JPH0527986B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a lead frame used in a resin-sealed semiconductor device, and particularly relates to an improvement in the island shape of a semiconductor chip mounting bed.

[Technical Background of the Invention] Lead frames for semiconductor devices are made by pressing or etching a single metal plate. A lead frame is a unit frame consisting of a bed for mounting a semiconductor chip, a plurality of leads for guiding electrodes of the chip to an external circuit, and a connecting bar for holding the bed and leads in a predetermined position during the manufacturing process,
It is a band-shaped metal plate made by arranging a large number of unit frames and connecting them with a transport frame. transistors and ICs
An automatic mechanized production method is used for assembly, and lead frames are generally widely used as a suitable member for this purpose.

A unit frame of a lead frame used in a conventional resin-sealed semiconductor device will be described below with reference to the drawings. FIGS. 3 and 4 are plan views of conventional resin-sealed transistors. In order to make the drawings easier to see, the upper resin of the resin envelope has been removed to expose the assembled semiconductor chip, leads, etc. FIG. FIG. 5 is a plan view of a unit lead frame used in the transistor of FIG. 4. Semiconductor chip 1 (transistor chip) is attached to chip mounting bed (island) 2.
It is attached and fixed with brazing material etc. 3 is a lead for leading out the electrodes of the chip to an external circuit. In this example, the lead 3 is composed of a lead 3a projecting from the chip mounting bed 2 and a lead 3b separated from the bed 2. Some varieties do not have leads that protrude beyond the tip mounting bed. 4 is a bonding wire made of Au, Al, etc., which electrically connects the semiconductor chip 1 and the leads 3b. The reason why the ends of the leads 3b are wide is mainly to facilitate wire bonding work. The one-dot chain line 7 is a center straight line 7 extending from the center line of the axis of the lead 3a projecting from the chip mounting bed 2. In the example shown in FIG. 3, a chip mounting bed is provided at a substantially symmetrical position with respect to the center line 7, and a semiconductor chip is mounted thereon. In the example shown in FIG. 4, the mounting is shifted to one side of the center straight line 7. A semiconductor chip 1 is mounted on a chip mounting bed 2, bonded with bonding wires 4, and then sealed with a resin such as epoxy by a transfer molding method or the like. 5 is this molded resin for sealing (resin envelope). Further, the resin-sealed portion of the lead 3 is called an internal lead (inner lead), and the portion that is not sealed with resin and is exposed to the outside is called an external lead (outer lead). In FIG. 5, a dam bar 8 prevents resin from flowing out during the resin sealing process and connects the leads to each other. Further, reference numeral 9 denotes a conveyance frame, and 10 denotes a conveyance hole, which has the function of moving the lead frame and positioning it at a predetermined position during the assembly process. The dam bar and transport frame are necessary only during the manufacturing process and are removed after resin sealing.
Copper, copper alloy, iron, or nickel-cobalt alloy is generally used as the material for the lead frame.

[Problems with Background Art] Generally, semiconductor devices are attached to a circuit board or the like using external leads. This attachment is carried out by inserting the external leads of the device into pin holes of the board or the like, or by soldering the external leads to a predetermined position on the printed wiring, or the like. Therefore, after the external leads of the semiconductor device are sealed with resin, it is necessary to mold them into a shape that is easy to attach. FIG. 2 is a perspective view showing an example of an external lead of a semiconductor device having the shape shown in FIGS. 3 and 4 after being formed.

On the other hand, during handling of semiconductor devices after resin sealing, lead spacing and lead straightness may deteriorate. Particularly in small devices where the mechanical strength of the external leads is weak, deviations from the normal shape often occur. In this case, immediately before installing the device, molding is performed to correct the misalignment using, for example, tweezers.

As described above, it is necessary to mold the external leads of a semiconductor device after resin sealing. During this molding operation, the external lead is subjected to tensile stress in its axial direction. In the case where the external lead is the lead 3a that protrudes from the chip device bed, stress is naturally applied to the chip that is attached and fixed to the same bed as the lead, and if that stress exceeds a certain limit, the chip will be destroyed. . Chip cracks during molding are a problem that cannot be ignored, especially in semiconductor devices with small external dimensions of several millimeters.

[Object of the Invention] An object of the present invention is to provide a semiconductor having a shape that reduces as much as possible the external force applied to the chip mounting bed through the external leads in the process of molding the external leads after resin molding of a resin-sealed semiconductor device. An object of the present invention is to provide a lead frame for a device.

[Summary of the Invention] The present invention provides a lead frame for a semiconductor device having a chip mounting bed for mounting a semiconductor chip and a lead protruding from the bed, in which the lead shaft protrudes from the bed at the center of the chip mounting bed. The bed has an overhanging beam portion (hereinafter also referred to as a protrusion) that is unevenly distributed on one side of the central straight line, and the bed projects to the other side of the central straight line, and the distance from the central straight line to the end of the overhanging beam portion is A lead frame for a semiconductor device characterized in that the distance is in the range of 1/3 to 1 of the distance from the center straight line to the edge of the bed, and a semiconductor chip is mounted in a central portion near the center of the bed. .

