JP2542428B2 - Method for manufacturing double-sided flexible printed circuit board - Google Patents

Description

The present invention relates to a method for manufacturing a double-sided flexible printed circuit board having metal conductors on both sides.

"Prior art" In recent years, with the miniaturization and high density of electronic devices,
Ultra-thin, light-weight, and high-density flexible printed circuit boards are required. Especially, in electronic device displays, thermal heads, etc., the high density of flexible printed circuit boards (FPC) is centered around these terminal connections. The miniaturization is being developed.

By the way, in high-density fine patterns, there are demands for the number of patterns to be 5 to 10 / mm or more.

In response to these requirements, in the field of electronic devices, double-sided flexible printed circuit boards have been investigated as technical liquids as an alternative to single-sided flexible printed circuit boards.

A double-sided flexible printed circuit board is generally a plastic film having conductor layers with thicknesses of 35 μm and 18 μm on both sides, and the plastic film is made of polyethylene terephthalate (PET), polyimide (PI), or the like. , The conductor layer of which is bonded to the surface of the plastic film via an adhesive, electrolytic copper foil,
It consists of a rolled copper foil, a physical metal vapor deposition means (PVD), a metal conductor directly formed on the plastic film surface through an electrochemical metal coating means, and the like.

By the way, in the invention disclosed in JP-A-61-47015, a conductor metal (conductor layer) is formed on the surface of a plastic film by using a physical metal vapor deposition means and an electrochemical metal coating means in combination. In the case of the prior art, since it is possible to form an extremely thin copper foil as the conductor layer, it is said to be preferable to the existing technology.

That is, in the flexible printed circuit board by the existing technology, it is not possible to obtain a sharp and fine circuit due to side etching when etching the copper foil, and the number of pattern lines is about 3 to 5 lines / mm. In the flexible printed circuit board according to the above-mentioned prior art, an extremely thin copper foil having a thickness of 1 to 9 μm is formed as a conductor layer, but such a problem is unlikely to occur.

The double-sided flexible printed circuit board produced in this way has a power source and ground circuit pattern on both sides.
A fine circuit pattern for a signal system is formed, and LSI, resistors, and other elements are mounted.

As described above, in the double-sided flexible printed circuit board according to the prior art, the copper foil as the conductor layer is extremely thin, 1 to 9 μm.

The ultra-thin conductor layer (copper foil) is most suitable as a fine circuit pattern (signal transmission line), and it is easy to route the lines during design, but the current capacity cannot be secured when it is driven into a power supply system or ground system. .

[Problems to be Solved by the Invention] As a countermeasure for the above-mentioned prior art, a circuit board having a low cost and a high utility value, in which a metal conductor made of copper foil is attached to one surface via an adhesive, that is, a single-sided flexible printed circuit board. It is possible to propose that a metal conductor (conductor layer) which is dense and has good adhesion and a uniform grain size is formed on the remaining one surface of the circuit board by means of sputtering, electrolytic plating or the like.

However, even with such means, the following problems occur.

For example, when an ultra-thin metal conductor is directly formed on one side of a plastic film remaining on a single-sided flexible printed circuit board by means of the sputtering method, the radiant heat at that time raises the temperature of the board to as high as 200 ° C. , As a result, not only the plastic film as the base material, but also the adhesive bonding the existing copper foil (metal conductor) is softened and deformed, and the adhesion between the copper foil and the plastic film is hindered, Deformation or dimensional change of the substrate occurs.

Even when an ultrathin metal conductor is formed on the surface of a plastic film via electrolytic plating means, the adhesion between the metal conductor and the plastic film is poor, and the surface of the metal conductor is rough due to the initial growth process of plating deposition. Become,
There are various problems such as poor bending resistance despite the fact that it is a thin film, and inability to finish to a fine pitch when forming a pattern.

In view of the above-mentioned problems, the present invention does not impair the adhesion between the metal conductor and the plastic film, the bending resistance of the substrate, the dimensions of the substrate, and the like, and has a relatively large electric capacity. It is intended to provide a method for manufacturing a double-sided flexible printed circuit board, which can satisfactorily form the above.

