JP2014201029A - Circuit board and thermal print head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermal print head whose circuit is formed with paste containing a silver particle.SOLUTION: In a circuit board and a thermal print head using the same, a circuit is formed on an insulating substrate by using conductive paste containing a conductor particle in which a palladium coat is formed on a surface of a silver particle and whose metallic element components are 80-99.5 wt.% of silver and 0.5-20 wt.% of palladium. Accordingly, an inexpensive silver paste can be used, and heat resistance and resistance to leakage due to migration are improved.

Description

The present invention relates to a circuit board and a thermal print head.
It proposes the use of inexpensive alternatives to expensive materials while reducing the number of processes when manufacturing circuit boards, and achieves low cost and high performance by achieving more efficient manufacturing processes and lower material costs. Of the present invention, and is particularly useful for a thermal print head.

  A thermal print head is configured by electrically connecting a resistor as a heating element between a pair of electrodes consisting of a common electrode made of a metal film and an individual electrode formed on an insulating substrate, and is necessary between both electrodes. The voltage is applied and controlled to generate heat, the heat is applied to the thermal recording paper, and thermal recording is performed according to the amount of heat generated by the resistor. Usually, a desired gold film is formed by repeating printing and baking, and a wiring pattern is formed by photolithography treatment.

Such thick film printing is characterized in that although the adhesion of the metal film is relatively small, it can be produced simply by applying and baking a metal paste on a substrate. That is, since a film can be continuously formed with a simple process and apparatus without requiring a large-scale facility, there is a merit of high productivity and low cost, and it is widely used. Thick film printing uses a paste that is a non-uniform viscous liquid in which various metals are dispersed in a solvent. For this reason, there has been a problem that even if it is applied to the substrate and fired, the metal particles are in a state of being in contact with the substrate and a uniform film is difficult to form.
Also, since pastes using gold are expensive, techniques for reducing the amount of gold used, or pastes mainly made of metals other than gold have been proposed. Many techniques have been proposed in which silver is employed as an inexpensive metal with good electrical conductivity.

  For example, in a paste for forming a conductor capable of forming a fine wiring pattern without reducing the interface resistance value between conductor layers and increasing the resistance value by sintering, an organic compound of silver and an organic compound of nickel And a resinate paste made by mixing these with a resin, and an alloy containing 95 to 99% by weight of silver and 1 to 5% by weight of nickel in the metal element component is used (patent) Reference 1). In addition, in providing a silver paste having a low resistivity and excellent resistance to electromigration and stress migration as compared with the prior art and a method for producing the same, the metal powder is mainly composed of Ag and 0.1 wt% of Pd. %, And a total of a plurality of elements selected from the group consisting of Al, Au, Pt, Cu, Ta, Cr, Ti, Ni, Co, and Si. A silver paste composed of a metal powder and a vehicle containing at most% is proposed (Patent Document 2).

  In the case of a silver-palladium alloy-containing paste, a uniform metal film is formed by applying the paste and then baking at a high temperature of about 950 ° C. to melt the metal fine particles. As a substrate, for example, there is a problem that only a high melting point material such as a ceramic substrate or a metal plate can be used. In addition, it has been proposed that a conductive material that can be baked at a high temperature of 1300 ° C. or higher using a conductive material made of powder in which silver particles are coated with a large number of fine palladium particles (Patent Document 3). ). However, since melting and firing are performed at a high temperature, there is a problem that a large firing furnace that can withstand high heat, peripheral facilities, and large energy are required.

  In addition, as a circuit board that can form bonding pads that do not require the use of gold paste more than necessary, an organic gold paste that constitutes a thermal print head is printed and fired on an insulating substrate to produce individual electrodes and common electrodes. Further, a heating resistor and a protective film are formed. In a bonding pad of an individual electrode, a mixed pad obtained by mixing an organic gold paste and an inorganic gold paste is laminated on the individual electrode to form a bonding pad. When this bonding pad and the bonding pad of the driver IC are connected by a gold wire, the required film thickness can be obtained by one printing and firing, and the cost can be reduced (Patent Document 4).