The lead frame according to the present invention has an improved shape of the semiconductor chip mounting bed (island). That is, the bed is provided on one side of the center straight line of the lead shaft, which is the same body as the bed, and the protrusion is provided on the other side. In addition, this protrusion is arranged in a direction opposite to the bed with respect to the center line, parallel to the surface of the resin envelope at the top of the bed, and has a size equal to or close to that of the bed, that is, 1/3 of the protruding distance from the center line. It has a protruding shape with a protrusion distance of ~1. If the mounting bed is shaped as a bed with protrusions in this way, even if tensile force is applied to the leads during external lead molding, part of the stress will be borne by the protrusions, and some of the shear stress will also be borne by the protrusions. canceled,
The external force applied to the tip mounting bed is significantly reduced compared to the conventional method.

[Embodiments of the Invention] This invention was made mainly based on conventional experience and trials, with some inferences added. In the case of the conventional semiconductor device shown in FIG. 3, when a tensile force is applied to the lead 3a, the chip mounting bed 2 is located on the center straight line 7 of the axis of the lead, that is, on the line of action of the tensile force. A stronger stress acts on the area than on other parts. In addition, in FIG. 4, the tip mounting bed 2 is on the right side of the center straight line 7 of the lead 3a (on the drawing).
Since the bed is biased toward the base, the bed is not subjected to direct tensile force, but is subjected to shear force due to bending moment. For this reason, it was inferred that in either case, the semiconductor pellets fixed to the mounting bed would be subject to strong stress and would be susceptible to destruction. To solve this problem, we decided to position the mounting bed off to the line of action of the tensile external force and provide a protrusion on the opposite side of the bed with respect to the center line of the lead shaft in order to offset the shear force caused by the bending moment.

FIG. 1 shows one embodiment of the lead frame of the present invention in which a protrusion is provided on the bed, and the drawing is a plan view of a semiconductor device using this lead frame. ,
The resin on the upper side of the chip mounting bed 2 and the like is not shown in the figure. In the drawings, the same reference numerals represent the same parts as in FIG. 3, etc. Chip mounting bed 2
The position of the center 11 of the lead 3a protruding from the bed 2 is unevenly distributed on one side (right side in this drawing) of the center line 7 of the axis of the lead 3a, and the center 11 is not necessarily located in the central part of the bed near the center 11. The semiconductor chip 1 is mounted, although the centers of the chips do not necessarily coincide. The chip mounting bed 2 also has a protrusion 6 that projects on the other side of the center straight line 7 (on the left side in this drawing).

An example in which a lead frame having the shape shown in FIG. 1 is applied to a resin-sealed product with a small envelope will be described. The outer dimensions of the resin envelope are 1.5 mm wide.
A piece with a width of 3 mm and a height of 1.1 mm was used. After being assembled and resin-sealed, the tips of the external leads of the above device (no external lead molding) are soldered and fixed to a predetermined position on a test board or the like. Next, external leads are molded into the shape shown in FIG. The characteristics of the device were examined before and after the external lead was molded, and changes in characteristics and chip breakage were investigated. As a result, there were no defects. This confirms that the protrusion bears a significant portion of the tensile force applied to the external lead and serves to stop the tensile stress.

[Effects of the Invention] In resin-sealed semiconductor devices using conventional lead frames, 5 to 30% of chips were destroyed and had poor characteristics due to lead molding. By using the lead frame according to the present invention, it was possible to completely eliminate defects caused by lead molding.

[Brief explanation of drawings]

FIG. 1 is a plan view of a semiconductor device using the lead frame of the present invention, FIG. 2 is a perspective view showing the external leads of the semiconductor devices shown in FIGS. 1, 3, and 4 after being formed. 3 and 4 are plan views of a conventional semiconductor device, and FIG. 5 is a plan view of a conventional lead frame. DESCRIPTION OF SYMBOLS 1... Semiconductor chip, 2... Chip mounting bed (island), 3... Lead, 3a... Lead protruding from the chip mounting bed, 5... Resin envelope, 6... Projection, 7... Center straight line of lead axis, 11... The center of the tip mounting bed.

Claims (1)

[Claims] 1. In a lead frame for a semiconductor device having a chip mounting bed for mounting a semiconductor chip and leads protruding from the bed, the center position of the chip mounting bed is on one side of the center line of the lead shaft protruding from the bed. The bed has an overhanging beam that is unevenly distributed and projects to the other side of the center line, and the distance from the center line to the end of the overhang beam is 1/3 of the distance from the center line to the end of the bed. 1. A lead frame for a semiconductor device, characterized in that the semiconductor chip is mounted in a central portion near the center of the bed.