JPH03283557A - Lead frame and semiconductor device using it - Google Patents

Abstract

PURPOSE:To reduce the position dependency of coefficient of stress magnification in the peeling face of a tab as well as the maximum value of coefficient of stress magnification and to suppress package cracks by arranging a tab constituted with smooth curve form. CONSTITUTION:A lead frame is constituted of a tab 1a shaped elliptical or circular, suspension pins 2, and a plurality of leads 3. The tab 1a has a shape formed not by straight lines as a rectangle, but by smooth curves as ellipse or circle. Hereupon, the sizes of the tab 1a and a semiconductor chip 4 are so set that at least part of the edge of the tab 1a is covered with the semiconductor chip 4. Terminal electrodes of the semiconductor chip 4 are connected to leads 3 with bonding wires 5. That is, shaping the tab 1a elliptical and setting sizes so that part of its edge is covered with the semiconductor chip 4 yield resin-sealed type semiconductor IC devices which hardly develop package cracks.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a semiconductor device constructed by mounting a semiconductor element on a lead frame and sealing it with resin, and particularly relates to an improvement of the lead frame. Regarding.

(Prior Art) In resin-sealed semiconductor devices, lead frames are used for assembly.

FIG. 6 shows an example of a conventional resin-sealed semiconductor device with a portion cut away.

The lead frame includes an element mounting portion (hereinafter referred to as a tab) 1b1 having a rectangular shape, a hanging rib 2, a plurality of leads 3, and the like.

The semiconductor element 4 is adhered and mounted on the tab 1b using a die bonding agent. Electrical connection between the electrode portion of the semiconductor element and the lead 3 is performed via a bonding wire 5 made of Au, AfI, or the like. The device is encapsulated by heating a thermosetting molding agent containing fillers, additives, and resins such as epoxy resin at a temperature of about 170° C., embedding the device therein, and molding the package 6. Complete.

Since the epoxy resin in the package 6 formed in this manner has hygroscopic properties, moisture accumulates at the bonding interface between the epoxy resin and the lead frame.

When surface mounting the package 6 on the board, the temperature is usually 200°C.
It is carried out at high temperatures.

As a result, there is a gap between the back surface of the tab 1b and the package resin, which has poor adhesion compared to between the semiconductor element 4 and the package resin, due to the water vapor pressure generated by the evaporation of water accumulated at the bonding interface during this process. Peeling occurs. It has been confirmed that as this peeling progresses, the entire surface eventually peels off and its shape becomes the same as that of the tab.

Along with this whole surface peeling, package cracks 7 occur in the area around the tab 1b where the strength is the weakest.

Once the package crack 7 occurs, it spreads in a circular shape as shown by the broken line in FIG. 4, starting from the tip 8 of the package crack 7.

The ease with which the package crack 7 develops at the tip of the peeling is determined by the stress intensity factor and the fracture toughness value of the resin. That is, when the stress intensity factor becomes larger than the fracture toughness value, package cracking progresses.

Next, the relationship between the stress intensity factor and package cracks when the peeled surface is rectangular in shape will be explained.

FIGS. 7(a), (b), and (c) are diagrams for explaining the position dependence of the stress intensity factor when the peeled surface has a rectangular shape. FIG. 7(a) is a diagram showing how a negative internal pressure 7 acts in a direction perpendicular to the peeling surface, forming a peeling portion 8 and causing full-scale peeling.

FIG. 7(b) shows the peeled surface seen from above the substrate surface.

The X-axis and y-axis are straight lines passing through the center of the rectangle and parallel to the long and short sides, respectively. θ is the angle formed between the X-axis and a straight line connecting the center of the rectangle and a point on the side, and corresponds one-to-one with the point on the side.

Due to symmetry, the stress intensity factor at each position on the side of the rectangle is
Since it is sufficient to consider only one of the four regions divided by the axes and y-axes, we will consider the range of 0''≦θ≦90°.Figure 7 (C) shows the peeling in such a range. It is a diagram showing the distribution of the stress intensity factor acting on a surface. As this diagram shows, the stress intensity factor varies depending on the position, and is maximum at θ-90'', that is, at the center of the long side. Therefore, if the fracture toughness value has no position dependence, package cracks will occur from the center of the long side of the peeled surface and will progress one after another from that point as a starting point.

As described above, the lead frame having the conventional structure has a problem in that the reliability of the semiconductor device decreases due to the occurrence of package cracks due to peeling of the entire surface.

(Problems to be Solved by the Invention) As described above, when surface-mounting a conventional resin-sealed package, peeling occurs across the entire surface between the tab backside and the package resin, starting from the portion of the tab where the stress intensity factor is large. There was a problem that package cracks occurred.

An object of the present invention is to provide a lead frame for a resin-sealed semiconductor device in which package cracks are less likely to occur.

Another object of the present invention is to provide a highly reliable resin-sealed semiconductor device using such a lead frame.

[Structure of the Invention] (Means for Solving the Problem) In order to achieve the above object, the lead frame according to the present invention is characterized by having a tab configured with a smooth curved shape.

Further, a semiconductor device according to the present invention includes a lead frame having a tab configured with a smooth curved shape, a +'- conductor element mounted on the tab of the lead frame,
It is characterized by having a package in which the semiconductor element is sealed with resin.

In this case, in the lead frame, it is desirable to set the size of the tab so that at least a portion of the edge of the tab is covered by the semiconductor element.

(Function) According to the present invention, the positional dependence of the stress intensity factor on the peeled surface of the tab is reduced, and the maximum value of the stress intensity factor is also reduced, making it difficult for package cracks to occur.

(Example) Hereinafter, details of the present invention will be explained with reference to illustrated embodiments.

FIG. 1 is a plan view of a lead frame according to an embodiment of the present invention.