JP-A-5-89716 JP 2001-307549 A JP 56-42910 A JP-A-6-132338

Conventionally, a paste mainly composed of gold has been used to form a wiring pattern used for a thermal print head or the like. However, gold paste is very expensive, which is the biggest factor pushing up manufacturing costs. Although silver paste using silver with similar properties has been tested and various proposals have been made as a substitute for gold paste, its heat resistance is weak, and there is a problem of leakage due to ion migration, so it has sufficient performance for practical use. It was not obtained.
The present invention solves the problems of the conventional gold paste or silver paste and makes it possible to provide a thermal printhead that is a circuit board using a conductor paste suitable for practical use and that exhibits excellent performance. Objective.
The present invention proposes the use of an inexpensive alternative to an expensive material while reducing the number of processes during the production of a circuit board, and achieving an increase in the efficiency of the manufacturing process and a reduction in material costs. An object of the present invention is to provide an inexpensive circuit board with excellent performance.

In particular, the present invention includes conductor particles in which a palladium film is formed on the surface of silver particles, and the metal element content is 80 to 99.5% by weight of silver and 0.5 to 20% by weight of palladium. It is possible to provide a thermal print head that eliminates the disadvantages of conventional conductive silver paste by using a conductive paste containing particles as a circuit board and using it in a thermal print head. Useful.
That is, the gist of the present invention is the following circuit boards (1) to (9).
(1) A circuit board in which a circuit is formed using a conductive paste containing conductive particles made of silver particles with a palladium film formed on an insulating substrate, each of silver and palladium in the conductive particles A circuit board comprising 80 to 99.5% by weight of silver and 0.5 to 20% by weight of palladium.
(2) The circuit board according to (1), wherein the conductive particles are conductive particles made of silver particles having a palladium coating formed on a surface area of 50 to 100%.
(3) The circuit board according to (1) or (2), wherein the conductor particles are conductor particles made of silver particles on which a palladium film having a thickness of 0.4 nm to 0.02 μm is formed.
(4) The circuit board according to any one of (1) to (3), wherein the conductive particles are conductive particles made of silver particles having a palladium film formed on a surface thereof by a plating operation.
(5) The circuit board according to any one of (1) to (4), wherein the conductor particles are conductor particles having a diameter of 30 nm to 1 μm.
(6) The circuit board is a circuit board in which a heating resistor is formed between the common electrode and the individual electrode on the insulating substrate, and the common electrode and the individual electrode are formed on the insulating substrate. The circuit board according to any one of (1) to (5), wherein the circuit board is a circuit.
(7) The circuit board according to (6), wherein each of the common electrode and the individual electrode includes a single conductor layer.
(8) The circuit board according to (6) or (7), wherein the common electrode and the individual electrodes are integrally formed.
(9) The circuit board according to any one of claims 1 to 8, wherein the circuit board is an insulating board or a circuit board having a circuit on the insulating board via a glaze layer.

Further, the gist of the present invention is the following thermal print head (10).
(10) A thermal print head using the circuit board according to any one of (1) to (9).

The present invention can provide an inexpensive circuit board with excellent performance and a thermal print head using the same by achieving efficiency in the manufacturing process and reducing the material cost.
That is, the following effects are exhibited by the present invention.
1. It is possible to provide a circuit board and a thermal print head using a silver paste having excellent characteristics without using a gold paste which is very expensive and increases the manufacturing cost.
2. The heat resistance that was a weak point of the conductive paste using silver is weak, and the problem of leakage due to ion migration is solved.
3. By covering the silver particles with palladium, which has high heat resistance, and using silver that has higher thermal conductivity than gold used in the past, the heat resistance of the heating element is improved, and the thermal response of the heating element is improved. Improvement is achieved.
4). Improvement of printing quality is achieved.
5. Since there is no need to provide the common electrode (second conductor layer), the manufacturing process of the circuit board or the thermal print head is reduced.
6). Since the number of individual electrodes (first conductor layers) can be eliminated, the manufacturing process can be reduced and the cost of the layer forming material can be reduced.
7). Partial deletion of the glaze layer is possible.

