JPH0737955B2 - Method of forming electrode terminal of sensor element - Google Patents

Description

The present invention relates to a method for forming an electrode terminal portion of a sensor element supported by a lead wire, particularly a gas sensor element, which receives a load of a gas sensor element, that is, a membrane electrode terminal is formed on an insulating substrate of the sensor element. And a method of joining a lead wire to the membrane electrode terminal.

[Prior art and its problems]

A gas sensor element in which a thin film or a thick film of a gas-sensitive material is formed on a ceramic insulating substrate is used to heat the sensor element by forming a heater on the back surface of the insulating substrate by a thin film method or a thin film method. I have to. This is because the temperature rise due to heating increases the adsorption / desorption rate of gas to / from the gas-sensitive material to improve the sensitivity characteristics and response speed characteristics of the sensor element, and the dirt adhering to the element surface is heated and burned to gas. The purpose is to always keep the detection part of.

In the sensor element that performs such heating, in order to reduce the power consumption of the heating heater, the chip of the sensor element is supported by floating in the air, and air is used as a heat insulating material to suppress heat radiation from the sensor element. The adiabatic mounting method is often used. In this adiabatic mounting type sensor, the sensor element is supported in the air by the lead wire, and therefore, for bonding the membrane electrode terminal on the ceramics insulating substrate to the ceramics insulating substrate and bonding the membrane electrode terminal to the lead wire, It is necessary to provide sufficient strength to hold the sensor element in place.

FIG. 5 shows the configuration of a general gas sensor. A lower electrode 2 and a membrane electrode terminal 3 are formed on a ceramic insulating substrate 1 by a normal thick film method. The gas-sensitive layer 4 is also formed on the lower electrode 2 by the thick film method, and the upper electrode 5 is also stacked by the thick film method, and the membrane electrode terminal 6 is formed on the ceramic substrate 1 so as to be continuous with the upper electrode 5. . Lead wires 7 and 8 are joined to the membrane electrode terminals 3 and 6. A heater 9 for heating is formed on the back surface of the ceramics substrate 1 by a thick film method as shown by a dotted line, and a film electrode terminal 10 formed in series with the heater 9 for heating is formed.
Lead wires 12 and 13 are joined to and 11. Lead wire 7,8,
12, 13 are welded to the stem pins 14, 15, 16, 17 by spot welding, respectively, and the sensor element is supported in the air by the lead wires 7, 8, 12, 13. As a result, a load due to the weight of the sensor element is applied to the joint between the insulating substrate 1 and the membrane electrode terminal and the joint between the membrane electrode terminal and the lead wire. FIG. 6 is an enlarged view of the joint between the membrane electrode terminal 3 and the lead wire 7.
The same applies to other membrane electrode terminals and lead wires.

In the prior art, the lower electrode 2 and the membrane electrode terminals 3, 6, 10 and 11 are all formed by screen-printing a platinum paste on the ceramic insulating substrate 1 to a thickness of 10 to 20 μm, followed by drying and firing. Platinum paste is also used for the upper electrode 5, and a part connected to the membrane electrode terminal 6 is screen-printed on the ceramic insulating substrate 1 and screen-printed on the gas-sensitive layer 4. Exactly the same. Referring to FIG. 6, a lead wire 7 made of platinum or a platinum alloy is joined to the membrane electrode terminal 3 after firing by spot welding. Spot welding is performed by joining while the lead wire is pressed against the membrane electrode terminal 3, and the membrane electrode terminal 3 and the lead wire 7 are materials of the same system, and heating / cooling of welding is performed between them. This is because there is an advantage that peeling or the like due to the difference in expansion coefficient does not occur.

