High temperature resistant conductive adhesive and preparation method thereof

Abstract

The present invention provides a high-temperature resistant conductive adhesive and a preparation method thereof, relating to the technical field of conductive adhesives. The high-temperature resistant conductive adhesive of the present invention comprises the following components by weight: 60-80% of a conductive filler, 15-30% of a bisphenol A cyanate ester resin, 3-8% of bismaleimide, 0.015-0.03% of a curing agent, 0.15-0.3% of a coupling agent, 0.15-0.3% of a silicone defoamer, and 1-3% of a diluent. The present invention uses bismaleimide as a toughening agent to modify the bisphenol A cyanate ester resin. By limiting its dosage and optimizing the types and contents of other components in the conductive adhesive composition, the prepared conductive adhesive has good mechanical properties and low resistivity, and also has high temperature resistance and excellent room temperature storage performance. The comprehensive performance is outstanding and has broad application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of conductive adhesives, and in particular to a high-temperature resistant conductive adhesive and a preparation method thereof. Background Art

[0002] Adhesive technology has become a major joining method alongside soldering and mechanical bonding. Conductive adhesives are composed of conductive fillers, solvents, and additives. Over the past few decades, the application of conductive adhesives in the microelectronics industry has rapidly expanded, and global and domestic production has also been steadily increasing. Room-temperature curing conductive adhesives are primarily used in high-tech fields such as aerospace, electronics, military, and the automotive, machinery, and vehicle industries. In certain specific applications, such as adhesives used in certain rocket engine components, not only must meet minimum temperature coefficient of resistivity requirements but must also withstand temperatures of 200°C or transients of 400-500°C. Spot welding adhesives must withstand temperatures below 240°C. In the aerospace and military industries, chips must be sealed with gold-tin sealant after bonding to ensure low gas density. However, the eutectic temperature of gold-tin solder reaches as high as 280°C, but conventional conductive adhesives cannot withstand temperatures exceeding 300°C and produce low gas.

[0003] Currently, most conductive adhesives on the market have poor overall performance. While meeting good mechanical properties and resistivity, they struggle to guarantee high-temperature resistance, making them unsuitable for applications such as precision instruments where high-temperature conductive adhesives are required. Alternatively, some high-temperature conductive adhesives exhibit unstable conductivity when used in high-temperature environments. Therefore, there is a need for a new high-temperature conductive adhesive that combines excellent mechanical properties (such as mechanical properties and adhesion) with excellent conductivity and heat resistance. Summary of the Invention

[0004] The purpose of the present invention is to provide a high-temperature resistant conductive adhesive and a preparation method thereof, which has good mechanical properties, excellent conductivity and heat resistance, in order to solve the technical problem of poor comprehensive performance of conductive adhesives in the prior art.

[0005] To solve the above problems, the present invention provides the following technical solutions:

[0006] In one aspect, the present invention provides a high-temperature resistant conductive adhesive, which includes the following components by weight: 60-80% conductive filler, 15-30% bisphenol A cyanate resin, 3-8% bismaleimide, 0.015-0.03% curing agent, 0.15-0.3% coupling agent, 0.15-0.3% silicone defoaming agent, and 1-3% diluent.

[0007] In the present invention, bismaleimide is used as a toughening agent.

[0008] Optionally, the conductive filler is flaky silver powder with a particle size of 5 μm-10 μm.

[0009] Optionally, the curing agent is selected from one or more of organotin, zinc acetylacetonate and nonylphenol.

[0010] Optionally, the coupling agent is KH570 coupling agent.

[0011] Optionally, the organosilicon defoamer is selected from any one of an oil-type defoamer, a solution-type defoamer, an emulsion-type defoamer, a solid-type defoamer and a modified silicone oil-type defoamer.

[0012] Optionally, the diluent is selected from acetone and / or butanone; and the solvent is selected from one or more of ethanol, propanol and isopropanol.

