JPH01265405A - Conductive paste and conductive circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a conductive paste containing copper powder as a filler, which has excellent printability, high conductivity, and high reliability, and which is used for printed wiring boards. Concerning a conductive circuit using.

(Prior Art and Problems to be Solved by the Invention) In recent years, copper pastes using copper powder as a filler have been developed, as copper has been evaluated for its resistance to electromigration and low cost compared to silver.

However, resin-curing copper paste, which is made by blending copper powder with thermosetting resin, tends to oxidize the surface of the copper powder when it is thermally cured after printing, so it cannot be used as a low-resistance conductive circuit. It has the disadvantage that it is difficult to obtain, and that circuit resistance is likely to increase due to subsequent changes over time. In order to prevent oxidation of copper powder, commercially available copper pastes have compositions that use reducing resins as binders or add reducing agents, but satisfactory results have not been obtained.

The copper powder in commercially available copper pastes is electrolytic copper powder or pulverized copper powder, and has a dendritic or flake shape. Because of their shape, they have a large surface area and a low tap density.

For this reason, when used as a conductive paste, it is difficult to obtain a low-resistance circuit because it is easily oxidized by heating during curing and has poor filling properties. Screens tend to clog during printing, which can cause chipping and disconnection of printed circuits, and high-mesh screens cannot be used, so they cannot be used to form high-density, high-precision circuits. It has the following problems.

Furthermore, copper paste circuits are usually not solderable, and as mentioned above, the circuit resistance is not very low. Therefore, electroless plating is applied on the copper paste circuits to reduce the circuit resistance and make it easier to solder. is being given gender. but,
In commercially available copper paste circuits that use electrolytic copper powder as described above or pulverized copper powder that has a dendritic or flake shape, the surface of the printed circuit is poor, and the copper powder that forms the core of the plating is stuck to the circuit surface. When electroless plating is applied, the adhesion between the copper paste circuit and the plated metal is poor, resulting in insufficient solder pull strength. In order to obtain good plating adhesion, the content of copper powder must be increased, but this sacrifices the conductivity of the circuit in the conductive paste and creates more voids in the circuit, which reduces the cohesive strength of the steel paste. There is a problem in that this decreases the solder pull strength.

(Means for Solving the Problems) As a result of intensive studies in view of the above-mentioned problems that conventional copper pastes have, we have discovered the characteristics of copper powder suitable for copper pastes having high conductivity and high reliability, and have invented the present invention. completed.

That is, the conductive paste of claim 1 of the present invention has a shape factor of 6 to 6.
7. A conductive filler made of granular copper powder having a specific surface area of 0.4 to 0.7II+278 and a tap density of 4.0 g/cm3 or more is blended into a thermosetting resin.

The shape factor of copper powder is represented by the value of K in equation (1) shown below, and takes a range of 6 to 12.

d=ni/(SXρ)...(1) (S=specific surface area (m2/g), d=average particle size (μm), ρ=copper powder density (8.93g/cm3)) True sphere It is 6 in the case of , and 12 in the dendritic case. The shape factor is selected to be a value that matches the average particle diameter obtained by substituting the specific surface area into equation (1) for the shape and average particle diameter observed and measured with an optical microscope, scanning electron microscope, or the like.

The copper powder in the present invention has a shape factor of 6 to 7, and is a pearl or a pearl-like copper powder. Spherical copper powder easily separates secondary agglomerates, so it is uniformly dispersed in the paste, and clogging during screen printing is less likely to occur. Furthermore, since it is spherical without anisotropy, excessive thixotropic properties can be suppressed when it is made into a paste, and a circuit coating film with a flat surface and good leveling properties during printing can be obtained.

The specific surface area of the copper powder in the conductive paste of the present invention is 0.4
~0.7 m2/g is preferred. When the shape factor is 6 to 7, the average particle size is approximately 1 to 2 μm, which is extremely fine as a conductive filler for a resin-curing conductive paste.

