1. Field of the Invention
The invention relates to a device and a method for fabricating a lead frame by press forming, and a lead frame fabricated using such a press-forming fabrication device.
2. Description of the Related Art
In a known technique, a lead frame W having a die-pad Wa, on which a semiconductor chip is to be mounted, and inner leads Wc inwardly extending toward the periphery of the die-pad Wa, as shown in FIG. 3, is fabricated by press forming a plurality of support bars Wb connected to the periphery of the die-pad Wa and supporting it, to thereby form a difference in level between the die-pad Wa and the inner lead Wc of the order of the thickness of the semiconductor chip to allow the chip to be stably mounted on the die-pad Wa while decreasing the difference in level and the distances between the external terminals of the semiconductor chip and the inner leads Wc to facilitate the bonding thereof.
A conventional device and method for fabricating a lead frame by press forming is described in Japanese Unexamined Patent Publication (Kokai) No. 9-27580 (JP 9-27580 A). The device described in this document performs the press forming of a lead frame W by holding the lead frame W between a die 2 and a punch 6 and forming the lead frame W to provide it with a shape along the die 2 and the punch 6, as seen from FIG. 1 of the document.
In the conventional device for fabricating a lead frame by press forming as described in JP 9-27580 A, there are problems such that the bent portions of the fabricated lead frame revert (a phenomenon of so-called spring-back), and the depth and the angle of the bending are uneven, leading to increased variation in fabricated shapes of products (lead frames).
To solve the problems, Shinko Electric Industries Co., Ltd., Nagano, Japan has developed a device Y for fabricating a lead frame by press forming, as illustrated in FIG. 8. The device Y is provided with a die 92 having a flat face 94a, on which a lead frame to be fabricated by press forming is placed, and a concavity 92a, which is dented relative to the flat face 94a, the concavity having a fabricating face 94 extending from the bottom 94c of the concavity 92a to the flat face 94a through a slant 94b, which is interposed between the bottom 94c and the flat face 94a, and the fabricating face 94 contributing the fabrication of a lead frame. The device Y is also provided with a punch 96, which can be relatively moved toward the die 92 and which has punching faces 98 formed so as to be opposite to the fabricating faces 94 of the die 92. The concavity 92a is formed such that when a lead frame W is placed on the flat face 94a, the die-pad Wa thereof is located over the concavity 92a and the respective support bars Wb straddle the margins of the concavity 92a. The fabricating faces 94 are formed at the locations of the concavity 92a which the support bars Wb straddle.
The device Y for fabricating a lead frame by press forming can fabricate the respective support bars Wb of the lead frame W by holding the support bars Wb between the fabricating faces 94 of the die 92 and the punching faces 98 of the punch 96 when the punch 96 is come close to the die 92.
In the device Y, the front ends 98a of the punch 96 opposite to the bottom 94c of the concavity 92a have an area smaller than the area of the bottom 94c of the concavity 92a, and somewhat squash the vicinities of the inner sides of the bent portions of the lead frame W (the support bars Wb), which are located between the front ends 98a of the punch 96 and the bottom 94c of the concavity 92a, during the fabrication by press forming. The shoulders 98b of the punching faces 98 opposite to the flat face 94a of the die 92 somewhat squash the vicinities of the outer sides of the bent portions of the lead frame W (the support bars Wb), which are located between the shoulders 98b of the punching faces 98 and the flat face 94a of the die 92, during the fabrication by press forming.
In the case of the use of the device Y for fabricating a lead frame W by press forming in FIG. 8, squashing the vicinities of the bent portions of the lead frame W by press-forming fabrication can prevent the spring-back of the fabricated lead frame W, and can render the depth and the angle of the bending even, resulting in the reduced variation in fabricated shapes of products (lead frames).
However, there is still a problem that the lead frame W having diagonally opposite support bars Wb, which has been press-formed by the conventional device described in JP 9-27580 A or the device Y of FIG. 8, is curved such that the intermediate section between the bent portions opposite to each other, including parts of the support bars Wb and the die pad Wa, is warped up at the side thereof having been in contact with the punch 96 during the fabrication.
Particularly recent miniaturization and increased integration of semiconductor devices has led to the increased demand for the formation of lead frames having very small thicknesses, such as 0.15 millimeter or smaller. The smaller the thickness of a lead frame, the larger the curvature of the lead frame. On the other hand, standards of planarity of a lead frame tend to be severe year by year. For example, a standard specifying a displacement of a die-pad and support bars by curvature of 0.015 millimeter or smaller has been established. However, there is a problem that conventional devicees for fabricating a lead frame by press forming cannot satisfy such a standard in some cases.