[0013] In another aspect, the present invention provides a method for preparing the above-mentioned high temperature resistant conductive adhesive, comprising the following steps:

[0014] S1: Add bismaleimide, curing agent and diluent to bisphenol A cyanate ester resin,

[0015] Mix and stir evenly to obtain a mixture;

[0016] S2: adding the conductive filler and the coupling agent into a solvent, dispersing and dissolving them, drying and grinding them to obtain a conductive filler powder;

[0017] S3: adding the conductive filler powder to the mixture to obtain a resin mixture;

[0018] S4: adding an organosilicon defoamer to the resin mixture and performing vacuum degassing to obtain a conductive adhesive;

[0019] S5: curing the obtained conductive adhesive and performing a temperature-raising treatment.

[0020] Optionally, the curing temperature rising conditions are: 0-30 min, 50°C; 31-60 min, 80°C; 61-90 min, 120°C; 91-120 min, 150°C.

[0021] Optionally, the vacuum degassing is carried out in a vacuum drying oven at 50-100°C.

[0022] Optionally, the viscosity of the organosilicon defoaming agent is 160-200 mPa·s.

[0023] The advantages of the high temperature resistant conductive adhesive and preparation method of the present invention over the prior art are:

[0024] Although bisphenol A type cyanate resin itself has good high temperature resistance, low moisture absorption rate and good dielectric property, but its toughness is very poor, can generate the very large triazine ring structure of cross-linking density in curing process, cause brittleness big, the present invention selects to add toughening agent to improve the toughness of bisphenol A type cyanate resin in bisphenol A type cyanate resin, particularly, select bismaleimide (BMI) to modify bisphenol A type cyanate resin as toughening agent, addition reaction has taken place after bismaleimide and bisphenol A type cyanate resin mix, can effectively improve the mechanical property of resin, and this reaction just can be carried out at a lower temperature. Bisphenol A type cyanate resin through bismaleimide modified had both the high heat resistance of cyanate resin, the advantage that dielectric property is good, have again the tensile strength of bismaleimide and the high characteristics of flexural strength, and most importantly, select bismaleimide not to influence the thermotolerance of cyanate resin as toughening agent.

[0025] The present invention optimizes the amount of toughening agent added, and the amount of toughening agent added is selected to be approximately 20-30% of the weight of the bisphenol A cyanate ester resin. When the bismaleimide content increases to a certain level, the density will become lower and lower, its modification effect will deteriorate, and the plasticity and toughness of the material after curing will decrease. When the toughening agent content reaches 40%, the bisphenol A cyanate ester resin will not provide sufficient adhesion to make the material a whole, and the conductive adhesive cannot be formed. Therefore, it is necessary to limit the content of the toughening agent to within the range of the present invention to achieve a good plasticity and toughness effect of the conductive adhesive after drying.

[0026] The present invention adopts a low-boiling-point diluent and controls the diluent dosage within 10 (wt)%. Controlling the diluent content within 10% will greatly improve the problem of conductive fillers (such as silver powder) precipitating and accumulating to cause non-conductivity, and can reduce the generation of bubbles. This plays an important role in preventing the generation of bubbles in the conductive adhesive, extending the storage time of the conductive adhesive, and improving the viscosity of the conductive adhesive.

[0027] The present invention limits the content of the conductive filler to 60-80 wt% of the conductive adhesive. Designing the conductive filler within this weight percentage range can avoid the problem of the resin being unable to cover all the silver particles due to excessive conductive filler, resulting in a significant decrease in shear strength; it can also avoid the problem of the conductive adhesive having excessively high resistivity and poor conductive performance due to insufficient conductive filler.

[0028] In summary, the present invention rationally arranges and optimizes the types and contents of raw material components, so that the conductive adhesive prepared has good mechanical strength and low resistivity, as well as high temperature resistance and excellent room temperature storage performance. Under the combined effect of various components, the volume resistivity of the conductive adhesive of the present invention is not higher than 4.5×10 -4 Ω·cm, the chip shear strength is greater than 12.25MPa, and it can withstand high temperatures of 300℃.

