JP2002162378A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor surely joining lead wires to even a minute area and stably-usable over the long term. SOLUTION: The gas sensor is provided with a heater lead 17 connected with a heater 12 formed on a low conduction substrate and an electrode lead 18 connected with two detector electrodes 15a/15b formed on an induction film 14. These leads 17 and 18 are made through thick film printing by using a conductive paste blended with a glass-based adhesive, joined to a lead wire 19 by joining the lead wire 19 by laser welding. Thus a glass coating is formed outside the joining point not only to protect contact points but also to produce firmer contact points by joining of this glass coating and a glass contained in the conductive paste, resulting in a long-term stable output.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for joining lead wires of a gas sensor for detecting gas leakage from gas-fired equipment, carbon monoxide generated by incomplete combustion of indoor-fired equipment, and various toxic gases and odors. It is about.

[0002]

2. Description of the Related Art There are various types and shapes of conventional gas sensors, one of which uses a solid electrolyte as shown in FIG. 6 as an example. In FIG. 7, reference numeral 1 denotes an insulator made of a ceramic plate such as alumina, and a heater 2 is set on the back surface. A solid electrolyte 3 is provided on the insulating plate 1 and has a pair of platinum electrodes 4a and 4b formed on the upper surface. Reference numeral 5 denotes a porous body having a large number of minute air holes of about 10 to 100 degrees inside.

An oxidation catalyst 6 is provided on the porous body 5 at a position corresponding to one platinum electrode 4a. 7 is an insulating plate 1,
While solid electrolyte 3 and porous body 5 are integrally fixed,
A sealing material that seals a gap between the solid electrolyte 3 and the porous body 5 and fixes the three members at the outer periphery. Reference numeral 8 denotes a lead wire, which is joined to the electrodes 4a and 4b and the heater 2 with a conductive paste.

[0004]

Generally, gas sensors are sensitive to carbon monoxide (hereinafter referred to as CO), methane, propane, hydrogen, odor, etc., and are used for gas leak alarms, CO alarms, odor sensors, and the like. Used. The solid electrolyte type sensor shown in FIG. 7 detects gas based on the following principle. In the following description, the case where the detection gas is CO will be described. In FIG. 7, power is supplied from a power supply (not shown) to the heater 2 to bring the solid electrolyte 3 to a predetermined temperature (40
0 ° C. to 500 ° C.), the electrode 4 and the solid electrolyte 2
Transfer of electrons is performed at the interface between the air and the air, and oxygen ions are generated.

Here, when CO is present, CO is oxidized by the catalyst 6 at the electrode 4a on which the catalyst 6 is mounted, and
CO does not reach a. At the other electrode 4, CO is oxidized to CO ₂ on the surface of the electrode 4. As a result, a difference occurs in the electrode reactions between the two electrodes, the equilibrium of oxygen ions is disrupted, and a potential difference occurs between the two electrodes. The potential difference is led to a detection circuit (not shown) by the lead wire 8, and the CO concentration can be detected. In the above-mentioned conventional method, the insulator 1, the solid electrolyte 3, and the porous body 5 are plate-shaped, and a power of several hundred mW to several W is required to heat them to a predetermined temperature. In recent years, there has been a remarkable trend of miniaturization and power saving of gas sensors, and development for battery driving has been spurred.

Specifically, if a semiconductor process technology is used, it is possible to form a gas-sensitive film in a region of, for example, 1 mm or less, thereby achieving miniaturization and power saving. In the case of joining a lead wire to such a minute area, a conventional joining method using a conductive paste cannot cope with the problem because a large joining area is required. It is well known that gold wire bonding is widely used in semiconductor manufacturing processes as a method for joining lead wires.However, wire bonding is limited to gold wire or aluminum wire, and directly on platinum electrodes or heaters. They cannot be joined. Further, since the strength of the gold wire is low, it is necessary to use a wire having a relatively large diameter when the gas sensor is held in a suspended state by a lead wire. The aluminum wire has a low melting point and cannot be used in a high-temperature atmosphere. In addition, the aluminum wire has problems such as a change in resistance due to formation of an oxide film.

An object of the present invention is to solve the above-mentioned conventional problems, and an object of the present invention is to provide a gas sensor which can be used stably for a long period by securely connecting a lead wire even to a minute area.

