield of semiconductor devices are reduced. Heat treatment is generally used as a method for removing this residual strain and stress.

However, in recent years, lead frames for semiconductor devices have tended to have a large number of pins, and the spacing between the tips of inner leads and outer leads has become extremely narrow, resulting in deformation of the leads such as distortion, twisting, warping, and misalignment. In addition, requirements for dimensional accuracy such as high precision positioning are becoming stricter. Therefore, the residual stress generated by the processing process has a great influence on the yield and long-term reliability of semiconductor devices. Heat treatment to remove this residual stress has become important, and demand for appropriate technological development is increasing.

An example of a conventional technique for removing residual distortion is
This is disclosed in Japanese Patent Publication No. 44422, Japanese Patent Publication No. 51-19970, etc.

[Problem to be solved by the invention]

However, when residual strain is removed using conventional heat treatment methods, slight dimensional changes occur in the required parts of the lead frame as the internal residual strain is removed, resulting in a high number of rejected products. .

This will be explained below using the lead frame strip C shown in FIG. 3 as an example.

That is, FIG. 3 is a plan view showing the strip C of the lj-deframe before being heat-treated by the conventional method. According to this diagram,
In the slitted band material for the lead frame, reference hole holes 2 and notches 6, which are used for positioning in each processing step, are first bored apart from each other on the edges in the width direction. After that, using this reference bottle hole 2, the element mounting stage 3. Inner lead 4. The strip C of the lead frame is formed by punching out the connecting pieces 8 and the like that connect the tips of the outer leads 5 and the inner leads into a desired shape in an appropriate order.

Here, FIG. 1 is a plan view showing a strip material A of a lead frame completed by performing heat treatment to remove residual strain and removing the remaining portions.

In this figure, the width dimension of the strip A is ■, the distance in the width direction of the reference bottle hole 2 is 2a
(equivalent to the center of ) is assumed to be ■, and the distances to the second, third, and fourth points below are assumed to be ■, ■, and ■, respectively.

Table 1 shows the lead frame strip C with the above pattern.
(Fig. 3) shows changes in the shape and dimensions of the strip material A of the lead frame manufactured by the conventional method and the dimensional values of the distances (1) to (2).

(Hereinafter, the margins of this page) Table 1 As shown in Table 1, when the strip material C of the lead frame (Fig. 3) is subjected to the conventional heat treatment to remove internal residual strain, the inner lead 4 Although the occurrence of rejections due to distortion of the tip, deformation such as twisting and warping, and misalignment is suppressed, extremely slight changes have occurred in the material width, the width of the positioning reference hole, the pitch dimension in the longitudinal direction, etc. I understand that.

As the requirements for dimensional accuracy become stricter as in recent years,
This extremely slight dimensional change falls outside the range of highly accurate dimensional tolerances required for positioning and the like required in semiconductor assembly processes, resulting in rejected products.

An object of the present invention is to solve the problem of extremely slight dimensional changes such as the material width, the width of the reference hole, and the pitch in the longitudinal direction that occur in the conventional method of removing residual strain, and to eliminate the distortion at the tip of the inner lead. It is an object of the present invention to provide a lead frame manufacturing method that is effective in preventing deformations such as torsion, warpage, and positional deviation, and at the same time preventing the occurrence of rejected products due to the above-mentioned dimensional changes.

[Means and actions for solving the problem] The extremely slight dimensional changes that occur during heat treatment to remove internal residual strain that occurs during slitting, press working, etc. This occurs because internal residual distortion is removed.

That is, when the front δ self-stripe material C (FIG. 3) is heated, the movement of metal atoms in this strip material becomes easy, and the atoms move in response to internal residual stress. The internal residual strain is removed by this movement of atoms, and at the same time the dimensions (1) to (4) mentioned above change.

The inventor conducted research on a method for preventing dimensional changes that occur when this internal residual strain is recovered, and established the present invention as the most practical and reliable method.

The feature of the present invention is that the apparent internal stress is 0.
A tension of 2 to 1.2 kg/mm2 is applied to the strip B (
In addition to the long and horizontal directions (Fig. 2), the pre-delta strip material B is heated in an atmosphere furnace at a temperature of 400 °C to 800 °C to remove internal residual stress while leaving residual internal strain in it. This is the way to do it.

Note that "apparent internal stress" as used in the specification of this application refers to the tension applied in the length direction of the strip B (Fig. 2) perpendicular to the length direction of the lead frame strip material immediately before the start of pressing Indicates the value divided by the cross-sectional area.