[0029] The preparation method of the present invention, on the one hand, utilizes a step-wise curing temperature ramp to allow the diluent to fully volatilize before the resin cures. The resulting conductive adhesive, produced by the step-wise heating, exhibits a smooth morphology and minimal bubbles. Furthermore, after obtaining the resin mixture, the present invention adds a silicone defoamer and performs vacuum degassing. This improved preparation process effectively eliminates the effects of bubbles on the conductive adhesive, resulting in improved mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of the method for preparing the high temperature resistant conductive adhesive of the present invention;

[0031] Figure 2 This is a thermal decomposition diagram of the cyanate ester resin provided by the present invention when heated to 400°C;

[0032] Figure 3 This is a thermal decomposition diagram of the high-temperature resistant conductive adhesive provided by the present invention after adding additives;

[0033] Figure 4 This is a graph showing the effect of different silver powder contents on the resistivity of the conductive adhesive provided by the present invention;

[0034] Figure 5 SEM images of conductive adhesives with different silver powder contents provided by the present invention;

[0035] Figure 6 The shear strength of the conductive adhesive with different silver powder contents provided by the present invention;

[0036] Figure 7 The microscopic morphology of the conductive adhesive with different silver contents (silver particles) provided by the present invention;

[0037] Figure 8 The scanning electron microscope images of the conductive adhesive provided by the present invention with different diluent contents are shown in FIG. (a) shows a diluent content greater than 10%; (b) shows a diluent content less than 10%.

[0038] Figure 9 Scanning electron microscope images of the conductive adhesive before and after adding the defoaming agent provided by the present invention. In the figure, (a) is before adding the defoaming agent, and (b) is after adding the defoaming agent;

[0039] Figure 10 DSC graphs of different curing agent resins provided by the present invention;

[0040] Figure 11 The change trend of cyanate gel time with temperature for different organotin dosages provided by the present invention;

[0041] Figure 12A schematic diagram of a cyanate resin provided by the present invention that cannot be molded after being cured with an excessive amount of toughening agent added;

[0042] Figure 13 This is the infrared spectrum of the silver powder with KH570 silane coupling agent added provided by the present invention;

[0043] Figure 14 The EDS elemental analysis results of the KH570 silane-coupled silver powder provided by the present invention;

[0044] Figure 15 This is a schematic diagram of the preparation process of high-temperature resistant conductive silver paste according to Example 1 of the present invention;

[0045] Figure 16 This is the curing temperature rise curve of the high temperature resistant conductive adhesive of Example 1 of the present invention;

[0046] Figure 17 The morphology of the conductive adhesive provided by the present invention is the step temperature curing and direct heating curing;

[0047] Figure 18 The morphology after shearing of the conductive adhesive treated with step temperature curing and direct heating curing provided by the present invention. DETAILED DESCRIPTION

[0048] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0049] Throughout this specification, references to terms such as "an embodiment," "one embodiment," "some embodiments," and "an implementation" indicate that the specific features, structures, materials, or characteristics described in connection with that embodiment or implementation are included in at least one embodiment or exemplary implementation of the invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or implementation. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or implementations.

[0050] One embodiment of the present invention provides a high-temperature resistant conductive adhesive, which includes the following components, by weight percentage: 60-80% conductive filler, 15-30% bisphenol A cyanate resin, 3-8% bismaleimide, 0.015-0.03% curing agent, 0.15-0.3% coupling agent, 0.15-0.3% silicone defoaming agent, and 1-3% diluent.

[0051] Resin compositions for high-temperature resistant conductive adhesives are generally divided into inorganic resin compositions and organic resin compositions. Among them, organic resins are more commonly used due to their good adhesion, good bonding strength, and very good environmental adaptability. The present invention uses high-temperature resistant bisphenol A cyanate resin, which is a thermosetting resin and the heat resistance of cyanate resin is significantly stronger than that of epoxy resin and phenolic resin. The cyanate group (-OCN) present in the cyanate resin can be cross-linked at high temperatures to form a triazine structure, with a decomposition temperature between 400°C and 410°C and low hygroscopicity.

[0052] The present invention selects bismaleimide (BMI) as a toughening agent to modify bisphenol A cyanate ester resin, so that the conductive adhesive has the advantages of high heat resistance and good dielectric properties of cyanate ester resin, and the high tensile and flexural strength of bismaleimide.

[0053] In some embodiments, the conductive filler is flaky silver powder with a particle size of 5 μm-10 μm.

[0054] In this invention, by screening and using different types of silver powder, it was discovered that silver flakes (flaky silver powder) can form better contact in the colloid than silver spheres (spherical silver powder particles). Silver flakes provide surface or line contact, which is superior to the point contact between silver spheres. Therefore, the conductive adhesive prepared with silver flakes has better conductivity than silver spheres. SEM images also show that the silver flakes are in full contact, while gaps exist between the silver particles.

