Plastic body surface-mounting semiconductor power device having dimensional characteristics optimized for use of standard shipping and testing modes

Abstract: A QFP plastic surface-mounting semiconductor power device, comprising a plastic package inside which there is provided a chip which is connected by means of conductors to terminals that protrude from the plastic package, and a heat sink plate (2) that is arranged on the bottom of the plastic package and has thinner ends (2'); its particularity is that the length (LH) of the heat sink plate is at least equal to the minimum length (Lp) of the plastic package, channels being formed in the plastic package in a position that is adjacent to the thinner ends of the heat sink plate.

Description: The present invention relates to a plastic body surface-mounting semiconductor power device having dimensional characteristics optimized for the use of standard shipping and testing modes. More particularly, the present invention relates to a surface-mounting QFP (Quad Flat Package) semiconductor power device having optimized dimensions and to a refinement by means of which it is possible to ship said QFPs in containers normally used to transport standard QFP devices. Integrated circuits and other active electronic devices or components are generally conventionally constituted by a chip made of a semiconductor material and require a special configuration for their mounting on an external electric circuit. A typical configuration of this kind is the one provided by a plastic package inside which a chip is arranged; the chip is connected, by thin metal conductors, to terminals which protrude from the package and are commonly termed leads. In integrated power circuits, which are designed to operate with high currents and are thus subject to intense heating, there is provided a small metal plate whereon the chip is mounted. This metal plate acts as a heat sink for the chip, since it transfers outside the plastic package the heat generated during the operation of the device. Figures 1 to 4 illustrate a conventional semiconductor power device known to the applicant. The configuration of the semiconductor power device is shown in particular in figures 1 and 2, where the reference numeral 1 designates the external plastic package, the reference numeral 2 designates the heat sink provided with ends 2', and the reference numeral 3 designates the leads. The length of the heat sink is designated by L, whereas the minimum length of the plastic package 1, i.e., of its bottom, is designated by L1. The figure shows that L is greater than L1, i.e., the heat sink protrudes lengthwise from the plastic package 1, whereas as shown in figure 3 it is not as wide as said package 1. In practice, the ends 2' of the heat sink 2 are exposed. It should be noted that since the device is preset for surface-mounting, as shown in figure 2, the leads 3 are bent so that their free ends reach the level of the plane that contains the heat sink 2. The power device is soldered to a printed circuit 4 by soldering the plate that constitutes the heat sink 2 and the leads 3 to preset metallic regions of said printed circuit 4. The exposed ends 2' of the heat sink 2 allow to inspect the chamfered edge 5 of the soldering, known as meniscus, after the power device has been soldered to the printed circuit 4, to check that the soldering is correct. Furthermore, the exposed ends 2' of the heat sink 2 allow to check its perfect alignment with the corresponding area of the printed circuit 4. In this regard, a process for producing the exposed ends 2' of the heat sink 2 is known which allows to obtain said ends 2' free of any plastic burr that would form both around the edges of the ends 2' and above them as a consequence of the molding of the plastic package 1 around the chip arranged on the heat sink 2. The need to have the ends 2' of the heat sink 2 exposed and most of all free from any residual plastic burr entails the production of protruding ends 6 of the plastic package 1. These protruding ends 6 are adjacent to the lateral surface of the exposed ends 2' of the heat sink 2, as shown in figures 3 and 4. Since it is indispensable to have burr-free lateral surfaces of the exposed ends 2', and since this burr removing operation entails using a tool, the need to make the ends 2' protrude from the plastic package 1 is evident. If the ends 2' did not protrude from said package 1, or if they were even recessed with respect thereto, the operation for eliminating the plastic burr would certainly damage the plastic package 1. In the case being considered, with the above mentioned conventional molding process no problems arise, since the elimination of the plastic burr from the lateral surface of the exposed ends 2' consists merely in eliminating the protruding ends 6 of the plastic package 1, which are located far enough from the edges of the plastic package 1. From the above description it is therefore evident that the exposed ends 2' of the heat sink 2 have an important functional purpose to which, however, drawbacks are linked. A first drawback is due to the fact that power devices having such a heat sink 2 cannot be placed in standard containers for shipping and cannot even be tested by means of electric contact pads and standard equipment. Semiconductor devices are in fact handled and shipped in standard plastic containers, wherein however it is not possible to place semiconductor