Metal heat sink baseplate for a resin-encapsulated semiconductor device, having raised portions for welding ground connection wires thereon

Abstract: A metal heat sink baseplate (3) of a resin-encapsulated semiconductor power device, onto which the semiconductor dice (1) is bonded, is provided with one or more raised portions (12) hear the perimeter of the bond area of the semiconductor dice, onto which ground wires (11) may be welded in order to increase the reliability of wire connections realized between ground connection pads (15) and the metallic heat sink baseplate itself. The raised portions are preferably obtained by deep drawing the metal of the baseplate. A Single-In-Line device is shown wherein the deep drawing of the metal heat sink baseplate for realizing a ridge on the assembly face, to be used for realizing wire ground connections, is formed in a separation zone between a portion (4) of the metallic baseplate which extends beyond the perimeter of the encapsulating resin body and onto which the fastening means of an external heat sink are arranged, and the remaining part of the baseplate onto which the semiconductor dice is bonded. The deep drawing performed in this zone is also instrumental in providing a mechanical decoupling between the two portions of the metallic baseplate for reducing transmission of flection stresses.

Description: FIELD OF THE INVENTION The present invention relates to a resin-encapsulated semiconductor device wherein the semiconductor die is directly mounted on the face of a heat sink metal baseplate, which is functionally part of an electric path between at least a pad for ground connection of the front of the semiconductor dice and a pin of the device. BACKGROUND OF THE INVENTION In resin-encapsulated integrated power devices, e.g. in so-called SIL (Single-In-Line) power devices, the dice is directly bonded onto the flat face of a heat sink metal baseplate, commonly a nickel or silver coated copper plate, which may or may not extend beyond the perimeter of the encapsulating resin body, which is molded on the face of the metal baseplate. Commonly, a projecting portion of the heat sink baseplate beyond the perimeter of the encapsulating resin body (or of an encapsulating metal enclosure) has the function of providing the means for mounting the device on an external heat sink or viceversa, with a good thermal coupling therebetween. In such a projecting portion of the baseplate one or more holes or slots are often formed for accommodating the stem of assembly screws or similar fastening means. The metal pins for the electrical connection of the integrated circuit are derived from a die-stamped metal frame, commonly of copper, which may be coated with nickel, silver or gold. During the assemblying stages of the device, the die-stamped metal frame is fastened to the heat sink metal baseplate by tab wedging or by laser welding, whereby at least one ground pin of the finished device is electrically in contact with the metallic heat sink baseplate, as is well known to the skilled technician. Therefore, the heat sink baseplate of the device is electrically connected through at least one of the pins of the device to a ground rail of the external printed card circuit, to which also the external heat sink plates mounted on the devices are customarily connected. The connection to ground of the metallic heat sink baseplate onto which the semiconductor dice is directly bonded by means of a conductive adhesive, loaded with conductive metal powder (e.g. silver powder), or an eutectic lead and tin alloy, is commonly exploited for closing, through the heat sink baseplate itself, one or more electrical paths toward ground, by connecting with metal wires the respective metallized pads for ground connection formed on the front of the dice to the baseplate itself. When the dice is relatively large, these ground connection wires tend to assume radiuses of curvature relatively small and because of the consequently increased mechanical stresses they often break near the welding point, either during the resin encapsulation molding or during the normal operation of the device. Statistically their vulnerability is dramatically higher than that of the connection wires between the other pads of the device and the respective pins of the patterned metal frame. US Patent No. 4,132,856 discloses a semiconductor device package assembly wherein a metal heat sink has a raised die-attach area onto which a semiconductor die is eventually bonded and pedestals, formed in the vicinity of the raised dies attach area for supporting a metal frame coplanarly with the die bonded on the raised die-attach area of the metal base plate. The metal leads (fingers) of the frame that rest on the pedestals may be friction-welded on the top surface of the supporting pedestals. OBJECTIVE AND SUMMARY OF THE INVENTION Objective of the present invention as set out in the present claims is the improvement of the reliability of the welded wire connections between ground connection pads of the semiconductor dice and the metallic heat sink baseplate onto which the semiconductor dice is bonded. This objective is fully met by making the electric connection by means of a metal wire welded at its extremities on a metallized ground connection pad on the front of the semiconductor dice and a purposely raised surface of the metallic heat sink baseplate which is substantially coplanar with the