[0055] In some embodiments, the curing agent is selected from one or more of organotin, zinc acetylacetonate and nonylphenol, preferably organotin.

[0056] In some preferred embodiments, the curing agent is dibutyltin dilaurate.

[0057] In the present invention, after adding different curing agents to the cyanate resin, the curing temperature of the resin system is greatly reduced, and the curing reaction effects of different curing agents are different. Under normal circumstances, the cyanate resin needs to be cured at about 300 ° C, but after adding the curing agent organotin, it only needs to be cured at about 120 ° C. After adding the curing agent zinc acetylacetonate, the curing temperature can also be reduced, but the effect is not as good as organotin. Therefore, the curing agent is preferably organotin (dibutyltin dilaurate). Organotin (dibutyltin dilaurate) can reduce the curing temperature of the resin as a curing agent, but the amount of organotin needs to be controlled within a certain range. The curing and gelation of the resin are controlled by the reactant concentration and the viscosity of the system. When the functional group concentration in the system is unchanged, increasing the organotin concentration can increase the reaction rate, accelerate the curing reaction, and rapidly increase the viscosity of the system. However, the increase in viscosity will in turn affect the reaction of the functional groups, thereby limiting the further increase in the reaction rate. Therefore, within a certain concentration range, the increase in the amount of organotin will increase the reaction rate. Beyond this range, the change in organotin concentration has no obvious effect on the reaction rate of the resin system.

[0058] In some embodiments, the coupling agent is KH570 silane coupling agent (γ-methacryloxypropyltrimethoxysilane).

[0059] In some embodiments, the diluent is selected from acetone and / or butanone. Preferably, acetone is used as the diluent, having a boiling point of 56.5°C. Preferably, the diluent is fully volatilized before heat curing. Diluent volatilization is related to the generation of bubbles during the surface curing of the conductive adhesive and the morphology of the prepared conductive adhesive. The more fully the diluent evaporates, the fewer bubbles are generated, and the more even the conductive adhesive morphology is.

[0060] In some embodiments, the solvent is selected from one or more of ethanol, propanol, and isopropanol.

[0061] In some embodiments, the organosilicon defoamer is selected from any one of an oil-type defoamer, a solution-type defoamer, an emulsion-type defoamer, a solid-type defoamer, and a modified silicone oil-type defoamer. For example, the organosilicon defoamer can be selected from any one of dimethylsilane, polysiloxane defoamer, and polyether-modified defoamer.

[0062] In some embodiments, the method for preparing the high temperature resistant conductive adhesive of the present invention comprises the following steps:

[0063] S1: adding bismaleimide, a curing agent, and a diluent to a bisphenol A cyanate ester resin, mixing and stirring to obtain a mixture;

[0064] S2: adding the conductive filler and the coupling agent into a solvent, dispersing and dissolving them, drying and grinding them to obtain a conductive filler powder;

[0065] S3: adding the conductive filler powder to the mixture to obtain a resin mixture;

[0066] S4: adding an organosilicon defoamer to the resin mixture and performing vacuum degassing to obtain a conductive adhesive;

[0067] S5: curing the obtained conductive adhesive and performing a temperature-raising treatment.

[0068] In some embodiments, in the preparation method, the order of step S1 and step S2 can be changed.

[0069] In some embodiments, during the preparation of the conductive filler, a coupling agent is coated on the conductive filler particles.

[0070] In some embodiments, in step S2 of the preparation method, the coupling agent is first dissolved in a solvent and mixed evenly by ultrasonic vibration, and then a conductive filler is added. After ultrasonic vibration, the conductive filler powder is obtained by filtering with distilled water, drying, and grinding.

[0071] In some specific embodiments, KH570 silane coupling agent is selected and the coupling agent is wrapped or adhered to the silver powder as follows:

[0072] KH570 silane coupling agent was dissolved in ethanol solvent to form a solution, and the solution was thoroughly mixed by ultrasonic vibration for 30 minutes; silver powder was added and ultrasonic vibration was performed for another 30 minutes; the solution was filtered with distilled water, dried in a vacuum drying oven at 100°C, and ground with a mortar to obtain conductive silver powder coated with the coupling agent.