power devices provided with a heat sink, due to the exposed ends 2'. Figure 5 shows the insertion of a surface-mounting semiconductor power device in a container 7 for shipping. The figure shows that the exposed ends 2' of the heat sink 2 interfere with the edges of the square hollow 8 provided for accommodating the device inside the standard plastic container 7. A second drawback shown in figures 6, 7a, and 7b arises from the observation that since the heat sink 2 is narrower than the plastic package 1 along its width and has exposed ends 2', the only point of contact with the square hollow 8 of the plastic container 7 is indeed at said exposed ends 2'. In this manner, as shown in figure 7b, the positioning of the power device inside the square hollow 8 is not stable enough, making its shipping unreliable. EP-0 548 496 discloses a mold and a method for manufacturing semiconductor devices of plastics incorporating an exposed heat sink of metal for inspecting the soldered joint. A principal aim of the present invention is therefore to provide a surface-mounting semiconductor power device having dimensional characteristics that are optimized for the use of standard shipping and testing modes. Within the scope of this aim, an object of the present invention is to provide a surface-mounting semiconductor power device the heat sink whereof is shorter than conventional embodiments while preserving their advantages. Another object of the present invention is to provide a surface-mounting semiconductor power device that can be stably placed on the container for shipping. Another object of the present invention is to provide a surface-mounting semiconductor power device that is highly reliable and relatively easy to manufacture at competitive costs. This aim, these objects, and others which will become apparent hereinafter are achieved by a QFP plastic surface-mounting semiconductor power device, as defined in claim 1. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of the device according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a side view of a conventional plastic body surface-mounting semiconductor power device; figure 2 is a side view of the semiconductor device of figure 1, soldered to a printed circuit; figure 3 is a plan view of the device of figure 1, with the corresponding supporting frame; figure 4 is a sectional view, taken along the plane IV-IV, of the device shown in figure 3; figure 5 is a sectional side view, illustrating the conventional device shown in figure 1 inserted in a container for shipping; figure 6 is a plan view of the conventional device inserted in the container for shipping; figure 7a is a partially sectional view of the conventional device, taken along the plane VII-VII of figure 6; figure 7b is a partially sectional view of the conventional device, taken along the plane VII-VII of figure 6; figure 8 is a bottom view of the device according to the invention, arranged on the corresponding supporting frame; figure 9 is a sectional view of the device according to the invention, taken along the plane IX-IX of figure 8; figure 10 is a plan view of the device according to the invention; figure 11a is a partially sectional view, taken along the plane XI-XI of figure 10; and figure 11b is a partially sectional view, taken along the plane XI-XI of figure 10; and figure 12 is a detail view of a portion of figure 11a. In all the different figures, identical reference numerals designate identical components. With reference to figures 8 and 9, the surface-mounting QFP semiconductor power device according to the invention comprises a plastic package 1, inside which the chip (not shown) is placed, and a heat sink that is formed by a metal plate 2 whereon the chip is placed. The overall length L H of the heat sink 2 is less than, or at the most equal to, the minimum dimension L P of the plastic package 1. At the regions of the plastic package 1, directly adjacent to each end 2' of the heat sink 2, two channels 10 are provided which are shaped as shown in figure 9. Four channels 10 are thus provided, two for each side of the plastic package, which are arranged at the ends 2' of the heat sink 2 and are closely adjacent to said ends. As shown in figure 9, the plastic package 1 protrudes with respect to the ends 2' of the heat sink 2 except at the regions adjacent to said ends 2', where the channels 10 are formed. Each channel 10 is formed by producing a hollow in the plastic package 1 that has a profile similar to the profile normally assumed by said plastic package 1, with the particularity that it has an inclined surface 11 that blends, in an upward region, with the normal profile of the package 1. In this manner it is possible to reduce the length of the heat sink 2 to the length of the plastic package 1, indeed by virtue of the presence of the four channels 10. At the four channels 10, the ends 2' of the heat sink 2 are in fact again exposed, as explained above, with the consequent advantage that it is possible to remove the protruding ends 6 of the plastic package 1 without damaging said package and to inspect the correctness of the soldering performed between the heat sink and the printed circuit whereon the QFP power device is surface-mounted. Outside the four channels 10, instead, the length L H of the heat sink is at least equal to, or smaller than (depending on the manufacturing choice that is made) the minimum length L P of the plastic package 