Plastic package for electronic devices

Abstract: A semiconductor electronic device comprises a chip of a semiconductor material, a set of metal conductors (33,38) adjacent to the plate, a set of wire leads joining selected points on the chip to the metal conductors, and a supporting metal plate (32) formed of three portions having a total surface area which is substantially less than the surface area of the chip, and forming a H-shaped supporting structure. All this, except the ends of the metal conductors, is encapsulated within a plastics material body.

Description: Field of the Invention This invention relates to a semiconductor electronic device, and more particularly, to plastics cases for surface-mount integrated circuits of the thin type. Prior Art As is known, a plastics case, or package, of the surface-mount thin type comprises a supporting plate for a chip of a semiconductor material, most commonly silicon, a plurality of rigid metal conductors connected electrically to selected areas of the chip by small-gauge wire leads, and a body of plastics, usually an epoxy resin, which encapsulates everything but the ends of the metal conductors that form the device rheophores. The rheophores (leads) are then bent with their ends lying in a common plane, which plane is substantially the plane of one of the major surfaces of the plastics body. A recent trend in plastics cases of this type has been toward decreasing thickness, changing from 3-4 mm for the packages of some years ago to 1.4 or 1 mm and even less for current packages. The supporting plate and metal conductors are formed, usually by a blanking process, from a single strip of sheet metal, as can be seen in Figure 1. This figure shows a sheet metal piece 1 which has been processed into the supporting plate 2 and two sets of metal conductors 3 and 4, each adjacent to one side of the plate. The structure denoted by 1 is called leadframe, and in general, includes a number of plate/conductor elements. The metal plate 2 is rectangular in shape and its dimensions are approximately the same as those of the chip received thereon. Also, this plate is slightly sunk with respect to the plane of the metal conductors 3 and 4, such that the chip can be located centrally in the plastics case and the package warping on cooling kept to a minimum. The chip is secured on the plate by means of a special adhesive, and the metal conductors on the leadframe are connected to selected areas on the chip by thin wire leads. Warpage is a frequent occurrence with ultra-thin packages, since the different thermal expansion coefficients of the resin, silicon and metal materials may cause the structure to distort out of plane, unless it is made symmetrical. Figure 2 shows a mold as used for forming the plastics case. This mold is a split type with an upper mold half 10 and a lower mold half 11, each having a corresponding hollow. The two mold halves are oriented with their hollows facing each other, thereby defining a mold cavity into which the resin will be injected. The portion of the leadframe that is to be encapsulated within the resin is clamped between the mold halves. A synthetic resin 20, e.g. an epoxy resin, is injected into the mold 12 at an elevated temperature through a gate 12. This process is best illustrated by Figures 3, 4, 5 and 6, which figures also highlight some more problems besetting this molding process. As shown particularly in Figure 4, the resin flows from the gate 12 parts along two paths, one to the upper part of the mold and the other to the lower part. The cross-section of the upper part is larger than that of the lower part; consequently, the resistance met by the resin flow in this part will be less, and the resin flow will be faster and reach the right end of the mold in less time. After reaching the mold end, the resin begins to flow toward the lower part, as shown in Figure 5, where there still is space unoccupied, and trap air 21 at a location whence it cannot be removed. This occurs especially in the center region of the package, as shown best in Figure 7, because the velocity of the resin flow admitted centrally through the gate 12 is high along the two sides of the mold and low in the center region. Thus, the package is bound to develop a fault that will unavoidably make a reject of it. A third problem with packages of this type is related to assembling the devices to the printed circuit boards. For surface mounting, the devices are welded to the printed circuit tracks using a process at a relatively high temperature, typically in the 200 to 250 degrees centigrade, and of relatively long duration (a few tens seconds). It has been found that this process originates phenomena within the plastics body which affect the physical characteristics and mechanical stability of the package. In particular, with a small ratio of the plastics body thickness to the surface area of the supporting metal plate, tied to the chip dimensions, these effects may result in case rupture. It would appear that this phenomenon, known to the skilled ones in the art as the "pop-corn effect", takes place as follows: the water absorbed by the resin from the environment and that is present in the adhesive used to join the chip to the plate migrates toward the outer and inner surfaces of the body; if the adhesion of the plastics body to the parts encapsulated therein is less than perfect or fails because of the different expansions of the materials during the thermal cycle, a void may be formed between the plastics and the chip and between the plastics and the underside of the supporting plate, wherein water can settle; due to the high temperature, the water is vaporized to exert a pressure against the walls of the void; the vapor pressure within the body distorts the package walls, and may ultimately rupture them. To obviate the destructive effect just described, various measures have been suggested, among which are: removing the absorbed water by baking the parts prior to the welding process, protecting the parts during storage so as to prevent water absorption, improving the adhesion of the resin to the device components within the body by chemical or mechanical treatment of the supporting plate surface. All of the measures listed above show more or less serious drawbacks and involve additional processing of considerable cost and difficulty. In view of the current manufacturing trend toward integrated circuits of growing complexity, and accordingly toward the use of chips of increasing size, albeit without increasing the package size, it will be appreciated that the above-described phenomena represent a serious problem in that they restrict the possibility of using certain standard packages. Japanese Patent JP 01 187841 published on July 27, 1989 describes a thick electronic device comprising a metal plate structure comprising three portions of reduced dimension. This metal plate structure is suitable to reduce cracks between the resin sealing parts and a metal parts glued together thereto. Moreover the Japanese Patent JP 03 163859 published on July 15, 1991 describes an electronic device comprising a metal plate structure comprising two portions connected by an elastic portion. The underlying technical problem of this invention is to provide a supporting plate having such construction and performance features as to enable a semiconductor electronic device to be fabricated within a case of the type outlined above which is particularly resistant to the degenerative effects of the welding process and involves no costly alterations of the fabrication process, thereby overcoming the aforementioned limitations of the prior art. The technical problem is solved by a semiconductor electronic device as indicated above and defined in the characterizing portions of Claims 1 to 4. The features of this invention will be more clearly apparent from the following detailed description of embodiments thereof, shown by way of non-limitative example in the accompanying drawings. Brief Description of the Drawings Figure 1 is an enlarged view of a portion of a sheet metal strip as blanked to a conventional pattern. Figure 2 is a sectional view of a mold for molding a plastics case. Figures 3, 4, 5 and 6 are sectional views illustrating a step in the molding of a conventional plastics case. Figure 7 shows a molding defect in a conventional plastics case. Figure 8 is an enlarged view of a sheet metal strip incorporating a blanked supporting plate. Figure 9 is an enlarged view of an embodiment of a blanked supporting plate according to the invention. Detailed Description With reference to the drawings, the supporting structure 30 in Figure 8 forms the termination end of a leadframe. In particular, the plate 32 comprises three major portions, indicated at 34, 35, 36, which are arranged into a H-shaped structure. This structure supports the semiconductor material chip along two of its sides and at a center portion, and has an overall surface area which amounts to about 50% of the chip surface area. A first portion 35 and a second portion 36 of the plate extend along two opposite sides of the chip, and a third portion 34 joins them at the middle. Conveniently, the first and second portions have on their ends, located at the chip corners, small extensions jutting perpendicularly inwards of the chip, thereby to form a support under each corner of the chip. The chip 32 and metal conductors 33, 38 are blanked off a single piece of sheet metal by conventional techniques. The subsequent fabrication steps of the device are common ones for packages of this type. In particular, they include: welding the chip, as suitably processed to provide an integrated circuit, on the supporting plate 32; welding thin wires to the metal conductors 33 and 34 and to specific metallized areas of the chip which constitute the electric terminals of the integrated circuit; forming an encapsulating body of plastics by injecting a heat-setting epoxy resin under pressure into a suitable mold; and cutting the sheet metal portions that hold together the metal conductors, outside the plastics case. The region below the chip unoccupied by the supporting plate is filled with resin during the molding operation. As a result, the volume of the empty portion in the lower mold will be larger than in the instance of a conventional device by a factor which is proportional to the gauge of the sheet metal used for the leadframe. This causes the resin flow velocity to be nearly the same through the upper and lower molds, so that no air can become trapped within the mold cavity. Furthermore, the amount of adhesive used for securing the chip on the plate is one half that used for a conventional device with a rectangular plate. This effectively lowers the rate of defects from water collected within the adhesive. An embodiment of the inventive device is shown in Figure 9. In particular, a supporting plate 42 is shown which has a modified central portion 44. The central portion of the plate includes an elastic center portion 44 which allows for partial elongation of the structure in the longitudinal direction of the central portion, which direction corresponds to the major axis of the chip. The elastic center portion 44 is formed by a trapezoid-shaped enlargement on whose inside an opening is formed, also trapezoidal in shape. This expedient confers some elasticity on the supporting plate, which affords partial compensation of the internal stresses to the device structure due to the different coefficients of expansion of the materials employed. Advantageously, to retain the structure own elasticity, the elastic portion 44 is adhesively unattached to the chip. It is noteworthy that the device of this invention has a unique advantage in that it requires no modifications to the fabrication process.

