TECHNICAL FIELD

This invention relates to packaging techniques for integrated circuits and, more particularly, to techniques for eliminating voids in a thermally enhanced plastic package.

BACKGROUND ART

FIGURE 1A shows a conventional quad flat package (QFP) assembly 10 for an integrated-circuit die 12. The integrated-circuit die 12 is attached to an upset die- attach paddle portion 14 which is at the center region of a conventional lead frame 16. Various inwardly-extending bonding fingers (typically shown as 18, 20) are connected to respective bonding pads on the integrated-circuit die 12 using respective bonding

wires 22, 24, as indicated in the Figure. The entire assembly described above is encapsulated in a molded plastic material, which forms a molded-plastic body 26 for the package.

FIGURE IB is a plan view of the lead frame 16 for the conventional quad flat package (QFP) assembly 10. Note that the ends of the bonding fingers are not connected to the die-attach paddle portion 14. This provides substantial spaces 30, 32, 34 between the ends of the bonding fingers (typically shown as 18, 20) and the die-attach paddle 14. The assembled die and lead frame combination is encapsulated in the plastic material by being placed in a cavity formed between two halves of a mold and by injecting plastic material into the top half of the mold at one corner of the lead frame. Air vents are provided in the mold at the other three corners of the package. Paths for the plastic material to flow into the bottom half of the mold from the top half of the mold are provided by the spaces 30, 32, 34 between the ends of the bonding fingers and the die- attach paddle as well as by the spaces between the leads of the lead frame. These flow paths permit the flow volume of the plastic molding material to be substantially balanced between the top half and the bottom half of the mold as the plastic material flows through the mold. Air is expelled out of the air vents at the three corners of the mold by the flowing plastic material so that no air remains trapped within the molded-plastic body. If air were to be trapped, it would cause the create voids, blow holes, or pin holes, in the molded-plastic body 26.

FIGURE 2A shows a package mold 30, which has a top half 32 and a bottom half 34, for molding a thermally-enhanced, quad flat package

(TE-QFP). A thermally-conductive, electrically-insulated substrate 36, which is formed of a material such as, for example, alumina nitride, has an integrated-circuit die 38 mounted thereto. The thermally conductive, electrically-insulated substrate 36 replaces a conventional die attach paddle and improves the thermal performance of a molded- plastic package. Bonding fingers (typically shown as 39, 40) of a lead frame 42 are attached to the margins of the thermally conductive, electrically-insulated substrate 36 with an adhesive film 44 formed of an material such as R-flex 1000. As in the case of a conventional quad flat package (QFP) assembly, the thermally conductive, electrically-insulated substrate 36 with its attached integrated-circuit die 38 are placed in the cavity formed between the two halves 32, 34 of the mold 30. Plastic material is injected into the top half the mold at the inlet gate 46. The plastic material enters the top half 32 of the mold and flows through the spaces between the bonding fingers of the lead frame into the bottom half 34 of the mold.

FIGURE 2B shows a plan view of the lead frame 42 and the thermally conductive, electrically-insulated substrate 36. Note that the ends of the bonding fingers are fixed to the substrate 36. There are no spaces between the ends of the bonding fingers and the substrate 36 to provide paths for the plastic material to flow into the bottom half of the mold from the top half of the mold. The only paths available are between the bonding fingers.

With reference to Figure 2A, the arrows indicate the flow of plastic molding material through the top half of the mold and the bottom half of

the mold. Note that the bulk of the thermally conductive, electrically- insulated substrate 36 is larger than that of a conventional die-attach paddle, such as shown in Figure 1. The use of the substrate 32 unbalances the flow of plastic material in several ways. One way is that the flow cross-sectional area in the lower space of the cavity is smaller. Another way is that no space is provided between the ends of the bonding fingers and the substrate. This results in the flow of molding material in the upper half of the mold being greater than the flow of molding material in the lower half of the mold. The differences in flows causes voids, blow holes, or pinholes in that portion of the body of the package which is formed in the lower half of the mold. The larger numbers of bonding wires for newer package designs with larger pin counts further restrict the flow of molding material through the bottom half of the mold. As a result of the differences in flow, air within the mold halves is expelled by the flowing plastic material at different rates so that some air remains trapped within the bottom mold, as indicated in the Figure. The trapped air creates voids, also called blow holes or pinholes, in the body of the package. A typical void 50 is created on the side of the package which is opposite the inlet gate 46.

