JP4951948B2 - Conductor formation method - Google Patents

Description

  The present invention relates to a method of forming a conductor containing at least conductive powder and a polymer resin, and more particularly to a method of forming a conductor used as a wiring on a circuit board or an electrode of an electronic component.

  2. Description of the Related Art Conventionally, a method using a thermosetting conductive paste is known as one of methods for forming a conductor for wiring of a circuit board or an electrode of an electronic component. In this case, the thermosetting conductive paste is applied to the substrate by means of screen printing or the like, and heat-treated at a temperature of about 100 to 300 ° C., thereby curing the resin and forming a conductive film.

  The thermosetting conductive paste is obtained by dispersing conductive powder such as noble metal powder, base metal powder or carbon powder in a resin composition containing a binder resin, a curing agent, a solvent, a catalyst, and other additives. is there. As the binder resin, an epoxy resin, a phenol resin, a melamine resin, an alkyd resin, an unsaturated polyester resin, an acrylic resin, an epoxy-modified acrylic resin, or the like is usually used.

  Such thermosetting conductive pastes are generally used to form printed circuit board jumper circuits and conductor circuits including through-hole conductors, various electronic components such as resistors and capacitors, and electrodes of various display elements, and electromagnetic wave shields. In addition to being used for the formation of conductive coatings for semiconductors, it is also used as a conductive adhesive for mounting semiconductor elements and electronic components on a substrate. Also, in recent years, when forming electrodes for solar cells, particularly those for solar cells that cannot be processed at high temperatures using amorphous silicon semiconductors, or outside chip-type ceramic electronic components such as multilayer ceramic capacitors, multilayer ceramic inductors, multilayer ceramic actuators, etc. Also when forming an electrode, a thermosetting conductive paste is used.

Conventionally, the conductivity in the thermosetting conductive paste has been realized only by contact between the conductive powders. Therefore, even when silver powder showing relatively high conductivity is used as the conductive powder, for example, 150 When cured under the heating condition of about 30 minutes at a temperature of 30 ° C., the specific resistance remained at most around 5 × 10 ⁻⁵ Ω · cm.

  Conventionally, when a circuit, an electrode, or the like is formed using a thermosetting conductive paste, it is desired to heat and cure the thermosetting conductive paste as quickly as possible from the viewpoint of increasing production efficiency. Therefore, heating at a high temperature is generally performed. On the other hand, heating at the lowest possible temperature is desired from the viewpoint of heat resistance of semiconductor elements and electronic components that are heated simultaneously with the thermosetting conductive paste. That is, it is difficult to satisfy these two requirements at the same time, and in addition, when the heat treatment is insufficient, it is difficult to ensure high conductivity required for circuits, electrodes, and the like.

  As described above, regarding the thermosetting conductive paste, generally, the heating time required from the production efficiency is shortened, the heating temperature required from the application to the semiconductor element or the electronic component is lowered, and the electric paste is electrically used. Matters such as high conductivity required from the characteristics are in a trade-off relationship, and a thermosetting conductive paste that satisfies all of the requirements has not been developed yet.

  For example, Patent Document 1 discloses a thermosetting conductive paste in which noble metal powder, particularly silver powder is used as a conductive powder, and this and a thermosetting resin component are blended in a weight ratio of 100: 5 to 100: 45. It is disclosed. Although the specific resistance value obtained from the thermosetting conductive paste of this document is unknown, at least in the examples, heat treatment at a relatively high temperature of 200 to 250 ° C. for 60 minutes is performed.

Patent Document 2 discloses a thermosetting conductive paste using a phthalic acid glycidyl ester type epoxy resin as a thermosetting resin. According to this thermosetting conductive paste, a specific resistance value of 2 × 10 ⁻⁵ Ω · cm or less can be obtained by heat curing at 100 to 300 ° C. Heat treatment is performed at a relatively high temperature of 30 minutes.
JP-A-6-267784 Japanese Patent Laid-Open No. 2005-19398

  As in these documents, in the conventional example, the final heating conditions are determined while looking at the balance between shortening the heating time, lowering the heating temperature, and high conductivity. For example, when production efficiency is important However, heating at a temperature close to the heat-resistant limit temperature shortens the heating time as much as possible and compromises with the resistance value obtained as a result.

