JP2007317782A - Flexible wiring board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To propose a method of improving flexibility of a plated board made of two copper layers. <P>SOLUTION: A flexible wiring board comprises a two-layered copper flexible wiring board having a copper-plated layer formed on a surface of an insulative film base material via a copper sputter film. A ratio (d/t) is 0.08 to 0.3 in an average crystal grain size (d) of a copper crystal in the copper-plated layer and a thickness (t) of the copper-plated layer. The thickness (t) of the copper-plated layer is preferably 4-18 μm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a copper flexible wiring board having a copper plating layer with improved refraction resistance.

The flexible wiring board is widely used for a refraction wiring in an electronic device or a digital camera that requires flexibility such as a read / write head of a hard disk or a printer head.
The flexible wiring board is commonly known as a three-layer board (copper layer / adhesive layer / insulating base film layer) in which electrolytic copper foil or rolled copper foil is bonded to a polyimide film with an adhesive, and copper is directly applied to the polyimide film by plating or the like. There are two types of commonly called two-layer substrates (copper layer / insulating base film layer) such as a plated substrate on which a layer is formed or a cast substrate in which a polyimide varnish is directly applied to a copper foil to form an insulating layer.

In recent years, the MIT folding endurance test standardized by JIS-P-8115 and ASTM-D2176 has been commercially used as an evaluation of the refraction resistance of copper foil used for electric circuit wiring of flexible wiring boards. It is. In this test, the refraction resistance of the wiring board is evaluated by the number of refractions until the copper pattern forming the circuit of the test piece is disconnected, and the greater the number of refractions, the better the refraction resistance.
Regarding the improvement of the refraction resistance of the copper foil, for example, heat treatment is performed on the surface of the copper foil (for example, see Patent Document 1), or the refraction resistance is improved by rolling (for example, Patent Document 2). reference.).
JP-A-8-283886 JP-A-6-269807

However, the methods described above are all rolled copper foil and electrolytic copper foil, and are for copper foil before the formation of the wiring board, and are compatible with three-layer boards and cast boards. A three-layer substrate or a cast substrate has a complicated manufacturing process, so there is a limit to cost reduction.
In that respect, the plating substrate is a simple manufacturing process, and cost reduction is easy. The plating substrate is also required to have a copper plating layer with refraction resistance, but the above-described technology for improving refraction resistance cannot be applied.
The present invention proposes a method for improving the refraction resistance better for a two-layer plated substrate made of a copper layer.

Therefore, the flexible wiring board of the present invention is a two-layer copper flexible wiring board in which a copper plating layer is formed on the surface of an insulating film base via a copper sputtered film, and the average crystal grains of the copper crystals of the copper plating layer A flexible wiring board having a ratio (d / t) of the diameter (d) to the copper plating layer thickness (t) of 0.08 to 0.3 was obtained.
In this invention, it is preferable that the layer thickness (t) of the said copper plating layer is 4-18 micrometers.
By configuring the flexible wiring board in this way, a two-layer plating flexible wiring board having good refraction resistance can be obtained.

  According to the present invention, in the copper-plated two-layer flexible wiring board, it is possible to improve the refraction resistance of the copper-plated layer, so that it is possible to expand the application range to electronic devices with a higher number of refractions.

As a result of various studies for improving the refraction resistance of the copper-plated two-layer flexible wiring board, the copper-plated layer thickness and the crystal size of the copper crystal of the copper-plated layer are within a certain range. It has been found that the refraction resistance of the copper plating layer is increased in a range where the ratio is constant.
That is, in order to solve the above-mentioned problems, the present invention is a flexible substrate in which a copper plating layer is formed on the surface of an insulating film substrate via a conductive film, and the average crystal grain size of copper crystals in the copper plating layer ( d) and the thickness (t) of the copper plating layer (hereinafter referred to as a crystal ratio) have a plating film having a ratio of 0.08 to 0.3. Here, the thickness of the copper plating layer is preferably a copper plating two-layer flexible wiring board having a thickness of 4 μm to 18 μm.
That is, assuming that crystal ratio = average crystal grain size d (unit: μm) / plating layer thickness t (unit: μm), d / t = 0.08 to 0.3, the resistance of the copper plating layer Refractive properties are dramatically improved. Here, t = 4-18 is preferable.

The present invention controls the size of crystal grains in a plating film so that the crystal ratio of the copper plating film is 0.08 to 0.3 in a copper-plated two-layer plating flexible wiring board having conductivity on the surface. By doing so, it is possible to obtain a two-layer flexible wiring board with good refraction resistance.
Specifically, after conducting the conductive treatment on the surface of the polyimide resin film to be a substrate, copper plating is performed to a desired thickness within a range of 4 to 18 μm in copper plating film thickness. Next, heat treatment is performed on the plated substrate with a heating device such as an oven, and the average crystal grain size of the cross section of the copper plating film after the heat treatment is measured, and the average crystal grain size and the film thickness are measured. The crystal grain size in the copper plating film may be controlled so that the crystal ratio is 0.08 to 0.3, and the heat treatment conditions are also determined here. In the present invention, since the refractive resistance of the substrate is determined by the crystal ratio, once the heat treatment conditions are determined, it is not necessary to measure the average crystal grain size of the plating film unless the plating thickness is changed.