1. Therefore, the reduction in the length of the heat sink 2 with respect to conventional solutions is combined with a local reduction (i.e., at the four channels 10) of the minimum length of the plastic package 1. An effect thereof is that it is possible to use, for the shipping of QFP power devices, standard containers 7 that are normally used to ship standard QFP devices, i.e., devices that do not have the heat sink 2. The reduction in the length of the heat sink 2 to the length of the plastic package 1 eliminates the interference between the ends 2' of the heat sink and the walls of the hollow 8 of the container 7 preset to accommodate said QFP power device. Furthermore, with reference to figures 10, 11a, 11b, and 12, the bottom surface of the plastic package 1 is provided, at its four corners, with four resting elements advantageously having a circular shape. These resting elements 11 allow the QFP power device to rest more stably inside the hollow 8 of the container 7 than it would occur if the device rested exclusively on the ends 2' of the heat sink. The thickness of the resting elements 11 is in any case smaller than the thickness of the heat sink 2 which protrudes from the bottom of the plastic package 1 (see the detail shown in figure 12). In this manner, the power device always rests mainly on the ends 2' of the heat sink 2, with the aid of additional resting points at the four corners of the plastic package 1 that considerably improve the stability of the behavior of the device accommodated in the hollow 8 with respect to what normally occurs with devices having a conventional configuration. Figure 11b shows the stable, tilt-free behavior of the QFP power device accommodated in the hollow 8 in comparison with the behavior shown in figure 7b. In practice it has been observed that the device according to the invention fully achieves the intended aim, since it allows to reduce the length of the heat sink, eliminating the above mentioned shipping drawbacks, without at the same time losing the advantages arising from the fact that the ends 2' of the heat sink 2 are exposed. Furthermore, the four resting elements 11 placed at the four corners of the plastic package 1 help to improve the shipping stability of the device. The device thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may furthermore be replaced with other technically equivalent elements. Thus, for example, the channels 10 and the resting elements 11 can of course also be provided on standard QFP devices if particular shipping and testing requirements suggest their use. Finally, it is also evident that the shape of the resting elements 11 can be altered at will without affecting their functionality in any way. In practice, the materials employed, so long as they are compatible with the specific use, as well as the dimensions, may be any according to the requirements and the state of the art. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A QFP plastic surface-mounting semiconductor power device, comprising a plastic package inside which there is provided a chip which is connected by means of conductors (9) to terminals that protrude from said plastic package (1), and a heat sink plate (2,2') at least partially embedded in the bottom of said plastic package and, in plan view, having two thinner ends (2') with respect to a central part thereof, said two thinner ends being exposed on respective opposing lateral faces of the plastic package, said ends (2') being narrower than the width of the respective face on which they are exposed, wherein the length (L H ) of said heat sink plate is less than or equal to the length (L P ) of said plastic package, said latter length being the overall length between said lateral faces of the plastic package, two vertical channels (10) being formed in said plastic package adjacent to each side of each of said two thinner ends of said heat sink plate, said channels causing a locally reduced length of said plastic package which is less than the length (L H ) of said heat sink plate. The device according to claim 1, characterized in that said heat sink plate is shorter than said length (L P ) of said plastic package. The device according to claim 1, characterized in that each one of said channels formed inside said plastic package has a profile that duplicates the profile of the region of said plastic package outside said channels, the upper region of each one of said channels being blended, by an inclined flat surface, with the profile of the region of said plastic package that lies outside said channels. The device according to claim 1, characterized in that it comprises a plurality of resting elements that are arranged on the bottom region of said plastic package and are adapted to allow stable resting inside the containers provided for the shipping of said power device. The device according to claim 4, characterized in that it comprises four resting elements arranged at the four corners of said plastic package. The device according to claim 5, characterized in that each one of said resting elements is thinner than said heat sink plate placed on the bottom of said plastic package. The device according to claim 6, characterized in that each one of said resting elements has a circular shape.

Inventors: MAZZOLA MAURO, POINELLI RENATO, MAZZOLA, MAURO, POINELLI, RENATO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H01L23
IPC codes: H01L23, H05K3, H05K1, H01L21