When the specific surface area of the copper powder exceeds 0.711I21g, that is, when the average particle size is less than about 1 μm, there is a problem in handling the powder, leading to combustion explosion and deterioration of the working environment due to rapid oxidation reaction. In addition, the specific surface area is less than 0.4 m2/g,
That is, when the average particle size exceeds about 2 μm, when printing a high-precision circuit pattern using a screen with a mesh size of 300 or more, the mesh opening is narrow and the screening effect causes a high circuit resistance, and this tendency is particularly high. 0.2m
It becomes noticeable in thin lines of m or less. In addition, the specific surface area of copper powder is
It can be determined by the nitrogen gas adsorption method (BET method).

The tap density of copper powder in the present invention is 4.0 g/cm
A high tap density of 3 or more is suitable. If the copper powder used has a high tap density of 4.0 g/cm3 or more, the printed conductive circuit will be highly filled with copper powder, making it possible to increase the number of contact points between the copper powders. As a result, a low resistance circuit can be obtained. On the other hand, the tap density is 4,0 g
If it is less than /am3, the conductive circuit will be coarsely filled with copper powder and the circuit resistance will increase. In addition, the circuit tends to contain voids, resulting in a circuit with many defects. Note that the tap density is one that is normally measured for powder for powder metallurgy.

The conductive paste of the present invention is obtained by kneading copper powder having the above characteristics with a thermosetting resin and additives.

As the thermosetting resin, phenol resin, epoxy resin, melamine resin, polyamideimide resin, etc. can be used.
Furthermore, these resins may be used in combination, and butyral resin, acrylonitrile butadiene rubber, and other resins may be mixed and used as necessary.

The content of the copper powder in the conductive paste is preferably 75 to 90% by weight, particularly preferably 80 to 85% by weight, based on the total weight of the solid content of the copper powder and thermosetting resin.

As additives, unsaturated fatty acids such as oleic acid, high boiling point solvents, antifoaming agents, and shaking agents can be used as appropriate.

The conductive paste of the present invention has been described above in detail. Next, a conductive circuit according to claim 2 of the present invention will be explained.

The conductive circuit according to claim 2 of the present invention is a circuit formed by printing a circuit on an insulating substrate by a screen printing method and then curing the circuit, and is characterized by using the conductive paste according to claim 1 of the present invention. shall be.

As the substrate for forming the conductive circuit, any commonly used substrate can be used, such as phenol resin, epoxy resin, etc. impregnated with paper, glass mat, glass cloth, etc.
Examples include synthetic resin substrates such as polyimide resin and diallyl phthalate resin, and metal substrates such as aluminum and iron whose surfaces are coated with resin or enamel.

The material of the screen plate is not particularly limited, and stainless steel or Tetron mesh can be used. Stainless steel mesh is preferred when printing precise patterns. The usable range of screen mesh is approximately 160-400 mesh.

In the conductive circuit according to claim 2 of the present invention, the copper powder that is the filler in the conductive paste used is spherical and fine, and is particularly applicable to high mesh screens, so it is precise and accurate, and has low circuit resistance. It is characterized by low

A conductive circuit according to a third aspect of the present invention is characterized in that electroless plating is applied to a part or the entire surface of the conductive circuit according to the second aspect.

There are no particular restrictions on the metal type for electroless plating in the present invention, but examples include gold, silver, copper, nickel, and tin. Moreover, a commercially available plating solution is sufficient. Pretreatment of the paste circuit surface when plating is usually performed by degreasing with an alkaline solution, washing with water, rinsing with pickling, and rinsing with water, as well as carrying out pretreatments designated by each plating solution.

(Function) In the conductive paste of the present invention, the shape factor of the copper powder used is 6 to 7, and the copper powder has a shape close to a pearl or a perfect sphere. Spherical copper powder easily separates secondary agglomerates, so it is uniformly dispersed in the paste, and clogging during screen printing is less likely to occur. Furthermore, since it is spherical without anisotropy, excessive thixotropic properties can be suppressed when it is made into a paste, and a circuit coating film with a flat surface and good leveling properties during printing can be obtained. In addition, the specific surface area of the copper powder is 0.4 to 0.7 m2/g, that is, the average particle size is approximately 1 to 271 m, and the powder is easy to handle, and it can be used in high precision areas using a screen of 300 mesh or more. Even if the circuit pattern was printed, no increase in circuit resistance due to the screening effect associated with mesh opening was observed, and it was found that 0.2 m
It is fully applicable to circuits with thin wires of less than m. In general, the tap density of copper powder is high at 4.0 g/cm2 or more, so in order to fill the printed conductive circuit with high copper powder, there are many contact points between the copper powders. A low resistance circuit can be obtained.

