Abstract:
A method for connecting a first part, composed of a difficult to solder material, with a second part. A wetting of a first surface of the first part, to be connected with the second part, with a first solder, and connecting the first solder with the first surface of the first part, takes place by introducing heat and ultrasound energy. A wetting of a second surface of the second part, to be connected with the first part, with a second solder takes place. Subsequently, machining of the surface of the first solder is carried out for removal of an oxide layer. Then the first and the second solder covered surfaces are brought into contact with one another, to form a unit. This is followed by exposing the unit to a temperature within a predetermined temperature range, which has an upper temperature limit of less than 800° C.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the European patent application No. 15001526.1 filed on May 21, 2015, the entire disclosures of which are incorporated herein by way of reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for connecting a first part composed of a material that is difficult to solder, particularly a ceramic, a glass ceramic or a glass, with a second part. 
         [0003]    The connection of parts, at least one of which comprises a ceramic material, particularly a monolithic ceramic, or of glass or of a glass ceramic, such as Zerodur, places great demands on the quality of the connection method. These materials represent materials that are difficult to solder. 
         [0004]    In general, adhesives are used to connect them. However, the use of adhesives brings with it the problem that the dimensional stability achieved by the bond produced by gluing is not sufficient for producing dimensionally stable structures for space flight applications. Adhesives furthermore demonstrate the disadvantage that they can outgas, and this is also undesirable in space flight applications. 
         [0005]    Furthermore, hard soldering methods are known for connecting ceramic materials, for example, that have similar or different material properties. In this regard, temperatures of more than 800° C. are generally required for the connection process. This can result in problems with regard to the thermal expansion coefficient between the solder material used and the ceramic of the connection partners, if the soldering process is not performed correctly. 
         [0006]    In order to be able to undertake connection of two parts by means of a soldering method that makes do with lower temperatures, in order to minimize problems on the basis of the effects of thermal expansion coefficients and to keep the stress on the parts to be connected as low as possible, what are called Cerasolzer solders (Cerasolzer, for short) are used. These have a high adhesion capacity to materials that are difficult to solder. The adhesion capacity of a solder connection with Cerasolzer depends, on the one hand, on the properties of the solder alloy. Cerasolzer solders contain small proportions of elements such as Zn, Ti, Si, Al, Be, Sb, and the rare earth metals, which have a good affinity for oxygen. These rare earth metals combine with oxygen during the connection process and form oxides that chemically bond with the surface of glass, ceramic, glass ceramics, etc. On the other hand, the introduction of heat is not enough to achieve wetting of the surface. Aside from heat, the additional introduction of strong ultrasound vibrations therefore takes place. However, the adhesion of the connection of two surfaces wetted with solder does not meet the criteria required for space flight demands. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the present invention to provide a functionally improved method for the connection of parts, at least one of which comprises a ceramic material or glass or a glass ceramic, which method puts as little structural stress as possible on the parts to be connected, and, at the same time, allows great dimensional stability of the structure produced. 
         [0008]    A method for connecting a first part composed of a material that is difficult to solder, particularly a ceramic, a glass ceramic or a glass, with a second part is proposed, wherein the following steps are performed: 
         [0009]    a) wetting of a first surface of the first part, to be connected with the second part, with a first solder, and connecting the first solder with the first surface of the first part by introducing heat and ultrasound energy; 
         [0010]    b) wetting of a second surface of the second part, to be connected with the first part, with a second solder; 
         [0011]    c) machining of the surface of the first solder, to be connected with the second solder, for removal of an oxide layer produced in Step a); optionally, machining of the surface of the second solder, to be connected with the first solder, can also be performed, in order to remove an oxide layer produced in Step b); 
         [0012]    d) producing a connection of the first surface of the first part, wetted with the first solder, and of the second surface of the second part, wetted with the second solder, by means of bringing the first and the second surface into contact with one another to form a unit; 
         [0013]    e) performing a temperature step in which the unit is exposed to a temperature within a predetermined temperature range, which has a lower temperature limit and an upper temperature limit, wherein the upper temperature limit is less than 800° C. 
