Patent Application
20060145317
This invention relates to leadframe designs used in plastic cavity packages for transistors, integrated circuits (ICs), and related devices.
A common form of packaging for electronic devices such as transistor devices is a plastic housing. In the most typical IC plastic package, electronic components are assembled on a metal leadframe and a polymer is molded over the assembly to encapsulate the device. The leadframe serves not only to support the electronic components, but has metal tabs that extend from the overmolded plastic and provide a means to electrically connect to the encapsulated electronic components.
Recent modifications of the molded plastic IC package include an air cavity design wherein the housing for the package is plastic but is pre-molded over a lead frame before the IC device is assembled into the package. This design offers the advantage that the IC chip may be encapsulated with over mold material with a lower dielectric constant than that of the plastic encapsulating the leadframe. Lower dielectric encapsulant materials offer better RF performance. In this design the IC device environment may be an air cavity (the best dielectric constant—1.0), or the cavity may be filled after the IC chip is die attached and wire bonded. The cavity filling may be any polymer, including polymers that cure at low temperatures. The choice of filling material is wider than the choices available in the case of overmolded plastic packages, since the choice is independent of the material used for the pre-molded plastic housing. Thus in a pre-molded plastic cavity package, the IC chip may still be polymer encapsulated, but the dielectric constant of the material maybe chosen for the best RF circuit performance.
Leadframes of the prior art typically are square or rectangular and have a center paddle to which the semiconductor chip is die bonded. Leadframes are typically stamped from copper or copper alloy sheets. The leads that provide electrical interconnection extend from the sides of the paddle, often along two opposing edges of the leadframe. The number of leads may vary widely. Common RF power devices, for example, RFLDMOS devices, may have only a few leads, one per side for each transistor. A typical RFLDMOS package has from 1-4 transistors.
The conventional method for attaching the IC die to the leadframe employs solder as the bonding medium. The specific bonding operation may take a variety of forms. Solder preforms are commonly used. A conventional method for electrically connecting the transistor die to the leads that extend from the sides of the plastic cavity is to use wire bonds from the transistor die to the top surface of the leads. The wire bonds typically are contained within the cavity, and are encapsulated when the cavity is filled with polymer.
In plastic cavity packages with many leads per side, the plastic body that forms the cavity flows around the leads and forms a very integral structure. A leadframe with 10 leads per side for example provides 9 spaces where the plastic joins. If the same space is occupied by only three leads for example, only two spaces where the plastic joins are provided. Thus as the number of leads are reduced, the mechanical integrity of the package may become an issue.
The mechanical integrity issue is especially relevant to the retention forces provided for the leads. A common failure mechanism for these kinds of packages are where the leads are pulled away from the package. Another lead failure mode occurs when the leads are bent on the outside of the package causing a bending moment on the portion of the lead inside the plastic cavity. Even though the wire bonds are protected with polymer filling material, if the bending of the portion of the leads on the outside of the package is severe, the wire bonds may be damaged or broken, especially if the filling material is soft, e.g. silica gel.
We have developed a leadframe design that provides additional mechanical integrity for the leads in a plastic cavity package. The portions of the leads that are housed either within the walls of the plastic cavity or within the plastic cavity itself are provided with cutouts. When the cavity is molded, or alternatively is filled with polymer, the polymer fills the cutouts, and joins to the mass of plastic fill on either side of the cutouts, thus forming a continuous integral mass of plastic. The end result is that the plastic in the cutouts, coupled to the main plastic mass, and to the rigid package sidewall, forms an effective anchor against pulling and bending forces the leads experience in manufacture or use.
The invention may be better understood when considered in conjunction with the drawing in which:
The invention will be described in more detail using as a prototypical package a plastic cavity RFLDMOS power transistor package. The invention was developed around this type of package and it represents a preferred embodiment. However, it should be understood that other kinds of IC devices may be packaged using the approach described.
Referring to
The plastic housing is typically insert molded to the chip support member 11 and the leads 13 by a conventional molding/extrusion process. Anchoring methods may be used to increase the integrity of the attachment. For example, tabs or holes may be formed in the chip support member 11 through which the molded plastic penetrates during molding. These act as anchors after the mold compound cures.
The plastic used for the insert molding process may be selected from a wide variety of polymers. It is particularly desirable to choose a material that will result in a plastic body capable of withstanding high temperatures, so as to facilitate a high temperature die attach process.
A main feature of the plastic cavity approach to IC device packaging is that the plastic housing for the package is formed prior to assembling the IC component on the metal support. In the most typical prior art plastic package, the plastic overmolded package, the IC die are attached to a metal lead frame prior to molding the plastic encapsulant around the die and leadframe. This versatile approach has been used to manufacture the vast majority of IC device packages. However, recent trends in IC packaging are toward pre-molded plastic housings, where the plastic housing can be shaped with precision, choice of the material of the plastic housing can be made from a wider selection, the plastic for the housing may be different from the polymer used to encapsulate the IC device, and the IC device after die attach and wirebonding may be encapsulated with a encapsulant with a chosen dielectric constant for a required RF performance.
A section view of
The IC chip 21 is bonded to the region 11′ of
After die bonding IC chip 21 to the plastic cavity package, the IC chip is connected to leads 13 by wire bonds 23, as shown in
With assembly of the IC device in the plastic cavity completed, the cavity may be filled with a protective fill 31, as shown in
In the design as shown in
A consequence of this design, where tab-like leads are used, is that lateral forces that tend to pull the tab away from the plastic cavity walls, or bend the tab thus distorting the package, are not uncommon. This is described in more detail in conjunction with
In
To provide anchoring for the leads, in accordance with the invention, cutouts are formed on the inner portions of the leads, i.e. the portions that reside inside the cavity. In principle, the cutouts may have any shape. However, in the preferred case the cutouts are apertures, defined here as openings through the leads, the openings having a surrounding wall.
Four different forms of cutouts are illustrated in
The size of the cutouts in this embodiment is not critical, as long as it is large enough for the polymer fill to flow through from each side and join. The ability of the polymer to flow through the cutout is also a function of the aspect ratio of the cutout. It is recommended that the aspect ratio be at least 1 to ensure flow through. Stated otherwise, the widest dimension of the cutout should be at least equal to the thickness of the lead.
The number of cutouts in each tab lead may vary from one to several or many. An advantage of having two or more cutouts, as compared with a single cutout, is that the anchor is more effective in preventing rotational movement, e.g. movement about the axis of a single cutout.
A preferred embodiment of the invention is shown in
The cutouts will generally be most effective, if contained completely with the sidewalls as shown in these figures. In that case, if the walls have a thickness W, the cutouts will have a maximum width of less than W, and preferably less than 0.9 W. A typical plastic cavity package has a plastic cavity wall thickness of, for example, 0.035″. Thus the widest dimension of the cutouts in this case would be less than 0.035″. The aspect ratio mentioned earlier is easily met since the lead thickness is typically less than 0.01″.
When reference is made herein to the cutout being located within the plastic housing, that reference is to be interpreted as meaning either within the wall of the plastic housing, or inside the cavity. That is, the expression “within the plastic housing” is to be interpreted as generic to the two specific embodiments of the invention that are shown in the figures.
Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.