  The present invention has been made in view of such a point, and in particular, in a conductor forming method using a conductive paste containing a thermosetting resin, a thermosetting process is performed at a low temperature in a short time. Another object of the present invention is to provide a conductor forming method capable of obtaining a conductor having sufficiently low resistance.

In order to solve the above problems, the present invention provides a conductor forming method using a conductive paste containing at least a conductive powder and a resin.
The conductive paste is at least semi-cured by heating at a first heating temperature that is 10 ° C. or more lower than the heat-resistant limit temperature of the substrate on which the conductor is formed , and is above the glass transition point of the resin and above the first heating temperature. Heating is performed at a low second heating temperature.

  Further, the resin is a thermosetting resin.

  The thermosetting resin is a silicone-modified epoxy resin.

  The heating at the second heating temperature is continued until the rate of decrease in the resistance of the conductor due to the heating is reduced.

  Furthermore, the conductive powder is a silver-based powder containing silver as a main component.

  According to the present invention, in a conductor forming method using a thermosetting conductive paste, it is possible to sufficiently reduce resistance by a thermosetting treatment at a low temperature. Therefore, it is possible to form conductors for various substrates and electronic components.

The conductive paste used in the conductor forming method according to this embodiment includes at least a conductive powder and a resin described below.
<Conductive powder>

  As the conductive powder, metal powders such as silver, gold, platinum, and palladium, and alloys containing these metals, for example, silver-copper alloys and silver-palladium-copper alloy powders are used. Moreover, what coated the above-mentioned metal or alloy on the surface of inorganic powders, such as a metal, a metal compound, glass, a ceramic, carbon, and organic powders, such as resin, can also be used. In particular, silver-based conductive powder is preferable because of its low specific resistance. Examples thereof include silver powder, alloy powder containing silver as a main component, and silver-coated powder.

  In addition, the conductive powder may be used in a mixture of two or more kinds, and may be used after being surface-treated with various fatty acids or a coupling agent by a conventional method.

The shape of the conductive powder is not particularly limited, such as spherical, flake, dendritic, and fibrous,
Since the powder particles are easily in contact with each other and advantageous in terms of conductivity, a flaky powder having an average particle size of about 0.1 to 20 μm is preferable.
<Resin>

  Although there is no restriction | limiting in particular as resin, In this embodiment, it is preferable to use a thermosetting resin as binder resin.

  Conventionally, as the thermosetting resin used in the conductive paste, a resin that exhibits a glass transition point higher than the temperature during the heat treatment has been generally selected in order to increase the heat resistance of the conductor after heat curing. For example, when the thermosetting treatment is performed at 150 ° C., an epoxy resin having a glass transition point of 160 ° C. or more has been used.

  In the conductor formation method according to the present invention, the temperature of the first heat treatment is set based on the heat-resistant limit temperature of the substrate on which the conductor is formed as in the conventional case, for example, when a thermosetting resin is used for the conductive paste Is characterized in that a thermosetting resin exhibiting a glass transition point lower than the set temperature of the first heat treatment is used. Further, it is preferable to use a thermosetting resin exhibiting a glass transition point that is 10 ° C. or more lower than the temperature of the first heat treatment to be set. For example, when the first heat treatment is set to be performed at 150 ° C., it is preferable to use a thermosetting resin exhibiting a glass transition point of 140 ° C. or lower.

  Here, the glass transition point of the thermosetting resin can be measured by a known method, for example, differential scanning calorimetry (DSC) or the like, but a viscoelastic property measuring apparatus (Orientec Co., Ltd. Rheobibron DDV). It can also be obtained from a loss curve (tan δ) measured at −01 FP). As an example, when a loss curve as shown in FIG. 1 is obtained, the temperature at which the value rapidly changes can be used as the glass transition point.

