CN101952902B - Conductive paste, electromagnetic wave-shielding film using same, and electromagnetic wave-shielding flexible printed wiring board - Google Patents

Abstract

The present invention provides a conductive paste comprising a conductive metal powder, a urethane-modified polyester resin and a blocked isocyanate, wherein the urethane-modified polyester resin is prepared by making an acid component, an alcohol component, and an aromatic isocyanate-containing and the total amount of aromatic components contained in the acid component, alcohol component and isocyanate component is 5mol% to 50mol% of the total amount of the acid component, alcohol component and isocyanate component. The conductive paste has a good balance between flexibility and heat resistance, and is capable of forming a shield layer with excellent bending resistance. The invention also discloses an electromagnetic shielding film and an electromagnetic shielding flexible printed wiring board using the conductive paste.

Description

Conductive paste, electromagnetic shielding film and electromagnetic shielding flexible printed wiring board using the same

technical field

The present invention relates to a conductive paste, and an electromagnetic shielding film and an electromagnetic shielding flexible printed wiring board using the conductive paste, and more particularly, the present invention relates to a flexible printed wiring board that needs to have bending resistance.

Background technique

The conductive paste is made into paste by mixing conductive fillers such as carbon black, graphite powder, noble metal powder, copper powder or nickel powder, binder resin and solvent. By methods such as screen printing, conductive paste is applied to a film or substrate to achieve pattern formation. The resin is cured to form conductive wiring. Recently, with the miniaturization and weight reduction of electronic components, conductive pastes with high conductivity are required for such applications.

Patent Document 1 discloses a conductive silver paste using silver as a conductive filler. Examples of the shape of the silver powder used as a conductive filler include, but are not limited to, granular, scaly, plate-like, dendritic, miliary, and cube-shaped. Silver powder with a particle size of 0.1 to 100 μm is used. A saturated copolymerized polyester resin and blocked isocyanate are used as the binder resin. Patent Document 2 discloses that, in order to improve the bending resistance of a conductive paste, a conductive paste mainly containing silver powder in which primary particles having a particle size of 0.1 to 5 μm are three-dimensionally connected, a binder having a number average molecular weight of 3000 or more, Hardeners and solvents. Examples of binders include polyurethane resins and polyester resins.

Conductive paste is also used as electromagnetic shielding material. In particular, frequencies in the high-frequency band are used today for high-speed transmission of intelligence. This requires conductive pastes with better electromagnetic shielding properties. Patent Document 3 discloses a conductive silver paste having improved conductivity and shielding properties, which is made by combining silver powders having a specific particle size, and an electromagnetic shielding film using the conductive silver paste.

Contents of the invention

Technical problem to be solved by the present invention

When conductive paste is used for the shielding layer in flexible printed wiring boards, the shielding layer formed by applying and curing the conductive paste needs to have heat resistance, surface smoothness, and bending resistance in addition to conductivity and electromagnetic shielding properties sex. In particular, when it is used for a movable part such as a hinge part of a mobile phone, the miniaturization of the machine requires durability at a small bending radius. Therefore, the purpose is to improve bending resistance.

In the conductive paste described in Patent Document 3, it is described that a polyester resin is preferably used as a binder resin in order to achieve a balance between heat resistance and flexibility after coating and curing. However, in order to achieve the currently required bending resistance, it is necessary to further improve the bending resistance.

In order to improve bending resistance, flexibility is required. Therefore, polyester resins belonging to soft resins have been used as binder resins. Polyester resins are resins prepared by polycondensation of acid components such as polyvalent carboxylic acids or acid anhydrides and alcohol components such as polyols. Proper selection of the types of acid components and alcohol components enables appropriate control of properties. For example, the use of high amounts of soft ingredients such as aliphatic dicarboxylic acids increases flexibility. However, use of a large amount of soft components lowers heat resistance, so that desired properties cannot be satisfied. In order to improve heat resistance, it is necessary to increase the proportion of rigid aromatic components such as terephthalic acid. In this case, flexibility is reduced.

In view of the above-mentioned problems, an object of the present invention is to provide a conductive paste capable of forming an electromagnetic shielding layer that balances flexibility and heat resistance and has an excellent Bending resistance.

means of solving problems

The conductive paste includes a conductive metal powder, a urethane-modified polyester resin, and a blocked isocyanate, wherein the urethane-modified polyester resin is passed through an acid component, an alcohol component, and an isocyanate component containing an aromatic isocyanate. It is produced by reaction, and the total amount of aromatic components contained in acid component, alcohol component and isocyanate component is 5 mol% to 50 mol% of the total amount of acid component, alcohol component and isocyanate component (first invention).

