JPS63232361A - Lead frame and semiconductor device - Google Patents

Abstract

PURPOSE:To prevent the generation of resin crack by heat in case of mounting a semiconductor device on a printed substrate by forming the section of a tab at a tab reverse element placing surface in a shape in which the thickness of the tab is gradually increased from both ends of short axis direction toward the center. CONSTITUTION:A tapered surface 14 is so formed at a reverse element placing surface 2a of a tab 2 that the sectional shape of the tab 2 is gradually increased in thickness from both ends 2e of the tab 2 in a short axis direction toward the center. Since the angle of the end 2e of the tab 2 of this sectional shape in the short axis direction becomes 90 deg. or larger, a stress concentration rate can be reduced. Accordingly, since the thickness of resin is increased to provide a stress reducing effect and it can prevent a local stress concentration, it can prevent the resin from cracking from both ends 2e of the tab 2 in the short axis direction.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a lead frame and a semiconductor device, and in particular to a lead frame structure suitable for preventing the occurrence of resin cracks in a resin-sealed semiconductor device and the use of this lead frame. related to semiconductor devices.

[Conventional technology]

In recent years, due to the demand for high-density packaging of semiconductor devices, there has been an increase in demand from pin-insertion type devices, in which leads are inserted into through-holes on printed circuit boards, to surface-mount type devices, in which leads are directly mounted on the surface of the printed circuit board. . To mount a semiconductor device on the surface of a printed circuit board, solder is applied to the printed circuit board 7, the semiconductor device is placed on the printed circuit board, and the semiconductor device is heated using methods such as infrared rays or paper reflow to melt the solder. This is done by connecting the semiconductor device and the printed circuit board. At this time, in these heating methods, not only the connection portion between the semiconductor device and the printed circuit board but also the entire semiconductor device is heated.

Usually, a semiconductor element is fixed onto a tab using an adhesive or the like, and terminals on the semiconductor element are electrically connected to a plurality of leads arranged around the tab by thin metal wires. The lead frame is formed of a group of leads, a tab, and a tab suspension lead that holds the tab (not shown),
After sealing with the sealing resin part, it is separated from the connected outer frame.

Silicon (Si) is used for the semiconductor element, and its coefficient of linear expansion is about 3 x 10-8/''C.

As the lead frame material, 4270I (linear expansion coefficient of about 5 x 10-'/'C) or copper alloy (linear expansion coefficient of about 17 x 10-'/'C) is usually used.
In addition, the linear expansion coefficient of the sealing resin is 20 to 30X10-6/'
It is C. In this way, since the linear expansion coefficients of the materials that make up the resin-sealed semiconductor device differ from each other,
Thermal stress acts inside the semiconductor device due to cooling after the semiconductor device is encapsulated with resin or a temperature drop during a temperature cycle test.

by this.

Peeling may occur at resin adhesive interfaces such as the surface opposite to the element mounting surface and the side surface of the tab.

When a semiconductor device whose adhesive interface between the tab and the resin part has peeled off is heated in order to be mounted on a printed circuit board, the entire semiconductor device is heated to a high temperature of about 200 to 250 degrees Celsius, so moisture absorbed by the resin and the lead frame are heated. Moisture that has entered from the interface between the tab and the resin is vaporized at the peeled part of the adhesive interface between the tab and the resin, and a large amount of pressure is applied from inside. Therefore, a large stress is generated in the resin at the lower part of the anti-element mounting surface of the tab, and resin cracks occur from both ends of the tab anti-element mounting surface in the short axis direction, where stress is concentrated.

As a measure to prevent such flow cracks, JP-A-61
There is a technique described in Japanese Patent No.-142760.

This has a convex portion protruding downward from the surface of the tab opposite to the element mounting surface.

[Problem that the invention seeks to solve]

In the above-mentioned conventional technology, when performing a temperature cycle test on a semiconductor device, the stress caused by the difference in coefficient of linear expansion between the lead frame material and the resin material is dispersed to both ends in the short axis direction of the tab and the end of the convex part.

It is effective in preventing resin cracks that occur during temperature cycle tests.

However, if such a convex portion is formed, the maximum stress will occur at the end of the convex portion during heating for mounting, and since the end portion is an acute angle or a right angle, stress concentration is unavoidable.

On the other hand, if the external dimensions of the semiconductor device are increased so that the thickness of the resin does not decrease even if the convex portion is provided, the generated stress can be reduced. However, increasing the thickness of the semiconductor device itself in this way cannot be applied to a surface-mounted semiconductor device, which is characterized by its thinness, such as a flat package.

