JP2965815B2 - Solderable conductive paste for film formation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive paste used for manufacturing an inexpensive solderable printed wiring board which forms a wiring by printing a conductive paste by a screen printing method or the like.

[0002]

2. Description of the Related Art It has been a practice in the field of printed wiring boards to form a solderable coating film by screen-printing a conductive paste, and conductive paste such as silver powder and copper powder has been used. A paste was prepared by mixing and dispersing a conductive filler, a binder resin composed of a thermosetting resin, a solvent for controlling printing characteristics, and additives. Then, this conductive paste is printed on a substrate made of a resin laminate or a plastic film by screen printing, dried and heated as appropriate to remove the solvent, and the thermosetting resin as a binder resin is cured and solidified. A wiring pattern consisting of a thin film was obtained.

[0003] Typical combinations of a binder resin and a conductive filler in a conventional conductive paste include epoxy resin and copper powder, epoxy resin and silver powder, phenol resin and silver-coated copper powder, phenol resin and silver powder, and the like.

[0004]

However, since these conventional conductive pastes use a thermosetting resin such as epoxy or phenol as a binder resin, the conductive paste contained in the formed coating film is not included. It is difficult to increase the ratio, so the surface of the conductive filler is covered with a layer of thermosetting resin, so-called poor wettability of solder, solder is difficult to adhere, and the reliability of soldering is low. there were.

[0005] The reason that the binder resin being a thermosetting resin results in the inability to increase the content of the conductive filler is considered as follows. The conductive paste must have screen printing suitability, generally a viscosity in the range of 60 to 300 poise, and moderately thixotropic flow properties. The major factors affecting printability, i.e., viscosity and flow characteristics, are the viscosity of the binder resin, the type and amount of the solvent, and the type and amount of the conductive filler. Various additives are used to adjust printability, but it is basically necessary to appropriately select and combine the above factors. The curable resin is designed to have a large molecular weight and three-dimensionally crosslink after curing,
Before curing, that is, at the material stage (compared with the thermoplastic resin of the present invention), the molecular weight is small, so when a resin solution having a constant viscosity is prepared by adding a solvent, the resin content in the solution,
That is, the solid content concentration is high. Furthermore, the conductive filler is dispersed to adjust the viscosity to a range suitable for screen printing, but the solid content concentration in the resin solution is high, and as a result, the solid content concentration in the conductive paste must be increased. Absent. When a thermosetting resin is used as the binder resin, the content of the conductive filler in the solid content is at most 50 vol.
%. In order to increase the amount of the conductive filler, a method of increasing the amount of the solvent to lower the solid content concentration of the resin solution and simultaneously lowering the viscosity may be considered. It is settled and the state where the conductive filler is dispersed cannot be maintained.

As described above, when the curable resin is used as a binder resin, the content of the conductive filler is only 50 vol% in the formed coating film (solid content), so that the periphery of the conductive filler is thermoset. Since the resin layer is covered with the conductive resin layer and the conductive filler near the surface of the coating film is also formed with a considerably thick resin layer, it is difficult for the solder to come into contact with the conductive filler.

Further, since the content of the conductive filler in the solid content cannot be increased, the type of the conductive filler is limited. That is, since the content of the conductive filler cannot be increased, the specific resistance of the coating film to be the wiring constituting the circuit tends to increase, and therefore, the specific resistance of the coating film can be reduced with the same content. For example, flake-like silver powder must be used, but flake-like silver powder has a disadvantage that solder erosion is likely to occur. Use of the conductive paste having a high specific resistance has been limited.

Accordingly, the present invention solves the above-mentioned problems of the prior art, and forms a conductive paste having a high content of conductive filler, capable of forming a coating film having good conductivity and good solder wettability, and having good printability. The purpose is to provide.

[0009]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, a conductive pen for forming a coating film capable of being soldered according to the present invention is provided.
The paste is made of thermoplastic saturated poly-
90-99 wt% of ester resin and crosslinking agent or thermosetting
A binder resin composed of 1 to 10% by weight of the resin contained a conductive filler in an amount of 60 to 80% by volume.

Furthermore , at least 70 of the conductive filler
% Is preferably composed of metal powder having a particle size of 1 to 10 μm.
No.

