Abstract:
A diffusion-soldered joint and a method for making diffusion-soldered joints includes a particularly actively diffusing, low-melting-point intermediate layer, applied in the molten state, introduced in the form of a solder carrier between at least two joint components. The solder carrier includes a metal foil that is equipped on both sides with solder layers, wherein the solder layers may include multiple layers.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a diffusion-soldered joint and to a method for making diffusion-soldered joints. 
     BACKGROUND INFORMATION 
     Diffusion-soldering is based on the fact that two base materials present in solid phases can diffuse into one another even at a temperature well below the melting point of a joint component. A suitable solder is placed between the soldering surfaces of the base materials, and the joint components are pressed together and heated for a long period. The result is a vacuum-tight, nondetachable joint. It is also known, instead of a separate solder, to produce diffusion-promoting layers, partially or over the entire surface, on the base materials by galvanic deposition, metallization, or vacuum sputtering, or by plasma spraying. The base materials are then pressed together and heated. Layers of pure tin, pure indium, or pure bismuth are known as diffusion-promoting layers of this kind. Solder joints made in this fashion have, however, only limited mechanical and thermal strength. 
     A method for joining planar workpieces, in which the workpieces participate in a direct joint by means of soldering or diffusion joining under vacuum, is already known from German Unexamined Patent Application No. 44 12 792. For this purpose, the workpieces are held in a sealed vacuum chamber, a melting intermediate layer being applied between the workpieces being joined. The intermediate layers are comprised of solders known in the art, which are introduced in the form of foils, powders, or granules. 
     Conventional joining methods such as soft soldering, brazing, and welding are used to produce joints which disadvantageously either have insufficient strength or require excessive joining temperatures. Such methods are therefore unsuitable when requirements exist for high thermal and mechanical strength over large areas (e.g., wafers) and for the lowest possible joining temperature in order to retain the original strength of the base materials of the joint components while maintaining very close dimensional tolerances. 
     SUMMARY OF THE INVENTION 
     The diffusion-soldered joint according to the present invention has the advantage that at a comparatively low joining temperature, at which the original strength of the joined materials is maintained, high thermal and mechanical strength of the components being joined (joint components) is achieved. The strength attained in the diffusion-soldered joint according to the present invention is comparable to that of brazed joints; the joining temperature (e.g., 250° C. to 450° C.) is considerably lower than in the case of brazed joints, so that, for example, the work-hardening produced by rolling, and the spring properties of the toughened joined materials are retained. As a consequence, very close dimensional tolerances for the joint components can be maintained. 
     With the solder carrier introduced between two joint components, it is possible in a simple manner to apply particularly actively diffusing, low-melting-point intermediate layers, applied in the molten state, to specific areas of the components being joined. The solder carriers allow very reliable, uncomplicated, and precisely targeted application of the aforesaid intermediate layers. Particularly advantageous in this context is the possibility of also applying a solder carrier of this kind to soldered parts that cannot be immersed in solder. 
     It is particularly advantageous to solder-coat a thin metal foil in a solder bath, using a passthrough method, in order to obtain a suitable solder carrier. Low-melting-point soft-solder alloys are suitable in this context as solder layers. The low-melting-point coating alloys can also, advantageously, be applied as multilayers in the form of their respective individual constituents, galvanically or by vacuum metallization, layer thicknesses of 1 to 10 micrometers in each case being preferred. 
     The method according to the present invention for making diffusion-soldered joints has the advantage of being applicable very reliably and precisely to produce high-strength joints in the case of joint components having relatively large areas. When large-area components are being joined, for example onto a wafer, it is advantageous to arrange an expendable, plastically deformable, solder-rejecting, and thermally conductive insert foil between a ram of the diffusion-soldering tool and the joint components. The insert foil allows compensation for dimensional tolerances in the height of the individual parts being joined, and thus approximately uniform distribution of the ram pressure. Lastly, joints of approximately uniform strength over a large area are obtained in this fashion. The aluminum or chromium layers of the insert foils guarantee good thermal conduction and distribution, and solder rejection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention are illustrated in the drawings and explained in greater detail hereafter. 
