Patent Application
20200051719
The invention concerns a method for producing a layer structure on a substrate using a paste based on a resistance alloy, as well as the resulting layer structure and its use.
Especially for the production of precision resistors alloys with a low temperature coefficient of electrical resistance (TCR) are used. Such alloys with a low TCR value are called resistance alloys within the scope of the invention. A typical resistance alloy with a low TCR value is e.g. ISOTAN® (also known as CuNi44, material no. 2.0842). To produce precision resistors, the alloy layers are applied to a substrate with a surface of a glass or ceramic material. Resistance alloys in the form of foils or sheets are usually bonded by roll cladding or lamination to substrate materials commonly used in electrical engineering. There is a need to apply resistance alloys as pastes to substrate materials using simple printing techniques, in particular screen printing or stencil printing, as this enables more flexible layer geometries. For this purpose it is necessary to provide resistance alloys in the form of printable pastes which can be burned in after application to the substrate. Such pastes consist at least of a powder of the resistance alloy concerned and an organic medium. During burning, the components of the organic medium volatilize and the molten or sintered powder of the resistance alloy remains. A wide range of organic media is available in which powders of these resistance alloys can be formulated and which basically guarantee printability. However, it has turned out that pastes consisting only of resistance alloy powder and organic medium show only low adhesion on the ceramic substrates used after burning. An improved adhesion of printed resistance alloys on glass or ceramic surfaces can basically be achieved by adding a glass frit to a resistance alloy paste. Layer structures consisting of a ceramic substrate and a glassy resistance alloy paste, or the resulting layer structures after burning, are state of the art. EP 0 829 886 A2, for example, teaches a resistance alloy paste containing glass frit, which is applied to an Al2O3 substrate. However, if a glass frit is added to the resistance alloy paste, this has the disadvantage that the TCR value of the layer formed after burning can differ from the TCR value of the bulk resistance alloy, so that the advantageous electrical properties of the resistance alloy cannot be exploited in the composite formed in this way.
The task underlying this invention is to provide a method for the production of resistance alloy layers on glass or ceramic surfaces by which resistance alloys can be applied by printing a paste and allow strong adhesion of the resistance alloys to the ceramic substrate without affecting the electrical properties of the resistance alloys in the layer structure produced. Furthermore, the task is to provide a layer structure in which the resistance alloy is mechanically stably bonded to the glass or ceramic surface of a substrate after burning.
These tasks are solved by a method for producing a layer structure comprising the successive steps:
The person skilled in the art knows from the previous formulation that the order of the steps must be adhered to, although it cannot be ruled out that further steps can optionally be carried out between the mentioned steps as long as the order is not changed.
It was found that the method according to the invention can be used to produce a layer structure with improved mechanical stability, in particular better long-term stability, without essentially altering the TCR of the resistance alloy.
Surprisingly, it was found that particularly good layer structures can be produced if a paste A is applied to the glass or ceramic surface of a substrate before the paste B is applied and, at the same time, the proportion by weight of glass frit in paste B is adjusted so that the paste B does not contain more than 15% by weight.
In step a), a substrate with a glass or ceramic surface is provided. The substrate thus has a surface comprising a ceramic or a glass, wherein the ceramic material of the surface may preferably be selected from the group consisting of oxide ceramics, nitride ceramics and carbide ceramics. Examples of suitable ceramics are forsterite, mullite, steatite, aluminium oxide, aluminium nitride, silicon carbide and hard porcelain. In particular, the ceramic surface contains aluminium oxide or consists of aluminium oxide. The glass of the glass surface is preferably a silicate glass.
In step b), a paste A is applied to at least part of the glass or ceramic surface of the substrate. It can be applied by screen printing, stencil printing, doctoring or spraying. A layer of paste A is obtained by the application. Paste A contains at least one glass frit and one organic medium or consists of at least one glass frit and one organic medium. Paste A preferably contains 50-90% by weight glass frit and 10-50% by weight organic medium, based on the total weight of Paste A.
The glass frit of paste A contains at least two different elements as oxides. These elements may be selected from the group consisting of Li, Na, K, Ca, Mg, Sr, Ba, B, Al, Si, Sn, Pb, P, Sb, Bi, Te, La, Ti, Zr, V, Nb, Mn, Fe, Co, Ni, Cu, Ag, Zn, and Cd. The glass frit can be made of oxides, fluorides or other salts (e.g. carbonates, nitrates, phosphates) of these elements. Examples of starting compounds for glass frit production may be selected from the group consisting of B2O3, H3BO3, Al2O3, SiO2, PbO, P2O5, Pb3O4, PbF2, MgO, MgCO3, CaO, CaCO3, SrO, SrCO3, BaO, BaCO3, Ba(NO3)2, Na2B4O7, ZnO, ZnF2, Bi2O3, Li2O, Li2CO3, Na2O, NaCO3, NaF, K2O, K2CO3, KF, TiO2, Nb2Os, Fe2O3, ZrO2 CuO, Cu2O, MnO, MnO2, Mn3O4, CdO, SnO2, TeO2, Sb2O3, Co3O4, Co2O3, CoO, La2O3, Ag2O, NiO, V2Os, Li3PO4, Na3PO4, K3PO4, Ca3(PO4)2, Mg3(PO4)2, Sr3(PO4)2, Ba3(PO4)2 and complex minerals, such as e.g. colemanite and dolomite.