[0008]

In order to solve the above-mentioned conventional problems, a gas sensor according to the present invention is formed by sequentially laminating a heater, an insulating film, a sensitive part, and a detection electrode on a substrate having low thermal conductivity. A heater paste formed by thick-film printing of a conductive paste mixed with a glass-based adhesive was connected to the heater, and a conductive paste formed by mixing a glass-based adhesive with the detection electrode was formed by thick-film printing. Electrode leads are connected, and lead wires are joined to the heater leads and the electrode leads by laser welding.

Thus, at the junction between the lead wire, the electrode lead and the heater lead, the lead wire and the conductive paste are instantaneously melted and joined by laser irradiation, and further, the junction is changed due to a difference in melting point and surface tension. A glass coating is formed on the outside of the substrate. Therefore, the contact is protected by the glass coating, and the glass coating and the glass contained in the conductive paste are joined to form a strong contact, so that a stable output can be obtained for a long period of time.

[0010]

According to the first aspect of the present invention, a gas sensor is formed by stacking a heater formed on a substrate having low thermal conductivity, an insulating film formed on the heater, and the insulating film. A sensing part, and a detection electrode formed on the sensing part,
A heater lead formed by thick-film printing of a conductive paste formed by mixing a glass-based adhesive connected to the heater film, and a conductive paste formed by mixing a glass-based adhesive connected to the detection electrode. By using a configuration including an electrode lead formed by thick-film printing and a lead wire joined to the heater lead and the electrode lead by laser welding, laser irradiation is performed at the junction between the lead wire, the electrode lead, and the heater lead. In addition to the instantaneous melting and joining of the lead wire and the conductive paste, a glass coating is formed outside the joint due to differences in melting point and surface tension. Therefore, the contact is protected by the glass coating, and the glass coating and the glass contained in the conductive paste are joined to form a strong contact, so that a stable output can be obtained for a long period of time.

According to a second aspect of the present invention, in the first aspect of the present invention, the heater lead, the electrode lead and the lead wire have the same main component, so that the same main component is melted by laser irradiation. By joining together, a stronger joint is formed.

According to a third aspect of the present invention, in the first and second aspects of the invention, the heater lead and the electrode lead are formed by thick film printing using a paste formed by mixing a glass adhesive with a noble metal such as gold or platinum. With such a configuration, it is difficult to form an oxide film, and it has stability against corrosive gas, so that a stable output can be obtained for a long period of time.

[0013] The invention according to claim 4 is the invention according to claims 1 to 3.
In the invention described, the heater lead and the electrode lead have a multilayer structure of at least two or more layers, and the first layer has a configuration containing more glass than the multilayer, so that the first layer maintains the adhesion to the substrate, and the further layers Since the bonding with the lead wire is performed, the electrical conductivity can be kept good and the bonding strength can be maintained.

According to a fifth aspect of the present invention, in the fourth aspect, the first layer is heat-treated at a temperature higher than the melting point of the glass, and the second layer is heat-treated at a temperature higher than the glass transition point of the glass and lower than the melting point. Since the first layer and the further layers are clearly separated from each other, the adhesion to the substrate and the good electrical conductivity can be secured.

According to a sixth aspect of the present invention, in the first to fifth aspects of the present invention, the irradiation position of the laser beam is in contact with the outer peripheral edge of the laser beam so as to include the lead wire, and the center of the laser beam is closer to the sensitive portion. Since the laser beam intensity is too strong because the laser beam is too strong, it prevents the lead wire and the sensitive part or the lead wire and the heater from being insulated, so that the laser beam is too strong. A stable state can be secured.

According to a seventh aspect of the present invention, in the first to fifth aspects of the present invention, the laser beam diameter is set to be substantially the same as the lead wire diameter. Since the electrode lead is prevented from being damaged by the laser beam, a favorable bonding state can be realized.

According to an eighth aspect of the present invention, in the first to fifth aspects of the present invention, the shape of the lead wire at the joining portion is rectangular in cross section. Since it is possible to prevent the laser beam from coming off and to reduce the intensity of the laser beam, a stable bonding state can be ensured.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a perspective view of a gas sensor according to a first embodiment of the present invention. In this embodiment, a solid electrolyte type CO sensor will be described.