A urethane-modified polyester resin modified with an isocyanate component containing an aromatic isocyanate is used as a binder resin. Also, the total amount of aromatic components contained in the acid component, alcohol component, and isocyanate component is 5 mol% to 50 mol% of the total amount of the acid component, alcohol component, and isocyanate component. This molecular structure achieves a good balance between the flexibility required for bending resistance and heat resistance.

The urethane-modified polyester resin preferably has a hydroxyl value of 5 mgKOH/g to 60 mgKOH/g (second invention). The hydroxyl value is an index of the molecular weight of the urethane-modified polyester resin relative to the number of crosslinking points (hydroxyl groups that react with blocked isocyanate). Higher hydroxyl values result in lower molecular weights. Lower hydroxyl numbers result in higher molecular weights. A hydroxyl value of less than 5 mg KOH/g results in high molecular weight and excellent flexibility, but also results in a reduced number of crosslinking points required for reaction with blocked isocyanate, resulting in reduced heat resistance. On the contrary, a hydroxyl value exceeding 60 mg KOH/g leads to a decrease in molecular weight. In this case, although heat resistance is improved, flexibility is lowered.

The blocked isocyanate preferably has a number average molecular weight of 500 to 3000, and is a polyfunctional blocked polyisocyanate compound in which the end of an addition type isocyanate consisting of isocyanate monomers and Polyol formation (third invention). The addition-type isocyanate has many functional groups (isocyanate groups) in its molecule, thereby increasing the crosslinking density of the urethane-modified polyester resin after the reaction, and improving heat resistance.

In terms of the molar ratio (NCO/OH) of the isocyanate group (NCO) of the blocked isocyanate to the hydroxyl group (OH) of the carbamate-modified polyester resin (NCO/OH), the blocked isocyanate modifies the carbamate The mixing ratio of the polyester resin is preferably 0.8 to 3.0 (fourth invention). If the amount of blocked isocyanate is lower than the above range, the urethane-modified polyester resin has low crosslink density and low heat resistance. If the amount of the blocked isocyanate exceeds the above range, isocyanate that does not participate in the reaction may remain in the binder resin, thereby reducing heat resistance. The molar ratio is more preferably in the range of 1.0 to 2.0.

The conductive metal powder is preferably formed of metal powder A having an average particle size of 0.5 μm to 20 μm and metal powder B having an average particle size of 100 nm or less, the weight content ratio of the metal powder A to the metal powder B being 99.5:0.5 to 70: 30, and the content ratio of the conductive metal powder to the solid content of the conductive paste is 50% by weight to 85% by weight (fifth invention). A higher conductive metal powder content ratio results in improved conductivity. However, an excessively high content ratio of the conductive metal powder results in decreased flexibility of the conductive paste, thereby deteriorating bending resistance. Therefore, in order to achieve a balance between conductivity and bending resistance, the content ratio of the conductive metal powder is preferably 50% by weight to 85% by weight.

In order to achieve satisfactory conductivity, it is preferable to use the metal powder A and the metal powder B having different particle sizes in combination at a specific content ratio. The nanoscale metal powder B is used to fill the gaps between the particles of the metal powder A with a larger particle size, thereby improving the conductivity and the surface smoothness after the conductive paste is coated. Surface smoothness and conductivity affect electromagnetic shielding performance. Therefore, improving conductivity and surface smoothness leads to further improving electromagnetic shielding performance.

The present invention also provides an electromagnetic shielding film comprising a layer made of the above-mentioned conductive paste on a substrate (sixth invention). Furthermore, the present invention provides an electromagnetic shielding flexible printed wiring board including a layer made of the above-mentioned conductive paste (seventh invention). Both the electromagnetic shielding film and the electromagnetic shielding flexible printed wiring board have excellent heat resistance, electrical conductivity, and electromagnetic shielding performance in addition to excellent bending resistance.

Invention effect

According to the present invention, there can be provided: an electroconductive paste capable of forming an electromagnetic shielding layer which balances flexibility and heat resistance and has excellent bending resistance; an electromagnetic shielding film and an electromagnetic shielding film using the electroconductive paste Shielded flexible printed wiring boards.