The transformation temperature Tg of the glass frit of the paste A is in the range of 600-750° C., particularly in the range of 690-740° C. The transformation temperature Tg can be determined for the purpose of the invention according to DIN ISO 7884-8:1998-02.
The glass frit contained in paste A preferably comprises silicon, aluminium, boron and at least one alkaline earth metal as oxide. The alkaline earth metal calcium is particularly preferred.
In order to achieve a particularly good adhesion, the glass frit can be produced in a preferred embodiment from:
a. 25-55 weight percent silicon oxide,
b. 20-45 weight percent calcium carbonate,
c. 10-30 weight percent of aluminium oxide; and
d. 1-10 weight percent boron oxide.
The organic medium may contain at least one organic solvent and at least one binder. The organic solvent may be selected from the group consisting of texanol, terpineol and other high boiling organic solvents having a boiling point of at least 140° C. The binder can be selected from acrylate resins, ethyl celluloses and other polymers such as butyrals. Optionally the organic medium of the paste A can contain further components, which can be selected from the group consisting of thixotropic agents, stabilizers and emulsifiers. The addition of these components can, for example, improve the printability or storage stability of pastes.
In step c), a drying step is carried out and, if necessary, the layer of paste A is burned. Drying can take place at temperatures in the range of 20-180° C., particularly in the range of 120-180° C., e.g. in a drying cabinet. By drying, the layer of paste A can be fixed on the substrate. The dried layer of paste A can already be so mechanically robust that a layer of paste B can be applied directly.
The layer of paste A can optionally be burned after drying. The burning can be carried out at temperatures in the range of 750-950° C. The layer of paste A is preferably burned in such a way that the organic medium is essentially removed and the glass frit is sintered together as homogeneously as possible. The burned layer of paste A contains at least one glass or consists of one glass. The burned layer of paste A can also be called layer A. Burning can take place either under atmospheric conditions or under inert gas conditions (e.g. N2 atmosphere). In a preferred embodiment of the invention, the layer of paste A is first dried in step c) and then burned. If the layer of paste A in step c) is already burned, it may be better to apply paste B in the following step d).
In step d), paste B is applied to at least a part of the layer from step c. while retaining a layer of paste B. The paste B is then applied to at least a part of the layer from step c. The paste B of this invention contains at least one resistance alloy powder and one organic medium. Optionally, paste B may also contain a glass frit. However, it may also be preferred that paste B does not contain glass frit. A glass-free paste B can have the advantage that the electrical properties of the resistance alloy, in particular the TCR value, are not negatively influenced by the presence of glass.
In order to further improve the adhesion of layer B to layer A in the finished layer structure, it may also be preferable for paste B to contain a glass frit. However, paste B does not contain more than 15 weight percent, preferably not more than 12 weight percent glass frit, based on the total weight of paste B. As can be seen in Table 5, a glass frit in paste B can improve the adhesion of the layer structure during frequent temperature changes (T-shock storage). Paste B preferably contains at least 3 percent by weight glass frit, in particular at least 5 percent by weight based on the total weight of paste B. Preferably, paste B may contain glass frit in an amount of 3-15 weight percent, more preferred glass frit in an amount of 5-12 weight percent, based on the total weight of Paste B. The content of resistance alloy in paste B may preferably be in the range of 60-98 percent by weight and the content of organic medium may be in the range of 2-40 percent by weight, in particular in the range of 2-37 percent by weight, based on the total weight of paste B in each case.
The resistance alloys used for the powder have a temperature coefficient of electrical resistance of less than 150 ppm/K, preferably less than 100 ppm/K and particularly preferred less than 50 ppm/K. The temperature coefficient of electrical resistance indicated in the invention refers to the measurement of the bulk alloy and can be determined in the invention on a wire or foil of the corresponding alloy in accordance with DIN EN 60115-1:2016-03 (with drying method I).
For example, the resistance alloy may contain elements selected from the group consisting of chromium, aluminium, silicon, manganese, iron, nickel and copper. The resistance alloy may preferably be selected from the group consisting of CuNi, CuNiMn, CuSnMn and NiCuAISiMnFe. In a particularly preferred embodiment, the resistance alloy can be selected from the group consisting of the alloys:
or
The powder of the resistance alloy can be produced by methods known to the person skilled in the art, such as gas nozzles under inert gas, water nozzles or grinding. The mean particle diameter d50 of the powder of the resistance alloy is preferably 0.2 μm-15 μm.
