Semiconductor package with high density i/o lead connection

Background

of

the

Invention

0001This invention relates to semiconductor device packages, and in particular to a package requiring a high density of connections from the semiconductor to external leads.

0002In standard semiconductor device packaging, the semiconductor chip is electrically connected to outside circuitry by means of a lead frame. The lead frame may be thought of as a solid metal picture frame with fingers radiating inward from either two opposing sides or from all four sides toward the geometric center of the frame. Also radiating inward from each of the four corners of the frame is a finger which terminates on a square or rectangular piece of metal, typically referred to as a paddle, which occupies a portion of the geometric center. One major surface of the chip is bonded to the paddle, and contact pads on the opposite surface are electrically coupled to the lead frame fingers by bonding conductive wires to the pads and fingers. The chip and a portion of the lead frame fingers are then encapsulated or molded in a material such as an epoxy or plastic molding compound, and the molded package body and lead frame fingers are cut from the frame. The lead frame fingers are then formed to provide a means of electrically connecting the package to the second level interconnection board, which is typically a printed circuit board. The severed fingers therefore constitute the I/O (Input/Output) leads for the semiconductor chip.

0003This standard packaging scheme generally has been satisfactory. However, a problem exists where a high density of connections is required between the chip contact pads and the lead frame fingers due to a great number of such contacts and/or smaller than standard ship sizes. In such cases, it is difficult to fabricate the fingers sufficiently close together to accommodate the high density of interconnections. For example, lead frame fingers cannot usually be made with less than a 16 mil (.4 mm) pitch (the distance between the center of two adjacent fingers). In order to provide connection to a chip with 50 pads on a chip side having standard dimensions (320 mils or 8 mm), the ends of the fingers can only be brought to within approximately 240 mils (6.1 mm) of the chip due to the limit on pitch. This tends to cause sagging and shorting of the wires between the pads and fingers.

0004One solution to the problem of high density I/O connections involves use of multi-level ceramic or glass packages (see, e.g., U.S. Patent 4,498,122). While adequate, such packages are fairly expensive. One possible alternative which has been proposed recently is to provide a multi-level lead frame structure for connecting the chip. It is desirable to provide a further alternative package for high density connections.

0005A further problem which exists in many semiconductor packages is inadequate heat removal from the chip during operation. This is more troublesome as chips become more complex and are required to do more functions. It is, therefore, also desirable to provide a semiconductor package with efficient heat dissipation.

0006EP-A-0 247 644 discloses a lead frame assembly with a semiconductor chip mounted on a central paddle. Conductor pads on the chip are connected to conductors of the lead frame by 3 sets of conductors.

0007In Patent Abstracts of Japan Vol. 7 No.233 (E-204) [1378] 15th October 1983, there is disclosed a form of lead frame with a central insulating member. The latter supports conductors which are connected to the inner ends of lead frame conductors. A chip is mounted on the insulating member and the supported conductors are connected by wires to the chip.

0008According to the present invention there is provided a device package as defined in claim 1.

<b>Brief Description of the Drawing</b>

0009In the drawing: <ul id="ul0001" list-style="none"><li>FIG. 1 is a plan view of a semiconductor device package at one stage of fabrication in accordance with one embodiment of the invention:</li><li>FIGS. 2 and 3 are magnified portions of a package, partially broken away, during further stages of fabrication in accordance with the same embodiment;</li><li>FIG. 4 is a perspective view of a portion of a semiconductor package in accordance with a further embodiment of the invention; and</li><li>FIG. 5 is a perspective view of the semiconductor package in a final stage of manufacture.</li></ul>

0010It will be appreciated that for purposes of illustration, these figures are not necessarily drawn to scale.

