JPH11181378A - Conductive adhesive - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a conductive adhesive which is free from oozing or change in shape or size even when screen printed and has an adhesive strength almost equal to a cream solder. SOLUTION: This conductive adhesive is thixotropic fluid and comprises a resin and a metal powder dispersed therein. The conductive adhesive which is a thixotropic fluid is free from oozing or change in shape or size in printing, whereas a conductive adhesive which is not a thixotropic fluid oozes in printing and impossible to carry out proper printing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive adhesive used for mounting an electronic component on a printed wiring board or a flexible printed board, and more particularly to a conductive adhesive for screen printing. is there.

[0002]

2. Description of the Related Art Generally, conductive adhesives are epoxy resins,
Phenolic resin, polyurethane resin, acrylic resin,
A conductive metal powder such as a silver powder, a copper powder, and a nickel powder is dispersed in a resin such as a vinyl chloride resin, and a solvent and an additive are added as necessary.

In particular, epoxy-based conductive adhesives are used at room temperature to
It cures under relatively mild conditions of 200 ° C, and since the cured product has excellent adhesiveness and heat resistance, it is widely used as a die bond for bonding semiconductor chips such as ICs and LSIs to lead frames and as an electromagnetic shielding material. Used.

[0004] Separately, in order to bond an electronic component such as an LED, a chip capacitor or a chip resistor to a printed wiring board or a flexible printed board, it is necessary to use a 0.1. Cream solder capable of printing a fine pattern with a pitch of several mm is exclusively used.

However, cream solder has many problems, such as the necessity of cleaning flux to prevent migration and the use of lead as a material. Therefore, recently, the use of a conductive adhesive as a substitute for lead-based solder has also been studied for bonding electronic components on a printed wiring board.

[0006]

However, in order to use the conductive adhesive for die bonding for bonding electronic components onto a printed wiring board, it is necessary to transfer a fine pattern to the printed wiring board by screen printing. However, even when screen printing was performed using a conductive adhesive for die bonding, a fine pattern could not be printed.

In the conventional conductive adhesive, the conductive adhesive bleeds in the printing direction from the gap between the metal mask of the printing press and the printed wiring board during printing, or becomes conductive in the horizontal direction when the metal mask is separated from the substrate. The adhesive spread, and the shape and dimensions could not be maintained. In order to adhere electronic components to a printed wiring board, it is necessary to form an electrode having a thickness of at least 100 μm in order to absorb variations in leads. However, since the shape and dimensional stability cannot be maintained, printing is not possible. The pattern was sagged, and a film thickness of 60 μm or more could not be secured.

[0008] In view of the above problems, an object of the present invention is to provide a conductive adhesive which does not cause bleeding or change in shape and dimensions even when screen printing is performed.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies on the relationship between each property of the conductive adhesive and printability in order to enable printing of a fine pattern by the conductive adhesive. It has been found that printability can be greatly improved by controlling the viscosity characteristics, and the present invention has been achieved.

That is, the conductive adhesive of the present invention has a temperature of 25 ° C.
When a shear rate of 10 s ^-1 is given under the following conditions,
It is characterized by exhibiting a thixotropic property in which the viscosity gradually decreases with time.

When a shearing speed is given to a fluid such as a conductive adhesive in which a metal powder is dispersed in a resin for a certain period of time, the viscosity may change as time passes. At that time, the viscosity can be described by equation (1).

Η = a (ΔT) exp (−t / τ) + c (1) where a and c are constants, ΔT is a change in torque, t is time, τ
Is the time constant of the viscosity change.

The viscosity has a time dependency, that is, the equation (1)
, The conductive adhesive is a thixotropic fluid.

The conductive adhesive which is a thixotropic fluid does not bleed or change in shape and dimensions during printing, but the conductive adhesive which is not a thixotropic fluid causes bleeding of the conductive adhesive during printing, and You cannot print.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The conductive adhesive of the present invention contains silver powder, copper powder, and the like in a resin such as an epoxy resin, a phenol resin, a polyurethane resin, an acrylic resin, and a vinyl chloride resin.
It is a dispersion of conductive metal powder such as nickel powder. The viscosity characteristics can be adjusted by changing the type and amount of the solvent or by adding an additive such as a thickener.

The type and molecular weight of the resin used in the present invention are not particularly limited, but an epoxy resin is preferred from the viewpoint of adhesive strength.