[0011]

According to the present invention, the above-mentioned structure is provided, and more specifically, a thermoplastic saturated polyether is used as a binder resin.
By using the ester resin, the content of the conductive filler in the coating film formed by using the conductive paste is 60%.
vol%, it can be extremely large, so that the surface of the filler located on the surface of the coating film is not covered with a relatively thick and strong resin film. The resin is softened by
As a result, the contact between the solder and the conductive filler is facilitated, so that the wettability is improved. At the same time, the conductivity of the coating is improved. In addition, since the content of the conductive filler is set to 80 vol% or less, good printability is maintained. By providing the latter configuration, specifically, 10 w
By replacing t% or less with a curable resin, the effect of the former configuration can be substantially maintained, and the heat resistance can be improved.

[0012]

EXAMPLE A thermoplastic polyester resin soluble in a solvent is used as a binder resin. Examples of the acidic content of the polyester resin include terephthalic acid, isophthalic acid, adipic acid, sebacic acid, eicosaninic acid, caprolactone,
Trimellitic acid, naphthalenedicarboxylic acid, glycol components such as ethylene glycol, propanediol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol, neopentyl glycol,
Polyethylene glycol, polytetramethylene glycol, trimethylolpropane, bisphenol A diethylene glycol, and the like.
Those having a melt viscosity at 0 ° C of 2900 poise or more and a glass transition point of 65 ° C or more can be used, and those having a glass transition point of 90 ° C or more are preferable. When the melt viscosity is low, an excessively large amount of solvent is not required to prepare a resin solution having a constant viscosity, so that the content of the conductive filler in the solid of the conductive paste cannot be increased. If the glass transition point is lower than 65 ° C., the heat resistance of the internal cohesive force of the solid material of the conductive paste is poor, and it is not practical.

As a secondary component of the binder resin, a crosslinking agent or a curable resin may be used. As a crosslinking agent, toluene diisocyanate, a block such as diphenylmethane diisocyanate, an iron-acetylacetonate complex,
Metal complexes such as a nickel acetylacetonate complex, a copper acetylacetonate complex, and a copper-8 oxyquinoline complex are used. Melamine and epoxy resin are used as the curable resin. However, when the curable resin is blended by 10% or more, the solder wettability is reduced.

As the solvent, methyl ethyl ketone, cellosolve acetate, butyl cellosolve acetate, carbitol acetate, isophorone and the like are used alone or in combination.

As the conductive filler, silver powder, silver-plated copper powder, silver-coated copper powder, and silver / copper alloy powder are used. At least 70% of the conductive filler has a particle size of 1 to 10 μm.
m, preferably 2 to 5 μm, is used. The spherical shape is used because the spherical shape has the smallest specific surface area and the content can be increased. When the particle size is less than 1 μm, the influence of so-called solder erosion increases. If the particle size exceeds 10 μm, it tends to settle out in the conductive paste and makes screen printing difficult. Also, the conductivity of the coating film is not good .

The content of the conductive filler in the solid content is 60
If the content is less than vol %, the conductivity will be significantly deteriorated. 80
If the content exceeds vol %, the fluidity of the conductive paste becomes dilatant, and the paste is not suitable for screen printing.

In addition to the above, other additives such as a silane coupling agent for enhancing the interface strength between the binder resin and the conductive filler, a coupling agent such as a titanate coupling agent, and the dispersion of the conductive filler in the paste may be used. A dispersing agent for assisting and a thixotropic agent for improving printability are appropriately added and used.

Examples 1 to 7 A saturated polyester resin (Kemit K-1294, manufactured by Toray Industries, Ltd., melt viscosity 4300 poise, glass transition point 67 ° C.) as a binder resin was dissolved in carbitol acetate as a solvent by heating to dissolve the resin solution. Produced. Furthermore, spherical silver powder (Silvest G-1 manufactured by Tokuri Chemical Co., Ltd.) having a particle size of 2 to 3 μm was mixed as the conductive filler at a different mixing ratio for each example, and dispersed by a three-roll mill to obtain a conductive paste. Was. Next, this conductive paste was applied to a paper-phenol laminate by screen printing, and the solvent was removed by heating to obtain a printed wiring board. The magnitude of the viscosity of the conductive paste due to the different mixing ratio of the conductive filler was adjusted by changing the amount of the solvent and the amount of the additive in advance. The content of the conductive filler in the solid content and the evaluation results of printability, solder wettability and conductivity in each example were as follows in each example. In the experimental example, the content of silver powder in the solid content was out of the range of the example, and a practical problem occurred.

[0019]

[Table 1] As described above, when a saturated polyester resin is used as the binder resin, the silver powder content in the solid content is 60 to 80.
%, Good solder wettability and printability were obtained in a high conductive filler content range as high as 100%, and good conductivity was also obtained.