In FIG. 1, reference numeral 11 denotes a substrate having low thermal conductivity, which is made of quartz glass of about 2 mm × 2 mm × 0.3 mm. Reference numeral 12 denotes a platinum heater whose resistance is set to a predetermined temperature by a sputtering method, an electron beam evaporation method, or the like. Reference numeral 13 denotes an insulating film formed of a thin film of an insulating material such as alumina, silica, or silicon nitride so as to cover the heater 12 by a sputtering method, an electron beam evaporation method, or the like. Reference numeral 14 denotes a solid electrolyte film which is a sensitive film formed on the insulating film 13, and is formed by sputtering a solid electrolyte having oxygen ion conductivity (8% yttria-stabilized zirconia) by about 0.5 mm ×
It is formed in a size of 0.5 mm. 15a and 15b are detection electrodes, and platinum is formed on the sensitive part by sputtering. Precious metals such as palladium, ruthenium, and rhodium may be partially mixed with platinum. 16 is one detection electrode 1
The catalyst 16 may be any of the catalysts set on 5a as long as it oxidizes and decomposes the gas to be measured. However, noble metals such as platinum, palladium, ruthenium, and rhodium and metal oxides such as tin oxide and zinc oxide can be used. The catalyst 16 is formed by a screen printing method, or by impregnating a porous material in the form of a nitrate or the like, followed by firing. Reference numeral 17 denotes a heater lead connected to the heater 12, and reference numeral 18 denotes an electrode lead connected to the detection electrodes 15a and 15b. The heater lead 17 and the electrode lead 18 are formed by mixing glass as an adhesive with a noble metal, forming a paste with an organic solvent, and forming a thick film by a screen printing method.
Reference numeral 19 denotes a lead wire joined to the heater lead 17 and the electrode lead 18 and joined by laser welding.

In the above arrangement, when power is supplied from a power source (not shown) to the heater 12 via the lead wire 18 to heat the solid electrolyte 13 to a predetermined temperature (400 ° C. to 500 ° C.), the detection electrodes 15a and 15b Electrons are transferred at the interface between the solid electrolyte membrane 14 and air, and oxygen ions are generated.
Here, if CO is present, CO is oxidized by the catalyst 16 at the detection electrode 15a on which the catalyst 16 is mounted, and does not reach the detection electrode 15a. The other detection electrode 15
In b, CO is oxidized to CO ₂ on the surface of the detection electrode 15.
As a result, a difference occurs in the electrode reactions between the two electrodes, the equilibrium of oxygen ions is disrupted, and a potential difference occurs between the two electrodes. By detecting this potential difference, the CO concentration can be detected. Here, quartz glass is used for the substrate 11. Quartz glass has a thermal conductivity of 1.5 W / mK and an insulating film 13 (35 to
45 W / mK) or smaller than the sensitive film 14 (6 W / mK), and therefore, when heated by the heater 12, the temperature of the substrate 11 hardly rises and the temperature of only the area of the sensitive film 14 immediately above the heater 12. Can be increased, so that the power consumption for heating can be greatly reduced.

Further, since the thermal shock strength is high, the temperature can be raised to a predetermined temperature in a short time. With the above configuration, it was possible to raise the temperature up to 450 ° C. with a power amount of 15 mWsec. The heater lead 17 and the electrode lead are usually
Use glass as a glue for precious metals such as gold and platinum.
% Of the mixture and paste-formed with an organic solvent is formed into a film by screen printing, and is heated to a temperature equal to or higher than the melting point of the glass to adhere to the substrate 11. Here, the lead wire 19 is usually made of gold or platinum, but the heater lead 17 and the electrode lead 18 are used.
It is desirable that the lead wire 19 and the lead wire 19 have the same main component. For example, when the lead electrode is made of gold and the lead wire 19 is made of platinum, or vice versa, when used in a high-temperature atmosphere, gold may diffuse into platinum and change the state of the film.

When gold is used for the lead wire 19,
Since gold has low rigidity, it is difficult to use when the gas sensor is suspended and fixed with the lead wire 19, and gold is limited to the case where the gas sensor is fixed to the base. From the above, platinum is optimal for the lead wire 19, the electrode lead 18, and the heater lead 17. Since the heater 12 and the detection electrode 15 can also be formed of platinum, it is advantageous in that the state of joining with them can be kept good.