Although the composition and shape of the conductive metal powder are not particularly limited, it is desirable to combine spherical powder and flake powder. Flake powder is excellent from the viewpoint of electrical resistance,
From the viewpoint of printability, a spherical powder having small anisotropy is excellent, and by mixing both, electrical resistance and printability can be compatible in a favorable state.

[0018]

The present invention will be described in more detail with reference to the following examples.

(1) Production of conductive adhesive A total of 16 g composed of 8 g of spherical silver powder having an average particle size of about 0.5 μm and 8 g of flake silver powder having an average particle size of about 10 μm
Silver powder, 2.6 g of a phenol novolak-type epoxy resin containing dicyandiamide as a curing agent, and 1.4 g of glycidyl phenyl ether as a solvent were preliminarily kneaded on a watch glass, and then passed through a three-roll mill seven times. Was obtained.

The viscosity of the obtained conductive adhesive is 10 s ⁻¹
Give the shear rate, using a cone plate type viscometer,
The measurement was performed at 25 ° C. When the time constant of the change in viscosity was determined according to equation (1), τ was 7 s, indicating that the conductive adhesive of the example was a thixotropic fluid.

For comparison, a conductive adhesive of Comparative Example was obtained in the same manner as in Example except that 1.4 g of alkyl glycidyl ether (C ₈ ) was used instead of glycidyl phenyl ether as a solvent. When the viscosity of the conductive adhesive of the comparative example was measured in the same manner as in the example, the viscosity did not show time dependency and was not a thixotropic fluid.

(2) Evaluation of printability A 150 μm-thick metal mask and a metal squeegee were used for printing, and a printed wiring board made of glass epoxy resin was used for the printed board. The metal mask used in this example had the following sizes. A rectangular opening having a width of 210 μm and a length of 1270 μm is provided in the metal mask, and these openings are arranged in an L-shape at a pitch of 0.4 mm. Note that the printing machine has a ModW
An el-810 printing machine was used.

The printability was evaluated based on four characteristics of print size ratio, transfer ratio, anisotropic ratio, and continuous printability. In the printed conductive adhesive, the size of the printed pattern differs between a direction parallel to the printing direction (a moving direction of the metal squeegee) and a direction perpendicular to the printing direction (the moving direction of the metal squeegee) due to anisotropy of printing. Therefore, the dimensions of the print pattern were measured and evaluated for two types of patterns, that is, a horizontal pattern in which the patterns were arranged in the printing direction and a vertical pattern in which the patterns were arranged in the direction perpendicular to the printing direction. Unless otherwise specified, the average of the two types of patterns was used in the evaluation of the print patterns described below.

The print size ratio was the ratio of the width of the print pattern to the width (210 μm) of the opening of the metal mask. It can be said that the closer the print size ratio is to 100, the more accurately the opening shape of the metal mask is reflected in the print pattern.

The transfer rate is determined by the volume of the opening of the metal mask (150 μm × 260 μm × 1270 μm = 4.95).
3 × 10 ⁻¹¹ m ³ ) was defined as the ratio of the transfer amount of the printed conductive adhesive. The transfer amount was the product of the maximum value of the width, the maximum value of the length, and the average value of the height of the print pattern. Even if the print size ratio is good, a print pattern with a desired film thickness cannot be obtained if the transfer ratio is low.

The anisotropy is defined by the following equation (2), using the ratio of the difference between the width of the vertical pattern and the width of the horizontal pattern to the sum of the width w1 of the vertical pattern and the width w2 of the horizontal pattern as the anisotropy.

Anisotropy (%) = {| w1-w2 | / (w1 + w2)} × 100 (2) If the anisotropy is large, the size of the print pattern will be uneven in the metal mask surface.

The continuous printability is defined as three or more times when the adjacent print patterns are not contacted due to shape deformation such as bleeding or sagging, or when the conductive adhesive is not transferred and the print patterns are not formed. A case where continuous printing was possible was regarded as good, and five or more times as excellent.

Table 1 shows the results of the above four types of property evaluations for the conductive adhesives of the examples and comparative examples.

[0030]

[Table 1]

As can be seen from Table 1, the conductive adhesives of the examples showed good results in any of the properties, but the conductive adhesives of the comparative examples could not satisfy all the properties. .

[0032]

As described above, by using the conductive adhesive of the present invention, it is possible to perform good screen printing even with a 0.4 mm pitch printing pattern.

Claims (1)

[Claims] 1. A conductive adhesive in which a metal powder is dispersed in a resin, wherein the conductive adhesive is a thixotropic fluid.