Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1999-12-08
2001-03-27
Beck, Shrive (Department: 1762)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C427S229000, C427S282000, C427S287000, C427S383700
Reexamination Certificate
active
06207300
ABSTRACT:
This invention relates to a solder paste for the production of geometric metal structures having precise contours on metallic substrates by solder coating.
Solder pastes have long been used in the prior art for providing a material bond between various materials. Depending upon the base material and particular application, various solder alloys in powder form are bound for this purpose with an organic binder as the matrix material and applied onto the workpieces to be joined in the form of a pasty composition. Application may, for example, be performed here, depending upon the requirement for uniformity and thickness of the layer, by dispensing, screen printing, dipping or spraying. On subsequent heat treatment, the organic binder is completely combusted and, once the solder forms a liquid melt phase, it creates a material bond between the workpieces. Wetting of the base materials by the liquid solder may be promoted by the addition of fluxes to the solder paste.
The same method may be used for providing a materially bonded surface coating on an individual workpiece surface. A uniform layer of solder may be produced on the base material by so-called solder coating. This technique is preferably used if the base material is to be protected from harmful environmental effects, such as when tinning copper, steel or brass sheet to provide corrosion protection, when coating turbine blades to prevent abrasive wear. The solder used for coating may here be applied either in the molten state (for example solder baths, drag soldering, flame spraying etc.) or be applied in the form of a suspension or solder paste. DE 43 15 475 A1 describes a solder suspension for applying thin solder layers on substrates. In this case, solder powders having a specific grain size distribution are suspended in an organic binder, applied onto the surfaces to be coated by dipping, spraying or brushing, and then soldered to the substrate. However, it is only possible to produce structures which are coarse or of a large area by dipping, brushing or spraying. Using known solder pastes, the viscosity and rheological properties of which have been specially adjusted, it is possible to apply micrometre magnitude structures having sharp contours for example by screen printing. However, during the subsequent fusion process, the contour, edge steepness and edge definition of the printed solder layer are lost, as, due to the wetting process, the liquid solder spreads out irreversibly on the surface of the base material. The extent of solder spreading is determined by the wettability of the base material and the surface tension of the molten solder. This behaviour is observed to a greater or lesser extent with all combinations of solder and base material. The flow behaviour of the solder once the liquidus temperature has been reached is especially undesirable when the screen printed solder layers with precise contours ought at most to shrink normal to the material surface during the soldering process, i.e. the reduction in the volume of the paste determined by combustion of the binder and the sintering and fusion process may only be discernible from a reduction in layer thickness, and spreading of the molten solder on the substrate beyond the predetermined limits of the structure must be excluded. One typical application is, for example, the application of spatially delimited spacers or thickened portions of a precisely defined height, which are materially bonded with metallic substrates. Such spacers or thickened portions produced by solder coating may find industrial application in the production of multi-ply steel seals. For this application, spatially delimited elevated portions having precise contours must be applied to the metal plies, ensuring reliable sealing in the multi-ply steel seal. DE 44 40 503 C1 and DE 195 28 031 describe single or multi-ply flat seals, which have spatially defined thickened rings to provide the sealing action, which rings are to be produced by sintering or fusion of solder material. These patents do not, however, reveal the composition of any screen-printable solder pastes usable for this purpose, nor the criteria which must be fulfilled in order permit the production of defined structures with precise contours using these pastes.
The object of the present invention was accordingly to provide a solder paste for the production of metallic structures with precise contours, which paste may be applied with sharp contours by screen printing onto a metallic substrate and precisely retains the two-dimensional extent of the predefined structure during the soldering process. Apart from this essential requirement of the solder paste, surface roughness, layer thickness and planarity of the resultant structures are of great significance for certain applications. This is especially the case if these layers must perform a sealing function, as in the case of multiply steel seals. In this case, the average surface roughness after the soldering process at a layer thickness of 25 to 250 &mgr;m must not exceed a limit value of 15 &mgr;m roughness height (mean roughness height R
z
to DIN 4768). The soldered structures must additionally retain stable contours up to maximum temperatures of 300° C., even under load and mechanical stress, which may only be achieved if the solder paste according to the invention leaves no pores or other deformable fractions in the layer after the soldering operation.
It has now been found that these requirements are fulfilled by a screen-printable solder paste which, in an organic binder system, contains 80 to 95 wt. % of a mixture of a nickel-based solder and a pulverulent alloy of nickel with one of the elements chromium, molybdenum, tungsten, manganese or iron as a higher-melting metallic filler, and which is characterised in that the weight ratio of solder to filler is 2-6:1, the average grain size of the solder is between 10 and 50 &mgr;m and the grain size ratio, relative to the average grain size, of solder to filler is 0.5-2.5:1.
The present invention accordingly provides a solder paste as characterised above for the production of geometric metal structures with precise contours.
The present invention furthermore provides the use of such a solder paste for the production of geometric metal structures with precise contours on metallic substrates by solder coating.
The present invention furthermore provides a process for the production of geometric metal structures with precise contours on metallic substrates by solder coating, wherein structures of such a solder paste are applied by screen printing onto the substrate, the structures are dried and the organic binder system present in the solder paste is decomposed without leaving residues in a subsequent heat treatment and the temperature is then raised until the solder material liquefies.
The solder paste according to the invention contains a pulverulent solder and a higher-melting pulverulent metallic filler in an organic binder system. The solder is a pulverulent, nickel-based solder having an average grain size of between 10 and 50 &mgr;m, preferably between 25 and 45 &mgr;m. Any nickel-based solders which melt within a range between 850 and 1100° C. are in principle suitable. Nickel-based solders are known to the person skilled in the art, who may straightforwardly select them in accordance with the requirements for the purpose according to the invention. The metallic filler is a pulverulent alloy of nickel with one of the elements chromium, molybdenum, tungsten, manganese or iron. According to the invention, the weight ratio of solder to filler is 2-6:1 and the grain size ratio, relative to the average grain size, is 0.5-2.5:1. The weight ratio is preferably 3.5-4.5:1 and the grain size ratio 1.0-2.0:1. The particles of the solder and filler powder should be largely of a spherical shape. Such metal powders with a spherical particle shape and a defined grain size may be produced using per se known processes, for example by spraying of suitable alloy melts and subsequent screening or classification. The filler alloys
Koch Jurgen
Koschlig Manfred
Krappitz Harald
Lonne Klaus
Schmitt Klaus
Beck Shrive
Crockford Kristen A.
Federal-Mogul Sealing Systems GmbH
Smith , Gambrell & Russell, LLP
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