Description of drawings

Fig. 1 is a schematic cross-sectional view of an electromagnetic shielding film of the present invention.

Fig. 2 is a schematic cross-sectional view of an electromagnetic shielding flexible printed wiring board of the present invention.

Detailed ways

Embodiments of the present invention will be described below. In the description of the drawings, the same symbols are used to denote the same elements, and repeated descriptions are omitted. The dimensional ratios in the drawings do not always agree with the dimensional ratios of actual items depicted in the drawings.

The urethane-modified polyester resin used in the present invention will be described. The urethane-modified polyester resin is prepared by reacting an acid component, an alcohol component, and an isocyanate component. Generally, the polyester resin is produced by polycondensation reaction of an acid component such as polyvalent carboxylic acid or anhydride and an alcohol component such as polyol. The terminal hydroxyl groups of the resulting polyester resin react with an isocyanate component to obtain a urethane-modified polyester resin. In this method, it is preferable to add and react the isocyanate component after the acid component and the alcohol component have reacted. Alternatively, the acid component, alcohol component and isocyanate component can be reacted simultaneously.

The acid component is not particularly limited as long as it is a polyvalent carboxylic acid or an anhydride thereof. Examples include aromatic dicarboxylic acids and their anhydrides, such as phthalic acid, isophthalic acid, terephthalic acid, and phthalic acid; aliphatic dicarboxylic acids and their anhydrides, such as succinic acid, adipic acid, pentanoic acid, diacids and sebacic acids; and unsaturated dicarboxylic acids and their anhydrides, such as maleic acid, fumaric acid, and itaconic acid. They can be used in combination of two or more.

The alcohol component is not particularly limited as long as it is a polyhydric alcohol. Examples include aliphatic alcohols such as ethylene glycol, diethylene glycol, triethylene glycol, neopentyl glycol, 1,3-propanediol, 1,4-butanediol and 1,4-cyclohexanediol; aromatic dihydric alcohols; alicyclic dihydric alcohols; and polyhydric alcohols higher than trihydric, such as trimethylolpropane and pentaerythritol. They can be used in combination of two or more.

The isocyanate component has two or more isocyanate groups in its molecule. An aromatic isocyanate having an aromatic ring in its molecule is essential. Examples of the aromatic isocyanate include xylene diisocyanate, toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, naphthalene diisocyanate and biphenylene diisocyanate. They can be used in combination of two or more. Furthermore, aliphatic diisocyanates such as trimethylhexamethylene diisocyanate, hexamethylene diisocyanate or trimethylene diisocyanate, or alicyclic family of diisocyanates such as cyclohexane diisocyanate.

As described above, the total amount of aromatic components contained in the acid component, alcohol component, and isocyanate component is 5 mol% to 50 mol% of the total amount of the acid component, alcohol component, and isocyanate component. These materials are reacted by a conventional method to obtain a urethane-modified polyester resin.

The blocked isocyanate used in the present invention is a compound in which the terminal isocyanate group of polyfunctional isocyanate is blocked with a blocking agent. The blocking agent is decomposed by heat to form isocyanate groups. The isocyanate group reacts with the hydroxyl group of the urethane-modified polyester resin to crosslink the urethane-modified polyester resin.

Examples of blocking agents include compounds having active hydroxyl groups such as alcohols, phenols, amides, oximes, and active methylene groups.

Any isocyanate such as trimethylene diisocyanate, hexamethylene diisocyanate (HDI) or diphenylmethane diisocyanate (MDI) can be used as polyfunctional isocyanate. In particular, an addition-type isocyanate represented by the general formula (I) and formed from an isocyanate monomer and a polyol is preferred.

[chemical formula 1]

(wherein R1 to R3 each represent a group excluding an isocyanate group from an aliphatic, alicyclic or aromatic diisocyanate; and R4 represents a group excluding a hydroxyl group from a polyol)

A polyol is a compound having two or more hydroxyl groups in its molecule. Examples thereof include glycerin, trimethylolethane, trimethylolpropane, 1,4-butanediol, neopentyl glycol and 1,6-hexanediol. Examples of diisocyanates include trimethylene diisocyanate, hexamethylene diisocyanate and diphenylmethane diisocyanate.

Any metal such as copper, gold, silver, platinum and nickel and alloys thereof can be used as the conductive metal powder used in the present invention. Silver powder having excellent conductivity is preferably used. Examples of its shape include, but are not particularly limited to: spherical, scaly, and granular.

