EP1023735B1 - Heating element and method for producing the same - Google Patents

Description

The invention relates to a method for producing a heating element with a predetermined resistance value, which is characterized by a high heating rate, as is required, for example, to ignite propellants for airbag systems.
Such heating elements are currently manufactured with resistance wires, the diameter of which must be selected very small in order to achieve a high heating rate (approximately 10 μm). For a given wire length, the resistance value for a certain resistance wire can only be varied over the wire cross-section. If a wide range of resistance values is to be covered, the technical limits in terms of heating rate, handling and installation options for the wire will soon be reached.

From US 3,998,980 A is a thick-film resistor as a pixel element with a predetermined Resistance value known for thermal printer applications, which in several print layers a ceramic substrate coated with a crystallizing glass as a heat barrier is applied, the thickness of the resistor being in the range from 12.5 μm to 254 μm. A bismuth thruthenate paste system is used as the resistance material. To the one To obtain the required plane resistance surface of the printer element, the resistance lapped, the lapping process after each pressure or as the last Process step can be applied. The lapping process also serves to adjust the Resistance value and the resistance temperature coefficient. A subsequent one Annealing process is said to contribute to the formation of microcracks in the Avoid resistance layer, which in the course of aging leads to an increase in resistance could lead. A disadvantage of this embodiment of a heating element is that the Resistance is designed as a thick-film component and not as a thin-film structure, because of its heat capacity, it does not fall below a certain heating rate can be.

EP 0 471 138 A2 describes a method for producing an electrical Measuring resistance with a predetermined temperature coefficient is known, in which a Alumina ceramic substrate is provided with a platinum thin film on which then using a screen printing process a layer made of a preparation containing platinum and Rhodium resinate contains is applied, its rhodium content for the desired Temperature coefficient is decisive. The coated carrier becomes one Heat treatment in the range of 1000 to 1400 ° C until the Rhodium evenly distributed in the resistance layer that forms. The The rhodium content of the layer is in the range from 0.1% to 12% based on the content of Platinum and rhodium. By varying the rhodium content of the resistance layer the temperature coefficient of measuring resistors based on platinum alloys in Set the range from 1600 to 3850 ppm / K exactly. This procedure is not based on it aligned to set the specific surface resistance of the resistance layer exactly.

WO 96/01983 A1 describes a method for producing a sensor for detecting Temperature and / or flow described, the sensor being structured Resistance layer is formed on a carrier. This is a platinum-rhodium layer that consists of a tempered platinum resinate / rhodium resinate mixture. So can for example, by mixing 99% platinum resinate paste and 1% rhodium paste Platinum-rhodium resistance layer with a temperature coefficient of 3500 ppm / ° C will be realized. This procedure is also not aimed at the specific Adjust the surface resistance of the resistance layer exactly.

EP 0 576 017 A2 describes a method for producing an inkjet print head known, wherein a thin film layer forms a heating element that in a period of a few microseconds to a temperature of 300 ° C and then on again Room temperature is cooled. The contact areas for the thin film heating elements are with Au or Pt resinate pastes. These contact areas cannot be soldered. The Thin film is formed from a resinate paste, such as metal alloys WNi, ZrCr, TaIr, TaFe or ZrNi contains. The focus is on compatibility with the ink, whereas possibilities of varying the specific sheet resistance be disregarded.

From DE-OS-2 020 016 a metal layer ignition agent is known which is based on a Insulating body made of glass or ceramic. On this are screen printed two contact surfaces, for example with palladium-palladium silver, Palladium-gold, platinum-silver, nickel or silver-aluminum thick-film conductor paste applied a sintering process at a temperature between 1000 ° C and 1100 ° C. be subjected. Then a tantalum or tantalum nitride layer is evaporated, which is structured in a photolithographic process to form an ignition bridge, wherein this overlaps the edge zones of the two contact surfaces. The length and width of the Ignition bridge preferably vary between 50 and 100 µm and the thickness between 0.2 µm and 1.5µm. The disadvantage of this process is the high technological effort involved through the use of two different technologies, namely thick-film technology (Screen printing process) and thin-film technology (vapor deposition technology). Furthermore, the photolithographic process for structuring the ignition bridge is not easy to use because the applied thick-film contact surfaces ensure the flatness of the Surface affected. Because of these bumps, it can occur in the contact copying process Under-radiation occurs, which has an adverse effect on the structural fidelity of the ignition bridge element.