In addition to the powder of the resistance alloy, paste B contains an organic medium. In a preferred embodiment, paste B contains an organic medium in an amount of 2-40% by weight. The organic medium of paste B may contain at least one organic solvent and at least one binder. The organic solvent may be selected from the group consisting of texanol, terpineol, isotridecyl alcohol or other high-boiling organic solvents having a boiling point of at least 140° C. The binder may be selected from acrylate resins, ethyl celluloses or other polymers. Optionally, the organic medium of the paste B may contain further components which may be selected from the group consisting of thixotropic agents, stabilizers and emulsifiers. By adding these components, the printability or storage stability of the paste, for example, can be improved.
The optional glass frit of paste B contains at least two different elements as oxides. The elements can be selected from the group consisting of Li, Na, K, Ca, Mg, Sr, Ba, B, Al, Si, Sn, Pb, P, Sb, Bi, Te, La, Ti, Zr, V, Nb, Mn, Fe, Co, Ni, Cu, Ag, Zn, and Cd. The glass frit can be produced from oxides, fluorides or other salts (e.g. carbonates, nitrates, phosphates) of these elements. Examples of glass frit starting compounds may be selected from the group consisting of B2O3, H3BO3, Al2O3, SiO2, PbO, P2O5, Pb3O4, PbF2, MgO, MnCO3, CaO, CaCO3, SrO, SrCO3, BaO, BaCO3, Ba(NO3)2, Na2B4O7, ZnO, ZnF2, Bi2O3, Li2O, Li2CO3, Na2O, NaCO3, NaF, K2O, K2CO3, KF, TiO2, Nb2Os, Fe2O3, ZrO2 CuO, MnO, Mn3O4, MnO2, CdO, SnO2, TeO2, Sb2O3, Co3O4, Co2O3, CoO, La2O3, Ag2O, NiO, V2Os, Li3PO4, Na3PO4, K3PO4, Ca3(PO4)2, Mg3(PO4)2, Sr3(PO4)2, Ba3(PO4)2 and complex minerals such as colemanite and dolomite.
In a preferred embodiment the glass frit of paste B can contain silicon, aluminium, boron and at least one alkaline earth metal as oxide. The glass frit of the paste B can be the same as the glass frit of the paste A or different. The glass frit of paste B can contain at least two elements as oxides, which are contained in the glass frit of paste A. In a preferred embodiment, the glass frits of pastes A and B are the same, as this can improve the compatibility of layers A and B with each other.
In case the layer of paste A in step c) has already been burned to layer A, the layer of paste B is applied to layer A accordingly. By applying the paste B to the layer from step c), a precursor is produced. The precursor thus contains a substrate on which a layer of paste A is applied, which can optionally already be burned (then also called layer A). Furthermore, the precursor contains a layer of paste B on the layer of paste A, whereby the layer of paste B is not burned. In a preferred embodiment, the paste B is applied to a layer A which has already been burned in step c. In one embodiment, the precursor can be designed so that the layer of paste B completely covers the layer of paste A.
In step e), the precursor is burned and the layer structure according to the invention is obtained. Optionally, a drying step can be carried out prior to burning. Drying can take place at a temperature in the range of 20-180° C., particularly in the range of 120-180° C., e.g. in a drying tap or an infrared belt dryer.
The precursor is preferably burned at a temperature in the range of 700-1000′C, particularly in the range of 850-900° C. The precursor is preferably burned so that the components of the organic medium in the precursor volatilize and the powder of the resistance alloy and the glass frit are sintered together. Burning can take place either under atmospheric conditions in the presence of O2 or under inert gas conditions (e.g. N2 atmosphere). By burning the layer of paste A, layer A is obtained, as explained above, and by burning the layer of paste B, layer B is obtained. If the layer of paste A has not already been burned in step c), the layers of paste A and paste B are burned simultaneously by burning the precursor. If the layer of paste A has already been burned in step c), layer A will inevitably be burned again when the layer of paste B is burned.
The layer structure according to the invention, which exists after step e) contains:
Layer A, which at least partially covers the glass or ceramic surface of the substrate, comprises the glass obtained by burning the glass frit from paste A. Typically, the glass in layer A contains sintered glass frit of paste A. Preferably, this glass frit is sintered homogeneously to the glass over the entire expansion of layer A and has no non-sintered areas.
In the layer structure, layer B has the resistance alloy of paste B and is mechanically firmly bonded to layer A. The mechanical strength of the adhesion can be determined by various tests. Layer B of the layer structure can have a TCR value that essentially corresponds to the bulk value of the resistance alloy.
