Abstract:
A process comprises the following operations: forming a structure of metal elements with functions of support and electrical connection, these metal elements having a high degree of surface finish; fixing a chip of semiconductor material, containing active parts and contact pads, to an area of a metal element of the structure acting as a support; electrically connecting the contact pads of the chip to predetermined metal elements of the structure acting as terminal conductors; and incorporating in plastic the chip of semiconductor material and part of the structure of metal elements. To improve the adhesion between the structure and the plastic, at least part of the surface of the metal elements is roughened by irradiation with a laser light beam.

Description:
TECHNICAL FIELD 
     The present invention relates to the manufacture of electronic semiconductor devices and, more particularly, to a process for making an electronic semiconductor device comprising a structure of metal elements partially incorporated in a plastic body. 
     BACKGROUND OF THE INVENTION 
     As is known, integrated circuits and other active electronic devices or components are made on “chips” of semiconductor material having a surface area of a few mm 2 , and require, for their connection to an external electrical circuit, suitable supporting, containment and electrical interconnection structures. A typical structure (package) suitable for the purpose includes a plastic body which encloses a chip which is soldered to an area of a metal supporting element of a metal structure formed by punching from a thin sheet and is connected, through thin metal wires soldered to suitable metallized areas (pads) provided on its surface, to corresponding electrical conductors which emerge from the plastic body and which also form part of the metal structure. 
     In the case of integrated power circuits, in other words devices intended to operate with high currents and, therefore, subject to considerable heating, these structures also comprise a metal plate through which the chip, which is fixed to it, can transfer to the exterior the heat produced during its operation. 
     The principal stages of the manufacture of the latter structures will now be described. The plate is formed by punching from a metal sheet, made for example of copper or copper alloys, possibly nickel-plated, possibly together with other identical plates which remain joined together by portions of the initial sheet which are designed to be removed by punching in a subsequent stage of processing. The chip is fixed on the metal plate by soldering with a low-melting-point alloy, for example a lead-tin alloy, or by gluing with a suitable adhesive, for example an epoxy adhesive. A set of metal strips, formed by punching from a thin sheet but still joined together by connecting portions and designed to become the terminal conductors of the device, are fixed to the plate, with at least part of the metal strips insulated electrically from the plate. Thin wires, usually made from gold, are welded at one end to the metallized areas of the chip with a low-melting-point alloy and at the other end to the ends of the metal strips, using a process, called “thermosonic”, in which heat and ultrasound are applied simultaneously. The structure formed in this way, together with other identical structures having their corresponding plates and corresponding sets of interconnected metal strips, is then placed in a suitable mould, into which a plastic material in the liquid state, for example a thermosetting epoxy resin, is injected. After the polymerization of the resin, a multiplicity of structures is obtained, each comprising a solid plastic body which incorporates the elements described above with the exception of one face of the metal plate and part of the metal strips, in other words the terminal conductors of the device, and the interconnecting portions between them. These portions, together with the interconnecting portions between the plates, are then cut off and in this way the finished electronic devices are obtained. 
     A typical problem with the structures (packages) described above, and in general of any structure consisting of a plastic body and metal elements formed from sheets having a high level of surface finish, in other words a low roughness (for example, R a ≦0.1 μm in the case of nickel, R a ≦0.4 μm in the case of copper and copper alloys), is the low reliability, due to insufficient adhesion of the plastic body to the metal parts. It has been found that many of the failures of these devices are due to the ingress of moisture into the body through interstices between the metal terminals and the plastic body which are formed as a result of the cutting force, as a result of the difference between the coefficients of thermal expansion of the metal and the plastic, which is manifested between the metal and plastic in the cooling phase after the molding operation and during the normal thermal cycles of operation. 
     To resolve this problem at least partially, in other words to provide greater reliability and a more moisture-resistant seal, a known method is to treat the metal surfaces in such a way as to increase their roughness, for example by subjecting them to compression with molds having surfaces of predetermined roughness, or by subjecting them to sandblasting. 