For ignition elements of propellants for airbag systems is due to the specifications for the Installation in housing specifies the length of the heating element. An increase in The resistance value of the resistance track is therefore only above given layer thickness a reduction in the web width is possible. This reduces the web width Limits that a minimum resistance area for reliable ignition of the propellant for heat transfer must not be undercut.

A heating element in which both the contact surfaces and the ignition bridge in the Screen printing processes are known from document EP-A-0 248 977.

The object of the invention is a method for producing a heating element specify after which an AuPd resinate resistance layer of a given layer thickness is treated by doping with Pd atoms so that a sheet resistance with Desired specific surface resistance adjustable in the range from 300mΩ to approx. 3Ω is. This object is achieved by the characterizing features of Claim 1 solved.

• Drucken einer Glas- oder Glaskeramikpaste (Sintertemperatur: 850°C bis 1100°C) mittels Siebdruckverfahrens zur Realisierung einer thermischen Isolationslage auf einem Aluminiumoxid- oder Stahlsubstrat;
• Trocknen der aufgedruckten Paste (bei ca. 150°C);
• Sintern der Paste;
• Wiederholen der genannten Prozeßschritte auf dem gleichen Trägerkörper bis die gewünschte Gesamtschichtdicke erreicht ist;
• Bei Bedarf (zu hohe Oberflächenrauhigkeit) Läppen und Polieren der gesinterten Glas- oder Glaskeramikbeschichtung bis eine spiegelnde Oberfläche erreicht ist;
• Tempern des Substrats mit der geläppten und polierten Glas- oder Glaskeramikbeschichtung, um mechanische Spannungen abzubauen, die zu Mikrorissen führen könnten;
• Drucken der Resinatpaste mittels Siebdruckverfahrens auf die Glas- oder Glaskeramikbeschichtung;
• Trocknen der aufgedruckten Paste (bei zwischen 80°C und 150°C);
• Sintern der Paste (850°C);
• Strukturieren der Widerstandsbahnen durch naßätztechnische Verfahren oder Sputterätzen;
• Drucken der Pd-hältigen Dickschichtleiterpasten zur Kontaktierung der Resinatwiderstandsbahnen mittels Siebdruckverfahrens auf die geläppte und polierte Glasbzw. Glaskeramikbeschichtung;
• Trocknen der aufgedruckten Paste (bei ca. 150°C);
• Sintern der Paste (zwischen 850°C und 950°C);

In a preferred embodiment of the method, an aluminum oxide ceramic is used as the carrier; it is also possible to use a carrier made of a steel substrate. A glass or glass-ceramic coating is applied as a thermal or also as an electrically insulating intermediate layer, the latter consisting of SiO ₂ , BaO, Al ₂ O ₃ and an inorganic dye compound, for example as a paste system under the designation IP 211 or available as unfired ceramic film under the name HERATAPE T5 or T211 from WC Heraeus GmbH, Hanau. The invention is based on the knowledge that with a view to uniform layer production and reproducible wet-chemical structuring of the resinate resistance layer, the glass or glass ceramic coating, which is applied as a heat barrier to the ceramic or steel substrate, may have to be lapped and polished. In this case, the dried and sintered glass or glass ceramic layer is lapped and polished until a reflective surface is achieved. The AuPd thin film resistor coating is then screen printed onto the carrier. The preparation to be applied is preferably a resinate system consisting of 22% by mass Au and 1% by mass Pd, which are distributed in a solution of synthetic resin and organic binders, and which is available under the designation RP 26001/59 from WC Heraeus GmbH, Hanau. After the resinate layer has been applied by screen printing, it is dried at a temperature in the range from 100 to 150 ° C. and then fired at a temperature in the range between 850 and 900 ° C., the organic solvents evaporating or burning. The layer produced by this method has a thickness in the range from 0.1 to 1.5 μm. In a subsequent process step, the resistance layer is structured, for example, by wet chemical etching processes or sputter etching in the form of a strip which has a path constriction. The invention is based on the finding that, according to the arrangement and extent of the path narrowing, the temperature distribution on the resistance path can be changed in such a way that the peak temperature occurs at desired points and areas of the resistance path. Contact fields for the external connections are provided at both ends of the sheet resistance. The contact fields are also applied using the screen printing process, using AgPd conductor pastes with different Pd contents (Ag: Pd ratio between 1.7: 1 and 26: 1). These are, for example, AgPd conductor pastes of the C1200 series from WC Heraeus GmbH, Hanau. The resistance path is doped with Pd via the AgPd contact. The invention is based on the finding that the resistance value of a resistance track built up with an AuPd resinate layer can be modified in a targeted manner by contacting it with an AgPd thick-layer conductor metallization with a different Pd content. Depending on the palladium content in the AgPd conductor paste, the specific surface resistance of the resistance track can be set for a resistance length of 1mm in the range from 310mOhm to 3Ohm after an annealing process: only the palladium content of the AuPd alloy of the thin film resistor is varied without changing the layer thickness. For technological reasons, it is not possible to produce an AuPd resinate paste in its basic composition with a higher proportion of Pd.
The invention also provides a method for producing a heating element with a high heating rate with a base body, a thermal insulation layer and a structured resistance layer with contacts which are arranged on the insulation layer, solved by the following method steps:

