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.
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.
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.
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.
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.
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.
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.
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:
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;
b) wetting of a second surface of the second part, to be connected with the first part, with a second solder;
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);
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;
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
The invention will be explained in greater detail below, using the description of exemplary embodiments in the drawing:
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.
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.
As is shown schematically in
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.
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.
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.
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
The processing device 300, which is shown schematically in
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
If the unit 30 is now introduced, together with the processing device 300, as shown in
According to
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.
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.
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.
Number | Date | Country | Kind |
---|---|---|---|
15001526.1 | May 2015 | EP | regional |