Process for assembling and resin-encapsulating a semiconductor power device mounted on a heat sink

Abstract: A die-stamped frame (2) is fastened to a heat sink metal baseplate (1) by wedging flexible tabs (3,4) into receiving indentations (5,6) of the baseplate (1) while keeping the frame (2) substantially in contact with the surface of the baseplate (1). The wire welding operations may then take place on the end of the fingers (2b) of the patterned metal frame (2) while the same are solidly resting on the surface of the baseplate (1) thus facilitating the welding. The backing-off of the metal frame (2) from the surface of the heat sink baseplate (1) takes place upon the closing of the mold (9,11) used for encapsulating in resin the device. The injection of the resin and its solidification "freezes" the pins (2b) in the backed-off position imposed by the mold (9,11) upon closing, thus ensuring the electrical isolation between the pins (2b) and the integral heat sink baseplate (1).

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to an assembly and resin encapsulation process for a semiconductor power device comprising an integral metal heat sink baseplate on a face of which the encapsulating resin body is formed. 2. Description of the prior art The assembly and resin-encapsulation of a semiconductor power device commonly comprises the following steps: fastening a die-stamped metal frame or strip of frames to a flat metal heat sink baseplate by wedging at least two tabs formed in two opposite sides of each die-stamped frame, into indentations purposely formed in opposite sides of the baseplate, so that once the. wedging has been made the metal frame or the strip of metal frames is supported by said wedged tabs parallel above the face of the baseplate and at a separation distance therefrom of some tenth of millimeter; bonding the semiconductor die or dies onto the face of the heat sink baseplate, in a central area in respect to the relative die-stamped metal frame so as to be surrounded by the inner ends of a plurality of coplanar fingers patterned in the die-stamped metal frame which will constitute external connection pins, functionally isolated among each other and from the metal heat sink in the finished device; electrically connecting the metal pads of the front of the semiconductor die to the fingers of the metal frame, respectively, by means of a metal wire welded to a pad and to the inner end of a finger of the metal frame; transferring the assembly inside a mold and injecting an incapsulating resin which, upon solidification, "freezes" the various parts of the assembly in their respective spacial position, thus isolating the fingers and the relative wire connections among each other as well as from the heat sink baseplate on which the encapsulating resin body is molded; separating the various devices which may have been simultaneously encapsulated in a single mold, from the outer perimetral band of the die-stamped metal frame or frames, and separating the various external pins (i.e. the outer portions of the fingers of the die-stamped metal frame extending out of the cavity of the encapsulation mold, passing between the opposed mating surfaces of the mold and of the counter-mold). Customarily, in accordance with modern fabrication techniques, the welding of the connecting wires to the metallized pads on the front of the semiconductor dies and to the respective finger or pin of the metal frame, is performed by a thermo-sonic mechanism. Practically the wire is pinned by means of a heated metal electrode against the metallic surface of the pad or of the finger 5 end and through the pinning electrode mechanical vibrations of sonic and ultrasonic frequency are transmitted to the pinned wire, causing a frictional localized overheating which is sufficient to momentarily melt the metal so that upon resolidification a permanent welding between the wire and the metallized surface of the pad or the surface of the metal finger is established. US-A-4,215,360 describes a method for assembling a power semiconductor device package employing an interlocking subassembly for displacing the lead frame to facilitate the wire bonding steps. Subsequently the interlock element is displaced in a second position before proceeding with the resin encapsulation process of the assembled device. In the Patent Abstracts of Japan, Vol. 009, No. 123, (E-317), 28 May, 1985, & JP-A-60 010 632, an assembly process employing a resin encapsulation mold, the lower part of which has an oblique lip portion for promoting an upward slanting of the inner portion of the leads away from the surface of a heat dissipating plate onto which the semiconductor die is set. The flexibility of the fingers' inner end portion, which extend in a cantilever manner from an outer perimetral band of the die-stamped frame, under the force, though relatively small, which is exerted by the welding electrode, often represents a problem for reliably welding the wires. In some cases, this problem is overcome by