• Printing a glass or glass ceramic paste (sintering temperature: 850 ° C to 1100 ° C) using a screen printing process to create a thermal insulation layer on an aluminum oxide or steel substrate;
• Drying the printed paste (at approx. 150 ° C);
• Sintering the paste;
• Repeating the above-mentioned process steps on the same carrier body until the desired total layer thickness is reached;
• If necessary (surface roughness too high) lapping and polishing the sintered glass or glass ceramic coating until a reflective surface is reached;
• Annealing the substrate with the lapped and polished glass or glass ceramic coating to relieve mechanical stresses that could lead to microcracks;
• Printing the resinate paste on the glass or glass ceramic coating using a screen printing process;
• Drying the printed paste (at between 80 ° C and 150 ° C);
• Sintering the paste (850 ° C);
• Structuring the resistance tracks by wet etching processes or sputter etching;
• Printing the Pd-containing thick-film conductor pastes for contacting the resinate resistance tracks by means of screen printing on the lapped and polished glass or Ceramic coating;
• Drying the printed paste (at approx. 150 ° C);
• Sintering the paste (between 850 ° C and 950 ° C);

A preferred embodiment of the present invention is described below Reference to the drawing explained in more detail.

Fig. 1 shows a sectional view of a heating element with a high heating rate according to a preferred embodiment of the present invention.

• Drucken der Glas oder Glaskeramikpaste mittels eines Siebdruckverfahrens auf ein Aluminiumoxidkeramiksubstrat mit einer Schichtdicke von etwa 80µm.
• Trocknen der aufgedruckten Paste bei 150°C über eine Zeitdauer von ca. 10Minuten.
• Sintern der getrockneten Paste in einem Durchlaufofen bei einer Temperatur von 850°C, wodurch sich eine Schichtdicke der Glas- oder Glaskeramikbeschichtung nach dem ersten Brennzyklus von 15µm ergibt.
• Wiederholen der ersten drei Prozeßschritte auf dem gleichen Substrat, bis eine Gesamtschichtdicke von ca. 45µm erzielt wird. Dies erfordert etwa drei Prozeßdurchläufe.
• Bei Bedarf Läppen und Polieren der Glas- oder Glaskeramikbeschichtung bis eine Oberflächenrauhigkeit <0,1µm erzielt wird.
• Tempern des Substrats bei einer hohen Temperatur, vorzugsweise bei 850°C über eine Zeitdauer von 1 Stunde.

The heating element (100) comprises a substrate (101), which can be provided with a lapped and polished glass or glass ceramic coating (102) on which a resinate resistance track (103) is arranged, which is coated with a thick-layer conductor track metallization (104, 104 ' ) is contacted with the solder stop dam (105, 105 '). In one embodiment, the substrate (101) is an aluminum oxide ceramic with a purity of 96-99%, the rest consisting of other oxides. A glass or glass ceramic coating (102) is applied to the substrate using commercially available paste systems from HERAEUS or ESL using the screen printing process. Pastes are preferably used which can be sintered at a temperature of 850 ° C. The surface roughness R _{a of} the coating is reduced from> 0.6 µm to <0.1 µm by a subsequent lapping and polishing process so that the resistance path (103) can be built up on it without pores and in a uniform layer thickness. With this glass or glass goods coating, a heat barrier is built up for the heating element, the following process steps being used.