Voids on the surface of a plastic package results in that packaging being rejected for visual defects. Internal voids in a package can produce internal cracks in the body of a package. Voids and cracks near leads can provide a path for moisture to penetrate into the package and result in failure of the integrated circuit. Temperature cycling and high pressure steam tests can be used to identify the presence of voids in a package. For temperature cycling tests according to Mil. Std. 883C , a

device is cycled between a temperature of 150 degrees Centigrade for ten minutes and a temperature of minus 65 degrees for ten minutes. Under these conditions, device failure caused by voids is accelerated. For a high-performance package design, which has good thermal and electrical performance characteristics, a major production concern is the moldability of the package design, that is, whether packages can be molded in a production environment to be free of voids, pin holes, of blow holes. As described herein above, thermally-enhanced molded plastic package have moldability problems.

Consequently, a need exists for a technique to prevent voids in the body of a thermally-enhanced molded plastic package, which uses a thermally conductive, electrically-insulated substrate.

DISCLOSURE OF INVENTION

It is therefore an object of the invention to provide a void-free package for a thermally-enhanced, plastic quad flat package (TE-QFP).

In accordance with this and other objects of the invention, a package for a thermally-enhanced, molded-plastic quad flat package (TE-MQFP) is provided. An integrated-circuit die is attached to a thermally conductive, electrically-insulated substrate, which has a central region to which the integrated-circuit die is attached. A lead frame with inwardly extending bonding fingers attached to the outer margins of the thermally conductive, electrically-insulated substrate.

Bonding wires are connected between respective bonding pads formed on the integrated-circuit die and the inwardly extending bonding fingers of the lead frame. A molded package body is formed of a plastic material around the integrated-circuit die, the thermally conductive, electrically- insulated substrate, and the bonding fingers. The thermally conductive, electrically-insulated substrate has apertures formed through it with the apertures being located adjacent to the outer margins of the thermally conductive, electrically-insulated substrate. The apertures are formed in the thermally conductive, electrically-insulated substrate at locations which are between the integrated-circuit die and the ends of the inwardly extending bonding fingers to balance the flow of plastic material over the top and bottom of the substrate and to provide the molded package body substantially free of voids.

These apertures provide paths for the plastic material to evenly flow over the top and bottom sides of the substrate so that the flow of plastic material is balanced to provide the molded package body substantially free of voids. In one embodiment of the invention, the apertures formed in the thermally conductive, electrically-insulated substrate are formed as elongated slots in the thermally conductive, electrically-insulated substrate at locations between the integrated- circuit die and the ends of the inwardly extending bonding fingers.

A method is provided for forming a molded plastic package for an integrated-circuit die. The method includes the steps of attaching the integrated-circuit die to a central region of a thermally conductive, electrically-insulated substrate; attaching the inwardly extending

bonding fingers of a lead frame to the outer margins of the thermally conductive, electrically-insulated substrate; and molding a molded package body by flowing a plastic material into a mold placed around the integrated-circuit die, the thermally conductive, electrically-insulated substrate, and the bonding fingers of the leads frame. The step of molding the molded package body includes flowing plastic material through apertures formed in the thermally conductive, electrically- insulated substrate to balance the flow of plastic material over the top and bottom of the substrate to provide a molded package body substantially free of voids. According to one embodiment of the invention, the step of flowing plastic material through apertures formed in the thermally conductive, electrically-insulated substrate includes flowing plastic material through elongated slots which are formed in the thermally conductive, electrically-insulated substrate at locations between the integrated-circuit die and the ends of the inwardly extending bonding finger to balance the flow of plastic material over the top and bottom of the substrate to provide the molded package body substantially free of voids.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention:

FIGURE 1A is a sectional, elevation view of a conventional quad flat package (QFP) for an integrated circuit.

FIGURE IB is a plan view of a lead frame for a conventional quad flat package (QFP).

FIGURE 2A is a sectional, elevation view of a conventional thermally-enhanced, quad flat package (TE-QFP), which uses a thermally conductive, electrically-insulated substrate for mounting an integrated-circuit die.

