METHOD FOR MANUFACTURING A MODULAR SEMICONDUCTOR POWER DEVICE AND DEVICE OBTAINED THEREBY

Abstract: The components used in the method comprise a heat-­ dissipating base plate, one or more three-layer plates (the top layer consisting of copper plates, and strips) and a one-piece frame designed to constitute the termi­ nals (81-87). After the chips have been soldered onto the upper plates and connected to the strips, the inner ends of the frame are soldered to points of connection with the chips. This is followed by the encapsulation in resin and the shearing of the outer portions of the frame, which, during the process, serve to temporarily connect the terminals.

Description: This invention concerns a method for manufacturing a modular semiconductor power device and the device obtained by means of such a method. In the manufacture of modular semiconductor power devices, as in the manufacture of numerous other components, an important target is to succeed in producing extremely reliable products using simple and inexpensive procedures. The known semiconductor power devices involve complex and costly procedures, both from the point of view of the individual components necessary for constructing them and from the standpoint of their assembly and reciprocal insulation. Some of these known devices are described in the USA patent N. 4.518.982. Said patent gives a detailed description of a modular power device whose manufacturing process consists of soldering one or more semiconductor chips onto a flat portion of a first electrode (which also serves as a heat sink), soldering other electrodes (possibly containing other chips) onto said flat portion by means of a dielectric adhesive material, electrically connecting the various chips and electrodes, encapsulating the device in resin and electrically insulating the heat dissipating surface, by means of a further layer of insulating material. Another known device, particularly suited for testing it at various stages during the manufacture, is described in the document GB-A-1255073. It is manufactured by mounting a semiconductor device on a preformed conductor network of flat shape, comprising a lead frame formed from a metal sheet. After encapsulation of parts of the assembly in several discrete steps the conductors are separated from from the lead frame. A further known device, particularly suited for reducing wiring inductance, is described in the document DE-A-3336979. Its manufacturing steps include the soldering of inner plate-shaped parts of separate terminal conductors onto an alumina layer, and the subsequent soldering of semiconductors devices and bonding wires onto said plate-shaped parts. The purpose of this invention is to provide a particularly reliable modular power device, according to claim 1, obtained by means of an extremely simple and not much expensive assembling procedure, according to a highly flexible manufacturing method and with components which, although extremely limited in number, can be used to create various circuit arrangements and layouts, always using the same tools and always keeping an identical external geometrical configuration of the devices obtained. According to a particular feature of the manufacturing method, the reciprocal insulation of the electrodes and their encapsulation are carried out in a single step. The manufacturing method according to the invention, concerns a modular semiconductor power device comprising one or more semiconductor chips, a metal plate (11) for dissipating heat generated by Joule effect, a plurality of electrodes constituting the signal and power terminals of the device, and a resin encapsulation, and is characterized by the following steps: welding each chip onto a sheet (33) of a number of sheets (33,34,35,36) of electrically conductive material; positioning said sheets (33,34,35,36) on a plane substantially parallel to the aforesaid heat dissipating plate (11) and close to the latter; blanking, from a single sheet of conductive material, a one-piece frame (51) designed to constitute the power and signal terminals, said blanking enabling temporary mechanical connections (61) to be kept between the portions of the terminal conductors designed to remain outside of the resin encapsulation; soldering the inner ends of the terminals to connecting points of said chips or sheets of said number of sheets, electrically connected to said chips; encapsulating with insulating resin all the active parts of the device, leaving uncovered the outer surface of the plate (11) and the portions of the terminals involved with the aforesaid temporary connections (61); shearing the temporary connections (61). Other solutions relative to preferred embodiments are further on illustrated in the claims. The features of the invention will be more clearly evident from the following description and the accompanying drawings referring to a non-restrictive embodiment, in which: Fig. 1 : shows an exploded view of the base plate and the substrates supporting the chips; Figs. 2a and 2b : show a side view and a cross-sectional view of the base plate of Fig. 1; Figs. 3a, 3b and 3c : show bottom, top and cross-sectional views of a substrate of Fig. 1; Fig. 4 : shows a view of the base plate of Fig. 1 after the substrates and the chips have been joined to it, and after the electrical connections between chips and metal coating of the substrates have been made; Fig. 5 : shows the one-piece Frame for obtaining the external terminals and their connections with the metal-coating of the substrates; Fig. 6 : shows a plan view of the flattened frame from which the frame of Fig. 5 was obtained; Fig. 7 : shows a view of the modular device after the resin encapsulation has been carried out by