The conductive paste of the present invention has a high tapping density of copper powder, so the copper powder in the circuit is densely packed, resulting in low resistance.Moreover, the average particle size is small, so the number of nuclei to which plating adheres is small. is extremely large, and the growth of plating is carried out efficiently.

Therefore, the conductive circuit of the present invention, which is based on a printed circuit using this conductive paste and subjected to electroless plating, has extremely low plating pinholes, has a smooth surface, has few internal defects, and has high reliability and extremely low resistance. It is a conductive circuit.

(Example) Next, the present invention will be specifically explained using examples.

1 ~ Examples 1 and 2 and Comparative Examples 1 to 3 are spherical copper powders with a shape factor of 6, Comparative Examples 4 and 5 are dendritic electrolytic copper powders with a shape factor of 12,
Comparative Example 6 used copper powder with a shape factor of 10, which was made into flakes by crushing resin-like electrolytic copper powder with a ball mill.

A conductive paste was prepared by adding each of the above copper powders, a resol type phenol resin, a small amount of oleic acid, and an antifoaming agent and kneading them in a three-roll mill.

Note that the content of copper powder was determined in advance by determining the lowest value of specific resistance in each conductive paste, and adopted that value. The content of copper powder relative to the total amount of copper powder and resin solids was 83% in Examples 1 and 2 and Comparative Examples 1 to 3, 80% in Comparative Example 4, and 80% in Comparative Examples 5 and 6. It is 75%.

A pattern of each conductive paste was printed on an epoxy glass cloth substrate by a printing method using a stainless steel screen of 200 to 400 meshes. The pattern is
It consists of a zigzag pattern with a line width of 21III+1 and a total length of 368cm and a land with a corner size of 311III+.

The printed paste was cured for 30 minutes in a hot air constant temperature bath at 160° C., and the resistance value and film thickness of the circuit were measured to determine the specific resistance.

Furthermore, copper plating with a thickness of 5 μm was applied using a commercially available electroless plating solution. For those printed with a 200 mesh screen, the solder pull strength is n = 1 on a 3 mm square land.
It was measured at 0 and the average value was calculated.

Table 1 shows the copper powder characteristics and measurement results of each conductive paste.

As is clear from Table 1, the conductive pastes of Examples 1 and 2 have extremely excellent resistivity, and there is no difference in resistivity due to the screen mesh.

Comparative Examples 1 to 3 are spherical copper powders, but the tap density is 4,
Since it is less than 0 g7cm3, the specific resistance is high and the solder pull strength is low. In Comparative Example 3, the average particle diameter is more than twice as large as that in Examples, so there is a difference in specific resistance depending on the screen mesh.

Comparative Examples 4 to 6 are inferior to Examples in both specific resistance and solder pull strength, and there is a difference in specific resistance due to the screen mesh.

(Effects of the Invention) The conductive paste of the present invention is particularly excellent in printing fine patterns, and the conductive circuit using this conductive paste has high conductivity and has few circuit defects, so electroless plating is performed using this circuit as a base. The conductive circuit that has undergone this process has excellent solderability and is extremely useful as a printed wiring board.

Patent applicant: Furukawa Electric Co., Ltd.

Claims (3)

[Claims] (1) Shape factor 6-7, specific surface area 0.4-0.7m^2
/g and a tap density of 4.0 g/cm^3 or more, a conductive paste made by blending and mixing a conductive filler made of granular copper powder with a thermosetting resin. (2) A conductive circuit obtained by printing the conductive paste according to claim 1 on an insulating substrate by a screen printing method and then heating and curing the paste. (3) A conductive circuit obtained by subjecting part or all of the surface of the conductive circuit according to claim 2 to electroless metal plating.