The schematic diagram of the silver conductor particle contained in a silver paste. (1) and (2) are examples conventionally proposed, and (3) is an example of the present invention. 2 is a cross-sectional structure of the thermal print head of the present invention. Sectional structure of a conventional thermal print head. Comparison of the circuit formed using the present invention and a conventional silver paste. The schematic diagram which shows the change in the baking process of the circuit formed using this invention and the conventional silver paste. Results of ion migration test. The relationship of the resistance value drift rate with respect to an applied voltage is shown. The result of the transient test which measured the protective film surface temperature at the time of electricity supply and after electricity supply stop is shown.

The present invention is a conductive paste containing conductive particles in which a palladium film is formed on the surface of silver particles, wherein the conductive particles have a metal element content of 80 to 99.5% by weight of silver, 0.5% of palladium. The present invention relates to a circuit board and a thermal print head on which a circuit using a conductive paste of ˜20% by weight is formed. By using the conductive particles of the present invention, in manufacturing a thermal print head, the material cost required for forming the conductive layer is reduced, the manufacturing cost is reduced by reducing the manufacturing process, and the film quality is improved. .
The conductor particles of the present invention are those in which the surface of silver particles is coated with a thin film of palladium metal, and an example thereof is shown in FIG. 1 (3). The structure is covered with a palladium thin film.
Examples of conventionally proposed silver conductor particles are shown in FIGS. 1 (1) and (2). The conductive silver particles in FIG. 1 (2) have a structure in which a large number of fine palladium particles adhere to the surface. In such a structure, since the surface area of silver not covered with palladium particles is large, satisfactory performance as conductor particles cannot be exhibited. In the present invention, as shown in FIG. 1 (3), the surface of silver particles is covered with a thin film of palladium.
Comparing the melting point and thermal conductivity of each metal of silver, gold, and palladium, the melting point is 961 ° C for silver, 1064 ° C for gold, and 1555 ° C for palladium. The surface of silver with a low melting point is palladium with a high melting point. The melting point of the entire particle can be increased by coating. Regarding thermal conductivity, silver is 429 w / mk, gold is 318 w / mk, palladium is 71.8 w / mk, and the heat conduction of the entire conductor layer produced by using silver is substantially higher than that of gold particles. Improve. By improving the thermal conductivity of the conductor layer, the heat responsiveness of the heating element is improved, thereby improving the printing quality.

[Structure and composition of conductive particles]
The conductor particles of the present invention preferably have a particle diameter in the range of 30 nm to 1 μm. Further, it is preferably 0.2 to 0.6 μm, and if the particle diameter is below this range, the adhesion to the insulating substrate is reduced due to the generation of blisters, and if it exceeds, the film formability is reduced due to the generation of porous. In addition, the function as a conductive paste is reduced.
In the conductive particles, the metal elements are preferably 80 to 99.5% by weight of silver and 0.5 to 20% by weight of palladium, and more preferably 0.5 to 3% by weight of palladium. If the amount of palladium is less than this, the thermal properties and electrical properties of the conductive particles will be lowered, and if the amount is larger, these properties will not be further improved, and more expensive materials will be added.
It is preferable that 50 to 100% of the surface of the silver particles of the present invention is coated with palladium, more preferably 80 to 100% is coated with palladium, and these ranges are preferred for heat resistance This is because the migration resistance is good.
Moreover, it is suitable for manufacturing the thermal print head of the present invention that the coating with palladium has a thickness of 0.4 nm to 0.02 μm. Further, the palladium film thickness is preferably 0.4 nm to 3.0 nm, and these ranges are preferable because the heat resistance and migration resistance described above are good.

  From the viewpoint of the thermal conductivity, the silver particles are preferably pure silver, but may be an alloy of silver, for example, nickel (Ni), gold (Au), platinum (Pt), cobalt ( Co), zinc (Zn), chromium (Cr), tungsten (W), molybdenum (Mo), antimony (Sb), aluminum (Al), or the like may be included.