moulding; Figs. 8a, 8b and 8c : show perspective top and bottom views and a plan view of the finished device. In Figure 1 , reference 11 indicates the base plate of the device which, since it has to act as a heat sink as well as a support and fastener for the device itself, is made of heat-conductive metal with high mechanical strength. The holes A in it serve to secure it, by means of screws, onto the external heat spreader, while the grooves V serve to absorb any possible deformation of the base plate due to the high tightening torque, thereby preventing them from being transmitted to the central portion of the plate. The S-shaped drawings M, on the sides (see the side and cross-sectional views of the plate shown in Figs. 2a and 2b ) serve to ensure a better grip of the subsequent resin encapsulation, as is explained more clearly further on with reference to figure 8b. Figure 1 also shows the components 12 and 13, which are two chip-supporting substrates, which are soldered onto the base plate 11 by means of the layers of solder 14. As illustrated in Figures 3a, 3b and 3c (bottom, top and cross-sectional views of a substrate) each substrate is composed of a quadrangular-shaped thin medial layer 31 of alumina (less than 1 mm thick), with thin copper plates soldered directly onto its two lateral faces. More precisely, soldered onto the face destined to lie facing the upper face of the base plate 11 is a single copper plate 32, which is also quadrangular in shape but with slightly smaller dimensions than those of the layer of alumina, while the other face is provided with a rectangular plate 33, for supporting the chips and for the connections with an external electrode, and, on either side of said plate 33, narrower plates (lateral strips) 34, 35 and 36 designed both for soldering the conductor for connection with the chips, and for soldering other external electrodes. Figure 4 shows the device (41) as it appears after the two substrates have been soldered onto the base plate, the chips have been soldered onto the larger upper copper plates b and f, and the electrical connections have been carried out between the chips and the lateral strips a, c, d, e, g, h. The latter connections are obtained by ultrasound soldering of aluminium wire. Figure 5 shows a copper frame 51 designed to constitute the external terminals and the connections of the latter with the lateral strips and the plates f and b of figure 4. The frame of figure 5 is obtained by blanking from a copper sheet and by subsequently bending the terminal portions downwards, said terminal portions being subsequently soldered onto the lateral strips and the plates f and b. Figure 6 shows a plan view of the frame of figure 5, as it appears after being blanked from the copper sheet and before the bending of the terminal portions, which are indicated respectively by h, g, e, d, c, a, f, b. After being bent downwards, said terminal portions are then soldered respectively onto the strips h, g, e, d, c, a, and onto the plates f and b of 41. After the frame 51 has been soldered onto the device 41, the device is encapsulated by means of a moulding process with insulating resin (e.g. thermosetting epoxy resin), preferably of the low-stress type. After the moulding phase, the device appears as shown in Figure 7 . At this point, in order to complete the device, it is necessary to shear the external temporary connections between the terminals, corresponding to the portions (61) illustrated in dotted lines in figure 6, and then bend the signal terminals upwards to a vertical position, and the power terminals inwards. The finished device takes on the appearance of Figure 8a (top perspective view) and of Figure 8b (bottom perspective view), in which 81, 82, 83 and 84 indicate the signal terminals and 85, 86 and 87 indicate the power terminals. As shown in Figure 8b, the resin encapsulation ends, from below, flush with the lower surface of the base plate 11, which can consequently be secured in direct contact with the supporting metal structure on which it is designed to be placed, thereby ensuring efficient dissipation of the heat. The same figure clearly shows the function of the S-shaped drawings on the base plate (see figs. 1 and 2b). In fact, on completion of the device, they are completely embedded in the resin body, so as to constitute two areas for anchoring and ensuring an efficient grip of the resin. As shown in Figure 8c (plan view of the device), after the bending, the terminal holes of the three power terminals 85, 86 and 87 come to rest exactly above the three hexagonal nuts embedded in the resin, so as to enable the electrical connection with the external connecting rods. The foregoing description gives a clear idea both of the versatility of the invented method and of the simplicity of the assembling procedure. In fact, it is clear that: with the substrates of fig. 1 and fig. 3, it is possible to use chips of different number and sizes, to achieve different connections of the chips with the lateral strips, and to obtain soldered strips with different geometrical layouts; the one-piece frame of fig. 5 and fig. 6 can also be made with different geometrical layouts, to enable it to adapt to the different geometrical layouts of the aforesaid soldered strips, and to the different electrical functions of the device; the procedures for soldering the conductors connecting the chips to the metal strips and the external electrodes to the metal chip-supporting strips or plates are simplified due to the flat disposition of the soldering points and to the fact that the inner ends of the electrodes are soldered when they are still firmly secured to each other by the aforesaid temporary connections; due to the fact that the chips