FIG. 2 shows a cross section when the platinum heater lead 17 (or the electrode lead 18) and the platinum lead wire 19 are joined by laser welding. Lead wire 19
When a laser beam is irradiated on the heater lead,
The lead wire 19 and the heater lead 17 at the portion irradiated with the laser beam are instantaneously melted. At this time, since glass is mixed with platinum in the heater lead 17, platinum becomes spherical around the lead wire 19 due to a difference in surface tension.
And the glass forms a coating around the platinum. Here, when the irradiation of the laser beam is stopped, platinum having a high melting point is first solidified into a spherical shape and joined to the platinum of the heater lead 17.

Next, the glass having a low melting point solidifies while forming the coating 21 so as to cover the spherical platinum, and is fixed to the heater lead 17. Therefore, since the bonding is performed so as to protect the bonding point of platinum, strong bonding is realized, and stable use for a long period of time becomes possible.

Embodiment 2 FIG. 3 is a perspective view of a gas sensor according to Embodiment 2 of the present invention. Since the basic configuration is the same as that of FIG. 1, only different points will be described. In FIG. 3, the heater lead 17 and the electrode lead 18 are formed of first layers 17a and 18a having a high glass content and second layers 17b and 18b having a low glass content. When two or more layers are formed, only the first layer is formed by increasing the glass content.

As the glass content of the heater lead 17 and the electrode lead 18 increases, the adhesive force with the substrate 11 increases, but the resistance sharply increases, and the heater lead 17 and the electrode lead 18 cannot be used as the heater lead 17 and the electrode lead 18. Therefore, by dividing into two layers, the first layers 17a and 18a secure the adhesion to the substrate 11, and the second layers 17b and 18b secure the conductivity. As the glass content, the first layers 17a and 18a
10 wt%, and 5-20 wt% of the second layers 17b and 18b can be used. Also, the first layer 17a, 18a and the second layer 17
b, 18b are set to the first layer 17
a and 18a are heat-treated at a temperature higher than the melting point of glass, and the second layers 17b and 18b are heat-treated at a temperature higher than the glass transition temperature and lower than the melting point. For example, if the first layer has a glass content of 20
wt% and the second layer having a glass content of 8%, the first and second layers 17a, 18a are heat-treated at a temperature higher than the melting point of the glass.
And the second layers 17b and 18b are completely dissolved to form a single layer having a glass content of 14%. It is known that when glass is mixed with platinum, if the glass content exceeds 10, the electric resistance sharply increases, and the electric conductivity of the heater lead 17 and the electrode lead 18 is impaired at all. Therefore, the first layer 17a, 18a and the second layer 17b,
By changing the heat treatment temperature at 18b, the first layer 17
a, 18a and the second layers 17b, 18b are clearly separated from each other to secure adhesion to the plate 11 and to secure electrical conductivity. Since the second layers 17b and 18b are heat-treated at a melting point or lower, they are not completely melted.

(Embodiment 3) FIG. 4 is a plan view of a main part of a gas sensor according to Embodiment 3 of the present invention. Since the basic configuration is the same as that of FIG. 1, only different points will be described. In FIG. 5, the irradiation position of the laser beam 22 on the heater lead 17 or the electrode lead 18 is in contact with the outer peripheral edge of the laser beam 22 completely including the lead wire 19 and the center of the laser beam 22 is farther from the sensitive film 14. It is configured to come to The diameter of the laser beam 22 is
When the diameter is larger than the diameter of the lead wire 1, the heater lead 17 and the electrode lead 18 are also irradiated with the laser beam 22.
Since the thickness of the heater lead 17 and the electrode lead 18 is smaller than the diameter of the heater 9, the heater lead 17 and the electrode lead 18 are damaged. For example, if the center of the laser beam 22 matches the center of the lead wire 19,
In the extreme case, the platinum component is completely lost around the lead wire 19 because the platinum at the position irradiated with the metal melts and gathers on the lead wire 19, and the lead wire 19 and the heater lead 17 or the electrode lead 18 are joined. May be insulated despite the presence. The irradiation position of the laser beam 22 is set such that the outer peripheral edge of the laser beam 22 is completely in contact with the lead wire 19 and the center of the laser beam 22 is farther from the sensitive film 14, so that the sensitive film 14 is covered. The conductivity can be ensured by eliminating defects.