<b>Detailed Description</b>

0011Packaging of a semiconductor device starts with a standard type of lead frame such as illustrated in FIG. 1. The lead frame, 10, includes a first plurality of conductive fingers, such as 11, radiating inward from the solid picture-frame-like perimeter, 12. In this example, the fingers are present on all four sides of the perimeter, but may also be present on any lesser number of sides. The lead frame also includes a paddle, 13, located at the center, with a second plurality of conductive fingers, such as 14, coupled to the paddle and extending to the four comers of the perimeter. A gap, such as 15, is formed between the first plurality of conductive fingers (11) and the paddle on all four sides of the paddle. The paddle, fingers and perimeter are typically formed from a sheet of metal such as alloyed copper.

0012For a typical high-density interconnection package, the frame includes a total of 200 fingers, such as 11, extending to each side of the paddle (50 on each side). At their narrowest point, the fingers are approximately 8 mils (.2 mm) wide with a spacing of 8 mils (.2 mm) between each finger on a side ( thus producing a 16 mil (.4 mm) pitch). The paddle is typically approximately 350 mils x 350 mils (8.9 x 8.9 mm). The gap, 15, between the paddle and the fingers is approximately 10 mils (.25 mm). It would be desirable to bring the conductive fingers closer to the semiconductor device to be bonded to the paddle, but it is difficult to do this since the fingers cannot generally be made narrower than 8 mils (.2 mm). This width limitation is due primarily to the fact that a lead frame is etched from a sheet of metal approximately 8 mils (.2 mm) thick and the etching features are limited to the thickness. (The lead frame can also be stamped from sheet metal, but similar dimensional limitations apply).

0013Therefore, in order to provide for the high density interconnection, an additional element is provided to bridge the gap between the lead frame fingers and the paddle. As illustrated in FIG. 2, which is a magnified view of the central portions of the lead frame, this element comprises a third plurality of conductive fingers, such as 16, formed on an insulating layer 17 mounted on the paddle 13. These conductive fingers correspond in number and position to the conductive fingers 11 of the lead frame and have their outer ends 23 extending approximately 10 mils (.25 mm) beyond the insulating layer and bonded to the upper surface of an associated conductive finger, while their inner ends 24 either form a gap, 40, extending approximately 10 mils (.25mm) from the sides of a hole, 18, formed in the insulating layer, or extend right up to the sides of the hole, 18. Also, although the outer dimension of the insulating layer, 17, is shown as slightly smaller than the paddle, it could be equal to or greater than the sin of the paddle. The portion of the paddle, 13, exposed by the hole, 18, forms a mounting pad 41 for a semiconductor chip to be described. The fingers 16 preferably form a gap 40 with the pad which is less than the gap 42 between the lead frame fingers 11 and the pad.

0014In this example, the additional element was formed from a wire-bondable tape which was supplied by 3M Company for Tape Automated Bonding (TAB) packages. The conductive fingers were made of 99.9% copper plated to a thickness of approximately 2 mils (.05 mm) and each had a width of 8 mils (.2 mm) and a separation between fingers of approximately 2 mils (.05 mm) to produce a pitch of 10 mils (.25 mm). The fingers, 16, can be placed closer together than the lead frame fingers 11, since the former are fabricated by plating rather than etching or stamping. Further, even if fingers 16 were etched, they are thinner and so can be placed closer together. The insulating layer, 17, which provided mechanical backing for the fingers, was made of polyimide and was approximately 3 mils (.08 mm) thick. The standard tape was modified by punching the hole, 18, in the insulator 17 at the center. The hole measured approximately 250 mils x 250 mils (6.4 x 6.4 mm).

0015The insulating layer, 17, was bonded to the underlying paddle, 13, and the conductive fingers 16 on the insulator were bonded to their corresponding fingers 11 of the lead frame by thermocompression bonding. This typically involves heating the structure at a temperature of approximately 550 degrees C for .2 seconds while applying pressure of approximately 40 PSI (.28 M Pa).