FIGURE 2B is a plan view of the conventional thermally- enhanced, quad flat package (TE-QFP) of Figure 2A.

FIGURE 3 is a plan view of a lead frame connected to a modified thermally conductive, electrically-insulated substrate, where the substrate includes slots formed therein, according to the invention, for balancing the flow of plastic molding material in the upper and lower halves of the mold to eliminate formation of voids in the body of the package.

FIGURE 4 is a sectional view of an upper mold half and a lower mold half containing a thermally conductive, electrically-insulated substrate having slots formed therein for balancing the flow of plastic molding material in the upper and lower halves of the mold, where the arrows indicate that the same amount of flow of the plastic molding material through the top half of the mold and the bottom half of the mold, eliminating formation of voids in the body of the package.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims.

FIGURE 3 shows a thermally conductive, electrically-insulated substrate 102, which has apertures, or slots, 104, 106, 108, 110 formed therein, according to the invention. These apertures balance the flow of plastic molding material in the upper and lower halves of a mold to eliminate voids in the body of the package. The apertures formed in the thermally conductive, electrically-insulated substrate are formed as elongated slots through the thermally conductive, electrically-insulated substrate at locations inside the ends of the bonding fingers, that is, between the sides of the integrated-circuit die and the ends of the bonding fingers. Forming the slots 104, 106, 108, 110 at these locations avoids the situation where the ends of the bonding fingers might interfere with the flow of plastic molding material.

An integrated-circuit die 112 is fixed to a central region 114 of the thermally conductive, electrically-insulated substrate 102, which is formed of a material such as, for example, alumina nitride. A lead

frame has a number of inwardly extending bonding fingers 116 extending inwardly to overlap the outer margins of the thermally conductive, electrically-insulated substrate. The lead frame is formed of copper or alloy 42 material. The inwardly extending bonding fingers of the lead frame are attached to the outer margins of the thermally conductive, electrically-insulated substrate 102 using an adhesive film such as R-flex 1000. Respective bonding wires 118 are connected between respective bonding pads formed on the integrated-circuit die 112 and the inwardly extending bonding fingers of the lead frame.

FIGURE 4 shows the bonding fingers 116 of the lead frame 102, the integrated-circuit die 104, and the thermally-conductive, electrically- insulated substrate 102 positioned in a molding cavity formed between an upper mold half 120 and a lower mold half 122. The arrows show the flow of plastic molding material through the upper mold half 120 and through the lower mold half 122. The molding material is, for example, a standard molding compound such as provided by the Sumitomo Company as 6300 HS or HG molding compound or as 7320C low-viscosity molding compound. The apertures 104, 106, 108, 110 formed therein balance the flow of plastic molding material in the upper and lower halves of the mold to eliminate formation of voids in the molded body of the package. The arrows indicate the same amount of flow of the plastic molding material through the top half of the mold and the bottom half of the mold. Note that, since the flows of molding material through the top half of the mold is balanced by the flow of molding material through the bottom half of the mold, the top and bottom flows meet near the air vent

130. This eliminates voids, or blowholes, caused by air trapped in the mold.

The invention provides an improved method for forming a molded plastic package for an integrated-circuit die. The method includes attaching the integrated-circuit die to a central region of a thermally conductive, electrically-insulated substrate. The inwardly extending bonding fingers of the lead frame are attached to the outer margins of the thermally conductive, electrically-insulated substrate. A molded package body is formed by flowing a plastic material into a mold placed around the integrated-circuit die, the thermally conductive, electrically- insulated substrate, and the bonding fingers of the leads frame. The step of molding the molded package body includes flowing plastic material through apertures formed in the thermally conductive, electrically- insulated substrate to balance the flow of plastic material over the top and bottom of the substrate in order to provide the molded package body substantially free of voids.

In a preferred embodiment of the invention, the step of flowing plastic material through apertures formed in the thermally conductive, electrically-insulated substrate includes flowing plastic material through elongated slots which are formed in the thermally conductive, electrically-insulated substrate to balance the flow of plastic material over the top and bottom of the substrate to provide the molded package body substantially free of voids.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.