Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – With heat sink means
Reissue Patent
1996-07-02
2002-05-21
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
With heat sink means
C257S666000, C257S676000
Reissue Patent
active
RE037707
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved leadframe for packages of integrated power devices. In particular, the present invention is related to medium-power integrated circuits.
As is known, in the manufacture of integrated power devices, the chip of semiconductor material which integrates the electronic components is welded to a dissipator which is connected to a leadframe. Generally, the dissipator, which is thicker than the leadframe and is arranged on a different plane, is manufactured separately and is connected mechanically to the leadframe so that its face which is to be welded to the chip is directed toward the leads. Then, after the provision of the interconnections between the chip and the leads or terminals, a plastic case of epoxy resin, with a generally parallelepipedal shape, is molded on one face of the dissipator and completely embeds the chip. In particular, the plastic case only partially covers the face of the dissipator which is in contact with the chip (the inner face), whereas the opposite face of the dissipator must remain as free as possible from resin traces, in order to ensure the required heat dissipation, particularly when the dissipator is fixed to a further external dissipation structure. This entails on one hand the need to prevent as much as possible the resin of the plastic case from reaching the outer face of the dissipator during molding, and on the other hand the need to clean said face from any residues (deflashing).
In order to reduce the residues on the exposed or outer side of the dissipator, according to the known solutions, the dissipator is pressed against a bottom of the mold by means of pushers provided in the mold of the plastic case, so as to prevent the penetration of the resin, which is injected in the fluid state and under pressure, between the outer face of the dissipator and the bottom of said mold. However, this solution is disadvantageous. In order to press the dissipator flat so that it adheres as much as possible to the bottom of the mold without lifting in any point, three pushers which act on the “inner” face of the dissipator are in fact used: specifically, one of the pushers acts on a portion of the dissipator which is thus external to the perimeter of the plastic case, and the other two pass through the resin, so that the plastic case has notches or recesses which are not filled with resin and reach the inner face of the dissipator, exposing part thereof.
A finished package manufactured in the described manner can be seen for example in
FIG. 1
, wherein the reference numeral
1
indicates the package as a whole,
2
indicates the dissipator,
3
indicates the molded epoxy-resin plastic case, and
4
indicates the leads or terminals, which are already bent. The reference numeral
5
indicates the portion of the dissipator which is not covered by the plastic of the case
3
, since a first pusher acts thereon and laterally delimits the perimeter of the plastic case, whereas the numerals
6
indicate cavities in the resin which are caused by the presence of the pushers in the mold. Said cavities expose further portions of the inner face of the dissipator, only one of which, indicated by
7
, is visible in the figure. The reference numeral
8
indicates the hole for the passage of the screw for connection to an external structure or plate for increasing heat dissipation.
However, the exposure of even only part of the inner face of the dissipator is unwanted, since said face is not protected against external influences during the further manufacturing steps (e.g. nickel-plating for the coating of the leads), and in particular it facilitates the penetration of humidity inside the package, reducing the life and therefore the reliability of the device.
Due to the presence of the pushers, the mold furthermore has a relatively complicated and therefore expensive configuration.
Nonetheless, part of the resin still succeeds to infiltrate below the dissipator, so that it must be removed.
SUMMARY OF THE INVENTION
Given this situation, the aim of the present invention is to provide an improved leadframe for packages of integrated power devices which can solve the disadvantages of the prior art.
Within the scope of this aim, a particular object of the present invention is to provide an improved leadframe which has such a configuration that no pushers which pass through the resin and cause interruptions therein are required during the molding of the plastic case.
In particular, an object of the present invention is to provide a leadframe by virtue of which, after the molding of the plastic case, the inner face of the dissipator which is in contact with the chip is completely covered by the resin, so that it is protected against external contaminating agents, avoiding thus the deposit of resin residues on the outer side of the dissipator.
An important object of the present invention is to provide a leadframe which allows to obtain a significant simplification in the mold for the plastic case, with a reduction in manufacturing costs.
Not least object of the present invention is to provide a leadframe which does not entail modifications in the other steps of the manufacture of the integrated device.
This aim, these objects and others which will become apparent hereinafter are achieved by an improved leadframe for packages of integrated power devices.
The present furthermore relates to an intermediate structure for obtaining a non-planar leadframe, to a power package which comprises the non-planar leadframe according to the invention, and to a process for the manufacture of the non-planar leadframe and the molding of the plastic case.
REFERENCES:
patent: 3423516 (1969-01-01), Segerson
patent: 3629672 (1971-12-01), Van de Water
patent: 3839660 (1974-10-01), Stryker
patent: 4095253 (1978-06-01), Yoshimura et al.
patent: 4807018 (1989-02-01), Cellai
patent: 4809053 (1989-02-01), Kuraishi
patent: 4963975 (1990-10-01), Sawaya
patent: 4965642 (1990-10-01), Titterington
patent: 5073817 (1991-12-01), Ueda
patent: 0124678 (1979-09-01), None
patent: 0053752 (1986-03-01), None
Hitachi Power Mosfet Data Book, Aug., 1995, #D11, p. 201, 2SK511, Silicon N-Channel MOS FET.
International Rectifier, Product Digest, Short Form Catalog 90, HEXFET Power MOSFETs Case Outlines, pp. 1 and 140, 1990.
Bozzini Pieramedeo
Marchisi Giuseppe
Clark Sheila V.
Jorgenson Lisa K.
Morris James H.
STMicroelectronics S.r.l.
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