JPH02163150A - Electrically conductive paste - Google Patents

Abstract

PURPOSE:To obtain an electrically conductive paste useful for printed wiring boards, having high electrical conductivity and excellent adhesiveness to conductor such as copper foil by blending a binder resin comprising a phenol resin and a bisoxazoline compound as essential components with copper powder. CONSTITUTION:(A) 50-95wt.% resol type phenol resin or mixture of the resin and a novolak type phenol resin is blended with (B) 5-50wt.% bisoxazoline compound [e.g. 2,2'-(1,2-ethylene)-bis(2-oxazoline)] to give a binder resin, which is mixed with (C) copper powder having <=20mum average particle diameter, preferably atomized copper powder having about 10mum average particle diameter in such a way the amount of copper powder is 75-90wt.% based on total amounts of copper powder and the binder resin and kneaded by a triple roll mill, etc., to give an electrically conductive paste.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to a highly conductive and highly reliable conductive paste used for printed wiring boards. This invention relates to a copper paste that has excellent adhesion to conductors.

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

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.

Commercially available copper pastes require the use of a reducing resin such as phenol resin as the binder resin in order to prevent oxidation of the copper powder, and are also formulated with additives such as reducing agents. Not enough results have been obtained.

Furthermore, the 1.sup.1 paste is rarely used as a single positive paste to form a conductive circuit. Usually, it is often used for conductive circuits in which copper base I is interwoven with conductors made of copper foil or copper plating, such as for jumper circuits or for forming EMI shield layers.

The electrical reliability of these entire conductive circuits is copper based!・
It can be said that not only the characteristics of the individual but also the reliability of the connection between the copper paste and the conductor such as copper foil are extremely important.

However, since conventional copper paste uses a reducing resin as a binder resin as mentioned above, it has poor adhesion to conductors made of copper foil or copper plating, and
The paint film has poor flexibility and is prone to cracking, and
Adhesion is adversely affected by the addition of reducing agents, etc.
There are other problems.

It has been empirically found that adhesion can be improved by subjecting the conductor to oxidation treatment by heating before printing the copper paste. However, this method has many problems in terms of mass production, such as an increase in the number of steps and difficulty in controlling the oxidation state to a constant level.

(Means for Solving the Problem) The inventors conducted extensive studies in view of the above-mentioned problem that conventional copper paste has insufficient adhesion to conductors such as copper foil and lacks electrical reliability as a conductor circuit. As a result, we discovered a binder resin composition suitable for copper paste that has high conductivity and high reliability, and completed the present invention.

That is, the conductive paste of the present invention is characterized in that copper powder is blended into a binder resin containing a phenol resin and a bisoxazoline compound as essential components.

The phenolic resin in the present invention is a resol-type phenolic resin or a mixture of a resol-type phenolic resin and a novolac-type phenolic resin.

In addition, the bisoxazoline compounds in the present invention include 2.2'-(1,2-ethylene)-bis(2-oxazoline), 2.2'-(1,4-butylene)-bis(2-oxazoline) , 2.2'-(1,3-phenylene)-bis(2-oxazoline), 2.2'-(1,4-phenylene)-bis(2-oxazoline), 2.2'-(1,3 -phenylene)-bis(5-methylene-2-oxazoline) and the like.

The blending ratio (solid content) of the phenol resin and the bisoxazoline compound is preferably 50 to 95% by weight for the phenol resin and 5 to 50% by weight for the bisoxazoline compound. If the bisoxazoline compound is less than 5% by weight, the adhesion to conductors such as copper foil will not be improved. When the bisoxazoline compound is heated over 50 times, the adhesion is very good and there is no problem, but the conductivity tends to decrease.

Note that, if desired, other thermosetting resins or thermoplastic resins may be blended with the resins mainly composed of these phenol resins and bisoxazoline compounds.

The copper powder used in the conductive paste of the present invention is not particularly limited, and various manufacturing methods and shapes such as electrolytic copper powder, atomized copper powder, and pulverized copper powder can be used. The average particle size is preferably 20 μm or less since the conductive paste is screen printed. In particular, it is preferable to use atomized copper powder with an average particle diameter of about 1071 m, since it provides excellent printability and conductivity.