[Example]
As shown in FIG. 3, the conventional thermal print head has a glaze layer on an insulating substrate such as ceramics, and a first conductor layer serving as an individual electrode. For example, the first conductor layer is composed of four layers formed by printing and baking with a paste containing gold. A common electrode formed in a separate process is provided at the end of the first conductor layer. A heating resistor capable of individually controlling energization is provided on the individual electrode and the common electrode.
On the other hand, the structure of the thermal print head of the present invention is common to the first conductor layer which is provided with a glaze layer on a part of an insulating substrate such as ceramics and directly serves as an individual electrode, as shown in FIG. A second conductor layer serving as an electrode can be provided. The first conductor layer is formed of a single layer formed by printing and baking a silver-containing paste having a palladium coating.
The thickness of the 1st conductor layer in this invention is the range of 1-6 micrometers, More preferably, it is the range of 2-4 micrometers. By setting the layer thickness within this range, the object of the present invention is easily achieved.
Further, in the present invention, in order to improve the adhesion between the gold, which is the first conductor layer, and the insulating substrate, the glaze layer provided on the entire surface of the insulating substrate may be provided only on the lower layer of the heating element layer. . This is because glaze is not required because the adhesion between the insulating substrate and the first conductor layer is high.
The fact that the thermal print head of the present invention can be produced by a simple process is derived from the fact that the conductor particles of the present invention are excellent in thermal conductivity and at the same time can form a thick conductive layer.

[Reducing material costs]
In the production of the circuit board or thermal print head of the present invention, the production process can be reduced and the materials used can be reduced. A conventional thermal print head is provided with a first conductor layer composed of four layers using, for example, a gold paste. These conductor layers require a plurality of processes by printing a gold paste by screen printing and firing the paste in a firing furnace. In contrast, in the present invention, it is sufficient to form a single conductor layer by using silver microparticles coated with a 1% by weight palladium film, and the number of manufacturing steps can be drastically reduced. Become. The firing temperature and time are both about 770 to 820 ° C. and about 60 minutes. In the cost estimation calculated from the usage amount and unit price of the material used in the present invention and the conventional method, the present invention can significantly reduce the material cost compared to the conventional method.

[Heat resistance test]
The shape of the dot pattern formed using the conductor paste of the present invention was compared with a silver particle paste (resinate paste using a silver compound), a mixed paste of silver particles and palladium particles, and a palladium film-coated silver particle paste. The dot pattern had a width of 25 μm shown in FIG. 4 and was formed by photolithography after screen printing and baking. FIG. 5 schematically shows how the coating provided on the insulator by photolithography is changed by firing. In patterns provided on an insulating substrate using silver paste or silver particles mixed with palladium particles, firing creates random gaps between particles or coalescence between particles due to particle melting. Since it is difficult for the pattern to be uniform, the edge portion of the pattern does not become sharp.
When the fine pattern according to the present invention is compared with the conventional example, as shown in FIG. 4, it is clearly recognized that the sharpness of the edge portion of the pattern is greatly excellent in the present invention.

[Ion migration]
Using the circuit of the present invention, an ion migration test was performed to measure the time for occurrence of a short circuit, and the degree of ion migration was estimated. Ion migration is a phenomenon of metal electrochemical migration, and when this occurs, there is a problem in that when voltage is applied, silver migrates and grows in a dendritic shape due to electrolysis, and the insulation resistance between the electrodes decreases or shorts. It is generally known.
In this test, a paste containing conductive particles in which silver particles having a particle diameter of 0.6 μm were coated with 5 to 10% by weight of palladium by electrolytic plating to a film thickness of 4.5 nm to 9.0 nm was used. As a comparative example, a conductor in which 5 and 10% by weight of palladium fine particles having a particle diameter of 0.2 μm were mixed with 0.6 μm of silver particles was attached, and a deionized water dropping method (test condition power supply voltage: 1.00 V) was used. The amount of dropping: 3.0 μm, the measurement line L / S = 150/150 μm) was subjected to an ion migration test, and the results are shown in FIG. The longer the short-circuit time (definition: the time it takes for the resistance between patterns to reach 1 MΩ or less during energization), the better the resistance to migration. In actual use of this conductor, ion migration is performed using silver. There will be advantages that do not occur.
As a result of the test, the short-circuiting time tends to be longer by coating or mixing palladium, but it is clear from FIG. 6 that the silver particles forming the palladium film of the present invention are overwhelmingly superior in migration resistance. It is.