     The technique based on molding is rather expensive, since it requires the preparation of individual molds and frequent maintenance operations to restore the desired degree of roughness of the mold, and the technique based on sandblasting is not suitable for use when the metal elements to be treated are of very small size, as in the case of lead frames for integrated circuits with a large number of pins. Furthermore, when a selective treatment of the metal surfaces is desired (as is often required in the case of frames with areas which must retain the original degree of finish to enable a chip of semiconductor material to be soldered to them), both of these known techniques require the use of masks: the first during the preparation and maintenance of the mold, and the second during the actual treatment. This further complicates the process. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a manufacturing process that comprises a phase of roughening the metal surfaces which is simple and effective, both when the treatment has to be applied to the whole surface and when the treatment has to be selective, in other words when it has to be applied to only part of the surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following detailed description of one of its embodiments, provided by way of example and therefore not in any way respectively, in relation to the attached drawings. 
     FIG. 1 is a perspective view of a structure which can be made by a process according to the invention, with a part removed to shown its interior. 
     FIG. 2 is a plan view of the structure shown in FIG. 1, as it appears before the molding operation. 
     FIG. 3 is a sectional view as indicated by the line III—III in FIG.  2 . 
     FIG. 4 shows schematically an apparatus for the practical application of a phase characteristic of a process according to the invention and, in perspective, a metal plate subjected to a treatment with this apparatus. 
     FIG. 5 shows an enlarged part of the metal plate in cross-section after the treatment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The structure of the electronic semiconductor device shown in FIGS. 1 to  3  comprises a plastic body  10 , a multiplicity of metal strips or terminal conductors  11 , a metal plate  12 , and a chip  13  of semiconductor material in which an integrated circuit is formed and which is fixed to the metal plate  12  with a layer of alloy solder  14 . The metal plate  12  preferably is made from copper or copper alloys, preferably nickel-plated, and provides support and heat dissipation. The terminal conductors  11  are formed, in the normal way, by punching from a single piece of thin sheet. Before being completely separated from each other after molding, preferably by punching again, they are joined together by interconnecting portions  15 , in such a way as to form in combination a lead frame, indicated as a whole by the number  16 , as seen in FIG.  2 . 
     The metal plate  12  is also cut from a sheet which is thicker than the thin sheet used for the terminals, together with a certain number of other identical plates, not shown, which remain joined together by short interconnecting portions designed to be removed subsequently, by cutting or by another method, after the simultaneous formation of the plastic bodies in a single mould. 
     The frame  16  has two opposite lateral strips which form two bars  19  inside the plastic body and two pairs of terminals which project from the body. Each bar  19  has two holes  20  which enable the frame  16  to be fixed to the plate  12  by means of suitable posts  21  projecting from the plate through the holes  20 . After the insertion of the posts  21  into the holes  20 , the posts are clinched to fix the bars  19  and the whole frame  16  in position on the plate. In this way an intermediate product is formed, consisting of a multiplicity of interconnected structures, each consisting of a plate  12  and a frame  16 . 
     The ends of the terminal conductors inside the frame  16  and the central parts of the bars  19  are then connected by the known method described above to the metallized areas  17  of the chip by means of thin gold wires  18 . 
     After the formation of the plastic body  10  and the punching of the interconnecting portions  15  of the frame  16 , carried out in the usual way, the structure shown in FIG. 1 is obtained. 
     To improve the adhesion between the plastic body  10  and the metal plate  12 , according to a known technique, the plate is shaped along its periphery in such a way that it has undercut surfaces, indicated by the number  30 , and a lateral projection at an acute angle, indicated by  31 , substantially along the whole of its periphery. To simplify the drawing, in FIG. 1 the undercut surfaces are shown only along the longer sides of the plate. 
