Copper electrode slurry for NTC (Negative Temperature Coefficient) thermistor as well as preparation method and application of copper electrode slurry

Abstract

The invention relates to the technical field of electronic paste, in particular to copper electrode paste for an NTC thermistor and a preparation method and application of the copper electrode paste. A copper electrode slurry for an NTC thermistor comprises copper powder, an inorganic glass binder, an organic carrier and a dispersant, the inorganic glass binder comprises CuO and NiO, and the organic carrier comprises a polymer thickener. The copper electrode slurry can be coated on the surface of an NTC thermistor ceramic substrate through a silk-screen printing method, can be used as a metallized conductive layer after being sintered in nitrogen, is used for temperature measurement, temperature compensation, temperature control components and the like, and is wide in application. The copper electrode slurry for the NTC thermistor provided by the invention can be sintered in nitrogen at 550-650 DEG C, and has the advantages of wide sintering interval, good sintering appearance, excellent welding performance and strong adhesive force. After sintering, the ohmic contact is good, so that the prepared NTC thermistor can truly reflect the resistance value of a substrate, and the phenomenon of ion migration is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic pastes, and particularly relates to a copper electrode paste for NTC thermistors, a preparation method thereof, and an application thereof. Background Art

[0002] A negative temperature coefficient (NTC) thermistor is an electronic ceramic material whose resistance decreases as the temperature increases. It is mainly made of metal oxides such as manganese, cobalt, nickel, iron, and copper, and is manufactured by a ceramic process. It is widely used in temperature measurement, temperature compensation, surge current suppression, and other applications.

[0003] An NTC thermistor needs to have an electrode layer formed on its surface and then be connected to a circuit to realize its electrical properties. At present, the vast majority of the electrode layers used for NTC thermistor chips in the market are silver electrode layers, and a small number of high-end products use gold paste or palladium-silver paste as the surface electrode layer material for NTC thermistors.

[0004] However, in recent years, the prices of precious metals such as gold, palladium, and silver have been continuously high. The cost of precious metals is relatively high, and the resources are limited, making it difficult to reduce the production cost of NTCs.

[0005] The silver electrode layer also has certain defects. Due to the natural silver ion migration phenomenon of metallic silver, under the action of a direct current electric field, the silver ions generated by the reaction of silver with trace amounts of water molecules drift directionally, which will cause the resistance value to change and lead to the failure of the NTC. In addition, due to the need for high adhesion and good solderability, the sintering temperature of existing NTC thermistor silver pastes is generally above 750°C. The high temperature will cause certain damage to the thermistor ceramic substrate, increase the brittleness of the substrate, easily cause chemical reactions between the substrate and the paste, and easily generate cracks and chipping of the substrate during the dicing process, affecting the reliability.

[0006] Cu and the NTC thermistor ceramic substrate have good bonding strength. However, at present, there is no mature copper paste product applied to NTC thermistors in the market.

[0007] Patent CN104319043A discloses a manufacturing method for an electrode of a negative temperature coefficient thermistor chip. First, a layer of copper paste is printed and sintered on an NTC thermistor ceramic substrate, and then a layer of silver electrode is printed and sintered on the copper layer to improve the stability of the silver electrode. In this electrode layer preparation process, both copper paste and silver paste are used. Although the qualified rate of the resistance value accuracy of the product is improved, the two-print and two-sinter process is relatively complex, and actually increases the process difficulty.

[0008] The development and application of pure copper paste are mainly restricted by the following two factors: 1) During high-temperature sintering at 650 °C and above, chemical reactions easily occur at the interface between the copper layer and the NTC matrix during the copper paste sintering process, the ohmic contact rapidly deteriorates, and the resistance value increases exponentially; 2) During sintering below 600 °C, the adhesion between the copper layer and the NTC matrix is difficult to meet the requirements. Summary of the Invention

[0009] Aiming at the problems existing in the prior art, one of the purposes of this application is to provide a copper electrode paste for NTC thermistors.