         [0014]    A part composed of a material that is difficult to solder, particularly a ceramic, preferably a monolithic ceramic such as silicon carbide, silicon nitride or aluminum nitride, a glass ceramic such as Zerodur or a glass, may be processed, at least as the first part. Optionally, a part composed of one of the aforementioned materials that are difficult to solder may also be processed as a second part. 
         [0015]    A monolithic ceramic composed of silicon carbide (SiC), a chemical compound of silicon and carbon, which belongs to the group of carbides may be used, for example. Silicon carbide demonstrates great rigidity and hardness, as well as low thermal expansion. Silicon carbide furthermore demonstrates great chemical and thermal stability. The mechanical properties with regard to bending resistance and ductility hardly change with the temperature. Likewise, silicon nitride may be used as a monolithic ceramic. Silicon nitride is a chemical compound that comprises the elements silicon and nitrogen, with the formula Si3N4. It has great strength, in comparison with silicon carbide, and, for monolithic ceramics, great ductility, a low heat expansion coefficient, and a comparatively small modulus of elasticity, and is therefore particularly well suited for components subject to thermal shock stress. Likewise, Zerodur may be used as the material of the first part, and optionally of the second part. Zerodur is a glass ceramic material that is produced by means of controlled volume crystallization. Zerodur contains a crystalline phase and a residual glass phase, by means which phases an extremely low expansion coefficient, good material homogeneity, chemical resistance, and mechanical properties that vary only slightly are achieved. 
         [0016]    The present method for connecting the first and of the second part does not make use of the technology of hard soldering (called brazing), but instead uses the method of soft soldering, in which fewer stresses are introduced into the connection partners, i.e., the first and the second part, because of the significantly lower temperatures. 
         [0017]    These materials, which as such cannot be connected by means of soft soldering, or can only be connected with difficulty, become connectable by means of soldering, since at least in Step a) the production of the connection of the first solder with the first part takes place, by introducing heat and ultrasound energy, thereby making good adhesion between the first part and the first solder possible. In the event that the second part also comprises a material that is difficult to solder, production of the connection of the second solder with the second part takes place in corresponding manner, by introducing heat and ultrasound in Step b), as well. 
         [0018]    In this manner, the first and optionally the second surface of the first or second part, respectively, composed of ceramic or Zerodur, is tin-plated, so that in the subsequent temperature step, a solder connection of the two surfaces can occur. For the benefit of a planar or impurity-free solder connection between the first and the second solder, and for the benefit of it to have the required adhesion properties, removal of the oxide layers that form on their own during wetting of the first and optionally second solder with the respective first and second surface of the first and second part may take place. Machining of the respective surface of the first and/or second solder to remove the oxide layer produced in Step a) or b) may take place by means of grinding or milling, for example. Of course, other removal methods are also possible. 
         [0019]    An advantage of the method of procedure described comprises that hard soldering (brazing) connected with high temperatures of more than 800° C. can be avoided. As a result, lower thermal tensions are introduced into the region of the connection surface. For smaller parts and non-structural connections, particularly when using the connected part as a space flight component, a highly effective connection method can be made available in this way. In this regard, the connection structure, i.e., the unit formed from the first and the second part, demonstrates significantly better dimensional stability as compared with a connection using adhesives. 
         [0020]    According to a practical embodiment, a solder that contains components of one or more rare earth metals may be used as the first and/or as the second solder. For example, Cerasolzer, also called Cerasolzer solder, may be used as the first and/or as the second solder. Cerasolzer is known from the production of electronic components, in order to contact electrical materials or to contact glass or metallized glass types. Cerasolzer is a eutectic solder that is free of flux, corrosion-free, and can be processed at temperatures between 150° C. and 300° C. It has wetting properties that are suitable for glass, glass ceramics, and ceramics. 
         [0021]    As was described initially, it is advantageous to work with the lowest possible temperatures in the connection process of the first and second part. For this reason, it is advantageous if Step a) is carried out at temperatures of less than 300° C., particularly less than 260° C., and further preferably less than 200° C. This temperature range can be achieved by means of selection of a suitable solder. 