Examples 8 to 9 The adhesive strength of the soldered portion after soldering was measured by changing the binder resin of Example 4 above. The sample of Example 8 was prepared in the same manner as in Example 4 except that the binder resin was a saturated polyester resin having a glass transition point of 35 ° C. and a melt viscosity of 7000 poise. The sample of Example 9 was prepared in the same manner as in Example 4, except that the binder resin was a saturated polyester resin having a glass transition point of 94 ° C. and a melt viscosity of 13,000 poise. 26 for these samples
After contact with molten solder at 0 ° C for 3 seconds,
After giving a heat history at −30 ° C. for 30 minutes and then at −30 ° C., the peeling strength of the soldered portion from the substrate was measured. For comparison, the same test as that of Example 4 was performed.

[0021]

[Table 2] As described above, it has been found that when the glass transition point is low, the heat resistance is low, and the soldered portion falls off depending on the conditions of the storage environment.

[Examples 10 to 13] A part of the binder resin in Example 3 was replaced with a melamine resin (Sumimar M-100 manufactured by Sumitomo Chemical Co., Ltd.) or an epoxy resin (Tactics 785 manufactured by Dow Chemical) as a curable resin. The evaluation in each case was as follows.

[0023]

[Table 3] When a part of the binder resin (saturated polyester resin) is changed to a curable resin, the heat resistance is improved. However, when the binder resin is 10% or more, the solder wettability deteriorates.

[Examples 14 to 17] The evaluation results in the case where part of the spherical silver powder as the conductive filler of Example 2 was replaced with flaky silver powder (TCG-7 manufactured by Tokurika Chemical) were as follows. Was.

[0025]

[Table 4] When a part of the conductive filler (spherical silver powder) is changed to flaky silver powder, the conductivity is improved, but when the ratio exceeds 30%, solder erosion occurs.

[0026]

Comparative Example The same evaluation was performed on a conductive paste using a commercially available thermosetting resin.

Manufacturer and product number of comparative example Composition Material of Comparative Example The content of the conductive filler in the solid of the comparative example was approximately 50 vol % or less.

[0028]

[Table 5] Evaluation result of comparative example

[0029]

[Table 6] In the above description, a saturated polyester resin has been described as a typical example of the thermoplastic resin. However, it is apparent that the thermoplastic resin can be replaced with another thermoplastic resin that can be considered from the viewpoint of the operation and effect of the present invention. Also, the curable resin used in addition to the thermoplastic resin can be appropriately changed.
Although only the silver powder has been described as the conductive filler, other materials such as silver-coated copper powder and silver / copper alloy can be substituted. Also,
It is apparent that the effect of improving the solder wettability and the like can be obtained by increasing the content of even a highly oxidizable filler such as copper powder.

[0030]

The present invention exhibits, the binder resin is a glass transition point 90 ° C. or more thermoplastic saturated polyester resin 90 to
Crosslinking agent or thermosetting resin at 99 wt% 1 to 10 wt%
Since consisting content of the conductive filler in a binder resin
Since the ratio can be as large as 60 vol% or more, the surface of the filler located on the coating film surface is not covered with a thick and strong resin film, and even if the film remains, the resin is softened by heat during soldering. This facilitates the contact between the solder and the conductive filler and improves the wettability, and as a result, improves the conductivity of the coating film. In addition, since the content of the conductive filler is set to 80 vol% or less, good printability is maintained. Further, at least 70% of the conductive filler is made of metal powder having a particle size of 1 to 10 μm, so that the influence of solder erosion is small and screen printing can be easily performed. Further, since the binder resin contains 1 to 10% by weight of a thermosetting resin, it is possible to improve the heat resistance while maintaining the above-mentioned good solder wettability and the like. The glass transition point of the thermoplastic saturated polyester resin, which is 90 to 99 wt% of the binder resin, is 9 %.
Since the temperature is 0 ° C. or higher, the heat resistance is good and the peel strength of the soldered portion due to the storage environment can be prevented from deteriorating.

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01B 1/20-1/22 C09D 5/24 H05K 1/09 C09J 9/02 C08K 3/08

Claims (2)

(57) [Claims] 1. A thermoplastic saturation having a glass transition point of 90 ° C. or higher.
90-99% by weight of polyester resin and crosslinking agent or thermosetting
A conductive paste for forming a solderable coating film, comprising a binder resin comprising 1 to 10% by weight of a curable resin and 60 to 80% by volume of a conductive filler. 2. at least 70% of said conductive filler
2. The conductive paste for forming a solderable coating film according to claim 1, wherein the conductive paste comprises metal powder having a particle size of 1 to 10 [mu] m.