Electric heating – Metal heating – By arc
Patent
1996-05-14
1997-09-02
Walberg, Teresa J.
Electric heating
Metal heating
By arc
21912172, 21912173, 21912175, 21912183, B23K 2604, B23K 2606
Patent
active
056628226
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a dam bar cutting process to cut, by using a pulsed laser beam, dam bars of a lead frame in a semiconductor device wherein a semiconductor chip is mounted on the lead frame and integrally sealed by a resin mold with the lead frame, and more particularly to a dam bar cutting apparatus and method by which dam bars can be cut in the desired cut positions at high speed.
BACKGROUND ART
In a semiconductor device wherein a semiconductor chip is mounted on a lead frame and these lead frame and semiconductor chip are integrally sealed by a resin mold, dam bars connect leads of the lead frame together and serve to dam a mold resin from flowing out between the leads when the lead frame and the semiconductor chip are integrally sealed by the resin mold. The dam bars also serve to reinforce the leads. After the integral sealing by the resin mold, the dam bars are cut and removed so that the leads (outer leads) of the lead frame are separated from one another. While the semiconductor device wherein a semiconductor chip is mounted on a lead frame and these lead frame and semiconductor chip are integrally sealed by a resin mold is sometimes called a resin-molded semiconductor device, it will be referred to simply as a semiconductor device in the description below.
Heretofore, since the lead pitch of lead frames is not so fine and some margin is provided in dimensional accuracy of an outer lead portion, dam bars have been often punched and cut by using a punching press. With a recent increase in degree of integration and performance of semiconductor devices, however, lead frames have had even higher pin count and finer pitch, thus requiring more strict dimensional accuracy. Then, it has become difficult to achieve such strict dimensional accuracy by the conventional press punching method from the technical point of view.
For a high pin count, fine-pitch lead frame with the pitch of 0.3 mm, by way of example, a frame thickness is about 0.1 to 0.2 mm and a width of lead gaps (hereinafter referred to also as slits) is about 0.1 to 0.15 mm. This means that a dam bar portion requires to be cut with the comparable or finer dimensions to or than the frame thickness. Manufacturing a tool capable of punching lead frames with such fine dimensions entails a great difficulty. Even if such a tool is manufactured, it would be broken with much possibility because the tool must have a fine thickness. Further, since the resin (hereinafter referred to as within-dam resin) that has been dammed by the dam bars and the resin (hereinafter referred to as resin burrs) that have flown out over surfaces of the lead frame are deposited near the dam bars, the press punching method cannot positively cut the dam bars into satisfactory configurations.
Meanwhile, the method of cutting dam bars by utilizing a laser beam has been developed in recent years. Because a laser beam can be condensed to a very small spot suitable for fine processing, the dam bars can be cut in a non-contact manner with high dimensional accuracy just by irradiating the laser beam to the dam bars. The prior art in which such a cutting process utilizing a laser beam (hereinafter referred to also as laser cutting) is applied to cutting of dam bars is described in JP, A, 4-322454, for example.
In this prior art, the cross-section of a laser beam output from a laser oscillator is transformed into an oblong shape (elliptical shape) by a cylindrical lens, a focused position of the laser beam is determined by a galvano-mirror, and the laser beam is condensed by a large-aperture condensing lens to a dam bar for melting and cutting it. The dam bar is cut by one shot of the laser beam irradiated corresponding to a narrow slit width. (This prior art will be hereinafter referred to as first prior art).
Also, the prior art disclosed in JP, A, 5-211260 proposes a method comprising the steps of taking in cut positions of individual dam bars by a camera or the like beforehand, detecting actual positions of all the dam bars through image processi
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Minomoto Yasushi
Miyanagi Naoki
Nagano Yoshinari
Ogata Kojiro
Okumura Shinya
Hitachi Construction Machinery Co. Ltd.
Mills Gregory L.
Walberg Teresa J.
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