The content of the conductive metal powder can be appropriately selected according to desired properties. An increase in the content of conductive metal powder results in an increase in electrical conductivity. Excessively high conductive metal powder content results in decreased adhesion (cohesion) between the resin component and conductive metal powder, thus forming voids in the conductive paste after coating, affecting printability and reducing adhesion. Moreover, the conductivity is also lowered. Therefore, the content of the conductive metal powder is preferably 95% or less of the solid content of the conductive paste.

In addition, an increase in the content of conductive metal powder makes the conductive paste hard, thus reducing flexibility. In order to strike a balance between the conductivity and flexibility of the conductive paste, the conductive metal powder content is preferably 50% to 85% of the total solid content of the conductive paste.

Preferably, metal powder A with an average particle size of 0.5 μm to 20 μm and metal powder B with an average particle size of 100 nm or less are used in combination as the conductive metal powder, and the weight ratio of metal powder A to metal powder B is 99.5:0.5 to 70:30. When the content of metal powder A is increased until the content ratio of metal powder A to metal powder B exceeds 99.5:0.5, the combination effect is reduced, thereby reducing the conductivity. When the content of the metal powder B is increased until the content ratio exceeds 70:30, the increase in the amount of the metal powder B raises the cost, which is not preferable. The content ratio of the metal powder A to the metal powder B is more preferably 99:1 to 90:10.

Metal powder A preferably has an average particle size of 0.5 μm to 20 μm. An average particle size of 0.5 μm or less leads to a decrease in electrical conductivity. An average particle size exceeding 20 μm makes fine printing difficult. For the same reason, it is preferred to use powders which do not have an extremely large maximum particle size. Powders with a maximum particle size of 20 μm to 50 μm are preferred. It should be noted that the particle size is defined as the maximum diameter of each particle, and the average value thereof is defined as the average particle size. For example, measurement is performed using a scanning electron microscope (SEM).

Instead of using one type of powder, a plurality of powders having different average particle sizes and different shapes may also be used in combination as the metal powder A. For example, the combined use of flaky silver powder and spherical silver powder further improves conductivity and smoothness after coating.

Metal powder B is a metal powder having an average particle size of 100 nm or less. In nanoscale powders, the particles are able to agglomerate with each other. The term "average particle size" is used to indicate the average particle size of the starting particles. Moreover, this nanoscale powder has a large specific surface area and thus high surface activity. In order to protect the surface and suppress the occurrence of secondary aggregation, it is preferable to use a powder in which the surface of powder particles is covered with organic matter. Examples of organic substances include polycarboxylic acids and polyacrylic acids.

For example, metal powder B having an average particle size of 100 nm or less can be produced by dissolving silver nitrate in a mixed solvent of water and a lower alcohol. Adjust the pH to above 11 using ammonia water. A solution of L-ascorbic acid serving as a reducing agent and polyacrylic acid serving as a dispersant dissolved in the same mixed solvent was added thereto, whereby silver particles were precipitated. When a dispersant is used to suppress secondary aggregation, the precipitated silver particles are filtered, washed and dried. The average particle size of the silver particles can be changed by changing the pH, temperature, concentration of materials, mixing method, and the like.

In particular, in the silver particles produced through the above steps, a dispersant is used in the reaction step to produce silver particles whose surfaces are covered with the dispersant. During the formation phase, the surface of the silver particles is covered with a dispersant, thus rendering the silver particles less sensitive to air. Furthermore, secondary aggregation of silver particles is less likely to occur. Even if aggregation occurs, the aggregated particles can be easily broken using, for example, an organic solvent due to the presence of the dispersant. Also, in the resin, silver particles have satisfactory dispersibility.

The conductive metal powder, the urethane-modified polyester resin, and the blocked isocyanate were mixed to form a conductive paste. The urethane-modified polyester resin and the blocked isocyanate are dissolved in a solvent before use. Any solvent capable of dissolving the resin can be used. Examples thereof include esters, ethers, ketones, ether esters, alcohols, hydrocarbons and amine solvents. When the conductive paste is used by screen printing, a high boiling point solvent having satisfactory printability is preferable. Specifically, carbitol acetate and butyl carbitol acetate are particularly preferred.