An object of the present invention is to provide a lead frame that can prevent resin cracks caused by heating when a semiconductor device is mounted on a printed circuit board, and a semiconductor device using the lead frame.

[Means for solving problems]

The above object is achieved by forming the cross section of the tab on the surface opposite to the element mounting surface so that the thickness of the tab gradually increases from both ends in the short axis direction to the center.

The first invention of the present application relates to a lead frame, which includes a tab having a rectangular surface for mounting a semiconductor element, and a group of leads arranged apart from the tab and having an electrical connection part with the semiconductor element. The side of the tab opposite to the element mounting side has a tapered surface such that the thickness of the tab gradually increases from at least both ends of the long side toward the center.

The second invention of the present application also relates to a lead frame, in which at least two tapered surfaces are formed on the side opposite to the element mounting side of the tab so that the center of the tab is thicker than the peripheral edge of the tab, and the cross section of the tab in the width direction When you look at it, you can see that the corner formed by both ends and the anti-element mounting surface, the corner formed by the tapers on the anti-element mounting surface, or the corner formed by the taper and the central plane part of the anti-element mounting surface are It is also characterized by an obtuse angle.

The third invention of the present application relates to a semiconductor device, and includes a semiconductor element, a tab having a rectangular surface for mounting the semiconductor element,
The semiconductor device includes a lead group arranged apart from the tab and electrically connected to the semiconductor element, and a sealing resin part that covers a portion of the semiconductor element, the tab, and the lead group on the tab side. The side of the tab opposite to the element mounting side is characterized by having a tapered surface such that the tab gradually becomes thicker from at least both end portions of the longer side toward the center.

In any case, it is desirable that a surface parallel to the element mounting surface (central plane part) be formed at the center of the tab on the side opposite to the element mounting surface.

As mentioned above, the tapered surfaces require at least two opposing sides (-set), but it is also possible to form a taper from the other two sides, that is, form a tapered surface from each end of each of the four sides of the tab to the center. good.

A tab suspension lead may be provided at each of the short side ends of the tab.

In the above embodiment, the element mounting surface is a rectangle with long sides and short sides, but each side may be made equal, that is, a square. In this case, a tapered surface is formed from each of the four sides to the center. Furthermore, the above-mentioned central plane portion may be formed on the surface opposite to the element mounting surface.

In semiconductor devices, it is desirable to consider the thickness of the resin layer in order to prevent the occurrence of cracks in the resin part. If the resin thickness under the side edge is j, then this ratio It is desirable to satisfy the formula d≧(1-1/X)a/2.

Although it is desirable for the dimensions of the tapered surfaces to be equal in order to equalize stress, the present invention is not particularly limited to this. However, when the taper widths are different, d in the above formula.

It is preferable that the relationship between a and X be satisfied for each tapered surface.

[Effect]

When peeling occurs at the resin adhesive interface on the anti-element mounting surface of the tab and pressure is applied inside, the stress that occurs is (
As stated in the Japan Society of Mechanical Engineers, published June 25, 1980, 1 Mechanical Engineering Handbook, A4 Material Mechanics Fundamentals Edition, pages 59 to 61, the four sides shown in Figure 10 are fixed. It can be determined by modeling a uniformly distributed load on a rectangular plate. The maximum stress σa^χ at this time occurs at the center of both ends of the long side, and its value is given by the following equation.

Here, P is the uniformly distributed load, and a is the length of the short side of the tab.

h is the thickness of the resin, and β is the coefficient given by the ratio of the long side to the short side to the length.As is clear from equation (1), in order to reduce stress, the short side length a should be It is desirable to shorten the length or increase the resin thickness h.

As is clear from equation (1) above, the stress generated at both ends of the tab in the short axis direction due to peeling occurring at the resin adhesive interface on the surface opposite to the element mounting surface and pressure acting inside the tab is as follows: This can be reduced by increasing the thickness of the resin plate on the anti-element mounting surface.

For example, uniformly distributed load P = 1 kg/wr", short side length a = 2 am, coefficient β = 0.5, resin thickness h
= 1 m, the maximum stress generated at both ends of the tab in the short axis direction is 2 kg/m" from equation (1). If the resin thickness is increased by 0.1 wxa with the same tab shape, 1 will occur. The maximum stress is 1.65 kg/mm2, which can be reduced by about 20%.