0016As illustrated in FIG. 3, which is also a magnified view of the central portion of the lead frame, the semiconductor chip, 20, was placed in the hole 18 of the insulating layer 17 so that the back surface of the chip was in mechanical contact with the mounting portion of the underlying paddle, 13. The chip was bonded to the paddle by conductive epoxy. The paddle and connected fingers, 14, therefore provided a ground connection to the back surface of the chip. Further, the paddle provided an excellent heat sink during the operation of the chip. In particular, the calculated heat dissipation for a 148 pin package assuming natural convection cooling was approximately 32 degrees per watt for this embodiment, which was approximately 3 degrees per watt better than when the chip was bonded to a conductive pad on the tape (FIG. 4). The chip measured approximately 190 mils x 200 mils (4.8 x 5.1 mm).

0017As also illustrated in FIG. 3, the front surface of the chip, 20, included a plurality of bonding pads, such as 21, on its periphery. These pads were made of aluminum with a thickness of approximately 1 micron and measured approximately 4 mils x 4 mils (.1 x .1 mm). The pads were also spaced approximately 4 mils (.1 mm) apart. Electrical connection between each of these pads and their corresponding conductive fingers on the insulating layer was provided by wires 22 attached to the pad and finger by standard wire bonding techniques. This involved ball bonding one end of a wire to a pad and then wedge bonding the other end of the wire to the conductive finger while heating to a temperature of 200 degrees C.

0018It will be appreciated, therefore, that the smaller pitch of the conductive fingers 16 formed on the insulating layer 17 permits the fingers to be brought closer to the semiconductor device, 20, than is practical for the lead frame fingers for a high density interconnect package. This allows a relatively short span for the wires 22 to traverse and thereby reduces the possibility of failures in the wire connections. In general, it is desirable to have wires spanning less than 150 mils (3.8 mm) distance. In this example, the wires were approximately 70 mils (1.8 mm) long. Further, since the device is bonded directly to the underlying paddle, 13, heat dissipation is maximized.

0019While the embodiment of FIG. 3 appears preferable in terms of maximum heat dissipation, the embodiment illustrated in FIG. 4, where corresponding elements are similarly numbered, may also be useful. (For the sake of clarity, the lead frame is not shown in FIG. 4.) It will be noted that the major distinction in the embodiment of FIG. 4 is that the device, 20, is bonded to a conductive material pad, 30, which is formed on the insulating layer 17 at the center of the conductive finger pattern. Tabs 32-35 also extend from the corners of the pad 30 out beyond the ends of the insulator 17. The layer 17 is bonded to the paddle (13 of FIG. 2) and the fingers, 16, are bonded to the lead frame fingers (11 of Fig. 2) as before. In addition, the Tabs 32-35 are thermocompression bonded to corresponding fingers (14 of FIG. 2) coupled to the paddle. Ground connection to the device is, therefore, provided through pad 30 and tabs 32-35 rather than through the paddle 13. In fact, in this embodiment the paddle can be dispensed with and ground connections made by wire-bonding or thermocompression bonding the tabs 32-35 to appropriate conductors of the lead frame. In this example, the pad 30 was made of copper, with a thin layer (approximately 30 micro-inches or .75 micros) of gold on the surface. The pad was approximately 220 mils (5.6 mm) long, 220 mils (5.6 mm) wide and 2 mils (.05 mm) thick.

0020In the final fabrication steps, the structure of FIGS. 3 or 4 is encapsulated with a standard material, such as room temperature vulcanizing silicone rubber, which covered the device, conductive fingers on the insulator, the paddle, and at least a portion of the conductive fingers of the lead frame. The fingers of the lead frame are then cut from the perimeter 12 and appropriately formed so that each would comprise an I/O lead for the package suitable for connection for instance to printed circuit boards. A typical final package is shown in a perspective view in FIG. 5 with the encapsulant shown as element 31.

0021The package described herein appears most advantageous for semiconductor devices requiring a high number of I/O leads, i.e., at least 15 on a side. The invention is also advantageous where it is desired to reduce the size of the chip, for example to less than 100 mils (2.5 mm) on a side, so that a higher density of interconnection is required. In general, the invention is most advantageous where a pitch of less than 16 mils (.4 mm) is desirable for the ends of the conductive fingers nearest the chip so that wire spans of less than 150 mils (3.8 mm) can be achieved.