Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – Insulating material
Reexamination Certificate
1999-07-20
2001-07-17
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
Insulating material
C257S684000, C257S796000, C257S707000, C257S712000, C257S787000, C257S675000, C257S717000, C264S272150, C264S272170, C438S121000, C438S122000, C438S123000
Reexamination Certificate
active
06262480
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for forming a plastic package for an electronic device having a fully insulated heat sink. The method is of the type which provides for forming the plastic case within a mold on whose interior a heat sink has been placed which has a first major surface to be insulated by means of a plastic material layer with a first thickness, whereon a metal leadframe and at least one semiconductor material die having an electronic circuit formed thereon have been fixed, and a second major surface opposite from the first and to be insulated by means of a plastic material layer with a second thickness, thinner than said first thickness; and at least one supporting element adapted to be positioned inside the mold cavity facilitating properly spacing the second surface of the heat sink out from a facing wall of the mold cavity during the process of introducing the plastic material for molding.
BACKGROUND ART
As is well known, electronic semiconductor devices are encapsulated within a package serving as a protection therefor. In particular, reference will be made herein to packages which comprise a plastic material case.
The active portion of such devices is a plate or “die” of a semiconductor material which measures a few square millimeters in surface area and has an electronic circuit, usually an integrated circuit, formed thereon.
So-called power packages are used for those devices which are liable to develop heat in relatively large amounts, as may be due either to high density or large numbers of power components therein, or to operation with large currents, that is for electronic power devices. For this reason, a heat sink is included in the package to dissipate heat generated within the package. The heat sink should have special constructional, e.g. a large heat dissipation area, and thermal features, e.g. high thermal conductivity. In practice, it is an element made of a metal, or good heat conductor, whose mass is definitely larger than that of the die to which it is coupled thermally.
To convey a heat flow generated during the device operation toward the ambient air, the heat sink is only partly embedded within the plastic case, with a major surface of the heat sink being left exposed, i.e. uncovered by the plastic material. An external heat sink may optionally be arranged to contact this exposed surface in order to further enhance the transfer of heat to the package outside.
In certain applications where the device is to operate at a high voltage, e.g. audio apparatus such as car radio sets, high-fidelity devices, and stereo systems for household use, the heat sink is sometimes further connected to the metal framework of the apparatus as well, such as a radio own chassis, to produce a conveniently expanded dissipating surface. However, this arrangement may affect the operation of the integrated circuit in an adverse manner. In fact, the chassis of the apparatus would normally be grounded or at a predetermined electric potential.
Alternatively, the heat sink may be connected to a printed circuit board, or to an external heat sink of larger size, again for the purpose of enhancing the transfer of heat.
In all of the above cases, the voltage between the dissipating outer metal structure, the heat sink proper and the semiconductor material die may damage the power device irreparably.
In general, if discharge between the heat sink and the printed circuit board or the metal pins of the package is to be prevented, the heat sink must be insulated electrically from the metal structures in contact therewith, or from the external heat sink.
Certain prior arrangements for insulating the device electrically, such as the interposition of insulating materials (e.g. mica) between the exposed surface of the internal heat sink and the outside structure providing additional dissipation, result in added complication and cost for their assembly and testing.
Other solutions consist of forming an insulating layer over the exposed surface of the heat sink. Such processes provide, for example, for the deposition of an oxidizing metal layer, followed by an oxidation or by the deposition of an insulating layer by a chemical method. Drawbacks are encountered in carrying out such processes, especially when chemical treatments are used.
The present invention is related in particular to the methods most commonly used for insulation wherein the exposed surface of the heat sink is fully covered with the same plastic material as the package case. The heat sink obtained with these methods is fully embedded within the plastic case during the case molding step. The resultant package is referred to as having an insulated heat sink, it being understood that the insulation provided is electrical in nature.
To best compromise, satisfactorily and functionally, between a good electric insulation and optimum dissipation of the device heat, it is necessary for the layer of plastic material covering said surface of the heat sink to be quite thin.
Referring to
FIG. 1
, it is shown diagramatically a side view of a typical insulated heat sink power package, with its left-hand portion in section, such as it appears after the molding step. The package is generally and schematically shown at
1
.
In this and the following figures, has been considered illustratively the instance of a so-called “single-in-line” package wherein the pins protrude out of only one long side of the package body. This configuration is a frequently adopted one for power packages.
The package
1
for the electronic device comprises suitable supporting and electric interconnection means within a case
2
made of a plastic material, typically a resin. A die of a semiconductor material, on which the device has been formed, is mounted rigidly with such means so as to leave the surface where the circuit is formed unencumbered.
Specifically, for mounting the package to a printed circuit board, a lead leadframe
3
typically comprises, as shown by the cross-sectional portion, a plurality of electric connectors or leads, denoted by
4
, which have one end outside the case
2
. The leads
4
, once bent, will form the package pins.
The structure for supporting the die also functions to dissipate heat generated within the package and includes, for this purpose, a heat sink
5
.
The leadframe
3
is connected to the underlying heat sink
5
, usually by means of rivets, not shown in the figure. The leadframe
3
is held by the rivets slightly raised above the heat sink
5
, and their electric connection is only effected through suitable sunk areas of the leadframe, in this example.
The semiconductor material die, not shown in the figure, is fixed on the top surface of the heat sink
5
. For connecting the leadframe
3
electrically to the circuit, the inner ends of the leads are connected by thin metal wires to a corresponding metallized area provided on the exposed surface of the die.
The assembly formed of the leadframe
3
and the heat sink
5
is duplicated for a number of times forming a continuous strip. A peripheral structure of a single leadframe is part of a common holder structure of the whole strip. As shown in the Figure, directly after molding, the peripheral structure, denoted by
6
, will remain outside the case
6
while still joining the packages together in one strip.
As can be seen, the plastic case
2
encapsulates the leadframe
3
partially to leave only the ends of the leads
4
on the package outside, and fully encapsulates the heat sink
5
.
The bottom surface of the heat sink
5
is covered, in fact, by a resin layer
7
. This layer is provided quite thin, so that its thermal resistance can be kept low. In this situation, the transfer of heat from the heat sink to the ambient occurs unhindered. The term “thin” used herein in connection with the thickness of the layer
7
will indicate a negligible thickness compared to that of the plastic material covering the heat sink surface on the side of the die, i.e. at the top in the figure.
Shown in
FIG. 2
for the sake of clarity is a top p
Ferri Stefano
Rossi Roberto
Galanthay Theodore E.
Jenkens & Gilchrist P.C.
SGS--Thomson Microelectronics S.r.l.
Williams Alexander O.
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