     FIG. 1 depicts a two-part nozzle plate as defined by the method according to an embodiment of the present invention for making a diffusion-soldered joint. 
     FIG. 2 depicts the individual components of the nozzle plate according to an embodiment of the present invention, together with a solder carrier. 
     FIG. 3 depicts a schematic depiction of a tool for making diffusion-soldered joints according to an embodiment the present invention. 
     FIG. 4 is a schematic depiction of a tool for making diffusion-soldered joints on large-area joint components according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a two-part nozzle plate  1  in which an upper nozzle part  2  and a lower nozzle part  3 , as the two joint components, are immovably joined to one another according to an embodiment of the present invention by means of a diffusion-soldering method. Nozzle plate  1  depicted in FIG. 1 is only a selected example of how diffusion-soldered joints are made. Nozzle plate  1  is characterized, for example, by the presence of two extremely precisely shaped individual components, which must be joined with exact positional accuracy in order to form fine structures, for example a very narrow peripheral annular gap  5 . A nozzle plate  1  of this kind is particularly suitable for injection valves, atomization nozzles, painting nozzles, and other spraying devices, for example as described in German Patent No. 196 22 350.4. 
     Upper nozzle part  2  and lower nozzle part  3  are produced, for example, using the known MIGA (microstructuring, galvanoforming, shaping) or LIGA (lithography, galvanoforming, shaping) technique. Production can be accomplished on “wafers” having several hundred nozzle plates  1  arranged in a grid pattern, thereby greatly reducing the work required for each nozzle plate  1 . 
     Upper nozzle part  2  is comprised of, for example, two axially successive functional planes  10  and  11 . While the upper circular functional plane  10  contains a filter structure  12 , arranged for example in annular fashion with, for example, honeycomb-like pores and, except for the fine-pore filter structure  12 , is constituted entirely of material, the circular lower functional plane  11 , configured for example with a somewhat smaller outside diameter than upper functional plane  10 , possesses an inner material region (e.g., fitting element  15 ), an annular open region  16  adjacent radially to the outside, and an annular outer material region  17  which completely radially surrounds the open region  16 . 
     The central fitting element  15  provides for better joining of the two parts  2 ,  3  of nozzle plate  1 . When nozzle plate  1  is in the assembled state, lower nozzle part  3  largely fills up open region  16  of lower functional plane  11  of upper nozzle part  2 . Filter structure  12  opens, at a lower end surface  20  of upper functional plane  10 , into the annular channel-like open region  16 , which is thus delimited with respect to upper functional plane  10  and is open toward the bottom so that lower nozzle part  3  can be inserted into it and can constitute, for example, annular gap  5 . 
     Both upper nozzle part  2  and lower nozzle part  3  are made of, for example, NiCo, Ni, Fe, or Cu that has been galvanically deposited onto previously fabricated plastic negatives. The final dimensions of parts  2 ,  3  are achieved, for example, by grinding. The annular lower nozzle part  3  possesses a central inner through opening  22  into which fitting element  15  of upper nozzle part  2  can engage in dimensionally precise fashion. The outer dimensions of lower nozzle part  3  are defined by the size of open region  16  into which it is at least partially introduced. In addition to precise fabrication of through opening  22 , an upper contact shoulder  23 , projecting toward upper nozzle part  2  and having an upper end face  24 , must also be configured very precisely. A lower end face  25 , opposite contact shoulder  23 , of lower nozzle part  3  does not need to be precisely fashioned, since this region is not required during joining and lies outside the flow path. 
     The stepped outer contour of lower nozzle part  3  comprises, for example, a peripheral bevel  26  which is located in open region  16  when nozzle plate  1  is in the assembled state and allows improved flow of a fluid from filter structure  12  to the spray geometry (annular gap  5 ). Adjacent to bevel  26 , which widens in the direction of flow, is also, for example, a vertical delimiting surface  27  which is located downstream from annular gap  5  after assembly. 
     After production of the two individual parts  2  and  3 , the two parts  2  and  3  are assembled and joined into an annular-gap nozzle. The joining technique used for exact positionally accurate joining of parts  2  and  3  with the least possible negative mechanical and thermal effects is a diffusion-soldering method for making a diffusion-soldered joint according to the present invention. 
