Abstract:
A radiation-emitting or -receiving semiconductor chip  9  is soft-soldered for mounting on a leadframe  2  over which a prefabricated plastic encapsulant  5 , a so-called premolded package, is injection-molded. Through the use of a low-melting solder  3  applied in a layer thickness of less than 10 μm, the soldering process can be carried out largely without thermal damage to the plastic encapsulant  5.

Description:
The invention concerns a method as defined in the preamble to Claim  1  and an optoelectronic semiconductor component as defined in the preamble to Claim  24 . It further concerns a method for the production of such an optoelectronic semiconductor component. 
     BACKGROUND OF THE INVENTION 
     Radiation-emitting and/or -receiving semiconductor chips are conventionally attached by a gluing process to so-called pre-encapsulated leadframes, which are made by molding over a chip mounting area with a plastic encapsulant. 
     In the known package designs the semiconductor chip is mounted on the leadframe by gluing, since this eliminates the high temperatures that are necessary for soldering processes and that can damage the plastic encapsulant. 
     Due to the very good electrical and thermal conductivity required for the connection between the semiconductor chip and the leadframe, especially in the case of high-electrical-output components, a metal connection between the chip and the connector would be preferable over a glued connection. Particularly in the realm of high-output light-emitting diodes, a very good thermal connection is crucial in order to carry off the dissipated power brought in from the package and, where applicable, couple it to an external heat sink. Because of the above-cited potential for damage to the package, solder connections have heretofore not been used between the light-emitting diode (LED) chip and the leadframe in pre-injection-molded plastic encapsulants. 
     SUMMARY OF THE INVENTION 
     The object of the instant invention is to develop a method of the kind cited at the beginning hereof by which a semiconductor chip can be attached in a plastic encapsulant by means of a metallic connection between the chip and the connector, and the risk of impairing the functional capability of the plastic encapsulant is simultaneously reduced. A corresponding optoelectronic component and a method for the production thereof is further to be provided. 
     These objects are achieved by means of a method disclosed herein. 
     The use of very small amounts of a low-melting solder eliminates excessive heating of the metal parts contacted by the plastic encapsulant, which contains a thermoplast and/or a duroplast and is preferably made of a thermoplast. Especially preferred as thermoplasts are high-temperature thermoplasts such as PPA, polysulfone and LPC (liquid crystal polymer). The thermoplasts are preferably filled with titanium dioxide, glass fibers, mineral fillers and similar filler materials. A solder layer is preferably used that allows the soldering to be performed at temperatures of between 200 and 260° C. At the same time, the use of soft solder, as opposed to hard solder, largely prevents warping of the soldered chip, which can lead to chip damage. 
     The connector advantageously has a first and a second main surface. In a particularly advantageous variant of the method, the first and second main surfaces are enveloped by the plastic encapsulant, in which case the first and/or the second main surface border are directly adjacent the plastic encapsulant. 
     The low-melting soft solder used can be pure tin, an alloy whose principal component is tin, or alloys having a eutectic composition, such as AgSn, CuSn, PbSn or InPb or a mixture of these alloys. If an alloy is used as solder, it is advantageous to select the composition of the alloy so that it is to the greatest possible extent at the eutectic composition of the two- or multi-material system concerned, so that the melting point of the alloy is as low as possible. 
     It is advantageous if a thin layer is deposited locally on the points to be soldered, in a thickness of between 1 and 10 μm. It is particularly advantageous if a layer with a thickness of between 2 to 5 μm is deposited. This prevents wetting of the side surfaces of the LED chip on the side facing the leadframe, which in the case of flip-chip assembly (see below) would easily cause a short circuit of the active epitaxial layer sequence. 
     The thin layers of solder can preferably be deposited by conventional methods, such as vapor deposition, sputtering, etc. 
     If the solder is deposited on the leadframe, this preferably takes place before the leadframe is molded over with the plastic encapsulant. 
     It is particularly advantageous if thin layers of the individual components of an alloy are deposited repeatedly in alternation. Here again, the total thickness of the layer stack is advantageously between 1 and 10 μm or 2 and 5 μm. 
     The metals of the thin layers intermingle during the soldering process owing to the short diffusion paths to the respective adjacent layers. The rapid melting of the solder and thus the rapid soldering of chip and connector advantageously have the effect that the plastic encapsulant of the premolded package need be exposed to an elevated temperature for only a short period of time, and its functional capability is therefore is largely unimpaired. 
     Because of the short soldering time in combination with the low temperatures of the soldering process, it is therefore feasible to solder LED chips to leadframes in premolded packages, that is, to provide LED chips with metallic connections between the chip and the connector. 