[Means for Solving the Problems] In order to achieve a desired object, a method for manufacturing a double-sided flexible printed circuit board according to the present invention is a metal foil conductor having a thickness of 18 μm or more via an adhesive on one surface of a plastic film. A single-sided flexible printed circuit board bonded to each other is prepared, and a metal conductor film having a thickness of 0.1 to 0.5 μm is formed on the remaining one side of the plastic film in the single-sided flexible printed circuit board through a physical metal vapor deposition means, and A metal conductor layer having a thickness of 1 to 10 μm is formed on the metal conductor film via an electrochemical metal coating means.

[Operation] In the case of the method of the present invention, a one-sided flexible printed circuit board is prepared by adhering a metal foil conductor having a thickness of 18 μm or more to one side of a plastic film via an adhesive.

The reason for using such a single-sided flexible printed circuit board is that it is inexpensive and that the metal foil conductor of the circuit board has a thickness of 18 μm or more, so that a circuit pattern having a large electric capacity can be easily formed. Is.

In the above single-sided flexible printed circuit board, a metal conductor film is first formed on the remaining one side of the plastic film through a physical metal vapor deposition means.

When the thickness of the metal conductor film formed through the physical metal vapor deposition means is less than 0.1 μm, the metal conductor film becomes non-uniform with many pinholes and is formed on the metal conductor film. It does not serve as a base material for the metal conductor layer.

When the thickness of the metal conductor film formed by physical metal vapor deposition means exceeds 0.5 μm, the temperature of the plastic film during metal vapor deposition rises to 100 ° C. or higher due to radiant heat and the like, and the adhesive rises due to such temperature rise. Due to softening and deformation, the adhesion between the metal foil conductor and the plastic film is hindered, causing deformation and dimensional change of the substrate.

At this time, if it is only necessary to suppress the temperature rise of the plastic film, it is sufficient to slow down the metal vapor deposition rate, but this will reduce the productivity by the amount corresponding to the slower metal vapor deposition rate.

Therefore, when the metal conductor film is formed through the physical metal vapor deposition means, the thickness of the metal conductor film is 0.1 to
It is necessary to set within the range of 0.5 μm.

A metal conductor layer is formed on the metal conductor film through an electrochemical metal coating means having a high film formation rate and high productivity.

At this time, the metal conductor layer strongly adheres as a dense layer having a predetermined thickness while using the uniform and dense metal conductor film as a base material.

If the thickness of the metal conductor layer formed through the electrochemical metal coating means is less than 1 μm, it becomes difficult to increase the thickness of the metal conductor layer based on the metal conductor layer.

When the thickness of the metal conductor layer formed through the electrochemical metal coating means exceeds 10 μm, the internal stress of the metal conductor film as the base material increases, which lowers the flexibility of the substrate and further increases the excess. Such a thick film causes side etching during pattern formation (photo etching), making it difficult to obtain a high-density miniaturized pattern.

Therefore, when the metal conductor layer is formed through the electrochemical metal coating means, the thickness of the metal conductor layer needs to be set within the range of 1 to 10 μm.

[Examples] Hereinafter, a method for manufacturing a double-sided flexible printed circuit board according to the present invention will be described with reference to the illustrated examples.

In FIG. 1, single-sided flexible printed circuit board
11 is a plastic film (base film) 12,
A metal conductor foil (14) adhered to one side of the plastic film (12) with an adhesive (13).

The plastic film 12 is polyethylene terephthalate (PET), polyimide (PI), polyether aleketone (PEEK), polyphenylene sulfide (PPS).
Made of plastic such as.

The adhesive 13 is a known or well-known synthetic adhesive having heat resistance.

The metal conductor foil 14 is mainly made of a copper foil such as an electrolytic copper foil or a rolled copper foil, but may be made of a metal conductor such as silver or aluminum in some cases.

 The metal conductor foil 14 has a thickness of 18 μm or more.

In the one-sided flexible printed circuit board 11 described above, the remaining one side of the plastic film 12 has a thickness of 0.1 μm through a physical metal vapor deposition means (PVD) as shown in FIG.
A metal conductor film 15 having a thickness of not less than m and not more than 0.5 μm is formed.

As the physical metal vapor deposition means (PVD), sputtering, ion plating, ion cluster beam, etc. are adopted, and the metal conductor film formed through the PVD method 1
5 is made of a copper thin film as an example.

As shown in FIG. 3, a metal conductor layer 16 is formed on the metal conductor film 15 described above through an electrochemical metal coating means.

As the electrochemical metal coating means, for example, an electroplating method is adopted, and the metal conductor layer 16 formed through the electroplating method is mainly made of copper, but the metal conductor layer 16 is silver, It may be made of aluminum, nickel, chrome, or an alloy thereof (including a copper alloy).