pad surface, respectively. The surface of the metallic heat sink baseplate onto which one end of the connection wire is welded is the top surface of a raised portion or protrusion which is purposely formed on the otherwise flat surface of the baseplate in a position which is functionally adjacent or near one or more ground connections pads of the semiconductor dice. The height of the raised portion above the plane of the face of the baseplate onto which the semiconductor dice is bonded, may be substantially equal to the sum of the thickness of the semiconductor dice and of the layer of the conductive adhesive interposed between the dice and the face of the metal heat sink baseplate. Of course a height of the raised portion slightly lower or slightly higher than the thickness of the semiconductor dice and of the adhesive layer, may still be perfectly satisfactory, as far as this slight difference does not produce a substantial reduction of the radius of curvature of the connecting wire extending from the top surface of the raised portion of the metal plate and a ground connection pad present on the front of the semiconductor dice. In other words, the relative coplanarity between the two surfaces to be connected by the metal wire is to be intended a condition which is inclusive also of minor differences of height between the two surfaces which do not negatively affect in a substantial manner the radius of curvature assumed by the connecting wire. The raised portion is produced on the otherwise flat surface of the metallic heat sink baseplate by stamping, during the die stamping of strips of base-plates from a copper plate or bar. The raised portion is produced by a "deep drawing" or indenting action, by imposing, during the die stamping step of the baseplates, a punch of a suitably shape which acts on the metal in opposition to a recess which may be purposely formed on a die-stamping tool used for the die stamping. BRIEF DESCRIPTION OF THE DRAWINGS The different aspects and advantages of the invention will become evident through the following description of a preferred embodiment and reference to the drawing, wherein: Figure 1 is a schematic plan view of a device of the present invention during an advanced phase of assembly; Figure 2 is a sectional view in the plain A-A indicated in Fig. 1. The figures depict an embodiment of the invention, as implemented in manufacturing a semiconductor power device in a so-called Single-In-Line package, solely for illustrative and nonlimitative purposes. DESCRIPTION OF THE BEST EMBODIMENT Fig. 1 is a fragmentary simplified plan view of a device made in accordance with the present invention, during an advanced phase of assemblying, preceding the molding of an encapsulating resin body. At this point of the fabrication process, the welding of the connecting wires between the pads of the front of the semiconductor dice and the pins for the external connection of the device has been completed. As may be observed in the figures, the dice of semiconductor material 1 is bonded by means of a layer of conductive adhesive 2 , on the face of a heat sink metal baseplace 3 (often referred to as "slug") of copper or of another metal which is a good conductor of heat and electricity, most commonly of nickel coated copper. The metallic heat sink baseplate 3 has a portion 4 , which is destined to extend beyond the perimeter of the body of encapsulating resin 6 , which is indicated in Fig. 2 by means of the shadow profile drawn with a dash line. The projecting portion 4 is provided with a hole 5 for accommodating the stem of a screw or of a similar means of fastening of an external heat sink, not shown in the figures. Along the lower side of the slug 3 , an indentation 7 is present, inside which a purposely shaped tab end 8 of a pin is forcibly wedged, after having been bent at about 90° in respect to the plane of the die stamped metal frame 9 in which the pins 10 of the device are patterned. In order to improve intelligibility of the drawing, the face of the underlying heat sink baseplate has been patterned in the plan view of Fig. 1 . The die stamped assembly metal frame 9 is mechanically and electrically connected to the metallic heat sink baseplate 3 by wedging a tab portion of one of the patterned pins (the ground pin) into a slot or indentations 7 cut in the perimeter of the metallic baseplate 3 . The inner ends of the patterned pins are functionally disposed around the area of bonding the dice 1 and are parallel stand spaced from the face of the metal baseplate 3 . Upon completing the resin encapsulation, the different pins 10 will be electrically separated one from the other by cutting with a cutting wheel the temporary bridge portions between adjacent pins. The pin of which a portion of the inner extremity 8 has been bent and wedged by plastic deformation inside the indentations 7 present along the side of the metallic heat sink baseplate 3 , constitutes the ground connection pin of the integrated circuit. Although the remaining inner flat portion of this ground connection pin remains available for establishing a connection with a relative ground pad 15 of the front of the device 1 , by means of the wire 11 , it is advantageous in many integrated circuits to form several pads for ground connection, functionally disposed along the perimeter of the front of the dice 1 , were needed, and especially in a diametrically opposite position from the