     In the normal techniques, both the sheet from which the plates of the heat sinks are formed and the thin sheet from which the terminal conductors are formed have a high degree of surface finish, in other words a very low degree of roughness, for example R a ≦0.1 μm for nickel or R a ≦0.4 μm for copper and copper alloys. To improve the adhesion between the plastic and the metal, the plate and the frame of the terminal conductors, before being assembled, are treated in such a way as to roughen all or part of their surfaces. 
     In the example shown in the drawing, the surfaces with controlled roughness are peripheral areas of the plate  12 , more precisely those delimited by the undercut surface  30  and the edge  32  opposite this surface on the upper face, as shown by hatching in FIG. 2, and the areas  33  of the enlarged end parts, partially incorporated in the body  10 , of the terminal conductors  11 , also shown by hatching in FIG. 2, on both faces. The roughness of these areas is produced, according to the invention, by selective irradiation with a laser light beam. 
     The equipment used to apply the process according to the invention, as shown schematically in FIG. 4, consists substantially of a laser  40 , a unit  41  for the frequency conversion, selection and deflection of the laser beam, an optical scanning device  42  and a synchronizing unit  43 . 
     The laser  40  may be selected from industrial lasers which emit radiation which can be largely absorbed by metals, with wavelengths of between 200 and 1000 nm. 
     In practice, the laser used is of the neodymium YAG type, which basically consists of a crystal of yttrium and aluminum doped with neodymium in the form of a rod having mirrors at its ends, which radiates at a wavelength of 1064 nm (infrared light) with pulses whose duration is between 6 and 8 ns. The frequency of repetition of the pulses in this application was regulated to approximately 30 Hz. The output beam of the laser  40  is applied to a frequency converter  41   a , consisting of a crystal of KH 2 PO 4  (biacid phosphate of potassium deuterate) which, with a conversion efficiency of approximately 30-35%, supplies at its output a beam having a frequency equal to twice the input frequency, in other words a wavelength which is halved, equal to 532 nm (green visible light). This is advantageous, since metals commonly employed for forming the metal plate and the lead frame absorb the radiation at 532 nm significantly more than the radiation at 1064 nm, the latter being reflected for a 90% or more. A dichroic mirror  41   b  separates the beam having double frequency from the residual beam at the laser output frequency, by deflecting it to a mirror  41   c  which, in turn, deflects it to the scanning device  42 . This scanning device consists of an optical system capable of directing the 532 nm laser beam in a controllable way on to predetermined areas of the surface to be treated, in this example the edge areas of the plate  12 , indicated by the number  32  in FIG. 2, whose roughness has to be increased. The synchronization unit  43  synchronizes the emission of the laser pulses with the scanning. 
     The laser beam strikes the surface of the body to be treated, in this example the plate  12 , on a substantially circular elementary area  44 , with an energy which is regulated in such a way as to erode the metal causing the formation of plasma. Due to multimode oscillation, the Nd:YAG laser beam exhibits a non-uniform distribution of energy density, so that the energy density striking the elementary area  44  varies from point to point. In this way, micrometric roughness is obtained on the surface of the body stroke by the laser beam. 
     The scanning of the laser beam is controlled in such a way that the surface of the plate is struck by the beam along a predetermined path, in other words, in this example, along the edge of the plate, the elementary areas corresponding to successive pulsed beams being made to overlap partially. In a practical application of the process, the diameter of the elementary area  44  was approximately 2 mm, and the energy of the beam was regulated to a value of between 150 and 300 mJ. Preferably, the elementary areas overlapped by approximately 50%. 
     Adhesion tests carried out on specimens of different metals have shown that the process according to one embodiment of the invention is particularly effective when applied to nickel substrates with a degree of surface finish of approximately Ra=0.07 μm. After treatment, the roughness was approximately Ra=0.2 μm. In the case of copper or copper alloys substrates, starting from a degree of surface finish of approximately Ra=0.39 μm, the roughness after the treatment was approximately Ra=1.10 μm. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.