[0073] In the present invention, the coupling agent can improve the dispersibility of the conductive filler in the matrix and enhance the bonding performance.

[0074] The flow chart of the method for preparing high temperature resistant conductive adhesive of the present invention is as follows Figure 1 The present invention explores and optimizes the components of the conductive adhesive, the content of each component, and the preparation method, as follows:

[0075] 1. Effect of additives on high temperature resistance of cyanate ester resin

[0076] The present invention selects to add 25% of bismaleimide, 0.1% of KH570, 0.1% of organotin, and 1% of organosilicon defoamer, respectively, based on the weight of bisphenol A cyanate ester resin, to the cyanate ester resin, and tests the effects of these additives on the high temperature resistance of the cyanate ester resin. The cyanate ester resin with the additives is subjected to a thermogravimetric test.

[0077] like Figure 2 and Figure 3As shown, the results show that after adding the additive, the cyanate ester resin still has strong high temperature resistance, and only decomposes less than 2% at 300°C. Although the high temperature resistance is lower than that of the cyanate ester resin without additives, it meets the high temperature resistance condition of 300°C.

[0078] 2. Optimization of silver flake (i.e. flaky silver powder) content

[0079] The present invention selects silver flakes with a radius of 5μm and prepares conductive adhesives with 50%-90% flaky silver powder content to test the effect of different silver powder content on the performance of the conductive adhesive. The volume resistivity of the conductive adhesive is measured by adding different silver powder contents of 50%, 60%, 70%, 80%, and 90%. The resistivity is as follows: Figure 4 Conductivity testing results show that at a silver content of 50%, the resistivity of the conductive adhesive is high, reaching 520 mΩ.cm. However, there is a significant drop between 50% and 60%, increasing to 43.9 mΩ.cm at 60%, after which the resistivity changes gradually, with percolation occurring between 50% and 60%. Resistivity is relatively low when the silver content is between 60% and 90%.

[0080] Figure 5 The SEM images show the microscopic morphology of the conductive adhesive, including (a) 50% silver flake content; (b) 60% silver flake content; (c) 70% silver flake content; (d) 80% silver flake content; and (e) 90% silver flake content. It can be seen that the flaky silver powder is relatively evenly dispersed in the colloid. However, when the flaky silver powder content increases and the specific gravity is too high (90%), it will agglomerate in the conductive adhesive.

[0081] Figure 6 The shear strength test results of conductive adhesives with different silver powder contents show that the addition of silver powder particles will significantly reduce the shear strength of the resin. When the silver powder content is 90%, the strength drops to 16MPa. This is consistent with the previous scanning electron microscopy results. It is because the resin content is too low to cover the silver particles, resulting in a significant decrease in shear strength.

[0082] In summary, the content of silver flakes (i.e., flaky silver powder) should be selected within the range of 60%-80%.

[0083] 3. Effect of different silver powder filler types on conductive properties

[0084] The conductive filler is spherical silver powder (silver particles with a particle size of 5 μm).

[0085] from Figure 7As the silver content increases, the silver particles become increasingly densely distributed within the resin, improving contact. At 90% silver content, as shown in Figures (g)-(h), the resin, due to the low content, cannot completely cover the silver particles, resulting in an uneven surface. The finished product is found to be non-conductive.

[0086] and Figure 5 By comparison, it can be seen that silver flakes (flake-like silver powder) can form better contact in the colloid than silver sphere particles (spherical silver powder). The silver flakes are in surface contact or line contact, which is better than the point contact between silver sphere particles. Therefore, the conductivity of the conductive adhesive prepared from silver flakes is better than that of silver spheres. It can also be seen from the SEM image that the silver flakes are in full contact, while there are gaps between the silver particles.

[0087] 4. Optimization of diluent content

[0088] When the diluent content is greater than 10%, silver powder precipitation and accumulation will occur, resulting in non-conductivity and a large number of bubbles. When the diluent content is less than 10%, this phenomenon is improved and the shelf life is greatly extended.

[0089] Figure 8 Scanning electron microscope images of the conductive adhesive with different diluent contents provided in Example 1 of the present invention. In the figure, (a) is a diluent content greater than 10%; b is a diluent content less than 10%.

