Active solid-state devices (e.g. – transistors – solid-state diode – Encapsulated
Reexamination Certificate
1995-04-28
2002-05-14
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Encapsulated
C257S664000, C257S730000, C257S773000, C257S788000
Reexamination Certificate
active
06388338
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved method for forming plastic packages for electronic semiconductor devices, which is particularly useful in the instance of thin packages including a supporting and electric connection metal frame and a plastic body. The invention also relates to a package with improved characteristics. The invention is specifically directed to the use of a mold of improved shape for forming the plastic casing.
2. Description of the Prior Art
As is well known, integrated electronic semiconductor devices, comprising an integrated circuit, are formed on go-called “dies” of a semiconductor material which are mounted integrally to electric interconnection structures enclosed in a body of a synthetic plastic material serving a protective function.
In the specific field of this invention, such structures typically comprise a support, usually a thin metal foil (leadframe), the central portion whereof, usually depressed, accommodates the die, and which has terminal leads. The die is, across one of its large surfaces, in contact with the leadframe, the integrated circuit being exposed on the opposite die surface in electric contact with the leads. A plastic body fully encloses the integrated device, and in part the leadframe on which this is mounted. The terminal parts of the leads, or pins, are located on the body exterior and function as electric connections for connecting the device to an external circuit.
Within the scope of this invention, of special concern are reduced-thickness packages, having body length and width which are much greater than its thickness. These are commonly referred to as thin packages, and are supplied to the field in different types and for different applications. Usually, they are employed for devices which require no large power loss, that is, devices which are designed for operating on comparatively small currents. For a better understanding of the aspects of this invention, it pays to briefly review the usual steps of a conventional package forming process.
Typically, the dies are first placed on a certain number of leadframes formed on a single metal strip, and the electric connections to the leads are established using thin metal wires. The strip carrying the dies is then introduced into a mold having cavities corresponding to the individual devices, and a molten, electrically insulating material is injected at a high temperature to form the monolithic plastic body of the package. This material is usually a synthetic resin, such as an epoxy resin. The molding step includes a number of stages at which the temperature is gradually changed to avoid any risks of breaking the semiconductor material or, in any case, reducing the device reliability. On this account, moreover, the leadframe is heated before the molten material is poured in. After injecting the resin, that is after the molding in the proper sense of the word, and following a first cooling step with consequent partial polymerization, the resin is subjected to thermodynamic curing processes. The latter will improve the material properties by promoting thorough polymerization and stretching the long molecules which compose the polymeric material of the resin. The batch of packages thus formed is then taken out of the mold.
A known mold for injecting the resin is depicted in
FIG. 1
, which shows an exploded vertical section view taken through a single cavity, i.e. a single package, of the batch. A leadframe
1
having a semiconductor die mounted thereon is placed into the mold cavity with the ends of the terminal leads protruding outside the cavity. The mold comprises, in the embodiment shown, two parts: a lower half-mold or half-shell die
3
and an upper half-mold or half-shell
4
, each having a corresponding hollow therein. The two half molds are disposed with their hollows facing each other to define a cavity wherein to the resin is then injected through an inlet port
5
formed in the mold itself.
In accordance with the prior art, the large parametric walls delimiting the cavity (bottom wall
6
and top wall
7
) are made flat in order to ideally produce a body having flat parallel top and bottom surfaces. Actually, despite the precautions taken to avoid exposure of the whole device to abrupt temperature changes brought about by the aforementioned thermal treatments, the package leaves the molding and drawing steps with a bending or so-called warpage deformation in it. The large surfaces of the resultant body are not flat, but with the central region of each surface that is sunk below or raised above the plane described by the corresponding edges.
This behavior is due to the fact that the different materials which make up the complete structure of the package, i.e. the metal leadframe, the semiconductor die, and the resin, have different thermal properties (different expansion and shrinkage coefficients). In addition, the asymmetrical distribution about the horizontal axis A—A, as shown in
FIG. 1
, of the materials, i.e. the leadframe
1
and the semiconductor die
2
, inside the package body, generates internal stresses which contribute toward a warped package.
In other words, the ideal plane containing the leadframe—represented in section by the axis A—A—is subjected to compression in one of the directions shown by the arrows C
1
and C
2
in FIG.
1
. This compression causes a deflection to occur in said plane, and hence in the body as a whole. The actual shape of the package at room temperature is that shown in
FIGS. 2 and 3
, which are sectional views taken along either of the large dimensions of the package (vertical sections). The amount of the deformation undergone has been exaggerated in the drawing figures for clarity. Viewed in cross-section, the body outline is basically that of a parallelogram with two long sides showing a concavity in the same direction.
Shown in
FIG. 2
is a package with a concavity upwards, whereas in
FIG. 3
the package has a concavity downwards. The direction of the bending is individual to each package and not fully predictable, although a deformation of the kind shown in
FIG. 2
is more likely to occur with an internal structure of the package having the distribution of the materials shown in the figures.
As shown in both
FIGS. 2 and 3
, the device as a whole, i.e. both the leadframe
1
(together with the semiconductor die
2
mounted on it) and the plastic body, generally denoted by
8
, is bent at this intermediate stage of its forming process. In particular, the body departs from the ideal shape, with flat top and bottom surfaces, by a degree of depression or elevation, with respect to a plane described by the edges of each of these surfaces, of a plane tangent the corresponding surface in its central region.
This depression (or elevation) is known as warpage, designated W in
FIGS. 2 and 3
. With a body illustratively measuring about 18-20 mm in width and length and about 1 mm in thickness, as would be typical of current thin packages, warpage may amount to a few tens of microns. Although the amount of warpage deformation is proportionally small compared to the size of the body, still it can grow into a serious problem during the following steps for completing the forming of the device, and does lower the reliability of the finished device.
After removing the packages from the mold, according to the prior art, the individual packages are separated from one another (the so-called singulation step) by cutting up the metal strip on which the batch of leadframes has been formed. Thereafter, for each package, that portion of the leads which has been allowed to protrude out of the package body is bent to create pins (the so-called lead forming step) that will enable the electric connection of the device for a particular application, usually by welding the pins to a printed circuit.
During both of these operations, and as shown schematically in
FIG. 4
, the plastic body
8
of the package is held between a bottom clamping surface
9
and a top clamping surface
10
. Also, its
Romano′ Luigi
Tondelli Fulvio
STMicroelectronics
Williams Alexander O.
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