In general, in resin-sealed semiconductor devices, analysis using the finite element method has revealed that stress at the same level as the static strength of the sealing resin is generated at the end of the tab. The generation of resin cracks can be prevented even by reducing the stress generated by 20%.

Therefore, by making the cross section of the tab on the side opposite to the element mounting surface such that the thickness of the tab gradually increases from the edge to the center, the area under both ends in the short axis direction of the tab where the maximum stress occurs is reduced. The thickness of the resin can be increased and the stress generated can be reduced. In addition, since the thickness of the tab gradually increases from the end to the center, the angle of the end on the tab anti-element mounting surface is 90 degrees or more in the cross section of the tab. 9
The stress concentration rate is lower than that at 0 degrees.

Therefore, due to the synergistic effect of reducing the stress generated at both ends of the tab in the short axis direction and reducing the stress concentration rate,
It is possible to prevent resin cracks from occurring from both ends of the tab in the short axis direction.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view of a tab portion of a lead frame for a semiconductor device according to an embodiment of the present invention, viewed from an opposite element mounting surface 2a. The anti-element mounting surface 2a of the tab 2 has a cross-sectional shape (long side in the direction of the short axis of the tab 2) such that the plate thickness gradually increases from both ends 2e to the center. The tapered surface 14 is formed by known techniques such as electroforming described in Japanese Patent No. 59-222951 and high-pressure pressing. Note that 8 is a tab suspension lead.

FIG. 2 is a sectional view of a semiconductor device using the lead frame shown in FIG.
By making the plate thickness of the tab 2 gradually thicken from both ends 2e of the tab 2 in the short axis direction to the center as shown in the figure, both ends of the opposite side of the element mounting surface 2a of the tab 2 in the short axis direction Since the thickness of the resin 5 under the portion 2e increases, as is clear from equation (1) above, the anti-element mounting surface 2 of the tab 2 increases.
The stress generated at both ends 2e in the short axis direction of a can be reduced. In addition, 1 is a semiconductor element, 3 is a lead (a collection of leads 3 arranged along both long sides of the semiconductor element l, excluding the tab hanging lead 8, constitutes a lead group), and 4 is each lead 3 and the semiconductor. This is a wire that connects the pad on the element 1.

When the tab of the lead frame according to this embodiment is used, the stress generated at both ends 2e of the long side of the tab 2 is caused by the stress generated in the resin at the bottom of the surface opposite to the element mounting surface of the tab 2 in the short axis direction of the tab 2. Since the thickness changes continuously such that it is thick at the end 2e and thinnest at the center of the tab 2, the thickness of the resin in formula (1) may be slightly higher than when it is constant. however,
When the plate thickness of the tab 2 is uniform as shown in FIG. 8, the angle of both ends 2e in the short axis direction of the tab 2 is 90 degrees, whereas the cross-sectional shape of the lead frame of this embodiment is In the tab 2, since the angle of the end 2e in the short axis direction of the tab 2 is 90 degrees or more, the stress concentration rate can be reduced. Here, FIG. 8 shows the mounting structure of a conventional semiconductor device, in which reference numeral 6 indicates solder, 7 indicates a printed circuit board, 9 indicates a lead frame, and 10 indicates a solder connection location.

Therefore, as well as the stress reduction effect due to the increase in the thickness of the resin, it is possible to prevent stress from concentrating locally, and therefore, it is possible to prevent resin cracks from occurring from both ends 2e of the tab 2 in the short axis direction. Also.

In order to obtain the same effect as the lead frame according to this embodiment while keeping the thickness of the tab 2 constant, both ends 2e of the tab 2 in the short axis direction are The cross-sectional shape is such that the plate thickness is thinner at the center and the plate thickness increases towards the center.

The tab 2 can have sufficient rigidity. This prevents the tab 2 from warping or shifting when water vapor pressure acts on the surface of the tab 2 opposite to the element mounting surface due to heating during resin molding or mounting. 4. No defects such as wire breakage will occur.

FIG. 3 shows another embodiment of the present invention, and is a perspective view of the tab 2 portion of the lead frame for a semiconductor device viewed from the opposite element mounting surface 2a, and FIG. 4 is a lead according to the present invention shown in FIG. 2 is a cross-sectional view of a semiconductor device using a frame. In this embodiment, both ends 2 in the short axis direction of the anti-element mounting surface 2a of the tab 2
The cross-sectional portion (tapered surface 14) in which the plate thickness gradually increases from e to the center is at both ends 2e in the short axis direction of the tab 2, and has a predetermined length or more, and the central portion of the tab 2 is a plate. The central plane portion 15 was formed to have a cross-sectional shape with a constant thickness. Even with a lead frame having a cross-sectional shape as shown in Fig. 3, the length of the cross-sectional portion where the plate thickness gradually increases from both ends 2e in the short axis direction of the tab 2 satisfies the conditions described in the first section. effect can be obtained.