[0010] The technical solution adopted in this application is as follows:

[0011] A copper electrode paste for NTC thermistors, comprising the following raw materials in parts by mass:

[0012] Copper powder: 82 - 88 parts,

[0013] Inorganic glass binder: 4 - 8 parts,

[0014] Organic carrier: 8 - 14 parts,

[0015] Dispersant: 0 - 0.2 parts, but not zero;

[0016] The inorganic glass binder includes 1 - 10 wt% CuO and 0.5 - 5 wt% NiO, and the organic carrier includes a polymer thickener.

[0017] Preferably, the inorganic glass binder further includes any two or more combinations of B2O3, Bi2O3, SiO2, and ZnO.

[0018] Preferably, the formula of the inorganic glass binder is: 1 - 10 wt% CuO, 0.5 - 5 wt% NiO, 10 - 20 wt% B2O3, 60 - 80 wt% Bi2O3, 1 - 3 wt% SiO2, 0.5 - 3 wt% ZnO.

[0019] Preferably, the copper powder is composed of fine spherical copper powder and coarse spherical copper powder mixed in a mass ratio of (60 - 80):(40 - 20), and the specific surface area of the fine spherical copper powder is greater than 5 m 2 / g, and the specific surface area of the coarse spherical copper powder is 1 - 3.5 m 2 / g.

[0020] Preferably, the weight loss of the copper powder by burning in nitrogen is less than or equal to 1%.

[0021] Preferably, the organic carrier further includes an organic solvent; the polymer thickener is ethyl cellulose and / or acrylic resin, the organic solvent is a combination of one or more of terpineol, alcohol ester 12, and butyl carbitol; the polymer thickener and the organic solvent are mixed in a mass ratio of (4-20):(80-96).

[0022] Preferably, the dispersant is a combination of one or more of RE610, Disperbyk 655, and BYK111.

[0023] Preferably, the method for preparing the copper electrode paste for NTC thermistors includes the following steps:

[0024] S1. Prepare the raw materials separately:

[0025] Prepare the organic carrier: Take a set mass of the polymer thickener and the organic solvent, mix them, heat the mixing system to 90-120°C, stir at 60-120 r / min for 4-5 h to obtain the organic carrier;

[0026] Prepare the inorganic glass binder: Mix the raw materials of the inorganic glass binder and melt them at 1150-1300°C, quench them with cold water, then grind them to a particle size less than or equal to 10 μm, and dry them at 120°C for standby;

[0027] Prepare the copper powder: Take the required mass of fine spherical copper powder and coarse spherical copper powder and mix them to obtain the copper powder for standby;

[0028] S2. Prepare the paste

[0029] Mix the prepared copper powder, organic carrier, and inorganic glass binder according to a set mass ratio to obtain a mixed raw material; grind the mixed raw material until the particle size of the mixed raw material is less than or equal to 10 μm, then add the dispersant and mix evenly to obtain the copper electrode paste for NTC thermistors.

[0030] The second object of the present application is to provide an application of the copper electrode paste for NTC thermistors as described above in the preparation of NTC thermistors.

[0031] Preferably, the method for preparing an NTC thermistor includes the following steps:

[0032] S1. Print the copper electrode paste on the NTC ceramic substrate by screen printing, and dry it at 150°C for 5-10 min;

[0033] S2. The printed NTC ceramic substrate is sintered in a nitrogen atmosphere, and the sintering process is as follows: the initial temperature is 25°C, the nitrogen flow rate is 30 L / min, the heating rate is 20°C / min. After heating to 400°C, the heating rate is adjusted to 10 - 15°C / min, and continue to heat to 650°C, then hold for 10 - 15 min. After the holding ends, it is naturally cooled to room temperature in the furnace.

[0034] The beneficial effects of this application are as follows:

[0035] The copper electrode paste for NTC thermistors provided in this application can be sintered in nitrogen at 550°C - 650°C, has a wide firing range, good firing appearance, excellent welding performance, and strong adhesion. After sintering, the ohmic contact is good, enabling the prepared NTC thermistor to truly reflect the matrix resistance value and there is no ion migration phenomenon.

[0036] Using this copper electrode paste, it is coated on the surface of the NTC thermistor ceramic substrate by screen printing. After sintering in nitrogen, it can be used as a metallized conductive layer for temperature measurement, temperature compensation, temperature control components, etc., and has a wide range of applications.

[0037] In this copper electrode paste, the addition amount of the inorganic glass binder is determined, and Cu and Ni elements identical to the NTC matrix composition are introduced into the used inorganic glass binder, enabling the copper paste to form a good ohmic contact with the matrix. If the addition amount of the inorganic glass binder is too small, it is difficult to ensure the adhesion; if the addition amount is too large, the solderability becomes poor, the interfacial effect is obvious, and the resistance value deviation is large.

[0038] The softening point of the inorganic glass additive used in the paste of this application is 500 - 550°C, and the final firing range of the copper electrode paste is 550°C - 650°C. At this temperature, the inorganic glass additive can be better distributed between the copper electrode layer and the matrix. If the sintering temperature is too low, the inorganic glass additive cannot effectively provide adhesion; if the sintering temperature is too high, a chemical reaction occurs between the copper layer and the NTC matrix, the ohmic contact rapidly deteriorates, and the resistance value increases exponentially. Description of the Drawings

[0039] Figure 1 It is the firing appearance diagram of different sintering processes of the copper paste in Example 1. The upper row is the abnormal state of the firing appearance in Example 1, and the lower row is the normal state.

[0040] Figure 2 It is the welding effect diagram of Comparative Example 1 and Example 2. The left figure in the drawing is the welding state of Comparative Example 1, and the right figure is the welding state of Example 2.

[0041] Figure 3 It is the welding lead pull-off effect diagram of Comparative Example 2 and Example 3. The left figure in the drawing is the welding pull-off state of Comparative Example 2, and the right figure is the welding pull-off state of Example 3. Detailed implementation mode

[0042] The present application will be further described in detail below in conjunction with embodiments.

[0043] A copper electrode paste for NTC thermistors, and the preparation method includes the following steps:

[0044] S1. Prepare raw materials separately:

[0045] Prepare the organic carrier: The polymer thickener is ethyl cellulose and / or acrylic resin, and the organic solvent is a combination of one or more of terpineol, film-forming aid, and butyl carbitol;

[0046] Weigh 4-20 parts of the polymer thickener and 80-96 parts of the organic solvent, mix them, heat the mixing system to 90-120 °C and stir well, with a stirring speed of 60-120 r / min and a stirring time of 4-5 h. After stirring evenly, the organic carrier is obtained;

[0047] Prepare the inorganic glass binder: The formula of the inorganic glass binder is: 10-20 wt% B2O3, 60-80 wt% Bi2O3, 1-3 wt% SiO2, 0.5-3 wt% ZnO, 1-10 wt% CuO, 0.5-5 wt% NiO.

[0048] After mechanically mixing the raw materials of the inorganic glass binder, heat them to melting at 1150-1300 °C in a muffle furnace, keep them warm for 10 min, pour them into cold water for quenching, then ball mill them in a water medium for 48 h until the particle size of the powder is less than or equal to 10 μm, dry them at 120 °C, and test the softening point to be 500-550 °C for standby;

[0049] Prepare copper powder: Mix 60-80 parts of fine spherical copper powder and 20-40 parts of coarse spherical copper powder to obtain copper powder for standby; among them, the specific surface area of the fine spherical copper powder is greater than 5 m 2 / g, and the specific surface area of the coarse spherical copper powder is 1-3.5 m 2 / g, and the loss on ignition in nitrogen is ≤1%.

[0050] S2. Preparation of the paste

[0051] Mix and stir 82-88 parts of copper powder, 8-14 parts of the prepared organic carrier, and 4-8 parts of the prepared inorganic glass binder to obtain a mixed raw material;

[0052] Grind the mixed raw material on a three-roll mill until the particle size of the powder of the mixed raw material is less than or equal to 10 μm, then add a dispersant and mix evenly to obtain a copper electrode paste for NTC thermistors. The dispersant is any one or a combination of RE610, Disperbyk 655, and BYK111.

[0053] According to the above method, Examples 1-3 and Comparative Examples 1-4 were prepared respectively, resulting in a total of 7 groups of copper electrode pastes. The specific formulations are shown in Tables 1 and 2 below.

[0054] Table 1 Inorganic glass binder formulation / g

[0055]

[0056] Table 2 Copper electrode paste ratio in Examples / mass fraction

[0057]

[0058]

[0059] Table 3 Copper electrode paste ratio in Comparative Examples / mass fraction

[0060]

[0061] The copper electrode pastes of Examples 1-5 and Comparative Examples 1-6 were respectively subjected to metallization firing. The steps were as follows: The prepared copper electrode paste was printed on the NTC ceramic substrate by screen printing, dried at 150 °C and then put into a mesh belt furnace for firing. The nitrogen flow rate was 30 L / min. The sintering process was: The initial temperature was 25 °C, the nitrogen flow rate was 30 L / min, the heating rate was 20 °C / min. After heating to 400 °C, the heating rate was adjusted to 10-15 °C / min, and heating was continued to 650 °C, then held for 10-15 min to obtain the NTC ceramic copper metallization layer.

[0062] The performance comparison results are as follows:

[0063] The firing appearance of the copper pastes in Examples with different sintering processes is shown in Figure 1 , Figure 1 Among them, the upper row of pictures shows the abnormal firing appearance of Example 1, and the lower row of pictures shows the normal state. Among them, for the leftmost one in the upper row, the sintering process: the nitrogen flow rate is 20 l / min, the highest temperature is 650 °C. For the middle one in the upper row, the sintering process: the nitrogen flow rate is 10 / min, the highest temperature is 650 °C. For the rightmost one in the upper row, the sintering process: the nitrogen flow rate is 30 l / min, the highest temperature is 800 °C. For the normal state in the lower row, the sintering process: the nitrogen flow rate is 30 l / min, the highest temperature is 650 °C.

[0064] The welding effect of Comparative Example 5 and Example 1 is shown in Figure 2 , the left picture shows the welding state of Comparative Example 5, with poor weldability, and the right picture shows the welding state of Example 1, with excellent weldability.

[0065] The welding lead pull-off effect diagrams of Comparative Example 6 and Example 3 are shown in Figure 3, The left figure shows the welding pull-off state of Comparative Example 6, where the copper layer is pulled off by the lead wire and the bonding strength is poor. The right figure shows the welding pull-off state of Example 3, where the NTC substrate is directly pulled off by the lead wire and the film layer bonding strength is reliable.

[0066] Technical indicators achievable by Examples 1-5:

[0067] Appearance: paste, bright red, uniform and delicate

[0068] Particle size: ≤10μm

[0069] Viscosity: 100-250Pa·s (25℃, 5rpm)

[0070] Firing temperature: 550~650℃

[0071] Film thickness: 5~15μm

[0072] Adhesion: ≥10N / mm 2

[0073] Ohmic contact: good, truly reflecting the substrate resistance

[0074] Aging adhesion: adhesion reduction less than 50% (100℃, 168 hours)

[0075] Comparative Example 1: The inorganic glass powder formula contains no copper and nickel elements, the adhesion of the sample is small, and the resistance value shifts.

[0076] Comparative Example 2: The inorganic glass powder formula does not contain copper element, the adhesion of the sample is small, and the resistance value shifts.

[0077] Comparative Example 3: The inorganic glass powder formula does not contain nickel element, the adhesion of the sample is small, and the resistance value slightly shifts.

[0078] Comparative Example 4: The proportion of copper and nickel elements in the glass powder formula is relatively high, the adhesion of the sample is relatively small, the firing appearance is abnormal, and the resistance value deviates.

[0079] Comparative Example 5: The addition amount of inorganic glass powder is relatively large, the solderability of the sample is poor, the pull-off force is small, and the resistance value shifts.

[0080] Comparative Example 6: The addition amount of inorganic glass powder is relatively small, the adhesion of the sample is small, and the resistance value slightly shifts.

[0081] The specific test results of the resistance values (Ω) of the example samples, the reference silver paste samples, and the comparative example samples are shown in Table 4 below:

[0082] Table 4 Test results of resistance values (Ω)

[0083]

[0084]

[0085] As can be seen from Table 4, the NTC resistances tested with the copper pastes of Examples 1-5 are comparable to those of the benchmark silver paste, and the resistance concentration is high. In Comparative Examples 1-3, the addition amounts of copper and nickel elements in the inorganic glass powder formulation are insufficient, resulting in resistance deviation of the samples. In Comparative Example 4, the addition amount of copper and nickel elements in the inorganic glass powder formulation is too high, leading to serious resistance deviation of the samples. In Comparative Example 5, the addition amount of the inorganic glass powder is too much, resulting in relatively large resistance deviation of the samples. In Comparative Example 6, the addition amount of the inorganic glass powder is small, resulting in slight resistance deviation of the samples. The resistance values of the copper paste samples in Comparative Examples 4-5 are significantly higher than the standard resistance value, indicating that the introduction of copper and nickel elements in the glass formulation promotes the ohmic contact characteristics of the product, but the proportion cannot be too high.

[0086] Adhesion of the sample in the Example (N / mm 2 ) The specific test results are shown in Table 5 below:

[0087] Table 5 Adhesion of the sample in Example 2 (N / mm 2 ) Test results

[0088]

[0089] As can be seen from Table 5, the adhesion of the sample in Example 2 is good, and the adhesion decreases less after being placed in an oven at 100 °C for 168 hours.

[0090] Adhesion of the sample in the Example and the comparative example (N / mm 2 ) The specific test results are shown in Table 6 below:

[0091] Table 6 Adhesion of the samples in the Example and the comparative examples (N / mm 2 ) Test results

[0092]

[0093]

[0094] As can be seen from Table 6, the adhesion of the copper paste samples using Examples 1-5 is ≥10 N / mm 2 , meeting the index requirements. In Comparative Examples 1-3, the inorganic glass powder for the copper paste does not contain copper and nickel elements, resulting in relatively low adhesion of the samples. In Comparative Example 4, more copper and nickel elements are added to the inorganic glass powder for the copper paste, and the adhesion of the sample is improved compared to Comparative Examples 1-3, but the proportion of copper and nickel elements may be too high, resulting in significantly lower adhesion of the copper paste sample in Comparative Example 4 than that in the Example. In Comparative Example 5, the addition amount of the inorganic glass powder is large and the solderability is poor, resulting in the inability to truly test the adhesion. In Comparative Example 6, the addition amount of the inorganic glass powder is small, and it is difficult for the copper layer to be tightly bonded to the NTC matrix, resulting in poor adhesion of the sample.

[0095] The specific test results of the resistance values (Ω) of the sample in Example 1 after sintering at different temperatures are shown in Table 7 below:

[0096] Test Results of Resistance Values (Ω) after Sintering at Different Temperatures in Table 7

[0097]

[0098] As can be seen from Table 7, for the samples of Example 1, when sintered at 550 °C and 650 °C, the resistance values are normal. When sintered at 680 °C, the resistance values increase exponentially and are discrete. Therefore, 650 °C is selected as the optimal sintering temperature.

[0099] The above are only the preferred practical examples of the present invention and are not used to limit the present invention; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should all be included within the protection scope of the present invention.