The material of the lead frame is, for example, 4270I or copper alloy (copper as the main component, Fe, Sn, P depending on the purpose).
~ several types added in several percentages). In this embodiment, there is no particular restriction on the material, and it is sufficient to select and use one of these materials in consideration of the characteristics of various ICs, assembly method, package shape, etc.

This lead frame has tabs that are oval or circular in shape.
It is composed of an AS hanging bottle 2, a plurality of leads 3, and the like. This lead frame differs from conventional lead frames in the shape of the tab 1a. The tab 1a does not have a straight line shape such as a conventional rectangle, but has a smooth curved shape such as an ellipse or circle.

FIG. 2 is a partially cutaway view of a semiconductor device according to an embodiment of the present invention using the lead frame shown in FIG. A semiconductor element 4 having a rectangular or square shape is mounted on a tab 1a of a lead frame.

Here, the sizes of the tab 1a and the semiconductor element 4 are set so that at least a part of the edge of the tab 1a is covered by the semiconductor element 4. The terminal electrodes of the semiconductor element 4 and the leads 3 are connected by bonding wires 5. The semiconductor element 4 integrated into the lead frame in this manner is sealed with a resin package 6 made of thermosetting epoxy resin or the like, as in the conventional case.

Next, the reason why the above embodiment makes it difficult for package cracks to occur will be explained below.

FIGS. 3(a), 3(b), and 3(c) are diagrams for explaining the position dependence of the stress intensity factor in an embodiment in which the shape of the peeling surface between the tab 1a and the resin cage 6 is elliptical. Figure 3(a)
is the peeled surface seen from the top of the substrate surface, and its shape is an ellipse. The method of selecting the coordinate axes is the same as in FIG. Figure 3(b)
A solid line a is a diagram showing the position dependence of the stress intensity factor in the case of such a peeled surface. The broken line indicates the position dependence of the stress intensity factor of the peeled surface in the case of a rectangular tab having a peripheral length comparable to that of the example. As can be seen from this figure, in the case of the above embodiment in which the peeled surface has an elliptical shape, the stress intensity factor gradually increases as the angle θ increases. That is,
The position dependence is smaller when the peeled surface is elliptical than when it is rectangular. If the peripheral length of the tab is about the same as that of the conventional one, the maximum value of the stress intensity factor will be smaller than that of the conventional one. As a result, according to the above embodiment, package cracks can be effectively suppressed.

From the above, it can be seen that the shape of the tab in which package cracks are least likely to occur is a circle in which the stress intensity factor has no position dependence.

FIG. 3(c) is a diagram showing the position dependence of the stress intensity factor using the area as a parameter when the peeled surface has an elliptical shape. The solid line C is a curve showing the stress intensity factor when the area is smaller than the solid line a.

As can be seen from this figure, the shape of the curve showing the position dependence of the stress intensity factor is not very accurate, but the smaller the area, the smaller the stress intensity factor. That is, it can be seen that it is preferable to make the area of the tab as small as possible in order to suppress package cracks.

As described above, according to this embodiment, the shape of the tab 1a is made into an ellipse, and the size of the tab 1a is set so that a part of the edge of the tab 1a is covered with the semiconductor element 4. It is possible to provide a resin-sealed semiconductor integrated circuit device that is less prone to cracking.

Note that the relationship between the tab 1a and the semiconductor element 4 is not limited to the above embodiment. For example, FIGS. 4(a) to (d) show the relationship between the tab 1a and the semiconductor element 4 when viewed from the top surface of the semiconductor element.
) can be selected appropriately. In the figures, (a) and (b) show an example in which the body of the tab 1a is covered by the semiconductor element 4, and (c) and (d) show an example in which a part of the edge of the tab 1a is covered by the semiconductor element 4. An example of a case where it is hidden is shown. Furthermore, even when the tab 1a is not covered by the semiconductor element 4, the tab 1a is not covered with the semiconductor element 4 as shown in FIGS.
By making the shape of a as smooth as possible to match the shape of the semiconductor element, a similar effect can be expected.

[Effects of the Invention] As described above, according to the present invention, the strength of the resin package can be increased with a simple configuration, and the occurrence of package cracks that occur when surface mounting a resin-sealed semiconductor device on a substrate can be avoided. can be suppressed. Thereby, a highly reliable semiconductor device can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a plan view of a lead frame according to an embodiment of the present invention, Fig. 2 is a diagram showing an embodiment of a semiconductor device using this lead frame, and Figs. 3 (a) to (c) are exfoliation. Figures 4(a) to 4(a) are diagrams explaining the position dependence of the stress intensity factor when the surface shape is an ellipse.
d) is a diagram showing the relationship between various sizes and shapes of the tab and the semiconductor element; FIGS. 5(a) to (d) are diagrams illustrating the relationship between the tab and the semiconductor element of other embodiments; FIG. 7 is a diagram showing a conventional semiconductor device, and FIG. 7 is a diagram illustrating the position dependence of the stress intensity coefficient when the peeled surface has a rectangular shape. 1a...Tab (element mounting part), 2...Hanging pin. 3... Lead, 4... Semiconductor element, 5... Bonding wire, 6... Package.

Claims (4)

[Claims] (1) A lead frame characterized by having an element mounting portion configured with a smooth curved shape. (2) A lead frame having an element mounting part configured with a smooth curved shape, a semiconductor element mounted on the element mounting part of this lead frame, and a package in which this semiconductor element is sealed with resin. Characteristic semiconductor devices. (3) The semiconductor device according to claim 2, wherein the element mounting portion has a shape of a circle or an ellipse. (4) The semiconductor device according to claim 2, wherein the size of the element mounting part and the semiconductor element is set so that at least a part of the edge of the element mounting part is covered by the semiconductor element. .