     To prevent an oxide layer from forming on the solder or the to-be-soldered surfaces while the solder is being heated, it is advantageous, after depositing the solder on a surface for soldering, to apply a thin layer of gold to the solder and/or to the surfaces of the chip and leadframe that are to be soldered. 
     Low processing temperatures allow the chip to be soldered largely without warping. 
     The tiny volume of solder used makes it possible in particular to solder chips with the active epitaxial layer sequence facing the connector. This so-called “flip-chip” arrangement, also known as “face-down” or “top-down” assembly, is made feasible only by virtue of the very small volume of solder according to the technical teaching of the invention. 
     Wetting of the side walls of the chip also occurs in conventional gluing processes. This is why it is difficult in flip-chip assembly to mount a chip on a leadframe or another electrically conductive connector via a gluing process. 
     Especially with the use of conventional, silver-particle-filled conductive adhesives, there is also the problem that silver particles migrate relatively intensively in an electric field when moisture is present. This is the case in particular with infrared sensors and infrared radiators, which need high voltages to operate. To address this problem, in these cases gold particles are often used as conductive filler in conventional schemes. 
     In contradistinction to this, the use of soft solder is far more cost-effective. 
     Advantageous improvements and embodiments of the method according to the invention and the optoelectronic component are recited in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and preferred embodiments of the method of the invention and the optoelectronic component will emerge from the following embodiment examples described in greater detail in connection with  FIGS. 1 to 8 . 
       Therein: 
         FIG. 1  is a schematic diagram of an optoelectronic component comprising a chip that is mounted by the method of the invention, 
         FIG. 2  is a schematic diagram of an optoelectronic component comprising a chip that is flip-chip-mounted by the method of the invention, 
         FIG. 3  is a schematic diagram of an optoelectronic component comprising a chip that is flip-chip-mounted by a conventional gluing method, 
         FIG. 4  is a schematic diagram of a solder point prepared by a method according to the invention, 
         FIG. 5  is a flow chart illustrating the assembly sequence of a first embodiment example of the method according to the invention, 
         FIG. 6  is a flow chart illustrating the assembly sequence of a second embodiment example of the method according to the invention, 
         FIG. 7  is a flow chart illustrating the assembly sequence of a third embodiment example of the method according to the invention, 
         FIG. 8  is a flow chart illustrating the assembly sequence of a fourth embodiment example of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiment example depicted in  FIG. 1  comprises a leadframe  2  around which a plastic encapsulant  5  has been compression- or injection-molded and which comprises a first and a second main surface (premolded package). In a recess  6  in this plastic encapsulant  5  that leads from outside the base encapsulant to the leadframe  2 , an LED chip  9  is attached to a connector  12  of the metal leadframe  2  by means of a soft solder. The first and second main surfaces of the leadframe are covered with plastic. Thus, there is substantially no additional heat sink apart from the leadframe. 
     A metallization layer  22  deposited on plastic encapsulant  5  and comprising a connector  12  can alternatively be provided instead of the leadframe. As a further alternative, a heat sink embedded in the plastic encapsulant can serve as the connector. A solder layer  3  between LED chip  9  and connector  12  preferably has a thickness of between 1 and 10 μm. Especially preferably, the solder layer has a thickness of between 2 and 5 μm. The soft solder in this embodiment example is composed substantially of pure tin or of an alloy whose principal component is tin. The solder material for soft soldering can also, for example, be an alloy whose material system has a eutectic composition. Candidates for this purpose are AgSn, CuSn, PbSn or InPb or a mixture or layer sequence composed of at least two of these alloys. The plastic encapsulant contains a thermoplast, preferably a high-temperature thermoplast such as PPA. Polysulfone or LCPs (liquid crystal polymers) can also be used. Improved temperature resistance can be achieved by means of filler materials such as, for example, glass fibers, mineral fillers and TiO 2 . The plastic encapsulant can also, however, contain a duroplast. 
     The second embodiment example, depicted in  FIG. 2 , differs from the example just described in that the LED chip  9 , in a flip-chip assembly (see above), is attached to a connector by means of a soft solder. The connector in this case is a metallization layer  22  vapor-deposited on the base encapsulant  5 , but it can also, as in the first-cited case, be part of a leadframe. Due to the minuscule thickness of the solder, it is not pressed out from under the chip  9 . The risk of short-circuiting the epitaxial layer sequence is greatly reduced. 