In a lead frame with a cross-sectional shape as shown in Fig. 3, the locations where high stress occurs are at both ends 2e in the short axis direction shown in Fig. 3.
and at both ends 15e of the tab cross section where the thickness of the tab 2 is constant.

To find the stress generated at each location.

Here, the shapes will be replaced with the shapes shown in FIGS. 5 to 7 as follows. The stress σ1 generated at the end 2e is due to the tab 2 having a constant plate thickness Ll and the short side length 81 as shown in FIG. 6, and the stress σ2 generated at the end 15e is
As shown in the figure, each tab 2 is replaced with a tab 2 having a constant plate thickness Lz and a short side length az. Therefore, the stresses generated at the ends 2e and 15a are as follows from equation (1).

Here, al and al are the lengths of the short sides of the tabs, ht and hz are the thicknesses of the resin, P is the uniformly distributed load, and β1. β2 represents a coefficient given by the ratio of the lengths of the long side and the short side.

In addition, h 1 > h x, a 1 > a z, and β1
4β2.

By setting hl to 1 times hz (>1) and setting the stress σ1 of the end 2e and the stress σ2 of the end 15e to be equal, al
becomes as follows.

From this, when az<a1/X, β2>β1,
When az>al/x, β2<β1. That is,
In the lead frame having the shape shown in Fig. 3, a x”)
If a 1/X is not used, the stress generated at the end 15e will be higher than that at the end 2e. Therefore, in the lead frame according to the present embodiment shown in FIG. 3, the ratio of the resin thickness at the bottom of the cross section where the thickness of the tab 2 is constant to the resin thickness at the bottom of the end 2e of the tab 2 is set to X (>1 ), it is necessary to set the length d of the cross-sectional shape in which the plate thickness gradually increases from the end 2e to the center of the tab as follows.

d≧(1-1/x) a/2 (5) Stress generated in the end portion 15e when the length d of the cross-sectional shape where the plate thickness of the tab becomes thicker satisfies the condition of equation (5). is lower than the stress generated at the end portion 2e, making it possible to prevent resin cracks from occurring there.

FIG. 9 shows how cracks occur when the measures of the present invention are not taken. Reference numeral 11 indicates peeling.

Each of the embodiments of the present invention described above prevents the occurrence of racks.

FIG. 11 shows, as a comparative example, a structure in which a convex portion is formed on the surface opposite to the element mounting surface described in the prior art. FIG. 12 is a partially enlarged view of FIG. 11. In the resin-sealed semiconductor device in which the above-mentioned convex portion 13 is provided on the surface opposite to the element mounting surface of the tab 2, maximum stress is generated at the end portion 13e of the convex portion 13 due to heating during mounting on the surface of the printed circuit board. I come to do it. In addition, in order to ensure the thickness of the resin, if the protrusion is provided by cutting the vicinity of the end of the tab 2 to make the plate thinner, the thinner part of the tab 2 will have a lower rigidity. The tab 2 may be misaligned or warped due to water vapor pressure generated by heating during resin sealing or mounting, which may cause defects such as damage to the semiconductor element 1 or breakage of thin metal wires.

Furthermore, in the lead frame according to the above embodiment of the present invention,
Angle 11 of ends 2e and 15g of tab 2 as shown in FIG.
11. Since both θ2 are 90 degrees or more, the stress concentration rate at the tab end is lower than the angle θ3 at the end when the convex portion 13 as shown in FIG. 12 is provided. Therefore, local stress concentration is less likely to occur, which is effective in reducing stress and preventing the occurrence of resin cracks.

The present invention can have various embodiments shown in FIGS. 13 to 19 above. In each of the embodiments shown in FIGS. 13 and 14, the tab plane is square, and in this case, the tapered surface 14 is formed from all four side edges toward the center. In FIGS. 15 and 16, the tab plane is not square, but in this case, there is no problem in forming the tapered surface 14 from each of the four sides.However, in any of the embodiments in FIGS. It is preferable that the shapes and angles of the set of tapered surfaces (therefore, there are two sets of tapered surfaces on each side) to be the same so that the stress is not biased.