The adhesive strength can be checked by the following tests: A strip of Scotch® Magic adhesive film (3M Deutschland GmbH) is stuck onto the burned layer structure and firmly applied with a fingernail, for example. The adhesive film is then removed again. Resistance alloy layers with low adhesion to the glass or ceramic surface of the substrate adhere to the adhesive film. Layer structures with a medium adhesive strength partly remain on the adhesive film and layer structures with a high adhesive strength are not detached from the adhesive film.
In the layer structure, layer A can act as an adhesion promoter between the glass or ceramic surface of the substrate and layer B containing the resistance alloy. This invention can thus be used to obtain a layer of a resistance alloy that is mechanically stably bonded to the substrate surface. The layer B contains the resistance alloy in the quantity originally used in paste B. The layer B contains the resistance alloy in the quantity originally used in paste B.
In the optional case that layer B additionally contains a glass made from the glass frit of paste B, the adhesion of layer B to layer A can be further improved. The glass content of layer B is determined by the amount of glass frit used in paste B. In a preferred embodiment, layer B does not contain more than 20% by weight of glass, in particular not more than 15% by weight of glass, based on the total weight of layer B.
Optionally, the layer structure can be provided with a sealant (also called protective glaze or overglaze) after step e). Typically, this sealing consists of a glass. This sealing serves in particular to protect the layer structure from environmental influences such as moisture.
The layer structure according to the invention can be used, among other things, to produce precision resistors.
General Production of Paste A
Pastes A were prepared by mixing 22% by weight organic medium (85% by weight texanol, 15% by weight ethyl cellulose (75% N7, 25% N50)) and 78% by weight glass frit according to Table 1. The pastes were homogenized using a three-roll chair.
General Production Pastes B
A powder of the resistance alloy isotane (mean particle diameter d50: 8 μm, produced by gas atomization of a melt under N2 atmosphere), an organic medium (65 wt. % texanol and 35 wt. % acrylate binder) and, if necessary, a glass frit were added in the specified quantities and homogenized by means of a three-roll chair. The produced pastes have a viscosity of about 30-90 Pas at 20-25′C.
Production of the Layer Structure
The glass pastes A, containing the glass frits from Table 1, were applied by screen printing to Al2O3 substrates with a size of 101.6×101.6 mm and a thickness of 0.63 mm (Rubalit 708 S, CeramTec). A screen from Koenen GmbH, Germany was used with an EKRA Microtronic II printer (type M2H). The emulsion thickness was about 50 μm (sieve parameters: 80 mesh and 65 μm wire diameter (stainless steel)). Printing parameters: 63 N doctor blade pressure, doctor blade speed 100 mm/s and a jump of 1.0 mm. The layer thickness after printing (wet) was about 90 μm. 10 minutes after printing, the samples were dried in an infrared belt dryer (BTU international, type HHG-2) for 20 min at 150° C. The drying time was about 10 minutes. The layer thickness after drying was about 60 μm. The printed glass layers were burned under nitrogen atmosphere (N2 5.0) in a furnace (ATV Technologie GmbH, type PEO 603). The temperature was increased from 25° C. to 850° C., kept at 850° C. for 10 and then cooled down to 25° C. within 20 min. The layer thickness after burning was about 50 μm. The resistance alloy paste B was applied to the previously produced layer by screen printing. A screen from Koenen GmbH, Germany was used with an EKRA Microtronic II printer (type M2H). The emulsion thickness was about 50 μm, sieve parameters: 80 mesh and 65 μm wire diameter (stainless steel).
The printed resistance alloy pastes (including the precursor) were burned in a nitrogen atmosphere (N2 5.0) in a furnace (ATV Technologie GmbH, type PEO 603). The temperature was increased from 25° C. to 900° C., kept at 900° C. for 10 min and cooled down to 25° C. within 20 min (total cycle time 82 min). The layer thickness after burning was about 50 μm.
Adhesion Layer Structure as a Function of the Amount of Glass in Paste B
T-Shock Storage:
The manufactured layer structures were each stored for 15 min in a chamber with a temperature of −40° C. or +150° C. The temperature of the storage chamber was −40° C. or +150° C. respectively. The transition from one chamber to the other was automated and took approx. 4 s. One cycle includes one storage at −40° C. and one at +150° C. The other cycle was automated. The adhesion was checked after different numbers of cycles with an adhesive tape as described above.
For layer structure 9 and layer structure 12, the TCR values were measured in the temperature range 20-60° C. according to the standard DIN EN 60115-1:2016-03 (drying method I):
For comparison The TCR bulk value for isotane (as wire) is in the range of −80 to +40 ppm/K.
| Number | Date | Country | Kind |
|---|---|---|---|
| 16193341.1 | Oct 2016 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/073421 | 9/18/2017 | WO | 00 |