[0090] 5. The effect of adding defoamer on conductive adhesive

[0091] During the preparation of conductive adhesive, the addition of a silicone defoamer during vacuum degassing can have varying effects on the adhesive's performance. After the resin was prepared, it was placed in a vacuum drying oven at 50°C for vacuum degassing. The addition of a silicone defoamer with a viscosity of 160-200 mPa·s significantly improved the adhesive's performance.

[0092] Figure 9 The scanning electron microscope images of the conductive adhesive before and after adding the defoaming agent provided by the present invention are shown in FIG. 1 , (a) is before adding the defoaming agent, and (b) is after adding the defoaming agent.

[0093] 6. Optimization of curing agent type and content

[0094] The present invention compares and analyzes the DSC curves of the cyanate resin after adding different curing agents and heating at a constant speed (heating rate of 10°C / min) with the resin system without adding catalyst. Figure 10 It can be clearly seen that the curing temperature of the resin system is greatly reduced after adding the curing agent, and different curing agents have different catalytic effects on the cyanate ester curing reaction.

[0095] Normal cyanate resin needs to be cured at around 300°C, but with the addition of organotin curing agent, curing can be completed at around 120°C. Adding the curing agent zinc acetylacetonate can also lower the curing temperature, but the effect is not as good as organotin. Therefore, the present invention preferably uses organotin as the curing agent for the conductive adhesive.

[0096] Regarding the dosage of organotin (dibutyltin dilaurate) as a curing agent for the resin: Although the curing agent can lower the curing temperature of the resin, the dosage should not be too much.

[0097] Table 1. Relationship between curing agent dosage and gel time

[0098]

[0099]

[0100] From Table 1 and Figure 11 As can be seen from the figure, at the same organotin dosage, the resin gel time shortens with increasing temperature. At the same temperature, when the organotin dosage is low, the resin gel time decreases significantly with increasing dosage, and after reaching a certain value, the reduction in gel time decreases. This is mainly because before the resin's gel point, the reaction rate is controlled by the reactant concentration and the system viscosity. When the functional group concentration in the system remains unchanged, increasing the organotin concentration can increase the reaction rate, accelerate the curing reaction, and rapidly increase the system viscosity. However, increased viscosity will in turn affect the reaction of functional groups, thereby limiting further increases in the reaction rate. Therefore, within a certain concentration range, increasing the organotin dosage will increase the reaction rate. Beyond this range, changes in organotin concentration have little significant effect on the reaction rate of the resin system.

[0101] Although the increase of organotin dosage can shorten the gel time of cyanate ester and accelerate the reaction rate, thereby shortening the curing cycle and reducing production costs, the increase of its dosage also can bring certain negative effects. First, excessive organotin makes the cyclotrimerization reaction of cyanate ester extremely intense, and the reaction process is difficult to control, and the reaction time is also difficult to grasp, thereby affecting the selection of reaction process. In addition, the consumption of organotin also can have an impact on the performance of the final product. Moreover, the content of catalyst can make the process of resin application period shorter too much, and the shelf life is shortened. Considering factors such as comprehensive curing temperature and gel time, the content of organotin is selected to be 0.1% of the cyanate ester resin content.

[0102] 7. Optimization of toughening agent type and dosage

[0103] Since cyanate ester resin has poor toughness after curing, in order to improve the mechanical properties of the conductive adhesive, a toughening agent bismaleimide is added to it to enhance the toughness of the material. The higher the content of the toughening agent, the better. As shown in Table 2, with the increase in the amount of bismaleimide, the performance shows a trend of first improving and then decreasing.

[0104] Table 2. Toughening agent ratio experiment

[0105]

[0106]

[0107] There is no consensus on the curing mechanism of bismaleimide toughened cyanate resin. One explanation is that after curing, they aggregate and cross-link to form an interpenetrating polymer network; another explanation is that the cyanate group of the cyanate resin reacts with the double bond of bismaleimide to generate two types of structures, pyrimidine and pyridine. The reason for the initial rise and then the fall may be that the bismaleimide and cyanate resin networks will penetrate each other. After the bismaleimide content increases to a certain level, the density becomes lower and lower, and its modification effect deteriorates. The plasticity and toughness of the material after curing decreases. When the toughening agent content reaches 40%, the resin can no longer provide sufficient adhesion to make the material a whole and cannot be formed. Figure 12 Therefore, the amount of toughening agent added in the present invention is selected to be about 25% of the base resin.