pin side of the dice, in order to avoid the need of forming long ground connection lines on the dice. These additional wire ground connections to ground connection pads of the device would take place, in accordance with the prior art, by welding one end of the connecting wires on the surface of the metal heat sink baseplate 3 , thus incurring in nonnegligeable problems of reliability and overall yield because of the necessarily sharp curvature of the connecting wires from said other ground pads to the flat surface of the baseplate onto which the dice is bonded. By contrast, in accordance with the present invention, the heat sink baseplate 3 is provided with a purposely raised portion or ridge 12 in a position facing the side of the perimeter of the dice 1 along which said further metallized pads 13 for the ground connection are formed. The raised portion 12 has a top surface which may be advantageously flat and which is at the same height of the metallized pads present on the front of the dice 1 . The connections made by welding a wire on the different ground pads 13 and the metallic heat sink baseplate 3 which is connected to the ground pin through the wedged tab portion thereof, are realized between the pads and the top surface of the raised portion 12 . In this way, the connection wires 14 assume a radius of curvature which is comfortably large by connecting points which are substantially coplanar, or almost so, among each other. This elimitates the drawbacks and the reliability problems of the prior art which appear substantially imputable to the relatively sharp bends of the connecting wires. The ridge 12 is preferably formed on the face of the baseplate 3 during die stamping, by plastically deforming the metal by means of a punch and of a recessed counter tool suitably shaped for producing a partial deep drawing of the metal in the selected area. In the shown example, the deep drawing of the metal plate 3 is realized in a necked separation zone between the portion of the baseplate 3 onto which the semiconductor dice is bonded and the portion 4 which will extend beyond the perimeter of the encapsulating body of resin 6 , upon completion of the package, and which is provided with an assembly hole 5 for mounting an external heat sink. The elongated deep drawn ridge produced along this zone of reduced width for forming the desired raised portion 12 on the assembly face of the baseplate 3 , also provides a zone of mechanical weakening (a line of discontinuity in the baseplate) which "separates" the externally projecting portion 4 of the baseplate from the central, dice-assembly portion thereof. This zone of mechanical discontinuity or weakening is instrumental in providing a mechanical decoupling between the portion of the baseplate 3 onto which the dice is bonded and the portion (4) extending outside the perimeter of the resin encapsulation body, onto which the stresses due to the mounting of the external heat sink exert themselves, thus preventing the transmission of large flection stresses from the zone of the assembly hole 5 to the bond zone of the dice 1 on the face of the baseplate 3 . The deep drawn zone may also extend for the entire width of the baseplate in correspondence of this separation line, or, as shown in the example, the deep drawn zone may extend only for a portion of the width of the baseplate.

Claims: A semiconductor device comprising a metal heat sink baseplate (3) on a first face of which a semiconductor die (1) is bonded, said metal heat sink baseplate (3) having a portion (4) extending beyond the perimeter of an encapsulating resin body (6) molded thereon and said extending portion (4) being provided with means (5) for mechanically anchoring an external heat sink thereto, a plurality of metal pins (10) for external connection, the inner ends of which are respectively connected to respective pads (13, 15) of said semiconductor die (1) by means of metal wires (11, 14), and wherein a metal wire (14) connects at least a pad (13) to said metal heat sink baseplate (3) for establishing through the latter a ground connection path said heat sink metal baseplate (3) being provided on the first face with a protrusion (12) near said pad (13) having a top surface at a height substantially equal to the height of said pad (13) from the first face of said baseplate (3) and said metal wire (14) being welded onto said pad (13) and onto said top surface of said protrusion (12), characterized in that the second face of the base plate has an indentation opposite said protrusion (12) defining a separation zone between said extending portion (4) and the remaining portion of said baseplate (3), said protrusion and indentation providing a mechanically weakened decoupling zone between the two portions of said metal baseplate in order to reduce transmission of flection stresses from said extending portion (4) and the portion of said baseplate onto which the semiconductor die (1) is bonded. The device according to claim 1, wherein on said top surface of said protrusion a plurality of metal wires are welded each being welded on a respective ground connection pad on the front of the semiconductor die for establishing through said metal heat sink baseplate a plurality of electrical ground connection paths.

Inventors: DELLA BOSCA PIETRO, MASSIRONI ANGELO, DELLA BOSCA, PIETRO, MASSIRONI, ANGELO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H01L23
IPC codes: H01L23