[Pulse resistance]
For pulse resistance, an evaluation test was performed using palladium-coated silver (silver particle size: 0.5 μm, palladium addition rate: 1 wt%, palladium film thickness: 0.9 nm, single layer structure) as the first conductor layer material. As a relative comparison, a sample using organic gold (three-layer structure) as a first conductor layer material was prepared. As a test method, a pulse voltage was applied to the heating resistor, the applied voltage was increased stepwise, and the resistance value for each applied voltage was measured. FIG. 7 shows the relationship between the resistance value drift rate and the applied voltage, and Table 1 shows the results of the pulse tolerance test.
Palladium-coated silver tends to have a resistance value that is less likely to increase (high pulse resistance) than the current one. This is thought to be because the film thickness of the first conductor layer was five times and the thermal conductivity was improved. Palladium-coated silver has a conductor resistance value of 0.03 Ω · mm ² / μm, which is about 1/20 or less of the current conductor resistance value of 0.70 Ω · mm ² / μm. No common electrode layer (second conductor layer) is required as a countermeasure.

[Color development]
In this example, a transient test was performed. The tested sample was subjected to an evaluation test using palladium-coated silver (silver particle size 0.5 μm, palladium addition rate 1 wt%, palladium film thickness 0.9 nm, one layer structure) as the first conductor layer material. . As a relative comparison, a current product using organic gold (three-layer structure) as a first conductor layer material was prepared. FIG. 8 shows the results of measuring the surface temperature of the protective film during the energization and after the energization was stopped in the transient test.
As a result of the test, the temperature of the palladium-coated silver during the current stoppage time is larger than that of the current product due to the transient waveform. This is presumably because the heat radiation from the first conductor layer was increased due to the good thermal conductivity of the palladium-coated silver. Further, palladium-coated silver has a larger difference between the peak temperature and the bottom temperature than the current product. This is also thought to be due to the improvement in thermal conductivity.

  The circuit board and the thermal print head of the present invention are configured by electrically connecting a resistor as a heating element between a pair of electrodes consisting of a common electrode made of a metal film and an individual electrode formed on an insulating substrate, The required voltage is applied between both electrodes to generate heat and heat is applied to the thermal recording paper to perform thermal recording according to the amount of heat generated by the resistor. However, the electrode made of a metal film is inexpensive. It can be produced with a paste mainly composed of silver particles, and because it has excellent performance as a circuit board or thermal printhead, it is possible to supply a thermal printhead with better performance at a lower cost. It can be said that the fact that the consumption of gold with a small amount of output can be suppressed greatly contributes to the industry.

Claims (10)

  A circuit board in which a circuit is formed using a conductive paste containing conductive particles made of silver particles with a palladium film formed on an insulating substrate, wherein each component of silver and palladium in the conductive particles is silver A circuit board comprising 80 to 99.5% by weight and palladium of 0.5 to 20% by weight.   The circuit board according to claim 1, wherein the conductive particles are conductive particles made of silver particles having a palladium coating formed on a surface area of 50 to 100%.   The circuit board according to claim 1, wherein the conductive particles are conductive particles made of silver particles on which a palladium film having a thickness of 0.4 nm to 0.02 μm is formed.   The circuit board according to any one of claims 1 to 3, wherein the conductive particles are conductive particles made of silver particles with a palladium film formed on the surface of the conductive particles.    The circuit board according to claim 1, wherein the conductive particles are conductive particles having a diameter of 30 nm to 1 μm.   The circuit board is a circuit board in which a heating resistor is formed between a common electrode and an individual electrode on an insulating substrate, and the common electrode and the individual electrode are formed on the insulating board. The circuit board according to any one of claims 1 to 5.   The circuit board according to claim 6, wherein at least one of the common electrode and the individual electrode is formed of a single conductor layer.   The circuit board according to claim 6 or 7, wherein the common electrode and the individual electrode are integrally formed.   The circuit board according to claim 1, wherein the circuit board is a circuit board having a circuit through a glaze layer on the insulating substrate or the insulating substrate. A thermal print head using the circuit board according to claim 1.