A polyimide substrate can be used as the insulating film substrate. In more detail, the insulating base material is preferably a polyimide resin that is soft, heat resistant, and durable when formed into a film, but other resins may be used if the heat treatment temperature and time are appropriately adjusted.
The insulating substrate is subjected to a conductive treatment for imparting conductivity to the surface in order to facilitate plating. The conductive treatment can impart conductivity to the surface of the insulator by forming a very thin metal film by sputtering or the like. The metal film formed by sputtering or the like is preferably formed of the same copper metal as the copper plating layer forming the electric circuit. A sputtering film thickness of 1 μm or less is sufficient. An electric circuit is formed by performing copper plating on the surface of the conductive layer of the insulating base material after the conductive treatment.
The copper plating is preferably glossy plating with fine crystals and smooth plating surface, but is not particularly limited. The usual electroplating method can be used, and fine crystals are deposited by optimally selecting the plating bath temperature and current density.
When the thickness of the plating layer is less than 4 μm, the refraction resistance is lowered due to plating defects or the like, and no correlation with the crystal grain size is observed. On the other hand, if the thickness exceeds 18 μm, the tensile stress in the vicinity of the surface when refracted becomes large, so that it tends to break, and the resistance to refraction decreases and the correlation between the resistance to refraction and the crystal grain size is not observed. Therefore, the thickness of the plating layer to be deposited is suitably formed to be thicker than 4 μm and thinner than 18 μm.

  After forming a copper plating layer, the plating layer is heat-treated to adjust the crystal grain size. The heat treatment temperature and treatment time depend on the material of the substrate, but if a polyimide resin is used as the base material, it can be heat treated even at about 300 ° C. However, if it is performed in the air, the copper plating film will oxidize at 150 ° C. About 250 ° C is desirable. When the temperature is lower than this, it is necessary to lengthen the heat treatment time. When the temperature exceeds 250 ° C., it is necessary to prevent oxidation of the copper plating layer with nitrogen gas or the like in the heat treatment atmosphere. The heat treatment time is suitably 15 to 30 minutes at 200 ° C. If the time is long, oxidation of the copper plating layer occurs, so that it is necessary to take an anti-oxidation measure. The average crystal grain size of the copper crystal after the heat treatment is obtained by polishing a cross-section of the copper-plated portion of the heat-treated substrate, taking a micrograph of the copper-plated layer, and drawing a line at an arbitrary portion as shown in FIG. (See dotted line 1). Then, about five large crystal grains are selected from the crystal grains cut by the line, the crystal size in the direction perpendicular to the thickness direction is measured, and the average crystal grain size is obtained. Then, the average crystal grain size d (unit: μm) is divided by the plating layer thickness t (unit: μm) to obtain the crystal ratio. By controlling the heat treatment temperature and time so that this ratio (d / t) is 0.08 to 0.3, it is possible to obtain a two-layer plated substrate having good refraction resistance.

(Examples and comparative examples)
Next, the present invention will be described using examples and comparative examples.
In this example and comparative example, a commercially available additive was used in a copper sulfate plating bath, using a copper-polyimide substrate that was made conductive by sputtering copper to a thickness of 0.2 μm on the surface of a polyimide film with a thickness of 38 μm. The surface of the copper sputtered film of the copper-polyimide substrate was subjected to copper plating to produce substrates having different copper plating layer thicknesses. The copper plating conditions were as follows: the plating bath temperature was 25 ° C., the current density was 4 A / dm ^2, and plating layers with thicknesses of 8 μm and 10 μm were formed at different times.
Next, after heat-treating each substrate at different temperatures and times as shown in Table 1, the average crystal grain size and copper plating layer thickness of the copper crystals were measured, and the crystal ratio and resistance to resistance calculated from these measured values were measured. The number of refractions was examined.
The average crystal grain size and copper plating layer thickness were measured by a cross-section polishing method performed by observing the crystal structure of a normal metal material, and the number of refractions was examined by an MIT folding resistance test. The MIT folding endurance test conditions were R = 0.38 mm, a load of 500 g, a refractive rotation speed of 175 rpm, and was performed according to JIS-P-8115. FIG. 1 is a diagram for explaining how to obtain the cross-sectional structure and crystal ratio of a copper-plated two-layer flexible substrate.
FIG. 2 shows the relationship between the crystal ratio and the number of refractions.

  From this result, it can be seen that a high anti-refraction frequency exceeding 500 times was obtained when the crystal ratio was in the range of 0.08 to 0.3.

  According to the present invention, it is possible to provide a two-layer flexible wiring board having good refraction resistance by forming an electric wiring pattern portion using a two-layer plated wiring board that is easy to manufacture and adjusting the crystal grain size by heat treatment. Therefore, it becomes possible to provide a flexible wiring board having excellent durability at low cost.

It is a figure explaining how to obtain a cross-sectional structure and a crystal ratio. It is a figure which shows the relationship between a crystal ratio and the frequency | count of antirefracting.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Insulating flexible base material 2 Copper sputtered film 3 Copper plating layer 4 Copper crystal grain

Claims (3)

A two-layer copper flexible wiring board in which a copper plating layer is formed on a surface of an insulating film base via a copper sputtered film, wherein the copper crystal has an average crystal grain size (d) and a copper plating layer A flexible wiring board having a ratio (d / t) to a thickness (t) of 0.08 to 0.3. The flexible wiring board according to claim 1, wherein the copper plating layer has a thickness (t) of 4 to 18 μm. The flexible wiring substrate according to claim 1, wherein the insulating film base material is a polyimide substrate.

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