For example, a lead frame strip material with a thickness of 0.15 mm and a width of 50 mm is considered to have a virtual cross-sectional area of 7.5 mm, no matter what pattern it is pressed in, and a tension of 3 kg is applied to it. 0.4kg
/mm” is expressed as an apparent internal stress generated.

Furthermore, when performing heat treatment to remove internal residual stress by applying tension in the above range, the strip material C of the lead frame is
Among the areas to be punched out and removed when forming the lead frame (Fig. 1), there is an allowance for the additional tension that maintains the state where the tension is applied to restrain the recovery of the internal residual strain that remains in the strip material C of the lead frame. This method is characterized in that the strip material B of the lead frame is formed in a shape that leaves a part of the removed area unremoved so as to reduce stress in the cross section that generates stress exceeding the range.

The purpose of leaving a predetermined area out of the area to be punched and removed is to prevent the lead frame from being locally deformed or damaged by the tension applied depending on the shape of the lead frame, so the stress distribution state is changed. The purpose is to prevent local deformation and damage by adding a portion to be left uncut to improve the situation.

That is, it is important to leave a part of the region to be punched out so as to reduce the degree of local stress concentration occurring within the strip B as much as possible by applying tension.

In FIG. 2, the shaded area is an example of the area to be left uncut.

Whether it is preferable to leave the entire hatched area (Fig. 2) or a part of the shaded area (Fig. 2) to be left uncut depends on the pressing technique, plate thickness, and plate width. , varies depending on the magnitude of the applied tension, etc.

It should be noted that the amount of the portion to be left out can be easily determined based on experience and preliminary tests.

If the lead frame pattern is heat-treated with many areas left uncut, deformation and damage due to tension can be prevented, but when the remaining areas are removed after heat treatment, residual strain will remain and cause deformation. This results in the production of rejected products.

Moreover, if too little portion is left, the lead frame will be deformed or damaged during heat treatment due to the tension applied to the strip B (FIG. 2).

From the above background, the extent of the area to be left blank in the lead frame pattern is determined so that the apparent internal stress within the lead frame caused by the tension applied to strip B (Figure 2) is zero.
．． It is in the range of 2 to 1.2 kg/mm2 - it is desirable to keep it as low as possible without deforming or damaging the frame.

〔Example〕

Embodiments of the present invention will be described based on the drawings and tables attached to the specification.

FIG. 1 is a plan view showing a lead frame strip A completed by removing the uncut portion after heat treatment to remove residual strain as described above.

Further, FIG. 2 shows a plan view of a strip B as an example of a lead frame used in the manufacturing method of the present invention.

In the method of the present invention, since the shaded area in FIG. 2 is heat-treated by applying the tension without punching, the entire strip B and the tip of the inner lead 4 are prevented from being deformed during and after the heat treatment. Residual stress is removed in a suppressed state.

Therefore, even slight dimensional changes due to heat treatment after press working are prevented, making it possible to manufacture lead frames with extremely high precision. After the heat treatment, the shaded area (FIG. 2) is punched out to form the final lead frame strip A as shown in FIG. 1.

Next, we will discuss dimensional changes and product acceptance/rejection when conventional heat treatment is applied to remove internal residual strain.

In the case of Fig. 11, the product is rejected due to a dimensional change in the distance ■, which appears as the largest value among the dimensions and distances ■ to ■. Incidentally, when the method of the present invention is implemented, the dimensional change in the distances (■) to (2) becomes smaller in approximately proportion to the dimensional change in the distance (2).

Table 2 shows how the present invention is applied to the lead frame material strip B shown in FIG.
An example of a test applied to this is shown below.

Test example 1 in Table 2 of e-based alloys is a case where the applied tension is small, and the internal residual strain is removed relatively freely. , the dimensional change after heat treatment becomes large and the product is rejected.

This is because the internal residual strain is removed by the metal atoms moving relatively freely against the internal residual stress.

Test Examples 2 to 4 in Table 2 were heat treated according to the present invention, and the metal atoms behaved due to the composite stress of the internal residual stress and the stress generated by the externally applied tension. It goes without saying that the internal residual stress is removed while leaving the internal residual stress, and that no rejects such as distortion of the lead tip, deformation such as twisting or warping, or misalignment occur.
Since dimensional changes that occur when internal residual strain is removed by heat treatment do not occur, rejects due to dimensional changes do not occur.