     FIGS. 2 and 3 explain in more detail how a diffusion-soldered joint can be made. The technology is particularly suitable for joint components that cannot be coated partially and in a melt in an immersion bath, for example upper nozzle part  2  and lower nozzle part  3  of nozzle plate  1  depicted in FIG.  1 . For this purpose, for example, the two soldered parts to be joined (parts  2  and  3 ) are pretreated by pickling. A disk-shaped solder carrier  30 , coated on both sides with molten solder, which in the exemplified embodiment depicted is annular in shape, is placed between the two parts  2  and  3 , which in order to the make the joint are, for example, manipulated upside down with respect to the later installation position of nozzle plate  1  as shown in FIG.  1 . 
     In the exemplified embodiment of nozzle plate  1 , solder carrier  30  rests with one of its end surfaces in open region  16  against end surface  20  of upper nozzle part  2 , while lower nozzle part  3 , with its end surface  24  of contact shoulder  23 , contacts solder carrier  30  at its opposite end surface. The sandwich component made up of lower nozzle part  3 , solder carrier  30 , and upper nozzle part  2  is then heated under pressure (arrow  34 ) between a schematically indicated heating table  32  and a heated pressing ram  33 , and diffusion-soldered. Heating (e.g., at 250° to 450° C.) between the two tool parts  32  and  33  takes place under a pressure of, for example, 100 N/mm 2  to 300 N/mm 2 . 
     The starting material used for solder carrier  30  is, for example, a foil  35  that is solder-coated on both sides in a solder bath using the passthrough method, the coating thicknesses usually being between approximately 1 and 10 micrometers. Materials particularly suitable for film  35  are copper, nickel, iron, or alloys of copper, nickel and iron. A plurality of solder carriers  30  as shown in FIG. 2 can be punched out of a large-area solder-coated film. Low-melting-point (e.g., &lt;400° c.) soft-solder alloys or multilayer arrangements are provided as solder layers  36 . In multilayer arrangements, the individual constituents are applied onto one another, for example, galvanically or by vacuum metallization, the individual layers being a few micrometers thick. Low-melting-point coating alloys for solder layers  36  are, for example, tin/indium, tin/lead/indium, bismuth/indium/tin/lead, tin/silver or tin/copper alloys. 
     FIG. 4 depicts an arrangement which illustrates how diffusion soldering according to the present invention functions in the case of relatively large-area wafers having a plurality of small parts (in this case nozzle plates  1 ). In this context, for example, lower nozzle parts  3  continuously constitute the initially one-piece wafer, while solder carriers  30  and upper nozzle parts  2  are placed as individual parts in the respective desired positions with respect to lower nozzle parts  3 . Since parts  2  and  3  being joined, as well as solder carrier  30 , may exhibit height tolerances in the many nozzle plates  1  being produced, and heating table  32  and pressing ram  33  have flat pressure surfaces, an insert foil  38  is arranged between pressing ram  33  and the workpiece (nozzle plate  1 ) being joined. 
     The insert foil  38  serves as a plastically deformable, solder-rejecting, thermally conductive consumable spacer made of anodized aluminum, or of copper plated with chromium or metallized with titanium nitride, to compensate for height differences. An approximately identical pressure over the entire wafer surface is achieved in this fashion. In order to ensure a uniform component height, the linear stroke of pressing ram  33  is limited, for example, by means of a fixed stop (not shown). Arrows  34  in pressing ram  33  in turn illustrate the direction in which a compressive force is applied to the workpieces (e.g., perpendicular to the wafer plane). The thickness of insert foil  38  is in the range between, for example, 50 micrometers and 1 mm. After diffusion soldering, the wafers are divided into the individual components (e.g., nozzle plates  1 ) by laser cutting, etching, punching, or similar separating methods. 
     As already mentioned, nozzle plate  1  represents only one selected exemplified embodiment in which a diffusion-soldered joint is used to assemble two joint components. A joint of this kind can also be achieved on components of completely different configuration that require joining; it is particularly suitable for joining complex micromechanical components.