inserting spacing laminae of a dielectric material in the gap between the surface of the flat heat sink baseplate and the fingers of the die-stamped metal frame, in order to support the cantilever ends of the frame fingers during the welding of the wires. These dielectric supports may also be left in place and be encapsulated in resin body. The insertion of these dielectric spacers or supports remains a time consuming and,expensive practice in terms of production costs and is impractical altogether. OBJECTIVE AND SUMMARY OF THE INVENTION The present invention effectively overcomes this problem by permitting on one hand to carry out the welding of the wires on the finger ends while the latter are firmly suported by directly resting on the surface of the flat metal heat sink baseplate and on the other hand to positively ensure the separation and the permanency of the separation of the metal fingers of the frame from the metal heat sink baseplate after having completed the welding operations during the encapsulation in resin of the mounted assembly. This is accomplished by means of a mechanical positioning action which is exerted by a counter mold upon closing over the metal fingers and the wedged protrusions or tabs of the die-stamped metal frame. The functional permanent separation of the encapsulated metal parts is accomplished by injecting the encapsulating resin into the mold and solidifying it before opening the mold, thus practically eliminating the possibility of acidental intervening deformations of pre-assembled parts which, if undetected, could establish untolerable electrical shorts in the finished product. BRIEF DESCRIPTION OF THE DRAWINGS The different aspects and advantages of the invention will become more evident through the following detailed description of an embodiment thereof and by reference to the attached drawings, wherein: Figure 1 is a partial schematic view of a pre-mounted frame and heat sink assembly, according to the process of the invention; Figures 2 and 3 are partial sectional views which show the mechanism of separation of the heat sink baseplate from the coplanar fingers of the die-stamped metal frame, which is actuated upon closing the mold in which the encapsulating resin is injected. DESCRIPTION OF PREFERRED EMBODIMENT In accordance with the process of the invention, to a die-stamped strip of patterned frames, commonly of copper or of nickel coated copper, distinct heat sink metal baseplates for every patterned frame of the strip are fastened by wedging. These heat sinks may be flat metal plates of copper, nickel-coated copper, silver-coated copper or other suitable material, made in a definite form by die-stamping. The common practice of wedging a single metal strip to a strip of die-stamped frames and to separate the distinct encapsulated devices after the resin encapsulation by cutting the composite piece along border lines between adjacent devices of the composite strip assembly normally would not be compatible with the process of the present invention. With reference to the partial schematic view of Fig. 1, a flat metal heat sink baseplate 1 is fastened to a strip of die-stamped metal frames 2, facing a respective frame 2a of the strip of frames by wedging. The fastening of the heat sink 1 to the strip of frames 2 is effected by utilizing two purposely patterned tabs 3 and 4 of the die-stamped unit frame 2a . These tabs are commonly formed by die-stamping on opposite sides of each unit frame of a die-stamped strip. The wedging is effected by forcing the tabs 3 and 4 inside two indentations or slots 5 and 6 , respectively, which are purposely formed in the opposite sides of the heat sink 1 . The sizes of the tabs 3 and 4 and of the indentations 5 and 6 are interfering among each other so that upon forcibly inserting the metal tabs 3 and 4 into the respective indentations the metal of the tabs and/or of the heat sink undergo a plastic deformation which establishes a stable mechanical anchoring of the heat sink 1 to the unit frame 2a of the strip 2 . According to the process of the invention, the wedging is carried out without substantially bending the tabs 3 and 4 , that is leaving the major portion of the tabs 3 and 4 substantially coplanar to the other patterned portions of the die-stamped frame so that the fingers 2b of the frame remain abutting against the surface of the heat sink 1 . As it will be evident to the skilled technician, in the embodiment shown in Fig. 1, a typical metal frame configuration for a so-called Dual-In-Line, 16-pin package (16 pin DIP package), is depicted, wherein, after having completed the semiconductor die assembly, resin encapsulation and cutting of the external pins for separating and discarding the outer perimetral portion of the die-stamped frame (depicted in Fig. 1 only partially) are completed. In this type of package the pins of the device are disposed along two opposite parallel sides of the encapsulation resin body, which generally has a parallelepiped shape. Of course, the