         [0022]    It is furthermore advantageous if in Step a) and optionally b), the first and optionally the second solder is melted on the first or second surface, respectively, and the melted solder is brought into adhesion with the material of the first and second part, respectively, for example, using an ultrasound solder gun, causing respective oxide layers to be removed from the first and second surface by means of ultrasound. What is called the “Ultrasonic Cavitation Phenomenon” is utilized for removal of the respective oxide layers, by means of which oxidized layers on the first and second surface may be removed in a simple manner and, at the same time, the surfaces are cleaned. Micro-vibrations are produced during this process, by means of an intensive ultrasound bundle, which vibrations have a brushing effect that makes complete removal of the oxide layer possible for direct wetting with the solder. This results in the advantage that no kind of flux is required when the solder is applied to the first or second surface. As a result, in combination with the tin plating described above, soft soldering of aluminum, glass, ceramics, metals that are difficult to solder (such as stainless steel, titanium, metal oxides) is made possible in simple manner 
         [0023]    It may furthermore be advantageous if previous heating of the first part or of the second part takes place before the step of introducing heat and ultrasound energy for connection of the first solder with the first part and optionally of the second solder with the second part. This may be implemented, for example, by means of a temperature-adjustable heating plate. 
         [0024]    For producing the connection in Step d), it is advantageous if the first and the second part are oriented plane-parallel with reference to their first and second surface, before the first and the second surface are pressed against one another with a force in a predetermined force range, particularly 0.05 N/mm2 to 0.5 N/mm2. This may take place using a processing device, for example. Furthermore, it is possible to apply a uniform force to the first and the second surface, over their entire contact surface, using the processing device. In this way, the reliability of the mechanical connection between the first and the second part can be optimized. 
         [0025]    It is furthermore advantageous if a ductile material is introduced between the first and the second surface as a spacer, by means of which a predetermined distance, particularly between 0.1 mm and 0.3 mm, between the first and the second surface is produced after solidification of the first and the second solder. In this way, the strength and plane-parallelity of the connection can be optimized. 
         [0026]    According to a first embodiment variant, in Step e) the step of vapor phase soldering (sometimes also called condensation soldering method) may be carried out. For this purpose, the unit composed of the first and second part connected with one another may be introduced into a vapor phase soldering apparatus, which utilizes the condensation heat released during the phase change of a heat transfer medium from the gaseous to the liquid state for heating the unit. In this regard, condensation takes place at the surface of the unit, until the entire unit has reached the temperature of the vapor. When the liquid (the heat transfer medium) boils, a saturated, chemically inert vapor zone forms above it, the temperature of which is identical, to a great extent, to the boiling point of the liquid, so that an optimal inert atmosphere is formed and oxidations during the vapor phase soldering process are excluded. Perfluoropolyether (PFPE), for example, may be used as the heat transfer medium. The heat transfer in a vapor phase soldering apparatus is fast and independent of geometry. In particular, no cold zones occur in the shadow of larger components. No overheating of the components is possible because of the precisely defined soldering temperature and the uniform heating. 
         [0027]    According to another embodiment variant, the first and the second surface may be locally exposed to the temperature in Step e). For this purpose, a reactive auxiliary layer, which experiences a self-maintaining exothermic reaction after controlled activation, may be introduced between the first and the second surface in a Step c1) that is carried out after Step c) and before Step d), so that in Step d), the first and the second surface are connected with the auxiliary layer. 
         [0028]    In Step e), activation of the auxiliary layer by means of the introduction of energy may then take place, thereby causing materials of the auxiliary layer to react chemically and, on the basis of their reaction, to generate thermal energy for melting the first and the second solder. Because the thermal energy is generated by the auxiliary layer, merely local heating of the first and of the second solder then takes place, thereby guaranteeing a particularly gentle connection process with regard to the introduction of temperature. Activation of the auxiliary layer by means of the introduction of energy may take place optically, electrically or thermally. 