[0108] 8. Optimization of the dosage and usage of coupling agents

[0109] The present invention selects KH570 coupling agent to carry out coupling effect verification. The general amount of coupling agent is 0.1%.

[0110] In order to wrap the coupling agent on the flaky silver powder, the specific operation is as follows:

[0111] (1) Dissolve KH570 silane coupling agent in ethanol to prepare a solution of a certain concentration, and then thoroughly mix it by ultrasonic vibration for 30 minutes;

[0112] (2) Add silver powder and ultrasonically vibrate for 30 minutes;

[0113] (3) Filter with distilled water to obtain Ag powder;

[0114] (4) Dry in a vacuum oven at 100°C and grind with a mortar to obtain a sample.

[0115] The obtained samples were subjected to infrared spectroscopy test, such as Figure 13 It can be seen that there is an absorption peak of silicon-oxygen bond at 1110cm-1, indicating that the KH570 coupling agent is attached to the surface of the silver particles. The coupling agent can improve the dispersion of the filler in the matrix and improve the bonding performance. Figure 14 The EDS elemental analysis also shows that the silane coupling agent is attached to the silver powder particles.

[0116] Example 1

[0117] A high-temperature resistant conductive adhesive, comprising the following components, calculated by mass percentage: 75 (wt)% of a conductive filler, 19 (wt)% of a bisphenol A cyanate ester resin, 3 (wt)% of a toughening agent, 0.03 (wt)% of a curing agent, 0.15 (wt)% of a coupling agent, 0.3% of a silicone defoaming agent, and 2.52 (wt)% of a diluent;

[0118] Among them, the conductive filler is silver flake (i.e. flaky silver powder), the toughening agent is bismaleimide, the curing agent is organic tin (dibutyltin dilaurate), the coupling agent is KH570 silane coupling agent, the organic silicone defoaming agent is dimethyl silane, and the diluent is acetone.

[0119] The high temperature resistant conductive adhesive is prepared by the following steps. The preparation process diagram is shown in FIG. Figure 15 As shown:

[0120] (1) adding flaky silver powder and KH570 silane coupling agent to a propanol solution according to the above weight percentages, and dissolving the flaky silver powder and KH570 silane coupling agent in the propanol solution by ultrasonic dispersion; drying the resultant in a vacuum drying oven to obtain a dry powder, and then grinding the resultant to obtain a silver powder;

[0121] (2) mixing dibutyltin dilaurate, bismaleimide, and acetone in bisphenol A cyanate resin according to the above weight percentages, and stirring with a high-speed homogenizer to obtain a mixture;

[0122] (3) adding the silver powder obtained in step (1) to the mixture obtained in step (2) to obtain a resin mixture;

[0123] (4) vacuum degassing the product obtained in step (3): placing the product in a vacuum drying oven at 50° C. for vacuum degassing, and adding dimethylsilane, an organic silicon defoaming agent;

[0124] (5) curing the conductive adhesive after vacuum degassing in step (4) and heating it up; wherein the heating process is: 0-30min, 50°C; 35-60min, 80°C; 60-90min, 120°C; 90-120min, 150°C.

[0125] In this embodiment, acetone is added as a diluent, and its boiling point is 56.5°C. Figure 16 The step curve shown is used to prepare the conductive adhesive after the diluent is fully volatilized before the resin is cured. The present invention found that compared with the conductive adhesive heated directly, the conductive adhesive heated by the step temperature has a smoother morphology ( Figure 17 Left middle image), while directly heated conductive adhesive will produce a lot of bubbles ( Figure 17The reason is that the conductive adhesive surface cures quickly during direct heating, preventing acetone from evaporating. Therefore, the curing temperature ramp curve of the present invention is as follows. Figure 18 The morphology of the conductive adhesive after shearing is shown. The left picture shows step-by-step curing, and the right picture shows direct heating (rapidly heating to 150°C and keeping warm for 1 hour) curing.