process of the invention may similarly be utilized also in case of different types of metal frames relative to other standardized plastic packages for power devices. Returning to the case depicted in Fig. 1, after having fastened the heat sink 1 to the unit frame 2a , the semiconductor wafer chip 7 is bonded, usually by means of a conductive cement which typically may be constituted by an epoxidic adhesive loaded with conducting metal powders, typically with silver powder, capable of ensuring a good thermal coupling between the rear of the semiconductor chip 7 and the heat sink 1 within a central area of the face of the heat sink 1. Subsequently the welding of the connecting wires to the metallized pads 8 of the front of the semiconductor chip 7 and to the inner ends of the respective patterned fingers 2b of the die-stamped frame is performed. The welding is commonly carried out by means of automatic machines through a thermo-sonic welding process, whereby a thin metal wire of aluminum, gold, copper or the like, is pressed by a heated electrode of the welding machine on the metallized surface of the pad or on the surface of the end portion of a finger of the metal frame, and heat and mechanical energy in the form of sonic and/or ultrasonic frequency vibrations are transmitted through the thickness of the wire to the contact area between the wire and the metal surface in order to cause a localized melting of the metal for a very short time sufficient to establish a permanent welding between the wire and the metal surface. The welding process is highly automatized and is usually performed in a very short time by utilizing highly sophisticated welding machines. The welding of the wires on the end portions of the fingers 2a of the frames may thus take place in an ideal condition wherein the ends of the fingers 2b of the metal frame solidly rest on the face of the heat sink 1 and therefore are not subject to flections induced by the load which must necessarily be exerted by the welding electrode. This highly improves the reliability of the welds produced. On the other hand, it is functionally essential that upon completion of the assembly the fingers 2b constitute as many electrical connection leads isolated among each other as well as from the metal heat sink 1. Such a necessary condition is ensured, according to the process of the present invention, by positively causing separation of the coplanar metal fingers of the metal frame from the face of the metal heat sink 1 on which they were abutting during the welding step and maintaining the metal frame in this raised-off position in respect to the heat sink baseplate 1 in opposition to an elastic restoring force due to the elastic memory of the metal of the frame during the injection of the encapsulating resin in the mold and until the resin has fully solidified, in order to "freeze" the metal frame fingers in their forcibly assumed spatial position in relation to the heat sink baseplate 1. The release of the actuating means which had forcibly determined the separation and spacing-off of the fingers from the heat sink may then take place without the fingers being able to elastically return in contact with the metallic heat sink. Basically, the actuating means for raising and keeping the metal fingers of the frame separate from the heat sink are constituted by the cooperating halves of the mold used for encapsulating the assembly. The resin injection mold may be composed of a first part which is substantially flat and which may be provided with ridges or supports onto which the assembly face of the heat sink rests. These supports may be disposed essentially along the sides of the perimeter of the face of the heat sink wherein the fastening tabs of the metal frame are wedged and through which there are not fingers of the die-stamped metal frame extending. A counter-mold, having a substantially flat mating face, may be provided with a cavity of dimensions sufficient to completely contain within said cavity the heat sink. By closing on the assembly rested on said first part of the mold, the counter-mold causes the flection of the parts of the die-stamped metal frame which extend beyond the perimeter of the heat sink and these outer portions of the metal frame are henceforth pinned against the flat surface of the first part of the mold, thus positively separating the coplanar fingers of the metal frame from the face of the heat sink. At this point a fluid encapsulating resin may be injected in the space defined between the surface of the first part of the mold and the opposing surface of the heat sink baseplate and laterally by the counter-mold and partially by the inner surface of said support ridges of the first part of the mold. Schematically, the action of separation of the fingers of the metal frame from the heat sink exerted upon closing the resin injection mold is depicted in the two partial, sectional views of Figures 2 and 3. The first part of the mold 9 is provided with ridges 10 which may extend essentially