         [0029]    The auxiliary layer may comprise aluminum as a first material and nickel as a second material. Such an auxiliary layer is known under the name NanoFoil, for example. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The invention will be explained in greater detail below, using the description of exemplary embodiments in the drawing: 
           [0031]      FIG. 1  shows a schematic representation of two parts disposed one on top of the other, on the surfaces of which a solder has been applied by means of a soft soldering method, in preparation, which surfaces are to be connected, 
           [0032]      FIG. 2  shows a processing apparatus by means of which a connection of the tin-plated parts to be connected is made possible, 
           [0033]      FIG. 3  shows a schematic representation of a vapor phase soldering apparatus, and 
           [0034]      FIGS. 4 a -4 d    show consecutive processing steps for the production of a connection of parts composed of a material that cannot be soldered, by means of soft soldering. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]      FIG. 1  shows a schematic representation of two parts  10 ,  20  that are to be connected with one another. For the sake of simplicity, the first and the second part  10 ,  20  are structured as flat elements. It is understood, however, that the method described here can also be used for shapes that are configured to be more complex. 
         [0036]    Preferably, the method described below is used for the production of space flight components that are subject to great demands with regard to dimensional stability and permanent quality of the connection. For the reasons stated, the first and the second part generally comprise ceramic or glass ceramic materials, such as, for example, monolithic ceramics or Zerodur® ceramic. Fundamentally, however, the method is suitable for connection even if only one of the two parts comprises a ceramic material. A monolithic ceramic or Zerodur ceramic has the property of being inherently difficult to process by means of soft soldering methods. But since the soft soldering method can be carried out at significantly lower temperatures, as compared with hard soldering, and thereby at lower stresses for the connection partners, the method described below was developed, with which parts comprising ceramic can be connected by means of a soft soldering method. 
         [0037]    Silicon carbide (SiC) or silicon nitrides (Si3N4), for example, are used as ceramics for the first and/or the second part. The first and/or the second part  10 ,  20  may alternatively comprise Zerodur ceramic. 
         [0038]    As is shown schematically in  FIG. 1 , the first and the second part  10 ,  20  are supposed to be connected with one another in the region of a first surface  11  of the first part  10  and a second surface  21  of the second part  20 . To carry out a soft soldering process, tin plating of the first and of the second surface  11 ,  21  takes place. For this purpose, a first solder  12  is applied to the first surface  11 , and a second solder  22 , having a proportion of rare earth metals, for example Cerasolzer, is applied to the second surface; they are melted and connected with the first surface  11  and the second surface  21 , respectively, by means of the introduction of ultrasound energy. Melting of the first and second solder  12 ,  22  takes place at temperatures between 150° C. and 300° C., depending on the solder selected. It is advantageous if the solder is selected in such a manner that melting is possible at a temperature of less than 300° C., particularly less than 260° C., and further preferably less than 200° C., with the temperature being determined by the solder being used. 
         [0039]    During application and melting of the solder  12 ,  22  onto the first and second surface  11 ,  21 , removal of any oxide layers on the first or second surface  11 ,  21  takes place, in that micro-vibrations are produced by means of an intensive ultrasound bundle and a brushing effect is achieved. After removal of the oxide layer, the solder  12 ,  22  can connect with the first or second surface  11 ,  21  of the first or second part  10 ,  20 . Also on this basis, the use of an additional flux is not necessary. 
         [0040]    In order for the solder connection between the first and the second solder  12 ,  22  to have the required adhesion properties, removal of oxide layers that form during wetting of the first and second solder  12 ,  22  with the respective first and second surface  11 ,  21  of the first and second part  10 ,  20 , on the surfaces of the first and second solder  12 ,  22  themselves takes place. Machining of the respective surface of the first and/or second solder to remove the oxide layers may take place, particularly after the first and/or second solder has solidified, by means of grinding or milling, for example, thereby creating a plane-parallel surface for the subsequent connection process of the solders  12 ,  22 , at the same time. 