The copper powder content in the conductive paste is preferably 75 to 90% by weight based on the total weight of the solid content of the copper powder and binder resin. Furthermore, the optimum copper powder content varies depending on the properties of the copper powder used.

As additives, organic acids, acid salts, reducing agents, high-boiling solvents, antifoaming agents, shaking agents, and the like can be appropriately added to the binder resin. In particular, it is preferable to add a small amount of organic acid because a highly conductive conductive paste can be obtained.

The conductive paste of the present invention is printed on a substrate by screen printing, and can be used as a conductive circuit or an EMI shield layer as it is, or by plating the surface of the copper paste with a metal such as nickel or copper.

(Function) The conductive paste of the present invention contains a bisoxazoline compound in the binder resin, so it has excellent toughness and can be applied to conductors formed by copper foil or plating without impairing conductivity. It has excellent adhesive properties.

As a result, in conductive circuits and EMl shield layers using copper paste, the connection between the copper paste and the conductor formed by copper foil or plating is well bonded and strong, resulting in printing with excellent electrical reliability. You can get a wiring board.

Note that the effect of improving the adhesiveness of the bisoxazoline compound in the conductive paste of the present invention is remarkable when an organic acid is used as an additive.

(Example) The present invention will be explained below using examples.

Bisoxazoline compounds include 2. manufactured by Takeda Pharmaceutical Company Ltd.
Using 2'-(1,3-phenylene)-bis(2-oxazoline), commonly known as 1,3-PBO, a resol type phenolic resin was mixed in the proportions shown in Table 1, and a small amount of oleic acid and quencher were added. A foaming agent was added to form a binder resin.

Atomized spherical copper powder with an average particle size of 10 μm was used as the copper powder.

Copper powder and binder resin were mixed and then kneaded in a three-roll mill to form a conductive paste.

Note that the copper powder content was selected to be the value that would give the lowest specific resistance. It was set at 83% with respect to the total amount of copper powder and resin solid content.

Each of the conductive pastes of Examples 1 to 3 and Comparative Example 1 was printed on a glass epoxy substrate using a stainless steel screen to form a zigzag pattern with lines 'Ig2 mm and length 36.8 cm. After curing this for 30 minutes in a hot air constant temperature bath at 160° C., the resistance value and film thickness of the zigzag pattern circuit were measured to determine the specific resistance.

Furthermore, the copper-clad glass epoxy substrate was heated at 150℃3 in the atmosphere.
One was prepared by heating the copper foil for 0 minutes to oxidize the surface of the copper foil, and the other was not heated. A pattern of 5Qmm x 20mm was formed on both substrates by screen printing method, and 1
It was cured by heating at 60° C. for 30 minutes.

J [According to the base grain test of 5K5400, draw 11 vertical and horizontal parallel lines on the coating film at 1 mm intervals, perpendicular to each other, with a cutter knife to form the base grain shape so that 100 squares are formed in 1 cm2. I got a cut. Adhesion was evaluated by the number of substrates of the coating film remaining on the substrate when the coating film was peeled off from above using cellophane tape.

Table 1 shows each binder resin composition and measurement results.

In Comparative Example 1, 1.3-PBO was not mixed, so the adhesiveness was poor, and the adhesiveness could not be obtained unless the copper foil surface was oxidized.

Examples 1 and 2 have the same specific resistance as Comparative Example 1, but have extremely superior adhesion. Even without oxidation on the surface of the copper foil, it is extremely excellent, and the effect of blending 1.3-PBO is remarkable.

In Example 3, the specific resistance was slightly high, but the adhesiveness was extremely excellent.

(Blank below) (Effects of the present invention) As described above, the conductive circuit using the conductive paste of the present invention has high conductivity, and moreover, the connection between the copper paste and the metal conductor formed by copper foil or plating is Since the adhesion of the parts is extremely good and strong, it has high electrical reliability.

Claims (1)

[Claims] A conductive paste characterized by mixing copper powder with a binder resin containing a phenol resin and a bisoxazoline compound as essential components.