In order to improve printing processability, additives such as thickeners and leveling agents may be added to the silver paste of the present invention. Also, an inorganic filler composed of, for example, carbon and silica may be added without impairing the performance of the present invention. These materials are mixed and dispersed using, for example, a ball mill, a three-roll mill, a rotary mixer/defoamer, to form a uniform conductive paste.

The electromagnetic shielding film of the present invention includes a layer made of the above-mentioned conductive paste on a base material. After the above-mentioned conductive paste is coated on the substrate, dried and the solvent is removed, the conductive paste is cured, thereby obtaining an electromagnetic shielding film. A polyester film or a polyimide film can be used as a base material. In view of flexibility, a polyimide film is preferable. FIG. 1 is a schematic cross-sectional view of an exemplary electromagnetic shielding film. The electromagnetic shielding film includes a conductive paste layer 2 on a substrate 1 . In order to protect the conductive paste layer, a protective film 8 may be provided on the conductive paste layer 2 . In use, the protective film 8 is removed.

The electromagnetic shielding film is laminated to one or both surfaces of the flexible printed wiring board to provide the electromagnetic shielding flexible printed wiring board of the present invention. 2 is a schematic cross-sectional view of an exemplary electromagnetic shielding flexible printed wiring board. The flexible printed wiring board 7 includes wiring composed of copper foil 5 on the base material 4 , the wiring being covered with a cover layer. The cover layer is composed of a cover layer film 6 a made of, for example, polyimide and a cover layer adhesive 6 b. The electromagnetic shielding film 3 is pasted on the cover layer side of the flexible printed wiring board. Alternatively, the conductive paste may be directly coated on the flexible printed wiring board to form a conductive paste layer.

The coating thickness of the conductive paste is not particularly limited, and is preferably in the range of 10 μm to 50 μm. A thickness of 10 μm or less cannot obtain desired electromagnetic shielding properties. A thickness above 50 μm impairs the flexibility of the shielding layer, thus reducing the bending resistance.

Examples of coating methods of the conductive paste include screen printing, gravure printing, offset printing, and use of a dispenser. Screen printing is most preferably used in consideration of fineness of lines, film thickness, and productivity.

Alternatively, the conductive paste of the present invention may be directly coated on a flexible printed wiring board and cured to form an electromagnetic shielding layer. An electromagnetic shielding flexible printed wiring board including a layer made of the conductive paste of the present invention is also provided by the method. Likewise, the conductive paste is applied to the casings of electronic devices such as personal computers and mobile phones to provide electromagnetic shielding casings each including a layer made of the conductive paste of the present invention.

The best mode for carrying out the present invention will be described below by way of examples. The scope of the present invention is not limited to these Examples.

Example

(Examples 1-3 and Comparative Examples 1-3)

(Preparation of conductive paste)

Urethane-modified polyesters formed by the reaction of the acid component, alcohol component, and isocyanate component shown in Table 1 were prepared. Specifically, the acid component and alcohol component shown in Table 1, and a mixed solvent of butyl carbitol acetate and butyl carbitol were charged into a four-necked flask. Under nitrogen flow, the mixture was heated to 60°C. Then, an isocyanate compound was added thereto, and heated to 80° C. for 5 hours to synthesize a urethane-modified polyester.

Scaly silver powder with an average particle size of 3.0 μm and spherical silver powder with an average particle size of 25 nm were used as conductive metal powder and blocked isocyanate was mixed with the prepared urethane-modified polyester resin to form a conductive paste. The mixing ratio of the urethane-modified polyester to the blocked isocyanate was set to be an equivalent molar ratio. The mixing ratio of the conductive metal powder was set to the ratio described below.

Mixing ratio (weight) of flaky silver powder=(weight of urethane-modified polyester resin+weight of blocked isocyanate)×2.1

The mixing ratio (weight) of the spherical silver powder=(the weight of the polyester resin modified by carbamate+the weight of blocked isocyanate)×0.233

(Samples manufactured for evaluation)

Adhesive-free copper-clad laminates (two-layer CCL) were fabricated in which copper foil was laminated to polyimide film. The copper foil portion was selectively etched by a subtractive method to form a pattern with a line width of 50 μm. A coverlay film was stuck thereon to manufacture a flexible printed wiring board for evaluation. The conductive paste was coated on the cover layer side of the flexible printed wiring board by a screen printing method and thermally cured in an oven. A solder resist was coated on the conductive paste for the sample used for the slide bending evaluation, and heat-cured in an oven.