• Printing the glass or glass ceramic paste by means of a screen printing process on an aluminum oxide ceramic substrate with a layer thickness of approximately 80 μm.
• Drying the printed paste at 150 ° C for a period of about 10 minutes.
• Sintering the dried paste in a continuous furnace at a temperature of 850 ° C, which results in a layer thickness of the glass or glass ceramic coating after the first firing cycle of 15 µm.
• Repeat the first three process steps on the same substrate until a total layer thickness of approx. 45 µm is achieved. This requires approximately three process runs.
• If necessary, lapping and polishing the glass or glass ceramic coating until a surface roughness of <0.1 µm is achieved.
• Annealing the substrate at a high temperature, preferably at 850 ° C for a period of 1 hour.

• Drucken der Resinatpaste mittels Siebdruckverfahrens auf die unbearbeitete oder geläppte und polierte Glas- oder Glaskeramikbeschichtung mit einer Schichtdicke von etwa 10µm.
• Trocknen der aufgedruckten Paste bei 150°C über einer Zeitdauer von 10 Minuten.
• Brennen der Paste in einem Durchlaufofen bei 850°C, wobei sich nach dem Brennzyklus eine Metallisierungsschichtdicke von etwa 0,1µm ergibt.
• Strukturieren der Widerstandsbahn durch naßchemische Ätzverfahren oder durch Sputterätzen.

The temperature treatment causes the mechanical stresses induced by the lapping and polishing process to be reduced, which could lead to the formation of microcracks in the glass or glass ceramic coating and subsequently also in the resinate resistance track. In view of the rapid heating behavior, the resistance track may only have a low heat capacity. This is achieved on the one hand by selecting a metallization layer with a low specific heat capacity or by miniaturizing the resistance track. An AuPd or Au resinate paste is used to produce the resistance track (103), the following process steps being followed.

• Printing the resin paste using the screen printing process on the unprocessed or lapped and polished glass or glass ceramic coating with a layer thickness of about 10 µm.
• Drying the printed paste at 150 ° C for a period of 10 minutes.
• Firing the paste in a continuous furnace at 850 ° C, which results in a metallization layer thickness of about 0.1 µm after the firing cycle.
• Structuring the resistance track by wet chemical etching processes or by sputter etching.

• Drucken der AgPd- oder PdAu-Dickschichtleiterpaste mittels eines Siebdruckverfahrens überlappend die Widerstandsbahn mit einer Schichtdicke von etwa 30µm.
• Trocknen der aufgedruckten Paste bei 150°C über eine Zeitdauer von ca. 10Minuten.
• Sintern der getrockneten Paste in einem Durchlaufofen bei einer Temperatur von 850°C, wodurch sich für die Kontaktierung eine Schichtdicke von etwa 15µm ergibt.
• Tempern des Substrats bei einer hohen Temperatur, bevorzugterweise 850°C über eine Zeitdauer von etwa 1 Stunde.

The present invention is based on the finding that the resistance value of the AuPd or Au resinate resistance track can be controlled as a function of the Pd content by contacting it with thick film conductor pastes (104, 104 ') based on AgPd. The following process steps must be followed:

• Printing the AgPd or PdAu thick-film conductor paste by means of a screen printing process overlapping the resistance track with a layer thickness of about 30 µm.
• Drying the printed paste at 150 ° C for a period of about 10 minutes.
• Sintering the dried paste in a continuous furnace at a temperature of 850 ° C, which results in a layer thickness of about 15 µm for the contact.
• Annealing the substrate at a high temperature, preferably 850 ° C for a period of about 1 hour.

Annealing is a targeted change and then a stabilization of the Resistance value reached.

• Drucken einer Glaspaste mittels eines Siebdruckverfahrens als Stegstruktur auf die Leiterbahnkontakte mit einer Schichtdicke von etwa 40µm.
• Trocknen der aufgedruckten Paste bei 150°C über eine Zeitdauer von ca. 10Minuten.
• Sintern der getrockneten Paste in einem Durchlaufofen bei einer Temperatur von etwa 500°C-600°C, wodurch sich für die Kontaktierung eine Schichtdicke von etwa 25µm einstellt.

If required, solder dams (105, 105 ') are applied to the thick-film conductor contacts (104, 104') by means of a glass paste. When soldering wire connections, the solder stop dams should avoid wetting the resistance path with solder and flux, as this could lead to the resistance path being stripped or contaminated. The following process sequence is followed for applying the solder stop dam:

• Printing a glass paste using a screen printing process as a web structure onto the conductor track contacts with a layer thickness of approximately 40 µm.
• Drying the printed paste at 150 ° C for a period of about 10 minutes.
• Sintering the dried paste in a continuous furnace at a temperature of about 500 ° C-600 ° C, which results in a layer thickness of about 25 µm for the contact.