         [0041]    In order to ensure a uniform distance of the parts  10 ,  20 , after connection and cooling, between 0.1 mm and 0.3 mm, a ductile material may be introduced between the first and the second part  10 ,  20  as a spacer. In this way, a plane-parallel connection between the parts  10 ,  20  during the soldering process, in particular, is ensured, and this ensures a planar connection, free of impurities. 
         [0042]    Subsequently, the production of a connection in the region of the first and the second surface  11 ,  21  takes place. For this purpose, a processing device  300  as shown schematically in  FIG. 3  is preferably used, so that a plane-parallel connection between the first and the second part  10 ,  20  and a uniform pressure application over the entire connection are made possible. 
         [0043]    The processing device  300 , which is shown schematically in  FIG. 2 , comprises a base plate  302  that is connected with a cover plate  304  disposed in plane-parallel manner, by way of two or more connection supports  306 . The first part  10  is laid onto the base plate  302  with the side not to be connected (i.e., the side that lies opposite the first surface). As a result, the first surface  11  to be connected faces in the direction of the cover plate  304 . The second part  20  is laid onto the latter, with its second surface  21  facing in the direction of the first part  10 . A pressure plate  310 , which, for practical purposes, has at least one surface that corresponds to the surface to be connected, borders on the back side of the second part  20 . The pressure plate  310  is mechanically connected with a shaft  308  that projects through the cover plate  304  and can be displaced in the axial direction. A spring element  312  is disposed between the pressure plate  310  and the cover plate  304 . In this regard, the spring element  312  presses the pressure plate  310  uniformly against the second part  20 , so that a uniform force is achieved in the region of the first and second surface  11 ,  21  to be connected. A locking element  314  that surrounds the shaft  308  makes it possible to remove the pressure plate  310  from the second plate  20  by pulling on an engagement element  316 , and thereby to hold the shaft  308  counter to the spring force, so that no force acting in the direction of the base plate  302  can be exerted on the unit  30  by the pressure plate  310 . As a result, the unit  30  can be removed from the processing device and new parts  10 ,  20  can be laid into the processing device. 
         [0044]    The unit prepared in this manner, which is subsequently provided with the reference symbol  30 , can be introduced into a vapor phase soldering apparatus  100 , as shown in  FIG. 3 , together with the processing device  300 . After having passed through the vapor phase soldering apparatus  100 , the first and the second solder layer  12 ,  22  are connected with one another with material fit. 
         [0045]      FIG. 3  shows a fundamentally known vapor phase soldering apparatus  100 , which is used to subject a unit  30  prepared in the processing device  300  to a temperature step for carrying out the soft soldering process. The vapor phase soldering apparatus  100  comprises a container  102 , for example composed of a non-rusting stainless steel. A chemically inert liquid  106  is in the container  102  as a heat transfer medium. In this regard, the chemically inert liquid  106  takes up a first region  108  in the height direction of the container  102 . The chemically inert liquid  106  is brought to a boil by a heating element  104 , which is completely surrounded by the chemically inert liquid  106 . As a result, a second region of a primary vapor layer, indicated with  110 , forms. Lying above that is a third region  112 , which forms a secondary vapor layer. Perfluoropolyether (PFPE) can be used as the heat transfer medium. The vapor phase soldering apparatus  100  utilizes the condensation heat released during the phase change of the heat transfer medium  106  from the gaseous to the liquid state to heat the unit  30 , which is still disposed in the processing device  300 . In this regard, condensation takes place at the surface of the unit  30  until the entire unit has reached the temperature of the vapor. 
         [0046]    If the unit  30  is now introduced, together with the processing device  300 , as shown in  FIG. 2 , into the primary vapor layer of the second region  110 , by way of the secondary vapor layer of the third region  112 , for a predetermined period of time, during which the heat transfer medium (the chemically inert liquid  106 ) is present in the gaseous state, the temperature of which is essentially identical to the boiling point of the chemically inert liquid  106 , then fast and geometry-independent heat transfer to the unit  30  takes place. As a result, the unit  30  and the solder layer  12 ,  22  have a precisely defined soldering temperature applied to them, which depends on the liquid or the selected heat transfer medium. At the same time, it is ensured that the unit  30  heats up uniformly and no overheating of the components takes place. At the same time, as long as the unit  30  is situated in the second region  110 , an optimal protective atmosphere has formed, so that oxidations in the vapor phase soldering process can be excluded. Furthermore, the demand for preheating zones is lower. After the unit  30  has been removed from the container, the solder layers have been connected with one another. Subsequently, the unit  30  can also be removed from the processing device  300 . 