  As such a thermosetting resin, for example, an epoxy resin, a phenol resin, a melamine resin, an alkyd resin, or the like can be used. In particular, in the case of an epoxy resin, a novolac type epoxy resin and a polyfunctional group epoxy resin are preferably used because of good heat resistance. Moreover, even when the glass transition point is lowered, it is more preferable to use a silicone-modified epoxy resin so that the heat resistance of the conductor after the heat treatment can be maintained or improved.

  In the present embodiment, a binder resin obtained by blending another resin component with a thermosetting resin may be used in accordance with the use of the conductive paste and the required characteristics. As other resin components, different types of thermosetting resins and thermoplastic resins can also be used, for example, butyral resin, acrylic resin, methacrylic resin, acrylic styrene resin, modified acrylic resin, glycidyl methacrylate, melamine resin, Examples include alkyd resins, polyester resins, phenoxy resins, urethane resins, phenol resins, vinyl resins, and cellulose derivatives.

  The mixing ratio of the thermosetting resin and other resin components is preferably about 100: 0 to 40:60 by weight. When the weight ratio of the thermosetting resin is 40 or less, the fluidity is increased by heating, and it is difficult to maintain the coating shape.

The blending ratio of the binder resin is 2 to 30 parts by weight with respect to 100 parts by weight of the conductive powder. When the amount exceeds 30 parts by weight, the resistance increases. When the amount is less than 2 parts by weight, the resistance increases because the strength of the cured film decreases.
<Other ingredients>

  The conductive paste used in the conductor forming method according to the present invention may be mixed with a solvent as necessary in order to adjust the coating property to the substrate. As the solvent, for example, known solvents such as alcohol solvents, ester solvents, ether solvents, ketone solvents, hydrocarbon solvents, fatty acid solvents, reactive diluents and the like are used.

  Moreover, it is good also as mix | blending a hardening | curing agent with an electrically conductive paste. When an epoxy resin is used as the thermosetting resin, as the curing agent, for example, an imidazole curing agent, an acid anhydride curing agent, an amine curing agent, a phenol resin curing agent, and the like can be applied. Agents are preferred. Moreover, as a hardening | curing agent, you may mix and use 2 or more types as needed. The blending amount of the curing agent is preferably adjusted according to the epoxy equivalent of the binder resin. In addition to such a curing agent, a curing accelerator or a curing catalyst may be added.

  Furthermore, additives that are usually blended as necessary, such as surfactants, reducing agents, chelating agents, antifoaming agents, plasticizers, thixotropic agents, dispersants, inorganic fillers, etc., are added to the conductive paste as appropriate. May be. With these, the properties such as heat resistance, water resistance, environment resistance, flexibility, solderability, solder heat resistance, etc. of the coating property and the formed conductor film can be adjusted appropriately and applied to various applications. It becomes possible to do.

The conductive paste thus adjusted is applied to the substrate by various means such as screen printing, transfer printing, dipping, brush coating, and application using a dispenser. The substrate is a substrate, an electronic component, or the like, and specifically, various materials such as a resin such as a printed wiring board, PET film, ceramic, glass, silicon semiconductor, and compound semiconductor can be used. Further, by applying a conductive paste on the conductor circuit, dielectric, and resistor formed on these substrates, it can also be used for the purpose of protecting electrodes, adhesives, and circuits.
<Conductor formation method>

Next, the conductor forming method according to the present invention will be described.
In the conductor forming method according to the present invention, first, the conductive paste prepared as described above is applied to a substrate. And after performing the 1st heat processing which heats at a 1st heating temperature in order to semi-harden an electrically conductive paste at least, it heats at the 2nd heating temperature which is more than the glass transition point of resin and is lower than a 1st heating temperature. A conductor is formed by performing the second heat treatment.