(Evaluation of conductive paste: volume resistivity)

The samples were cut into 5 mm wide pieces. Resistance value measurement was performed by a four-probe method (distance between probes: 100 mm). The thickness of the cured film of the silver paste was measured using a surface roughness tester. Then calculate the volume resistivity.

(Evaluation of Conductive Paste: Adhesion Strength)

The samples were passed twice through a reflow oven set to a maximum temperature of 260°C. A notch is then formed between the cured film of the silver paste and the cover layer film. The cured film of the silver paste was bent in a direction of 180° C. and stretched at a speed of 50 mm/min to measure the adhesive strength.

(Evaluation of conductive paste: sliding bending test)

The sample was passed twice through a reflow oven set to a maximum temperature of 260° C. as described above. Then, under the conditions of a stroke of 100 mm, 15 seconds/cycle and a sliding bending radius of 1.0 mm, a sliding bending test was performed on the sample. The number of sliding and bending times when the line resistance increased by 20% was evaluated. The results are shown in Table 1. In the evaluation items, satisfactory ranges are as follows: adhesive strength of 0.8 N/cm or more, number of slide bending times of 70,000 or more, and volume resistivity of 90×10 ⁻⁶ Ω·cm or less.

In Comparative Example 1, the total amount of aromatic components contained in the acid component, alcohol component, and isocyanate component was less than 5 mol% of the total amount of the acid component, alcohol component, and isocyanate component, and the adhesive strength after reflow treatment was low , and poor heat resistance. Similarly, in Comparative Example 2 in which the isocyanate component does not contain an aromatic component, the adhesive strength after the reflow process was low, and the heat resistance was poor. In Comparative Example 3 in which the total amount of aromatic components exceeded 50 mol%, although the adhesive strength was good, the sliding bending performance was poor.

(Comparative Examples 4 to 8)

Urethane-modified polyester resins with different degrees of polymerization and different hydroxyl values were prepared. Conductive pastes were prepared using the resins in Examples 1-3. Adhesion strength, sliding bending performance, and volume resistivity after reflow processing were evaluated. The results are shown in Table 2.

[Table 2]

There is a correlation between the adhesion strength of urethane-modified polyester resins after reflow treatment and the hydroxyl value. Higher hydroxyl numbers lead to improved adhesion strength. In Example 4 having a hydroxyl value of less than 5 mg KOH/g, the adhesive strength was 0.8 N/cm, which was slightly low. In Example 8 in which the hydroxyl value exceeded 60 mgKOH/g, the flexibility was poor, and the number of times of sliding bending was slightly low.

(Examples 9 to 13)

As shown in Table 3, polyfunctional blocked polyisocyanate compounds having different number average molecular weights were prepared. The polyfunctional blocked polyisocyanate compound is a compound in which the end of an addition-type isocyanate formed from an isocyanate monomer and a polyol is blocked with a blocking agent. As in Examples 1 to 3, the urethane-modified polyester resin described in Example 1 was used in combination with a polyfunctional blocked polyisocyanate compound as a curing agent to prepare a conductive paste. Adhesion strength, sliding bending performance, and volume resistivity after reflow processing were evaluated.

[table 3]

In each sample, the adhesive strength, sliding bending property, and resistivity satisfied the required characteristics. In Example 9 in which the number-average molecular weight of the polyfunctional blocked polyisocyanate compound was less than 500, the number of sliding and bending times was 80,000, which was slightly lower. In Example 13 in which the number average molecular weight exceeded 3000, the volume resistivity was slightly higher.

(Examples 14 to 18)

The urethane-modified polyester resin and polyfunctional blocked polyisocyanate compound used in Example 1 were used. Conductive pastes were prepared as in Examples 1 to 3 except that the ratio of NCO/OH was changed by changing the mixing ratio of the two. Adhesion strength, sliding bending performance, and volume resistivity after reflow processing were evaluated. The results are shown in Table 4. In Example 14 in which the NCO/OH ratio was less than 0.8, the volume resistivity was slightly higher. It is speculated that this is because the low ratio of NCO/OH leads to a decrease in crosslink density. In Example 18 in which the NCO/OH ratio exceeded 3.0, the adhesive strength after the reflow treatment was slightly low. It is speculated that this is due to the high NCO/OH ratio resulting in excess curing agent remaining, resulting in reduced heat resistance.