By the way, it is stipulated that a gas leak alarm device using a gas sensor element can sufficiently withstand vibration and drop impact. The impact acceleration value applied in the most severe drop impact test is 170G. Since the sensor element uses a 0.5 mm-thick 2.5 mm square ceramics insulating substrate, its weight is about 12 mg, so the impact load applied to it is about 2 g. Since the safety factor is set to 5 times, if this is taken into consideration, the membrane electrode and the lead wire must have a strength that can withstand a load of 10 g and do not peel off. However, in the conventional technique of spot welding a platinum wire to a membrane electrode terminal formed using a platinum paste, the membrane electrode terminal and the lead wire are joined well as already described, but a thermal shock is applied during welding. Therefore, the bonding between the platinum membrane electrode terminal and the ceramics insulating substrate is weakened, and the membrane electrode terminal may be easily peeled off from the ceramics substrate, which may not satisfy the above-mentioned conditions capable of withstanding the impact load. There are wire bonding and brazing methods for joining lead wires that are less likely to be subjected to thermal shock, but the wire diameter of the wire that can be used is small enough to support the sensor element as a gas sensor in the wire bonding method. Cannot be applied to the lead wire. In addition, in the brazing method, it is necessary to coat the surface of the membrane electrode terminal and the lead wire with NiCr coating, vapor deposition or sputtering so that brazing can be performed easily. There is a problem that the joining work is troublesome, such as requiring a jig for fixing the electrode terminal surface.

[Object of the Invention]

An object of the present invention is to solve the above-mentioned problems and to provide a method for forming a membrane electrode terminal portion that can firmly maintain the bonding strength between a ceramics insulating substrate and a membrane electrode terminal of a sensor element without requiring any trouble. .

[Main points of the invention]

This invention uses a platinum rhodium paste, which has excellent bonding strength to a ceramics insulating substrate, as a material for a membrane electrode terminal, and spot-welds a gold or gold alloy lead wire to a fired membrane electrode terminal having a thickness of 5 μm or more and 50 μm or less. Bonding by a thermocompression bonding method that heat is applied more rapidly than that of the conventional method improves the bonding strength between the membrane electrode terminal and the ceramics insulating substrate, avoids applying a large thermal shock to the bonding part, and It is intended to improve the bondability with the membrane electrode.

Example of Invention

An embodiment of the present invention is shown in FIG. The relationship between the thickness of the platinum / rhodium film electrode terminal formed on the alumina substrate serving as the ceramics insulating substrate and the adhesion strength to the film electrode terminal is indicated by a line connecting black circles. The structure of the membrane electrode terminal portion is the same as that of FIG. 6 except that the material of the membrane electrode terminal is platinum rhodium, the lead wire is a gold wire, and the method of joining the lead wire to the membrane electrode terminal is thermocompression bonding. . Membrane electrode terminals are formed by screen-printing platinum rhodium paste on an alumina insulating substrate, evaporating the solvent by drying at room temperature, and then polymerizing and curing at 100 to 150 ° C, and further increasing the temperature to 180 ° C / hour up to 1400 ° C. It is performed by raising the temperature and baking at 1400 ° C. for 2 hours. A gold wire as a lead wire is joined to the membrane electrode terminal after firing by a thermocompression bonding method. The thermo-compression bonding method is a method in which a lead wire is brought into contact with a membrane electrode terminal, and a heater chip heated to the lead wire is pressed against the membrane electrode terminal to bond it to the membrane electrode terminal. Bonding is done by diffusion. Since this method has a longer heating time than the spot welding method, the thermal stress at the time of joining is relaxed, so that the joining state of the membrane electrode terminal and the alumina substrate is not hindered. Further, since the lead wire is pressed against the membrane electrode terminal by the heater chip at the time of joining, a jig is not particularly required to fix the lead wire. A load is applied to the gold wire bonded to the membrane electrode terminal by the above thermocompression bonding method, and the membrane electrode terminal is vertically peeled off from the alumina substrate, and the value of the load is defined as the adhesion strength in FIG. Since the adhesion strength varies, the average value of the values measured for a large number of membrane electrode terminals is shown by the solid line in FIG. 1, and the lower limit value that considers three times the standard deviation σ of the variations, that is, the value of -3σ is shown by the dotted line. It is indicated by. It can be seen that even if the lower limit of variation is taken into consideration, the condition of the required adhesion strength of 10 g is satisfied at a thickness of 5 μm. Therefore, the film thickness of 5 μm after firing is set as the lower limit value according to the method of the present invention. In FIG. 1, the measured values of the membrane electrode terminals formed by using the platinum paste are also shown by white circles for comparison. The one-dot chain line is the average value, and the two-dot chain line is -3σ. The conditions for forming the membrane electrode terminal using the platinum paste are exactly the same as the conditions for the membrane electrode terminal using the platinum rhodium paste described above, and the lead wire is also joined by the thermocompression bonding method of the gold wire. Therefore, it can be considered that the values shown here are in a state where there is no influence of thermal shock. It can be seen that even with the platinum film electrode terminal under such favorable conditions, a thickness of 9μ is required for the lower limit of the variation in adhesion strength to exceed 10 g, and the adhesion strength of the platinum rhodium film electrode terminal is far superior. Further, in the measurement range shown in FIG. 1, no abnormality was observed in the joining state between the lead wire and the membrane electrode terminal, and it has been proved that the joining by the thermocompression bonding method is sufficiently reliable.