Claims: A semiconductor electronic device, comprising: a chip of a semiconductor material having a first surface, a second surface, sides, and corners; a supporting metal plate (32) for the chip, having a major surface in contact with the first surface of the chip; a set of metal conductors (33,38) adjacent to the plate; a set of wire leads joining selected points on the second surface of the chip to the metal conductors; a body made of a plastics material which encapsulates everything but the ends of the metal conductors; characterized in that the metal plate (32) is made up of at least three portions the total metal plate surface area being substantially less extended than the surface area of the first chip surface, a first (35) and a second (36) of said portions extending each along one of opposing sides of the chip, and including on each of their ends, and located at the chip corners, an extension disposed along a perpendicular direction inwards of the chip and the third (34) of said portions extending perpendicularly to both the first and the second, being located centrally of the semiconductor material chip and joining the first and second portions together, and the third portion (34) has an elastic center portion (44) to allow partial elongation of the structure in the longitudinal direction of said third portion (34), said elastic center portion (44) having a trapezoidal-shape and an opening, also trapezoidal in shape. An electronic device according to Claim 1, characterized in that the three portions form a H-shaped structure. An electronic device according to Claim 2, characterized in that the first (35) and second (36) portions have the same or a greater length as/than the length of the chip sides adjacent thereto. An electronic device according to one of the preceding claims, characterized in that the plate (32) and conductors (33,38) are blanked off a single piece of sheet metal.

Inventors: MAGNI PIERANGELO, MAGNI, PIERANGELO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H01L23
IPC codes: H01L21, H01L23