along two opposite sides of the perimeter of the face of the heat sink 1 wherein the wedged fastening tabs 3 ( 4 ) of the frame are located. Preferably the ridges 10 may not extend for the entire length of the relative side of the perimeter of the heat sink 1 but have a length sufficient to provide at least two rest surfaces for the assembly and their position, coincides with the tab wedging points. The sectional profile of the fingers of the metal frame, arranged along sides of the heat sink 1 orthogonal to the sides in which the tabs 3 (and 4 ) are wedged in their respective seats, are indicated with 2b also in the partial elevation views of figures 2 and 3. The counter-mold 11 has a substantially flat surface 12 and a cavity 13 which has dimensions suitable to completely contain the heat sink 1 inside the cavity upon the closing of the mold. Upon closing of the mold (schematically shown in Fig. 2 by the arrow F ), the flat surface 12 of the counter-mold 11 acts on the parts of the metal frame which extend beyond the perimeter of the cavity, i.e. beyond the perimeter of the heat sink 1 , and lowers the coplanar fingers of the metal frame 2b until they abut against the surface of the first part of the mold 9 , thus positively separating and spacing them by an exactly pre-established distance from the face of the heat sink 1. Of course the displacement of the coplanar fingers of the metal frame is allowed by a sufficient length and flexibility of the connecting wires (not shown in the figures) which have already been welded as well as by the forced bending of the wedged tabs 3 ( 4 ) . As it will be evident to the skilled technician, the process of the invention may be carried out also in the case wherein a mold or counter-mold having cooperating comb-like ridged mating surfaces for accommodating the figures and sealing the spaces between adjacent fingers, along the respective sides, as well as in the case the metal frames are patterned by die-stamping with an uninterrupted transversal band, temporarily bridging the fingers and which may conveniently act as a gasketing portion for sealing the perimeter of the cavity of the mold and preventing the exit of the resin between adjacent fingers. This transversal band may then be cut between adjacent pins by a special tool after molding and separation of the outer perimetral band of the assembly frame in order to electrically separate the pins from each other.

Claims: A process for assembling and encapsulating in resin a die (7) of a semiconductor wafer having metallized pads (8) for wiring connections, said process comprising fastening together a patterned metal frame (2) and a flat metallic heat sink baseplate (1), disposed in respectively parallel planes, by wedging fastening tabs (3, 4) of said metal frame (2) into indentations (5, 6) formed on opposite sides of one surface of said heat sink (1), while maintaining coplanar fingers (2b) of the metal frame (2) resting on the surface of the heat sink (1) and successively cementing said die (7) on said heat sink (1), connecting said pads (8) to the ends of said coplanar fingers (2b) of said patterned frame (2) by welding metal wires on the ends of said coplanar fingers (2b) resting on the surface of the heat sink (1); placing the mounted assembly on a substantially flat mold (9) provided with raised portions (10) for supporting the heat sink (1) by said surface thereof, the raised portions (10) being arranged along the sides of the perimeter of the heat sink (1) on which said tabs (3, 4) of the metal frame (2) are anchored thereto; closing the mold by means of a counter-mold (11) having a substantially flat mating surface (12) provided with a cavity (13) capable of completely containing said metal heat sink (1), said die (7), said connecting wires and an inner portion of said coplanar fingers (2b), which extend beyond the perimeter of said cavity (13) by passing between perimetral mating surfaces of said mold (9) and of said counter-mold (11), said counter-mold (11) pushing said fingers (2b) while bending said fastening tabs (3, 4) against the surface of said flat mold (9) thus spacing said coplanar fingers (2b) from the surface of the heat sink (1); injecting an encapsulation resin in the cavity defined between the surface of said mold (9) and the opposed surface of the heat sink (1) and laterally by said counter-mold (11) and, partially, by the inner face of said raised supports (10) of the mold (9), and thus ensuring the electric isolation of each of said coplanar fingers (2b) from said metallic heat sink (1), and solidifying the injected resin within the space comprised between said surface of the heat sink (1) and said fingers (2b) of the metal frame (2) before opening the mold and recovering the encapsulated device.

Inventors: CASATI PAOLO, COGNETTI DE MARTIIS CARLO, MARCHISI GIUSEPPE, CASATI, PAOLO, COGNETTI DE MARTIIS, CARLO, MARCHISI, GIUSEPPE
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1991
Main classification: H01L23
IPC codes: H01L21, H01L23