(Embodiment 4) FIG. 5 is a sectional view of a main part of Embodiment 4 of the present invention. Since the basic configuration is the same as that of the above embodiment, only different points will be described. In FIG. 5, the laser beam 2
The two diameters are approximately equal to the diameter of the lead wire 19. With this configuration, it is possible to prevent the heater lead 17 and the electrode lead 18 having a small thickness with respect to the diameter of the lead wire 19 from being damaged by the laser beam 22, so that a favorable bonding state can be realized.

(Embodiment 5) FIG. 6 is a perspective view of a main part of Embodiment 5 of the present invention. Since the basic configuration is the same as that of the above embodiment, only different points will be described. In FIG. 5, the shape of the lead wire 19a at the joint is rectangular. By making the cross section rectangular, the width of the lead wire is widened, so that the laser beam 22 is prevented from coming off the lead wire, and the laser beam 22 is uniformly applied to the lead wire 19, so that a stable bonding state is ensured. be able to.

[0031]

As described above, according to the present invention, in addition to the fact that the lead wire and the conductive paste are instantaneously melted and joined by laser irradiation at the joining point of the lead wire, the electrode lead and the heater lead, A contact point is protected by the glass coat by forming a glass coat outside the joining point due to the difference in melting point and surface tension, and this glass coat and the glass contained in the conductive paste are joined to form a strong contact. As a result, a stable output can be obtained for a long period of time.

[Brief description of the drawings]

FIG. 1 is a perspective view of a gas sensor according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a main part of the gas sensor according to the first embodiment of the present invention.

FIG. 3 is an exploded perspective plan view of a gas sensor according to a second embodiment of the present invention.

FIG. 4 is a plan view of a main part of a gas sensor according to a third embodiment of the present invention.

FIG. 5 is a plan view of a main part of a gas sensor according to a fourth embodiment of the present invention.

FIG. 6 is a perspective view of a main part of a gas sensor according to a fourth embodiment of the present invention.

FIG. 7 is a perspective view of a conventional gas sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Substrate 12 Heater 13 Insulating film 14 Sensitive film 15a, 15b Detection electrode 16 Catalyst 17 Heater lead 18 Electrode lead 19 Lead wire 22 Laser beam

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Takashi Niwa, Inventor 1006 Kadoma, Kazuma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (72) Kunihiro Tsuruta 1006, Kadoma, Kazuma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. 72) Inventor Takahiro Umeda 1006 Kadoma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor Mitsuhiro Shiba 1006 Odaka Kadoma Kadoma City, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. 1006 Kadoma Kadoma Matsushita Electric Industrial Co., Ltd. F term (reference) 2G004 BB04 BD04 BE12 BE22 BF07 BH15 BJ03 BL08 BM05

Claims (8)

[Claims] 1. A heater formed on a substrate having low thermal conductivity, an insulating film formed on the heater, a sensitive part laminated on the insulating film, and a detection part formed on the sensitive part. An electrode, a heater lead formed by thick-film printing of a conductive paste formed by mixing a glass-based adhesive connected to the heater film, and a conductive lead formed by mixing a glass-based adhesive connected to the detection electrode. A gas sensor comprising: an electrode lead formed by thick film printing of a conductive paste; and a lead wire joined to the heater lead and the electrode lead by laser welding. 2. The gas sensor according to claim 1, wherein the heater lead, the electrode lead, and the lead wire have the same main component. 3. The gas sensor according to claim 1, wherein the heater lead and the electrode lead are formed by thick film printing using a paste formed by mixing a glass adhesive with a noble metal such as gold or platinum. 4. The gas sensor according to claim 3, wherein the heater lead and the electrode lead have a multilayer structure of at least two or more layers, and the first layer is configured to include more glass than the multilayer. 5. The gas sensor according to claim 4, wherein the first layer is heat-treated at a temperature higher than the melting point of the glass, and the second layer is heat-treated at a temperature higher than the glass transition point of the glass and lower than the melting point. 6. The laser beam irradiation position on the heater lead or the electrode lead is such that the outer peripheral edge of the laser beam is in contact with the lead wire and the center of the laser beam is farther from the sensitive portion. 6. The gas sensor according to claim 5. 7. The gas sensor according to claim 1, wherein a beam diameter of the laser is substantially equal to a lead wire diameter. 8. The gas sensor according to claim 7, wherein the shape of the lead wire at the joining portion is rectangular in cross section.