In FIG. 1, it is recognized that the adhesion strength tends to increase with the film thickness. This is because when the film thickness of the membrane electrode terminal is thin, the strength of the film itself against the load applied via the lead wire is not sufficient, so the film breaks at the boundary of the joint between the lead wire and the film, which is equivalent to the aforementioned joint. It is explained that the load is concentrated on a very small area, and on the other hand, when the film thickness increases, the film itself can withstand the load and the load is distributed to the region corresponding to the entire area of the membrane electrode terminal. There is. Therefore, when the film thickness is thin, the film at the bonding portion of the lead wire is peeled off, and when the film thickness is thick, the entire membrane electrode terminal is peeled off. If the area of the sensor element is larger than 2.5 mm square and the thickness is larger than 0.5 mm, the load applied to the membrane electrode terminal is also increased, so that the membrane electrode terminal needs to be thick. When increasing the film thickness, it is necessary to repeat screen printing a plurality of times depending on the film thickness. The film thickness after firing is reduced to about half of the film thickness after printing, and the degree of this reduction depends on the compounding ratio of the raw material conductor powder and the binder to be kneaded in the thick film conductor paste such as platinum rhodium paste. Also, the film thickness per screen printing depends on printing pressure, printing speed, screen mesh size and mesh thickness, etc. in addition to the above composition ratio. There is a big difference between the specific gravity of the conductor and the specific gravity of the binder,
This is because, for example, in the case of platinum paste, the specific gravity of the binder is about 1 with respect to the specific gravity of platinum 21, and the capacity ratio of the binder for keeping the paste state is extremely large. In some cases, a plasticizer may be added, which further reduces the capacitance ratio of the conductor. 10 after normal firing
A film thickness in the vicinity of μm can be obtained by screen printing once or twice, but a film thickness larger than that cannot be obtained unless the number of printing is increased. However, it is considered that the practical screen printing is limited to 4 to 5 times, and the film thickness of 50 μm corresponding to this is set as the upper limit of the film thickness according to the present invention. From Figure 1
It can be estimated that the adhesion strength at a film thickness of 50 μm gives sufficient adhesion strength.

Table 2 is a graph showing the change in adhesion strength over time. The thickness of the membrane electrode terminals is 14 μm, the temperature of the insulating substrate is 400 ° C., and the four-lead wires are fixed to the stem pins in an adiabatic mounting structure. Is the result of measurement.

This result was obtained by extracting 40 sensor elements from one lot and performing 10 destructive tests for each of the initial, 5 days, 10 days, and 20 days, and the average value for 10 is shown. There is. All of the breakdowns occurred at the joint between the membrane electrode terminal and the insulating substrate. Therefore, it can be concluded that the adhesion strength between the membrane electrode terminal formed by the method of the present invention and the insulating substrate and the joint strength between the membrane electrode terminal and the lead wire are not deteriorated.