  In the first heat treatment, it is sufficient that at least the conductive paste can be semi-cured, and more than that, for example, it may be completely cured. In the present invention, the semi-curing of the conductive paste is a state in which the surface of the conductive paste after application is cured but not completely cured. When a thermosetting resin is used as the resin, the conductive paste is cured by curing the thermosetting resin. Therefore, the curing of the conductive paste and the curing of the thermosetting resin are substantially synonymous.

  The optimal first heating temperature varies depending on the heat-resistant limit temperature of the substrate, semiconductor element, electronic component, etc. (hereinafter referred to as “substrate”) and the combination of resin and curing agent, but does not apply thermal stress to the substrate, etc. Thus, it is preferable to heat at a temperature that is 10 to 100 ° C. lower than the heat resistant limit temperature of the substrate or the like. There is no limitation in particular about a heating means, It can carry out by a well-known method.

  When heat-treating a thermosetting conductive paste, in order to obtain a low-resistance conductor in a short time, it is common to heat at as high a temperature as possible. Therefore, conventionally, heating has been performed at a high temperature that does not exceed the heat-resistant limit temperature of the substrate or the like, but even in this case, the higher the temperature, the more certain thermal stress is applied to the substrate or the like. . In order to avoid this, heating should be performed at a relatively low temperature (for example, 100 ° C.), but the production efficiency is lowered and a highly conductive conductor cannot be obtained.

  In the conductor forming method of the present invention, the heat treatment for the thermosetting conductive paste includes a first heat treatment for heating the conductive paste to at least a semi-cured state, and a second heat treatment for lowering the resistance value of the conductor. The heat treatment is divided into two stages.

  In the first heat treatment, since the heat treatment at which the conductive paste is at least semi-cured is sufficient, it is lower than the case where the heat treatment is performed at a temperature as close as possible to the heat-resistant limit temperature of the substrate or the like as in the prior art. Heat treatment can be performed at a temperature, and the heat treatment time can be shortened. Therefore, the thermal stress on the substrate or the like can be reduced.

  Since the conductive paste is at least semi-cured when the first heat treatment is completed, the second heat treatment can be performed while maintaining the applied shape.

  In the second heat treatment, the heat treatment for the thermosetting conductive paste is performed at a temperature not lower than the glass transition point of the resin and lower than the first heating temperature. Reduced. Since the second heat treatment is performed at a relatively low temperature, a large-scale production facility is not required as compared with the first heat treatment. Therefore, the second heat treatment is a production time such as a free time until the next process after the first heat treatment, for example, a time temporarily stored in a warehouse or the like before turning to an inspection process for inspecting a product. It can be performed using the dead time on the process. Therefore, a conductor having excellent electrical characteristics can be obtained without reducing the overall production efficiency.

  In the present invention, the second heating temperature is set to a temperature that is equal to or higher than the glass transition point of the resin used in the paste and lower than the first heating temperature. Therefore, when using a thermosetting resin, a thermosetting resin whose glass transition point is lower than the first heating temperature is used. In order not to apply thermal stress to the substrate or the like, the second heating temperature is preferably lower by about 10 to 50 ° C. than the first heating temperature. Further, in order to efficiently reduce the resistance value by the second heat treatment, the second heating temperature is desirably higher by 10 ° C. or more than the glass transition point of the resin.

  In the second heat treatment, the resistance value of the conductor rapidly decreases for a while after the heat treatment is started, but the rate of decrease gradually decreases and becomes dull. Therefore, in order to efficiently increase the conductivity of the conductor, it is desirable that the second heat treatment is continued at least until the rate of decrease in the conductor resistance value slows down. Even after the rate of decrease in the conductor resistance value has slowed down, the resistance value slightly decreases due to the second heat treatment, and thereafter, the resistance value may continue for a predetermined time, or may end at that point in terms of production efficiency.