are soldered onto coplanar plates and to the presence of the temporary connections between the electrodes it is also possible to carry out the encapsulation and reciprocal insulation of the electrodes in a single step. A further advantage, in addition to those mentioned previously, consists of that related to the particular structure of the substrate chosen for soldering the chips as well as that related to the type of resin used for the encapsulation. In fact, said substrates, which consist of a layer of alumina with copper plates soldered directly onto both faces, are characterized by thermal expansion coefficients very similar to those of silicon. This reduces to a minumum the thermomechanical stress which would otherwise be transmitted to the chips due to the differential expansions of silicon and copper (other embodiments envisage the insertion, between the chips and the supporting copper plates, of layers of material, such as for example molybdenum, having an expansion coefficient lying half way between those of silicon and copper, which however complicate the assembling and lower the thermal performance). The use of a low-stress type of resin helps to limit the stress transmitted to the chips even in the case of chips of very large dimensions. It is also clear that the following modification to the embodiments previously described may be made. For example, the wires connecting the chips to the metal strips of the substrates can be substituted by direct soldering between the inner terminal portions of the one-piece frame and the chips, whenever the latter have wettable metal coatings. These internal portions can then be soldered to points of connection with the chips situated on the aforesaid plates 33 and strips 34, 35, 36 (as in the case illustrated in Figs. 4 and 5), or situated on the same plates and on wettable metal coatings on the surface of the chips. Likewise, the chip-supporting substrates could have a different structure from that previously described and the insulation between the chips and the dissipator could be achieved by means of a layer of the encapsulating resin itself - which in this case should be of high thermal conductivity - instead of by a layer of alumina.

Claims: Method for manufacturing a modular semiconductor power device comprising one or more semiconductor chips, a metal plate (11) for dissipating heat generated by Joule effect, a plurality of electrodes constituting the signal and power terminals of the device and a resin encapsulation, comprising the following steps: welding each chip onto a sheet (33) of a number of sheets (33,34,35,36) of electrically conductive material; positioning said sheets (33,34,35,36) on a plane substantially parallel to the aforesaid heat dissipating plate (11) and close to the latter; blanking, from a single sheet of conductive material, a one-piece frame (51) designed to constitute the power and signal terminals, said blanking enabling temporary mechanical connections (61) to be kept between the portions of the terminal conductors designed to remain outside of the resin encapsulation; encapsulating with insulating resin all the active parts of the device, leaving uncovered the outer surface of the plate (11) and the portions of the terminals involved with the aforesaid temporary connections (61); shearing the temporary connections (61), characterized in that before the encapsulating step the inner ends of said terminals are bent in a direction perpendicular to the plane of said frame and soldered to connecting points of said chips or sheets of said number of sheets, electrically connected to said chips. Manufacturing method as claimed in claim 1, characterized by the fact that said sheets (33,34,35,36) are made of copper, that the ones of them (34,35,36) not supporting any chip are strip shaped, and that they belong to a plate (12, 13) consisting of three layers, the first of which is made up of the same sheets (33,34,35, 36), the intermediate consists of an alumina plate (31) onto which each one of said sheets is directly soldered, and the third one consists of a further sheet of copper (32) substantially equal in size or slightly smaller than the intermediate layer and directly soldered onto it, and by the fact that the soldering of the inner ends of the terminals to the aforesaid connecting points are preceded by the soldering of wires connecting each chip to the aforesaid strip shaped sheets (34, 35, 36). Manufacturing method as claimed in claim 2, characterized by the fact that said third layer (32) is soldered onto the internal surface of the plate (11) and that the connections between said chips and said strip shaped sheets (34, 35, 36) are achieved by means of ultrasound soldering of aluminium wires. Manufacturing method as claimed in claim 1, characterized by the fact that the aforesaid connecting points are located on the aforesaid sheets (33) supporting the chips and on metal coatings on the surface of the chips, wettable with respect to the solder used to solder the inner ends of said terminals. Manufacturing method as claimed in claim 1, characterized by the fact that, after the encapsulation and the shearing of the temporary connections (61) of the one-piece frame (51), the terminals designed to perform the function of signal terminals are bent in a direction perpendicular to the base plate, while the terminals designed to perform the function of power terminals are bent over the capsule in a direction parallel to the base plate itself.

Inventors: DI CRISTINA, NATALE, GANDOLFI, LUCIANO, MINOTTI, CARLO, PERNICIARO, SPATRISANO ANTONIO
Applicants: SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1988
Main classification: H01L25
IPC codes: H01L25, H01L21, C12N7, H01L23, A61K39