FIG. 3 is a sectional view showing a second embodiment of the present invention.
Increasing the thickness of the membrane electrode terminal 23 by superimposing the platinum rhodium membrane electrode 24 only on the location corresponding to the membrane electrode terminal 23 of the platinum rhodium electrode lead 22 printed on the ceramic insulating substrate 21.
After increasing the adhesion strength of this portion, a gold wire is used as a lead wire 25 and thermocompression bonded to the membrane electrode 24. This method is effective when it is not possible to provide the electrode lead 22 with a thickness that provides the necessary adhesion strength, for example, when there is an electrode lead connected to a heating heater whose film thickness is restricted by the relationship with the resistance value.

FIG. 4 shows a third embodiment of the present invention. Platinum rhodium film lead 27 is connected to the platinum film electrode in the same manner as the platinum film electrode lead 26, and the film electrode terminal 28 is required to have strong adhesion strength with the ceramic insulating substrate 21. Is formed of platinum rhodium, and a gold wire as a lead wire 29 is thermocompression bonded thereto. Platinum membrane electrodes are excellent as electrode materials because they have high conductivity except that they have a problem of adhesion strength when a load is applied. Further, since the material itself has a catalytic action, it has an action of promoting the activity of the gas sensitive film as an electrode of the gas sensor element. Therefore, the embodiment of FIG. 4 is effective in that the above advantages of platinum are effectively used and that sufficient adhesion strength is given to the membrane electrode terminal.

〔The invention's effect〕

According to the present invention, a film electrode terminal on a ceramics insulating substrate having a weight of about 12 mg, which constitutes a gas sensor element, is formed by screen-printing a platinum rhodium paste, and a gold or gold alloy lead wire is heated on a baked film electrode terminal of 5 μm or more and 50 μm. Since the bonding is performed by the crimping method, the adhesion strength between the membrane electrode terminal and the ceramics insulating substrate is higher than that of the conventional platinum membrane electrode, and the heating condition for joining the lead wire to the membrane electrode terminal is the spot welding method. It is relaxed compared to the conventional method and does not adversely affect the bonding state between the membrane electrode terminal and the ceramic substrate. Therefore, even if the lower limit of the variation in adhesion strength is 5
Adhesion strength of 10 g or more for a membrane electrode terminal having a thickness of 8 μm and 20 g or more for a film thickness of 8 μm or more is secured, and there is no change over time, and the strength is significantly improved over the conventional platinum membrane electrode terminal. Also, the connection between the lead wire and the membrane electrode terminal is good, which improves the reliability of the heat insulation mounting method, and the substrate is 2.5 mm.
It is possible to use small corners, and it has become possible to use the heater with low power consumption of about 0.5W.

[Brief description of drawings]

FIG. 1 is a relationship diagram between the film thickness and adhesion strength of a platinum rhodium film electrode terminal in an embodiment of the present invention, and FIG. 2 is a relationship diagram between adhesion strength of a film electrode terminal and elapsed time in an embodiment of the present invention. FIG. 3 is a sectional view of the second embodiment of the present invention, FIG. 4 is a perspective view of the third embodiment of the present invention, and FIG. 5 is a perspective view showing a heat insulating mounting state of a gas sensor element, and FIG. [FIG. 3] is an enlarged view of a membrane electrode terminal portion in the prior art. 1,21: Insulating substrate, 3,6,10,11,23,28: Membrane electrode terminal, 7,8,12,
13,25,29: Lead wire.

Claims (3)

[Claims] 1. An electrode terminal made of a platinum rhodium paste is screen-printed on a ceramics insulating substrate having a weight of about 12 mg to form a thickness after firing of 5 μm or more and 50 μm or less, and the electrode terminal and the ceramics insulating substrate are bonded together. Then, a method for forming an electrode terminal portion of a sensor element is characterized in that a lead wire is joined to the electrode terminal after firing by a thermocompression bonding method. 2. The method according to claim 1, wherein the lead wire is a gold wire, and the electrode terminal portion of the sensor element is formed. 3. A method for forming an electrode terminal portion of a sensor element according to claim 1, wherein the lead wire is an alloy gold wire.