Test Example 5 in Table 2 is a case in which the applied tension was too large, causing local deformation and resulting in a rejected product.

Furthermore, Test Example 6 in Table 2 is an example in which no heat treatment was performed at all, and although there were no rejected products due to dimensional changes, there was distortion and twisting of the lead tips.

The product was rejected due to deformation such as warping.

In order to apply the method of the present invention to a common Ni-Fe alloy lead frame strip and perform heat treatment to remove internal residual stress while applying tension to restrain the recovery of internal residual strain, Apparent internal stress is 0.4 to 0.8 kg
The heat treatment temperature varies depending on the shape of the strip, pressing conditions, required productivity, equipment, etc., but is most preferably within the range of 620 to 720°C.

Similarly, in order to apply the method of the present invention to a Cu-based alloy lead frame strip and perform heat treatment to remove internal residual stress while leaving internal residual strain, the apparent internal stress must be 0.
3-0. , 6 kg/mm', and the heat treatment temperature varies depending on the shape of the strip, the conditions of the processing process such as press working, the required productivity and equipment, etc. Most preferably, the heat treatment is carried out within a temperature range of -700<0>C.

In addition, lead frames can be made using the method of the present invention! ! The manufacturing process has the following advantages:

When a strip of a lead frame is heat-treated in a continuous furnace, the lower surface of the strip rubs against the bottom of the furnace, etc., resulting in scratches and often rejects. On the other hand, when the method of the present invention is applied, continuous heat treatment can be performed if the distance between the line connecting the outlet and the outlet of the heat treatment furnace and the bottom of the furnace is greater than the maximum deflection of the strip. This can prevent the occurrence of abrasions.

For example, when the method of the present invention is carried out on a 42Ni alloy strip with a NO. The maximum deflection between the supports of the material will be the values shown in Table 3.

Table 3 As can be seen from Table 3, the maximum deflection of the strip in this case is about 30 mm.

Therefore, in the continuous furnace in this case, the occurrence of scratches can be prevented by making the distance between the line connecting the inlet and the outlet to the bottom of the furnace 30 mm or more.

Furthermore, in the continuous heat treatment furnace to which the present invention is applied, the furnace atmosphere sealing section, which is one of the most important matters in continuous heat treatment, can be more accurately managed. For example, the quality of the frictional force in the seal portion can be determined from the tension applied from outside the furnace and the deflection of the lead frame inside the furnace.

Further, the appropriate appearance according to the design of the lead frame can also be selected in accordance with the internal stress and the allowable furnace atmosphere components and the frictional force of the seal portion. In addition, if the lead frame is deformed even though the furnace atmosphere and the lead frame deflection in the furnace are normal, it is necessary to change the parts that are punched out before heat treatment or the frictional force of the seal parts. Appropriate measures can be taken in consideration of the change in the amount of circulating Ii of the furnace atmosphere gas.

〔Effect of the invention〕

According to the present invention, by completely removing the internal residual stress while leaving the internal residual stress, any deformation due to the internal residual stress is prevented, and at the same time, the slight dimensional change that occurs during the heat treatment after conventional press forming is avoided. This also makes it possible to manufacture lead frames with extremely high precision.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing an example of a lead frame strip completed by removing residual strain, FIG. 2 is a plan view showing an example of a lead frame strip using the manufacturing method of the present invention, and FIG.
The figure is a plan view showing an example of a lead frame strip used in a conventional manufacturing method. 2: Reference bottle hole 4: Inner lead 6; Notches AB, C: Strip element mounting stage 5: Araku lead 8: Connection piece

Claims (1)

[Scope of Claims] 1. A method for manufacturing a lead frame in which heat treatment is performed to remove internal residual stress accumulated in the lead frame during the process of forming a required pattern of the lead frame by plastic working, the method comprising: When forming a lead frame, the lead frame is shaped so that part of the area to be removed is left by punching to reduce the stress in the cross section that generates stress that exceeds the allowable range in response to applied external forces. The apparent internal stress in the longitudinal direction of the lead frame strip relative to the cross-sectional area in the width direction is 0.2 to 1.2.
Applying a tension in the range of kg/mm^2, and performing heat treatment to remove the internal residual stress while applying the tension that restrains the recovery of the internal residual strain remaining in the strip of the lead frame, A method for manufacturing a lead frame, characterized in that the region left unextracted after heat treatment is removed to form a desired lead frame.