[0126] Example 2

[0127] A high-temperature resistant conductive adhesive, comprising the following components, calculated by mass percentage: 60 wt% of a conductive filler, 34 wt% of a bisphenol A cyanate ester resin, 3 wt% of a toughening agent, 0.02 wt% of a curing agent, 0.3 wt% of a coupling agent, 0.3 wt% of an organosilicon defoaming agent, and 2.38% of a diluent;

[0128] Among them, the conductive filler is silver flake (i.e., flaky silver powder), the toughening agent is bismaleimide, the curing agent is organic tin (dibutyltin dilaurate), the coupling agent is KH570 silane coupling agent, the organic silicone defoaming agent is dimethyl silane, and the diluent is isopropyl ketone.

[0129] The preparation method is the same as that of Example 1.

[0130] Example 3

[0131] A high-temperature resistant conductive adhesive, comprising the following components, calculated by mass percentage: 80 (wt)% of a conductive filler, 14 (wt)% of a bisphenol A cyanate ester resin, 3 (wt)% of a toughening agent, 0.015 (wt)% of a curing agent, 0.15 (wt)% of a coupling agent, 0.15 (wt)% of an organosilicon defoaming agent, and 2.685% of a diluent;

[0132] The conductive filler is silver flake (i.e., flaky silver powder), the toughening agent is bismaleimide, the curing agent is zinc acetylacetonate, the coupling agent is KH570 silane coupling agent, the organosilicon defoaming agent is dimethylsilane, and the diluent is acetone. The preparation method is the same as in Example 1.

[0133] The comprehensive performance of the conductive adhesive prepared in Examples 1-3 is outstanding. When the silver content is 60-90%, the volume resistivity is not higher than 4.5×10 -4 Ω·cm, the chip shear strength is greater than 12.25MPa, and it can withstand high temperatures of 300℃.

[0134] Although the present invention is disclosed as above, the protection scope of the present invention is not limited thereto. Those skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention, and these changes and modifications will fall within the protection scope of the present invention.

Claims

1. A high temperature resistant conductive adhesive, characterized in that: The high-temperature resistant conductive adhesive includes the following components by weight: 60-80% of a conductive filler, 15-30% of a bisphenol A cyanate resin, 3-8% of a bismaleimide, 0.015-0.03% of a curing agent, 0.15-0.3% of a coupling agent, 0.15-0.3% of a silicone defoaming agent, and 1-3% of a diluent; the conductive filler is flaky silver powder with a particle size of 5 μm-10 μm.

2. The high temperature resistant conductive adhesive according to claim 1, characterized in that: The curing agent is selected from one or more of organotin, zinc acetylacetonate and nonylphenol.

3. The high temperature resistant conductive adhesive according to claim 1, characterized in that: The coupling agent is KH570 coupling agent.

4. The high temperature resistant conductive adhesive according to claim 1, characterized in that: The organosilicon defoamer is selected from any one of an oil-type defoamer, a solution-type defoamer, an emulsion-type defoamer, a solid-type defoamer and a modified silicone oil-type defoamer.

5. The high temperature resistant conductive adhesive according to claim 1, characterized in that: The diluent is selected from acetone and / or butanone.

6. The method for preparing a high temperature resistant conductive adhesive according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1: adding bismaleimide, a curing agent, and a diluent to a bisphenol A cyanate ester resin, mixing and stirring to obtain a mixture; S2: adding the conductive filler and the coupling agent into a solvent, dispersing and dissolving them, drying and grinding them to obtain a conductive filler powder; S3: adding the conductive filler powder to the mixture to obtain a resin mixture; S4: adding an organosilicon defoamer to the resin mixture and performing vacuum degassing to obtain a conductive adhesive; S5: curing the obtained conductive adhesive and performing a temperature-raising treatment.

7. The method for preparing the high temperature resistant conductive adhesive according to claim 6, characterized in that: The curing temperature rising conditions are: 0-30min, 50°C; 31-60min, 80°C; 61-90min, 120°C; 91-120min, 150℃.

8. The method for preparing a high temperature resistant conductive adhesive according to claim 6, wherein: The vacuum degassing is carried out in a vacuum drying oven at 50-100°C.

9. The method for preparing a high temperature resistant conductive adhesive according to claim 6, wherein: The viscosity of the organosilicon defoamer is 160-200 mPa·s.

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