In the above embodiment, the number of tapered surfaces 14 is 2 in one cross section.
However, the present invention is not limited to this, and the tapered surface 14 may be formed with stepwise angles as shown in FIG. However, for the purpose of reducing stress concentration points, the cross section may be formed in a curved shape as shown in FIGS. 18 and 19.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to reduce the stress generated at both ends of the short axis direction of the anti-element mounting surface of the tab due to heating when surface mounting a semiconductor device, and to reduce the stress generated at both ends of the tab in the short axis direction. Since concentration can also be alleviated, there is an effect of preventing the occurrence of resin cracks.

[Brief explanation of drawings]

1, 3, 13, 14, 15, and 16 are all perspective views of the tab portion of the lead frame according to the embodiment of the present invention, and FIG. 4 is a sectional view of a semiconductor device according to an embodiment of the present invention to which the tab of FIG. 3 is applied, and FIG. 5 is a sectional view of a semiconductor device according to an embodiment of the invention to which the tab of FIG. FIGS. 6 and 7 are cross-sectional views of a model for stress calculation of the tab in FIG. are the same〈A cross-sectional view of a semiconductor device illustrating the occurrence of resin cracks in a conventional example, Fig. 10 is a perspective view of a stress calculation model, Fig. 11 is a cross-sectional view of a semiconductor device according to a conventional example, and Fig. 12 is a cross-sectional view of a semiconductor device according to a conventional example. FIG. 17 is a tab sectional view of the device of FIG. 11; 18 and 19 are cross-sectional views of a tab according to an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...Semiconductor device, 2...Tab, 2a...Anti-element mounting surface, 2e...Tab long side end, 3...Lead, 4
...Wire. 5...Resin, 8...Tab hanging lead, 9...Lead friend No. 1 Decoy $2 Prisoner! ... Ritsu conductor 4... Tapered surface No. 6: 6th No. 5 Prisoner 2... 2a... Tade Nagashieki Ekikui No. 6 Mouth rate 7 Figure/, ζe 乎 8 No. 1, ・5... 2...Tab 6...); 92nd 10th prisoner 11 country 1... 1st country 2... 13th country 14th day (Ding Iji-

Claims (1)

[Scope of Claims] 1. A lead frame comprising a tab having a rectangular surface for mounting a semiconductor element, and a group of leads disposed apart from the tab and having an electrical connection part with the semiconductor element, A lead frame characterized in that a side of the tab opposite to the element mounting side has a tapered surface such that the tab gradually becomes thicker from at least both ends of the long side toward the center. 2. The lead frame according to claim 1, wherein a surface parallel to the element mounting surface is formed at the center of the tab on the side opposite to the element mounting surface. 3. The lead frame according to claim 1, wherein the tapered surface is formed from each end of each of the four sides to the center. 4. The lead frame according to claim 1, wherein a tab suspension lead is provided at each of the short side ends of the tab. 5. The lead frame according to claim 3, wherein the element mounting surface is square and has a surface parallel to the element mounting surface at the center of the opposite element mounting surface. 6. In a lead frame comprising a tab having a rectangular surface for mounting a semiconductor element, and a group of leads disposed apart from the tab and having an electrical connection part with the semiconductor element, the tab is opposite to the element mounting part. At least two tapered surfaces are formed on the side so that the center of the tab is thicker than the peripheral edge of the tab, and when viewed in cross section of the tab in the width direction, both long side ends and the opposite element mounting surface are formed. A lead frame characterized in that the corner formed by each other, the corner formed by the tapers on the anti-element mounting surface, or the corner formed by the taper and the central plane part of the anti-element mounting surface are all obtuse angles. 7. A semiconductor element, a tab having a rectangular surface for mounting the semiconductor element, a group of leads arranged apart from the tab and electrically connected to the semiconductor element, the semiconductor element and the tab, and the tab. In the semiconductor device, the tab has a sealing resin portion that covers a portion of the lead group on the side, and the side of the tab opposite to the element mounting side is such that the thickness of the tab gradually increases from at least both ends of the long side toward the center. A semiconductor device characterized by having a tapered surface. 8. The semiconductor device according to claim 7, wherein a central plane portion parallel to the element mounting surface is formed at the center of the tab on the side opposite to the element mounting surface. 9. Ratio x of the resin thickness i under the central plane part and the resin thickness j under the long side end of the anti-element mounting surface in the same cross section
has a relationship of x=j/i>1, and x has a relationship of d≧(1-1/x)a/2 with the width a of the tab and the projected width d of each tapered surface. A semiconductor device according to claim 7, characterized in that the semiconductor device has: