Low temperature sintering aluminum paste for forming ptc thermistor aluminum electrode and preparation method thereof

Abstract

Low-temperature sintering aluminum paste for forming PTC thermistor aluminum electrode and preparation method, the present application relates to the field of conductor paste, in order to overcome the drawbacks brought by using glass powder in the traditional preparation method, different metals are used to form low melting point alloy composition according to certain proportion, and aluminum powder particles are used to form dense electrode layer, and glass powder is abandoned. The aluminum paste provided by the present application can realize good metallization at 450-600 DEG C, form good ohmic contact and mechanical combination with the PTC substrate, and improve the conductivity of the electrode layer. At the same time, it can also be used on the piezoresistor element to form ohmic contact electrode, after sintering at 500-600 DEG C, good ohmic contact and adhesion can also be formed, and the electrical performance and aging performance of the current general piezoresistor silver paste and copper paste can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of conductor paste, in particular to a low-temperature sintering aluminum paste for forming PTC thermistor aluminum electrode and a preparation method thereof. BACKGROUND

[0002] PTC (Positive Temperature Coefficient) thermistor is a special resistor whose resistance value increases with the increase of temperature. By utilizing its unique temperature sensitivity, the resistance value can be changed according to the change of temperature, so it is widely used in overcurrent protection, overheat protection, temperature measurement and control, etc. The current PTC thermistor electrode materials mainly have ohmic silver-zinc paste, electroplated nickel electrode, ohmic aluminum paste, etc. The ohmic silver-zinc paste generally contains 30-50% of silver and the rest is zinc, which has a high cost. With the market sensitivity to cost, silver-zinc paste is gradually being replaced by aluminum paste. Electroplated nickel electrode involves huge equipment investment and environmental protection problems, and its technical advantage is also shrinking. The cost advantage of ohmic aluminum paste is obvious, and the supply chain of raw materials including glass powder and aluminum powder is becoming more and more perfect due to the great development of solar aluminum paste, and its market share is also increasing.

[0003] The traditional aluminum paste used as PTC thermistor electrode is mainly composed of metal aluminum powder, glass powder and organic carrier, etc. The aluminum paste is prepared by mixing under the action of mechanical force, and is screen printed on a ceramic substrate to obtain an aluminum electrode after high-temperature sintering. The organic carrier can adjust the viscosity and process thixotropy of the paste, which is beneficial to screen printing and forming the electrode. In the process of high-temperature sintering, the organic carrier volatilizes and decomposes to separate from the paste system, and the glass powder gradually melts with the increase of temperature, penetrates and infiltrates into the PTC substrate, while the aluminum powder adheres to the substrate in the liquid glass and forms the electrode, and the aluminum powder effectively contacts with each other to play a conductive role.

[0004] The glass powder is a key material in the traditional aluminum paste, which is obtained by high-temperature melting, water quenching and ball milling of a series of oxides. As a binder in the aluminum paste, the glass powder also brings some negative effects to the aluminum paste with the deepening of application. One is that the glass powder will penetrate into the interior of the PTC ceramic substrate during the sintering process of the aluminum paste, thereby changing the overall composition distribution of the ceramic substrate and causing the PTC ceramic to not reach the designed electrical performance, such as large resistance value dispersion and large and small resistance values. Another is that the addition of the glass powder reduces the volume conductivity of the aluminum paste. In addition, most of the PTC thermistors requiring lead welding need to print a layer of surface silver paste, and the glass powder in the aluminum paste will also adversely affect the silver paste layer. The glass powder in the aluminum paste will react with the silver paste during the sintering of the silver paste, and transition to the silver paste layer, which will affect the matching of the silver paste and the aluminum electrode layer, and also affect the appearance and welding performance of the silver paste. SUMMARY

[0005] To solve the above problems, the ohmic aluminum paste described in the application discards the traditional glass powder, and utilizes different metals to form a low-melting alloy component in a certain proportion, and the aluminum powder particles are matched to form a dense electrode layer, and the application adopts the following technical solutions:

[0006] The application provides a low-temperature sintering aluminum paste for forming a PTC thermistor aluminum electrode, and the aluminum paste comprises metal powder (70-90% of the mass percentage of the paste), organic auxiliary agent (0.1-4.0% of the mass percentage of the paste), resin (0.5-5.0% of the mass percentage of the paste), and solvent (5-25% of the mass percentage of the paste).

[0007] The difference between the traditional ohmic aluminum paste and the application is that the application does not contain glass powder or oxide glass inorganic binder, and the problems of the change of the design electrical performance index caused by the penetration of the glass powder into the substrate during sintering, the decrease of the aluminum electrode conductivity, and the influence on the printed silver electrode are solved.

[0008] The metal powder comprises:

[0009] Aluminum powder (35-80wt% of the metal powder), zinc powder (5-40wt% of the metal powder), and

[0010] Nickel powder (2-20wt% of the metal powder);

[0011] And at least one of the following metal powders:

[0012] Bismuth powder (0-20wt% of the metal powder), magnesium powder (0-20wt% of the metal powder),

[0013] Iron powder (0-20wt% of the metal powder), ruthenium powder (0-20wt% of the metal powder), indium powder (0-20wt% of the metal powder), manganese powder (0-20wt% of the metal powder), titanium powder (0-20wt% of the metal powder), silver powder (0-20wt% of the metal powder), gold powder (0-20wt% of the metal powder), lead powder (0-10wt% of the metal powder), vanadium powder (0-10wt% of the metal powder), and / or alloy powder (0-30wt% of the metal powder);

[0014] The alloy powder refers to the pre-prepared alloy powder 1 and / or alloy powder 2, wherein the alloy powder 1 is a mixture of one or more of aluminum-copper alloy powder (copper accounts for 30-40wt% of the alloy powder), aluminum-magnesium alloy (magnesium accounts for 40-70wt% of the alloy powder), aluminum-zinc alloy powder (zinc accounts for 80-95wt% of the alloy powder), silver-bismuth alloy powder (bismuth accounts for 20-30wt% of the alloy powder), and the alloy powder 2 is an alloy powder composed of aluminum and one or more non-metallic elements such as silicon, boron, sulfur and phosphorus, wherein the aluminum accounts for 85-99wt%, and the rest is the non-metallic element. If the mixed ratio of the alloy powder 1 and the alloy powder 2 is 1:4-4:1.

[0015] The alloy powder is generally prepared by a conventional atomization method. First, the various metals and non-metallic elements are weighed according to the proportion, and the whole process is carried out under the protection of nitrogen atmosphere. The molten metal liquid is heated, and then sprayed out by high-pressure nitrogen to form small metal microdroplets. After the microdroplets are cooled, atomized metal alloy powder is obtained. By adjusting the process parameters of spraying, powders of different particle sizes are obtained. Subsequent classification treatment will be carried out to obtain alloy powder with ideal particle size.

[0016] The addition of the alloy powder makes the alloy powder and the low-temperature metal powder melt in advance during the sintering process, so that the un-melted metal powder particles are infiltrated and shrunk, filling the space left by the volatilization of the organic matter, and the powder particles are more likely to contact and penetrate each other, forming an ideal alloy structure. At the same time, the alloy powder can also control and guide the low-temperature metal and other metals to form a specific alloy composition and crystal structure, increase the density of the film layer and the adhesion to the ceramic matrix.

[0017] The morphology and particle size of the powder can be refined as follows: the aluminum powder is spherical, flaky or dendritic with D50 of 0.5-5.0 microns; the zinc powder is spherical, flaky or dendritic with D50 of 0.5-5.0 microns; the nickel powder is spherical, flaky or dendritic with D50 of 0.1-0.5 microns; and the other pure metal powders and alloy powders are spherical with D50 of 0.5-2.5 microns. The oxygen content of all metal powders and alloy powders ranges from 0 to 10000ppm.

[0018] The organic additive is one or more of the following: organosilicon additives (BYK-3451, BYK-3456, BYK-348, BYK-349), organoboron additives (triethanolamine borate, tributyl borate, tripropyl borate), organotin acids (dimethyl tin, dibutyl tin, di-zinc-based tin, stannous octoate), organotitanium acids (titanic acid tetrabutyl ester, tetraethyl titanate, tetraisopropyl titanate), organobismuth acids (bismuth isooctoate, bismuth laurate, bismuth neodecanoate, bismuth naphthenate), organozinc acids (zinc isooctoate, zinc stearate, zinc formate, zinc acetate).

[0019] The organic auxiliary agent provides printing performance support, facilitates film formation and drying adhesion, and decomposes to form corresponding metal oxides during the formation process (sintering process) of the metal electrode to improve the density and electrical properties of the electrode.

[0020] The resin is a mixture of two or more of ethyl cellulose, hydroxyethyl cellulose, polymethyl methacrylate, polyvinyl acetate, and styrene-maleic anhydride copolymer resin.

[0021] The solvent is a mixed solvent composed of two or more of terpineol, cyclohexane, diethylene glycol ethyl ether acetate, diol diacetate, and propylene glycol methyl ether acetate.

[0022] The application also provides a preparation method of the low-temperature sintering aluminum paste for forming the PTC thermistor aluminum electrode, and the method comprises the following steps:

[0023] 1) The various metal powders, organic auxiliary agents, resins, solvents and other materials are weighed according to the proportion, mixed in a kneader, and then sufficiently dispersed and uniformly distributed by a three-roll mill;

[0024] 2) The obtained material is placed in a vacuumized stainless steel sealed container and aged at 80-100°C for 48-168 hours, and then uniformly stirred to obtain the low-temperature sintering aluminum paste.

[0025] The aluminum paste provided by the application can realize good metallization at 450-600°C, form good ohmic contact and mechanical combination with the PTC substrate, and improve the electrical conductivity of the electrode layer. Meanwhile, the aluminum paste can also be used to form an ohmic contact electrode on a pressure-sensitive resistor element, and after sintering at 500-600°C, the aluminum paste can also form good ohmic contact and adhesion, and realize the electrical properties and aging properties of the commonly used pressure-sensitive silver paste and copper paste. DETAILED DESCRIPTION

[0026] First, the various metal powders, organic auxiliary agents, resins and solvents and other materials are weighed according to the formula proportion, mixed in a kneader for 1-5 hours, and then rolled by a three-roll mill for 1 hour to be sufficiently dispersed and uniformly distributed. The obtained paste is placed in a vacuumized stainless steel sealed container and aged at 80-100°C for 48-168 hours, and then uniformly stirred to obtain the low-temperature sintering aluminum paste.

[0027] The application has 8 embodiments in total, and the specific proportion formula of each embodiment is shown in Table 1 below:

[0028]

[0029] The composition of the metal powder of each embodiment (in mass percentage of the total metal powder) is as follows:

[0030]

[0031]

[0032] The composition of the organic auxiliary agent of each embodiment (in mass percentage of the slurry) is as follows:

[0033]

[0034]

[0035] The composition of the resin of each embodiment (in mass percentage of the slurry) is as follows:

[0036]

[0037] The composition of the solvent of each embodiment (in mass percentage of the slurry) is as follows:

[0038]

[0039]

[0040] The different ohmic aluminum paste obtained in Examples 1-8 was respectively printed on the front and back surfaces of a plurality of PTC thermistor substrates (provided by a domestic well-known PTC thermistor element manufacturer, with a resistance of 2.0-3.0 Ω) by means of screen printing, dried at 120°C, and sintered in a box furnace (560°C for 10 min). A plurality of PTC thermistor substrates with sintered aluminum electrodes were obtained. The resistance of each substrate at 25°C was measured using a multimeter, and the resistance R1 was obtained. Ten substrates were taken, the aluminum electrodes were polished off using sandpaper, indium-gallium electrodes were applied, and the resistance at 25°C was measured, and the bulk resistance R was obtained. Another ten substrates with sintered aluminum electrodes were taken, silver paste (a product currently available from Shenglong Special Materials Co., Ltd., with a product brand of SK-5570) was printed, and dried and sintered (540°C for 10 min), and the resistance of the silver-sintered substrate at 25°C was measured, and the resistance R2 after sintering with silver was obtained.

[0041] The following tests are described:

[0042] 1) Aluminum electrode sheet resistance test. The test was performed according to the method specified in the national standard GB / T1747.3-2008 “Test Methods for Noble Metal Paste for Microelectronic Technology-Sheet Resistance Determination”, and the film thickness was 20 microns.

[0043] 2) Change rate of the resistance of the PTC thermistor after sintering of the aluminum electrode = [(R1-R) / R]*100%

[0044] 3) The PTC thermistor after sintering aluminum electrode, the resistance value of the extreme difference = R1 最大值 - R1 最小值

[0045] 4) The PTC thermistor after sintering silver electrode, the resistance value change rate = [(R2-R1) / R1]*100%

[0046] 5) The PTC thermistor after sintering silver electrode, the resistance value of the extreme difference = R2 maximum value-R2 minimum value

[0047] 6) The resistance change rate after 30 days of storage at room temperature, for silver-based substrate, the resistance value change rate after 30 days of storage at room temperature in the natural condition in the room temperature warehouse. The resistance change rate after 30 days of storage = [(R2(30 days)-R2) / R2]*100%.

[0048] 7) On-off experiment, to simulate the actual use of PTC thermistor substrate conditions, to detect its stability and reliability, first 20 pieces of silver electrode after burning the substrate, fixture clamps the two electrodes of the substrate, fixed on the test machine, under 24V voltage, on 3 seconds off 60 seconds, for a cycle, 10,000 times on-off test cycle, after measuring the resistance value of the substrate is R3, the resistance value change rate = [(R3-R2) / R2]*100%.

[0049] The sheet resistance of aluminum electrode mΩ / □(electrode film thickness 20μm)

[0050] Example 1 2 3 4 5 6 7 Comparison 1 15 15 13 12.5 12 10 10.8 25

[0051] The glass powder is added in Comparative Example 1, and the sheet resistance is higher than that of Example 1, and the conductivity of Examples 1-7 is improved compared with Comparative Example 1.

[0052] The resistance value change rate of PTC thermistor after sintering aluminum electrode

[0053] Example 1 2 3 4 5 6 7 Comparison 1 2.56％ 2.50％ 2.26％ 2.05 1.89％ 1.05％ 1.15％ 3.50％

[0054] The resistance value of the extreme difference of PTC thermistor after sintering aluminum electrode (Ω)

[0055] Example 1 2 3 4 5 6 7 Comparison 1 0.62 0.58 0.59 0.55 0.6 0.56 0.57 1.51

[0056] The resistance value change rate of PTC thermistor after sintering silver electrode

[0057] Example 1 2 3 4 5 6 7 Comparison 1 1.50％ 1.34％ 1.25％ 1.10％ 1.15％ 1.02％ 1.08％ 2.45％

[0058] The resistance value of the extreme difference of PTC thermistor after sintering silver electrode (Ω)

[0059] Example 1 2 3 4 5 6 7 Comparison 1 0.63 0.58 0.58 0.57 0.6 0.56 0.57 1.75

[0060] Secondly, the resistance change rate of Comparative Example 1 is greater than that of Examples 1-7, and the resistance difference after the aluminum and silver sintering is also obviously greater, and the resistance dispersion is obvious. Especially, the resistance difference and change rate after the silver sintering, Comparative Example 1 is obviously greater than Examples 1-7, which shows that the matching of the aluminum electrode and the silver electrode in Examples 1-7 is better than that of Comparative Example 1.

[0061] Resistance change rate after storage for 30 days at room temperature

[0062] Example 1 2 3 4 5 6 7 Comparison 1 2.74％ 2.72％ 2.66％ 2.34％ 2.51％ 1.79％ 1.85％ 2.76％

[0063] Resistance change rate in on-off experiment (10000 times on-off experiment, 24V, on for 3 seconds and off for 60 seconds)

[0064] Example 1 2 3 4 5 6 7 Comparison 1 2.52％ 3.05％ 2.98％ 2.67％ 2.56％ 2.43％ 2.40％ 5.67％

[0065] Furthermore, the storage stability of Comparative Example 1 is similar to that of Example 1, but the on-off experiment result shows that the resistance stability under the simulated use condition is worse than that of Examples not containing glass powder. Moreover, by comparing the on-off experiment results of Examples 1, 6 and 7 containing alloy powder and Examples 2, 3, 4 and 5 not containing alloy powder, the resistance change rate of Examples 1, 6 and 7 is smaller than that of Examples 2, 3, 4 and 5, and the addition of the pre-prepared alloy powder has the effect of increasing the density of the film layer.

[0066] Weldability test and judgment

[0067] The silver electrode sintered substrate is inserted into the lead wire (0.6mm tinned copper wire) on the lead line, dipped in the flux, and then the sample is entirely immersed in the lead-free solder tin tank (solder material Sn96.5Ag3) with the temperature set at 245±3℃ for 3±0.3 seconds. The weldability inspection should be carried out under sufficient light conditions, using a 4╳ to 25╳ magnifying glass. The immersed workpiece silver brushed surface should be completely covered by the relevant solder material, without phenomena such as pinholes, surface non-wetting or excessive wetting, the coverage rate >95% and smooth and bright, which is good weldability. The coverage rate is 85%-95%, the tin layer is smooth, which is general weldability, the coverage rate is <80%, the tin layer is not smooth and dark, which is poor weldability. The test results of the examples and comparative examples are shown in the following table.

[0068] Adhesion test

[0069] The clamps of the tensile testing machine are used to clamp the lead wire of the round sheet resistance after the welding is completed, and the tensile testing machine is started to pull the lead wire in the opposite direction. When the lead wire is pulled off from the substrate, the maximum tensile force value is recorded, which is the adhesion value. The test results of the examples and comparative examples are shown in the following table.

[0070]

[0071] Due to the influence of glass powder, the welding performance of Comparative Example 1 (containing glass powder) is worse than that of Examples 1-7, although the adhesion is basically at the same level, but the minimum adhesion of Examples 1-7 is greater than that of Comparative Example 1, which also shows that the matching of aluminum electrode without glass powder with silver paste is better than that of aluminum electrode containing glass powder. Moreover, compared with Examples 1-7 which do not contain glass powder, the adhesion results of Examples 1, 6, 7 containing alloy powder and Examples 2, 3, 4, 5 not containing alloy powder, the adhesion of Examples 1, 6, 7 whether the maximum or minimum is greater than that of Examples 2, 3, 4, 5, which proves that the addition of pre-prepared alloy powder increases the density of the film layer and the adhesion with the substrate.

Claims

1. A low-temperature sintered aluminum paste for forming aluminum electrodes of PTC thermistors, characterized in that, The aluminum paste comprises 70-90% by weight of metal powder, 0.1-4.0% by weight of organic additives, and 0.5-5.0% by weight of resin, with the remainder being solvent; the metal powder comprises: The first component comprises aluminum powder (35-80 wt% of the total metal powder weight), zinc powder (5-40 wt% of the total metal powder weight), and nickel powder (2-20 wt% of the total metal powder weight); and The second component includes at least one of the following: bismuth powder, magnesium powder, iron powder, ruthenium powder, indium powder, manganese powder, titanium powder, silver powder, gold powder, lead powder, vanadium powder, etc., accounting for no more than 20 wt% of the total weight of the metal powder; and alloy powder accounting for no more than 30 wt% of the total weight of the metal powder. The alloy powder in the second component is one or more of pre-made alloy powder 1 or alloy powder 2. The alloy powder 1 is one or more of aluminum-copper alloy powder with copper accounting for 30-40 wt%, aluminum-magnesium alloy powder with magnesium accounting for 40-70 wt%, aluminum-zinc alloy powder with zinc accounting for 80-95 wt%, and silver-bismuth alloy powder with bismuth accounting for 20-30 wt%. The alloy powder 2 is an alloy powder composed of aluminum and one or more of the non-metallic elements silicon, boron, sulfur, and phosphorus. The aluminum content in the alloy powder 2 is 85-99 wt%, and the remainder is non-metallic elements.

2. The low-temperature sintering aluminum paste for forming aluminum electrodes of PTC thermistors as described in claim 1, characterized in that, The alloy powder is made by mixing alloy powder 1 and alloy powder 2 in a ratio of 1:4 to 4:

1.

3. The low-temperature sintering aluminum paste for forming aluminum electrodes of PTC thermistors as described in claim 1, characterized in that, The aluminum powder is a spherical, flake, or dendritic powder with a D50 of 0.5-5.0 micrometers; the zinc powder is a spherical, flake, or dendritic powder with a D50 of 0.5-5.0 micrometers; the nickel powder is a spherical, flake, or dendritic powder with a D50 of 0.1-0.5 micrometers; other metal powders and alloy powders are all spherical powders with a D50 of 0.5-2.5 micrometers; and the oxygen content of all metal powders and alloy powders ranges from 0 to 10,000 ppm.

4. The low-temperature sintering aluminum paste for forming aluminum electrodes of PTC thermistors as described in claim 1, characterized in that, The organic additive is one or more of the following: organosilicon additive, organoboron additive, organotin acid, organotitanium acid, organobismuth acid, and organozinc acid.

5. The low-temperature sintering aluminum paste for forming aluminum electrodes of PTC thermistors as described in claim 4, characterized in that, The organosilicon additive is one or more of BYK-3451, BYK-3456, BYK-348, and BYK-349; or, The organoboron additive is one or more of triethanolamine borate, tributyl borate, and tripropyl borate; or, The organic tin acid is one or more of dimethyltin, dibutyltin, dizincyltin, and stannous octoate; or, The organic acid titanium is one or more of tetrabutyl titanate, tetraethyl titanate, and tetraisopropyl titanate; or, The organic bismuth acid mentioned is one or more of bismuth isooctanoate, bismuth laurate, bismuth neodecanoate, and bismuth naphthenate; or, The organic zinc acid is one or more of zinc isooctanoate, zinc stearate, zinc formate, and zinc acetate.

6. The low-temperature sintering aluminum paste for forming aluminum electrodes of PTC thermistors as described in claim 1, characterized in that, The resin is a mixture of two or more of the following: ethyl cellulose, hydroxyethyl cellulose, polymethyl methacrylate, polyvinyl acetate, and styrene-maleic anhydride copolymer resin.

7. The low-temperature sintering aluminum paste for forming aluminum electrodes of PTC thermistors as described in claim 1, characterized in that, The solvent is a mixture of two or more of the following: terpineol, cyclohexane, diethylene glycol ethyl ether acetate, diol diacetate, and propylene glycol methyl ether acetate.

8. The method for preparing low-temperature sintered aluminum paste for forming aluminum electrodes of PTC thermistors as described in any one of claims 1-7, characterized in that, The preparation method includes the following steps: 1) Prepare materials according to the proportions, mix them in a kneader, and then fully disperse them evenly through a three-roll mill; 2) The obtained material is placed in a vacuum-sealed stainless steel container and aged at 80-100℃ for 48-168 hours, then stirred again to obtain the low-temperature sintered aluminum slurry.

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