  Even when the second heat treatment is performed at a temperature lower than the glass transition point of the resin, a phenomenon that the resistance of the conductor is reduced is observed, but the reduction rate in that case is not significant. Therefore, in order to reduce the resistance of the conductor, heating for a long time is required, and the production efficiency is lowered.

The mechanism by which the phenomenon in which the resistance of the conductor is reduced by the second heat treatment is not clear, but when the thermoviscoelastic properties of the cured product of the conductive paste and the cured product of the single resin were examined, the second Heat treatment increases the elasticity of the conductor, so heating and heating at temperatures higher than the glass transition point of the resin promotes bonding and fusion between the conductive powders, forming a network structure in the conductive paste. It is presumed that this is because of the progress.
<Use of conductor>

  The conductive paste and the cured film in the present embodiment can be used for various applications. Typical applications include jumper circuits and through-hole conductors for printed circuit boards, conductor materials for flexible substrates such as PET film, use for additive circuits, etc., use for conductor circuits for touch panels, use for resistance terminals, Examples thereof include use as an electromagnetic wave shield, adhesion between electronic components, use as a conductive adhesive for mounting a semiconductor element or electronic component on a substrate, and the like.

In addition, the conductive paste according to the present embodiment is heat-treated and cured by the above-described conductor formation method, thereby serving as an external electrode of a chip-type ceramic electronic component such as a multilayer ceramic capacitor, a multilayer ceramic inductor, or a multilayer ceramic actuator. It can also be used. For example, external electrodes of conventional chip-type ceramic electronic components can be obtained by baking a high-temperature firing type electrode on a component body and coating a thermosetting conductive paste on the component body, or a thermosetting conductive paste directly on the body. Is formed by a method of applying and heat-curing, and solder plating is applied if necessary. The conductive paste according to the present embodiment can be used as an alternative to a conventional thermosetting conductive paste.
In addition, the above-mentioned use is an illustration, The electroconductive paste which concerns on this embodiment is not limited to the use which concerns, It can be used for various uses in which electroconductivity and adhesiveness are requested | required.

  Bisphenol A type epoxy resin (Japan Epoxy Resin Co., Ltd. Epicoat 1001) is added to 100 parts by weight of silver powder (D50 = 2 μm) obtained by flaking silver powder (Ashei Chemical Industry Co., Ltd. Ag-026) with a ball mill. A silicone-modified glass transition point 110 ° C. resin 10 parts by weight, an imidazole curing agent 1 part by weight, and ethyl carbitol acetate 8 parts by weight were added and kneaded in a three-roll mill to prepare a thermosetting silver paste.

As a preliminary experiment, the obtained thermosetting silver paste was applied to a 0.5 mm × 5 mm line pattern on a glass substrate so that the thickness of the cured film was 20 μm, and 125 ° C., 150 ° C., 175 ° C. , 200 ° C., 225 ° C., and heat curing treatment was performed under the respective temperature conditions. At that time, the resistance value was measured using a digital multimeter (2002 model manufactured by Keithley Instruments Co., Ltd.), and the electrode thickness was measured using a surface roughness meter (Surfcom manufactured by Tokyo Seimitsu Co., Ltd.). The value of was obtained.
The obtained results are shown in FIG.
In FIG. 2, “x” indicates that the thermosetting silver paste is uncured, “◯” indicates the completion of curing, and “Δ” indicates the middle. Specifically, after performing a rubbing test (50 reciprocations) using a solvent of methyl ethyl ketone (MEK) on the coating film after the heat treatment, the appearance of the coating film is visually evaluated, and when there is no abnormality in the coating film Since the coating film is at least semi-cured, it is indicated as ◯ when the curing is completed, x when the coating film is dissolved and x when it is uncured, and when it is partially dissolved, it is indicated as △ between the completion of curing and uncured ing.
As shown in FIG. 2, the higher the heat treatment temperature is, the higher the conductivity is obtained even in a short time. For example, when heating is performed at 225 ° C., a low resistance value is obtained even by heating for 60 minutes. When heat treatment is performed at a relatively low temperature of 125 to 150 ° C., sufficient conductivity is not obtained even by heating for 120 minutes or more. However, when heating is performed at 225 ° C., thermal stress is applied to the substrate and the like.