     If solder layers  3  of very large thicknesses are applied, however, there is a risk, as illustrated in  FIG. 3 , that the solder will be pressed out from under the chip  9  placed on the leadframe  2 . This solder wets the connector and the side walls of the chip  9 . If the chip  9  is arranged as a flip chip with its active side  4  toward the leadframe  2 , the solder, forming a meniscus  13 , causes an electrical short circuit from connector  12  to chip substrate  1  of chip  9  at epitaxial layer sequence  4 . 
       FIG. 4  shows a particularly advantageous option for the arrangement of a solder layer ( 3 ) which during the soldering process forms an alloy whose material system has a eutectic composition. Thin layers ( 23 ,  33 ) of the individual components of the alloy are arranged alternatingly between the chip and the connector. Layers ( 23 ) are composed, for example, of tin and layers ( 33 ) of silver. 
     Due to the short diffusion paths to the respective adjacent layers, the individual metals intermingle during the soldering process and form an alloy. 
     The embodiment example illustrated schematically in  FIG. 5  concerns a mounting sequence for soft-soldering an LED chip in a premolded package as described exemplarily in the embodiment example of  FIG. 1 . A solder layer (for example having a thickness of 2 to 5 μm) composed substantially of tin or of an alloy whose principal component is tin is applied to a connector. The connector can, for example, be a leadframe, a heat sink embedded in the plastic encapsulant or a metallization layer deposited on a plastic encapsulant. In the case of the leadframe, a leadframe can in particular be embedded without the use of a heat sink, so that a first and a second main surface of the leadframe are covered with plastic. Flux is then applied to the solder and the chip is placed thereon. The premolded package then passes, preferably at 200 to 260° C., through a soldering furnace in which the solder connection is produced. Subsequently, the premolded package passes through a washing unit in which the residues, produced mainly by the flux, are rinsed away. 
     The flux is necessary to remove the oxide layer that forms on the solder and the chip in the through-type furnace and to keep an oxide layer from developing. To prevent the formation on the solder of an oxide layer that would sharply lower the quality of the solder connection, a gold film can also be deposited on the solder prior to the soldering operation. 
     As an alternative to the procedure described above, the solder layer can be deposited on the leadframe before the production of the plastic encapsulation. 
     The process according to the second embodiment example, which is illustrated schematically in  FIG. 6 , differs from the foregoing embodiment example essentially in that solder is used in the form of a solder paste. The solder paste can be applied to the connector alone or to the semiconductor chip alone or to both the connector and the semiconductor chip. An LED chip is placed at the connector site, which has been prepared with solder paste, inside a premolded package before the strip passes through the solder furnace. 
     To remove the chemicals such as flux that are present in the solder paste, a washing unit is also installed downstream of the soldering furnace in this embodiment example. 
     In the method according to the embodiment example of  FIG. 7 , the solder is applied to the back side of an LED chip by vapor deposition. The solder material used is an alloy whose material system has a eutectic composition. This alloy can be AgSn, CuSn, PbSn, InPb or a mixture of at least two of these alloys. A layer sequence of individual alloys or of individual metals forming these alloys can also be used. The chip is placed on a leadframe inside a premolded package. In contrast to the preceding embodiment example, here the leadframe and the chip are heated by means of a heating plate located under the leadframe in order to melt the soft solder. 
     Here the soldering is performed without the use of a flux. Consequently, there is no washing unit after the soldering furnace. The soldered semiconductor component  7  is thereby obtained largely without impurities in a particularly frugal manner. 
     In the method according to the embodiment example of  FIG. 8 , as in the previously described embodiment example, an LED chip with solder vapor-deposited thereon is placed on the leadframe inside a premolded package. After vapor deposition, the solder layer has a thickness of between 1 and 10 μm, preferably between 1 and 5 μm. The plastic encapsulation can be produced on the leadframe by injection molding or compression molding or another molding process for duroplasts or thermoplasts. In this embodiment example, flux is first deposited at the location on the leadframe that is to be soldered. The strip passes through a soldering furnace and then a washing unit. After the soldering operation, the thickness of the solder layer is between 0.1 and 10 μm, preferably between 0.1 and 5 μm. 
     In all the embodiment examples for the method of the invention, the LED chip can also be attached to the connector in a flip-chip assembly. The vapor deposition of solder on the LED chip then naturally has to be performed not on the back side of the chip, but on the active epitaxial layer sequence of the LED chip. 
     The connector can be enveloped by the plastic of the plastic encapsulant on two main surfaces, so that no further heat sink is present on the connector. 
     In addition to its use with radiation-emitting semiconductor chips, the method according to the invention is also, of course, applicable to the soldering of other semiconductor structures such as, in particular, infrared sensors or infrared radiators, as well as transistors.