         [0047]      FIGS. 4 a -4 d    show an alternative embodiment, in which heating of the solder layers  12 ,  22  is implemented using an auxiliary layer  40  disposed between the solder layers  12 ,  22 . A reactive multi-layer foil, such as one called NanoFoil®, composed of a plurality of aluminum and nickel layers, for example, can be used as an auxiliary layer  40 .  FIG. 4 a    shows the sequence in which the first part  10 , the first solder  12 , the auxiliary layer  40 , the second solder  22 , and the second part  20  are disposed one on top of the other and connected. According to  FIG. 4 b   , as has already been described, the first solder  12  is first applied to the first surface  11  of the first part  10  by means of the introduction of ultrasound. In a corresponding manner, the second solder  22  is applied to the second surface  21  of the second part  20  by means of the introduction of heat and ultrasound. The surfaces of the first solder layer  12  and of the second solder layer  22 , which are connected with the layer sequence  40 , are furthermore made flat, smooth, and clean by means of a suitable processing process, thereby causing the undesirable oxide layers  12 ,  22  to be removed. The first and second parts  10 ,  20  prepared in this manner are disposed to lie opposite one another, with their solder layers  12 ,  22  facing one another. The auxiliary layer  40  is provided between them. 
         [0048]    According to  FIG. 4 c   , the layer sequence prepared in this manner has a force F applied to it. The force F that is exerted should be selected in such a manner that the melted solder  12 ,  22  flows and sufficiently wets the component surfaces. The force preferably lies in a force range between 0.05 N/mm2 and 0.5 N/mm2. After this step, which ensures uniform wetting of the surfaces  11 ,  21 , activation of the layer sequence  40  takes place by means of the introduction of optical, electrical or thermal energy, thereby causing the layer sequence  40  to react chemically and to generate thermal energy for melting the first and second solder  12 ,  22 , on the basis of its exothermic reaction. The heating process and the cooling take place so rapidly, in this connection, that only part of the solder thickness is melted, and when the solder layers formed by removal of the oxide layers have a uniform thickness, the final distance between the parts  10 ,  20  also comes out to be uniform. 
         [0049]    As has been described, a reactive multi-layer foil, such as one called NanoFoil, may be used as an auxiliary layer, for example; this comprises thousands of what are called nano-layers composed of aluminum and nickel, which react exothermically after the reaction has been started with an energy pulse. The thermal reaction that has been triggered after activation serves as a fast and controllable local heat source, which melts the adjacent solder layers  12 ,  22  and thereby produces a connection of the components. This process is known under the name NanoBond®. In this regard, heat generation occurs so quickly that only the solder layers  12 ,  22  that border on the auxiliary layer  40  experience the introduction of heat. 
         [0050]    A connection produced in this manner demonstrates great reliability. In particular, great dimensional stability exists, so that the method is particularly well suited for the production of space flight components. 
         [0051]    While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority. 
       REFERENCE SYMBOL LIST 
       [0000]    
       
           10  first part 
           11  first surface 
           12  first solder 
           20  second part 
           21  second surface 
           22  second solder 
           30  unit composed of first part and second part 
           40  auxiliary layer 
           42  activation energy 
           44  activated auxiliary layer 
           100  vapor phase soldering apparatus 
           102  container (composed of stainless steel) 
           104  heating element 
           106  chemically inert liquid 
           108  first region with boiling inert liquid 
           110  second region (primary vapor layer) 
           112  third region (secondary vapor layer) 
           300  processing device 
           302  base plate 
           304  cover plate 
           306  connection supports 
           308  shaft 
           310  pressure plate 
           312  spring 
           314  locking device 
           316  engagement element