[Table 4]

(Examples 19 to 23)

Metal powder A having an average particle size of 4.8 μm and metal powder B having an average particle size of 30 nm were prepared as conductive metal powders. A conductive paste was prepared as in Examples 1 to 3 except that the content ratio of metal powder A and metal powder B was changed. Adhesion strength, sliding bending performance, and volume resistivity after reflow processing were evaluated. In this case, the total weight of the metal powder A and the metal powder B was set to be 2.333 times the total weight of the urethane-modified polyester resin and the blocked isocyanate.

The results are shown in Table 5. In Example 19 in which the content ratio of metal powder A to metal powder B was less than 99.5:0.5, the volume resistivity was slightly higher. In Example 23 in which the content of metal powder A to metal powder B exceeds 70:30, there is no particular effect of improving properties. Metal powders with an average particle size on the order of nanometers are expensive. Considering the properties and cost of the conductive paste, the content ratio of the metal powder A to the metal powder B is preferably in the range of 99.5:0.5 to 70:30.

[table 5]

Metal powder A having an average particle size of 4.8 μm and metal powder B having an average particle size of 30 nm were prepared as conductive metal powders. The conductive paste was prepared by fixing the content ratio of metal powder A to metal powder B at 90:10 and defining different content ratios of metal powders as the total amount of metal powder A and metal powder B. Adhesion strength, sliding bending performance, and volume resistivity after reflow processing were evaluated.

The results showed that the volume resistivity was slightly higher when the content ratio of the metal powder was less than 50% by weight. When the content ratio of the metal powder exceeds 85% by weight, the sliding bending performance is slightly lower. This is presumed to be because the flexibility decreased due to an excessively high metal powder content ratio.

It should be understood that the disclosed embodiments and examples of the present invention are illustrative and not restrictive in every respect. The scope of the present invention is defined by the scope of the claims, not by the above description, and is intended to include any modifications within the scope of the claims or equivalent meanings to the scope of the claims.

Industrial Applicability

The present invention relates to a conductive paste, an electromagnetic shielding film and an electromagnetic shielding flexible printed wiring board using the conductive paste. In particular, the present invention can be suitably used for flexible printed wiring boards that require bending resistance.

Symbol Description:

1 substrate

2 layers of conductive paste

3 Electromagnetic shielding film

4 base material

5 copper foil

6a Overlay film

6b Overlay Adhesive

7 Flexible printed wiring board

8 protective film

patent documents

Patent Document 1: Japanese Unexamined Patent Publication No. 1-159906

Patent Document 2: Japanese Unexamined Patent Publication No. 9-306240

Patent Document 3: JP-A-2005-294254

Claims (7)

1. A conductive paste comprising: Conductive metal powder, urethane-modified polyester resin and blocked isocyanate, wherein said urethane-modified polyester resin is prepared by reacting an acid component, an alcohol component, and an isocyanate component containing an aromatic isocyanate, and The total amount of aromatic components contained in the acid component, alcohol component and isocyanate component is 5 mol% to 50 mol% of the total amount of the acid component, alcohol component and isocyanate component. 2. The conductive paste according to claim 1, wherein the urethane-modified polyester resin has a hydroxyl value of 5 mg KOH/g to 60 mg KOH/g. 3. The conductive paste as claimed in claim 1, wherein the blocked isocyanate has a number-average molecular weight of 500 to 3000, and is a polyfunctional blocked polyisocyanate compound, wherein the terminal of the addition-type isocyanate is Blocking, the addition-type isocyanate is formed from isocyanate monomers and polyols. 4. The conductive paste as claimed in claim 1, wherein, the molar ratio (NCO/ In terms of OH), the mixing ratio of the blocked isocyanate to the urethane-modified polyester resin is 0.8 to 3.0. 5. The conductive paste according to claim 1, wherein the conductive metal powder is formed of a metal powder A having an average particle size of 0.5 μm to 20 μm and a metal powder B having an average particle size of 100 nm or less, and wherein the metal powder A is The weight content ratio of the metal powder B is 99.5:0.5 to 70:30, and the content ratio of the conductive metal powder to the solid component in the conductive paste is 50 wt% to 85 wt%. 6. An electromagnetic shielding film comprising a layer on a substrate, the layer being made of the conductive paste according to any one of claims 1 to 5. 7. An electromagnetic shielding flexible printed wiring board comprising a layer made of the conductive paste according to any one of claims 1 to 5.

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