Claims

1. A copper electrode paste for NTC thermistors, characterized in that, The raw materials contain the following parts by mass: Copper powder: 82 - 88 parts, Inorganic glass binder: 4 - 8 parts, Organic carrier: 8 - 14 parts, Dispersant: 0 - 0.2 parts, but not 0; The inorganic glass binder includes 1 - 10 wt% CuO and 0.5 - 5 wt% NiO; the organic carrier includes a polymer thickener.

2. The copper electrode paste for NTC thermistor according to claim 1, characterized in that The inorganic glass binder further includes a combination of any two or more of B2O3, Bi2O3, SiO2, and ZnO.

3. The copper electrode paste for NTC thermistor according to claim 2, characterized in that The formulation of the inorganic glass binder is: 1 - 10 wt% CuO, 0.5 - 5 wt% NiO, 10 - 20 wt% B2O3, 60 - 80 wt% Bi2O3, 1 - 3 wt% SiO2, 0.5 - 3 wt% ZnO.

4. The copper electrode paste for NTC thermistor according to claim 1, characterized in that, The copper powder is composed of fine spherical copper powder and coarse spherical copper powder mixed in a mass ratio of (60 - 80):(40 - 20), and the specific surface area of the fine spherical copper powder is greater than 5 m 2 / g, and the specific surface area of the coarse spherical copper powder is 1 - 3.5 m 2 / g.

5. The copper electrode paste for NTC thermistor according to claim 4, characterized in that The weight loss of the copper powder after calcination in nitrogen is less than or equal to 1%.

6. A copper electrode paste for an NTC thermistor as described in claim 1, characterized in that, The organic carrier further includes an organic solvent; the polymer thickener is ethyl cellulose and / or acrylic resin, the organic solvent is a combination of one or more of terpineol, alcohol ester twelve, and butyl carbitol; the polymer thickener and the organic solvent are mixed in a mass ratio of (4 - 20):(80 - 96).

7. The copper electrode paste for NTC thermistor according to claim 1, characterized in that, The dispersant is a combination of one or more of RE610, Disperbyk 655, and BYK111.

8. A copper electrode paste for an NTC thermistor according to any one of claims 1-7, characterized in that, The preparation method of the copper electrode paste for NTC thermistors includes the following steps: S1. Prepare the raw materials separately: Prepare the organic carrier: Take a set mass of the polymer thickener and the organic solvent, mix them, heat the mixing system to 90 - 120 °C, stir at 60 - 120 r / min for 4 - 5 h to obtain the organic carrier; Prepare the inorganic glass binder: Mix the raw materials of the inorganic glass binder and melt them at 1150 - 1300 °C, quench with cold water, then grind to a particle size less than or equal to 10 μm and dry at 120 °C for standby; Prepare the copper powder: Take the required mass of fine spherical copper powder and coarse spherical copper powder and mix them to obtain the copper powder for standby; S2. Prepare the paste Mix the prepared copper powder, organic carrier, and inorganic glass binder according to the set mass ratio to obtain a mixed raw material; grind the mixed raw material until the particle size of the mixed raw material is less than or equal to 10 μm, then add the dispersant and mix evenly to obtain the copper electrode paste for NTC thermistors.

9. Application of the copper electrode paste for NTC thermistors as described in claim 8 in the preparation of NTC thermistors.

10. The application according to claim 9, wherein The method for preparing an NTC thermistor includes the following steps: S1. Print the copper electrode paste on the NTC ceramic substrate by screen printing and dry at 150 °C for 5 - 10 min; S2. Sinter the printed NTC ceramic substrate in a nitrogen atmosphere. The sintering procedure is: the initial temperature is 25 °C, the nitrogen flow rate is 30 L / min, the heating rate is 20 °C / min. After heating to 400 °C, adjust the heating rate to 10 - 15 °C / min and continue heating to 650 °C, then hold for 10 - 15 min. After the holding ends, cool naturally to room temperature with the furnace.

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