Therefore, in this example, the first heat treatment was performed by heating at 150 ° C. for 60 minutes, and the cured product was lower than the first heating temperature and from the glass transition point of the thermosetting resin. The second heat treatment was performed at a high temperature of 125 ° C., and the change in specific resistance was measured in the same manner. The result is shown in FIG.
As shown in FIG. 3, even when the second heat treatment is performed at a relatively low temperature of 125 ° C., the specific resistance can be sufficiently reduced. The specific resistance rapidly decreases for a while after the second heat treatment is started, but the rate of decrease gradually decreases. Therefore, in the present invention, it is desirable to perform the second heat treatment at least from the start of the second heat treatment until the decrease rate of the specific resistance slows down. In the present invention, “from the start of the second heat treatment until the rate of decrease in specific resistance slows down” refers to the ratio before the blunting from the start of the second heat treatment, as shown in FIG. The time (arrow in the figure) until the tangent which shows the decreasing rate of resistance and the tangent which shows the decreasing rate of the specific resistance after blunting shall be shown.
Even after the decrease rate of the specific resistance has slowed down, the specific resistance decreases due to heating, so the second heat treatment may be continued. However, from the viewpoint of production efficiency, the second heat treatment may be terminated after a predetermined time (for example, 100 hours) has elapsed after the decrease rate of the specific resistance has slowed down.

  As is apparent from FIG. 3, in the second heat treatment, when heating is performed at a temperature (125 ° C.) exceeding the glass transition point (100 ° C.) of the resin used, the specific resistance is lowered even at a relatively low temperature. In addition, it is possible to avoid applying thermal stress to the substrate or the like. Specifically, the specific resistance value can be finally lowered to about 20 μΩ · cm, and as a result, a resistance value equivalent to that when heated at 225 ° C. for 60 minutes (see FIG. 2) can be obtained. .

For comparison, FIG. 4 shows the result of heat treatment at 65 ° C. lower than the glass transition point of the thermosetting resin for the same heat treatment performed at 150 ° C. for 60 minutes. Show.
As shown in FIG. 4, even when the temperature is lower than the glass transition point of the thermosetting resin, the resistance value tends to gradually decrease by performing the heat treatment. The resistance value decreased by about 20 μΩ · cm by the heat treatment for a period of time. Furthermore, although the specific resistance may decrease by performing the heat treatment for a long time, the specific resistance value with respect to the heating time is not sufficiently lowered, which is not desirable from the viewpoint of production efficiency.

It is the graph which calculated | required the glass transition point of a silicone modified epoxy resin from the loss curve (tan-delta) measured with the viscoelastic property measuring apparatus. It is a graph which shows the relationship between heating temperature, heating time, and specific resistance. It is a graph which shows resistance change at the time of performing a 2nd heat processing at temperature higher than the glass transition point of resin. It is a graph which shows resistance change at the time of heat-processing at temperature lower than the glass transition point of resin.

Claims (5)

In a conductor forming method using a conductive paste containing at least conductive powder and resin,
The conductive paste is at least semi-cured by heating at a first heating temperature that is 10 ° C. or more lower than the heat-resistant limit temperature of the substrate on which the conductor is formed , and is above the glass transition point of the resin and above the first heating temperature. A method for forming a conductor, comprising heating at a low second heating temperature.   The conductor forming method according to claim 1, wherein the resin is a thermosetting resin.   The conductor forming method according to claim 2, wherein the thermosetting resin is a silicone-modified epoxy resin.   The method for forming a conductor according to any one of claims 1 to 3, wherein the heating at the second heating temperature is continued until the rate of decrease in the resistance of the conductor due to the heating is reduced.   The conductor forming method according to any one of claims 1 to 4, wherein the conductive powder is a silver-based powder containing silver as a main component.

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