The invention is not restricted to the exemplary embodiment mentioned above. A ferritic high-temperature-resistant steel can also be used as the base body (101) instead of the aluminum oxide ceramic. The glass ceramic layer (102) can not only be applied using the screen printing method, but can also be laminated onto the base body in the form of a "green" (unfired) ceramic film and then sintered.
A glass / glass ceramic layer can be dispensed with if a glass ceramic or a ceramic with low thermal conductivity, such as zirconium oxide or magnesium oxide, is already used as the base body. However, the surface may need to be lapped and polished to achieve a surface roughness of <0.1 µm.

Claims (10)

1. Heating element (100) with a high heat up rate consisting of

   a base body (101)

   a structured resistance layer (103) which is arranged on a base body (101)

   contact fields (104, 104') which are arranged overlapping on both ends of the resistance track, characterised in that the heating element furthermore consists of

   bar-shaped solder stop dams (105, 105') which are applied on to the contact surfaces (104, 104'), that the resistance track (103) is an AuPd or Au resinate layer and that the contact surfaces (104, 104') overlapping the resistance track (103) consists of an AgPd or PdAu thick layer conductor path metallisation.
2. Heating element (100) according to Claim 1, characterised in that the base body (101) consists of 96% to 99% aluminium oxide with a residual proportion of other oxides.
3. Heating element (100) according to Claim 1, characterised in that the base body (101) consists of a high temperature resistant glass or a glass ceramic or a ceramic with low thermal conductivity such as, for example, zirconium oxide, the surface roughness of which is maintained if needed by lapping and polishing to <0.1µm.
4. Heating element (100) according to Claim 1, characterised in that a heat barrier (102) is applied to the base body (101) consisting of a glass or glass ceramic layer, the surface roughness of which is, if necessary, by means of lapping and polishing, <0.1µm.
5. Heating element (100) according to Claim 1 or Claim 4, characterised in that the base body (101) consists of a high temperature resistant carbon poor chromium-containing steel.
6. Process for manufacturing a heating element (100) with high heat up rate for igniting propellants with a base body (101), a thermal barrier (102), a structured resistance layer with or without necking (103) which is arranged on the thermal barrier (102), contact surfaces (104, 104') which are arranged overlapping on both ends of the resistance track (103) and bar-shaped solder stop dams (105, 105') which are applied on to the contact surfaces (104, 104'), characterised by the following process steps:

   printing of the glass or glass ceramic case by means of a screen printing process on to an aluminium oxide ceramic substrate;

   drying of the applied paste;

   sintering of the dried paste;

   repeating the first three process steps on the same substrate until the desired overall layer thickness is achieved;

   preferably lapping and polishing the glass or glass ceramic layer until the permissible surface roughness is achieved;

   tempering the substrate in order to avoid the formation of microcracks;

   printing of the AuPd or Au resistance resinate paste by means of a screen printing process on to the preferably lapped and polished glass or glass ceramic layer;

   drying the applied paste;

   firing the resinate paste layer;

   structuring the resistance layer by wet chemical etching processes or sputter etching;

   printing the conductor paste by means of a screen printing process overlapping the resistance track;

   drying the applied paste;

   sintering the dried paste;

   tempering the substrate in order to alter the resistance value in targeted fashion and thereafter to stabilise it;

   printing the glass paste by means of a screen printing process as a bar structure on to the conductive track contacts;

   drying the applied paste;

   sintering the dried paste.
7. Process for manufacturing a heating element (100) according to Claim 6, characterised in that in place of a glass or glass ceramic paste, a "green" (unfired) glass ceramic foil is used which is applied by lamination in place of a screen printing process on to the aluminium oxide or steel substrate.
8. Process for manufacturing a heating element (100) according to Claim 6 or 7, characterised in that for the resistive track, an AuPd resinate paste is used which contains 22 weight percent Au and 1 weight percent Pd, wherein the remainder of the paste is constituted by organics.
9. Process for manufacturing a heating element (100) according to Claim 6 or 7, characterised in that for the resistive track, an Au resinate paste is used which contains 12 weight percent Au wherein the remainder of the paste is constituted by organics.
10. Process for manufacturing a heating element (100) according to Claim 8 or 9, characterised in that for contacting the resistance track, an AgPd or PdAu thick layer conductive paste with a Pd proportion between 0 and 100 weight % is used, wherein the remainder falls out from Ag or Au, an organics base, a glass phase and/or oxidic additions.

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