Thus, when the predetermined process is completed, the required double-sided flexible printed circuit board 17 is obtained.

When the metal conductor film 15 is formed on the plastic film 12 as the base film through the PVD method, oxygen, argon, and dioxide are used for the purpose of further increasing the relative adhesion between the plastic film 12 and the metal conductor film 15. The low temperature plasma treatment with carbon, air, etc. may be performed in advance.

FIG. 4 shows an example of a double-sided flexible printed circuit formed by masking the necessary circuit portion of the double-sided flexible printed circuit board 17 thus obtained with photoresist and etching the other portions. .

In the circuit as shown in Fig. 4, the circuit pattern 18 for the power supply system and the ground system, which requires a large current capacity, is formed on one side that had the metal conductor foil 14 (example: 2 wires / mm), and is small. A signal system fine circuit pattern 19 having a sufficient current capacity is formed on the other side having the metal conductor film 16 (eg, 10 lines / mm).

Of course, if it is not necessary to be fine, on one side that had the metal conductor foil 14, a circuit pattern of a signal system may be formed, and depending on the width of the circuit pattern, it may be formed on both sides of the substrate. Through holes can be formed between the circuit patterns of the signal system.

Next, more specific examples of the method of the present invention will be described below.

Example 1 As a first step, a single-sided flexible printed circuit board (manufactured by Nikkan Kogyo Co., Ltd.) having a copper foil with a thickness of 18 μm was set in a sputtering apparatus equipped with a plasma processing mechanism, and the inside of the sputtering apparatus was sufficiently filled. After purging and introducing oxygen gas, at a gas pressure of 3 × 10 ^-2 Torr and an AC voltage of 800 V, the PI plastic film surface (the surface without the copper foil) of the above circuit board is kept at low temperature for 10 seconds. Plasma treatment was applied.

In the second step, after thoroughly purging the inside of the sputtering apparatus and introducing argon gas, gas pressure: 3 × 10 ⁻³ Torr,
Under the condition of DC voltage: 400 V, sputtering was performed toward the low temperature plasma treated surface of the circuit board, and a 0.5 μm thick copper film was deposited on the treated surface.

In the third step, the processed product after sputtering is degreased with Cleaner 160 (manufactured by Meltex Japan Co., Ltd.),
After washing with water, activation with sulfuric acid, and further washing with water, electrolytic plating was performed in a copper sulfate plating bath to form a copper layer having a thickness of 5 μm on the copper film.

The double-sided flexible printed circuit board thus obtained showed almost no deterioration (adhesion or poor adhesion) of the adhesive bonding the copper foil, deformation of the board, and dimensional change.

Example 2 As a first step, the same single-sided flexible printed circuit board as in Example 1 was subjected to low temperature plasma treatment as in Example 1.

In the second step, a chromium film having a thickness of 0.1 μm was deposited on the low temperature plasma-treated surface of the circuit board under the same conditions as in the example.

In the third step, the processed product after sputtering was degreased, washed with water, activated with sulfuric acid, washed with water in the same manner as in Example 1, and then electrolytically plated in a copper sulfate plating bath to give a thickness of 9 μm on the chromium film.
m copper layer was formed.

The double-sided flexible printed circuit board thus obtained was also good as in Example 1.

Comparative Example As the first step, the same single-sided flexible printed circuit board as in Example 1 was subjected to the low temperature plasma treatment as in Example 1.

In the second step, when a copper film having a thickness of 2 μm was deposited on the low temperature plasma treated surface of the circuit board under the same conditions as in the present invention 1, the surface temperature of the circuit board reached 200 ° C. or higher, and the copper foil was removed. The adhered adhesive was thermally deteriorated, and a good double-sided flexible printed circuit board could not be obtained.

[Advantages of the Invention] As described above, according to the method of the present invention, a good double-sided flexible printed circuit board can be obtained which does not interfere with the adhesion between the metal conductor and the plastic film, the bending resistance of the board, and the board size. Circuit patterns with a relatively large electric capacity on both sides of the circuit board,
Since it is possible to satisfactorily form a fine circuit pattern or the like, it is possible to contribute to making electronic devices extremely thin, light and compact.

[Brief description of drawings]

1, 2, and 3 are cross-sectional views schematically showing a method for manufacturing a double-sided flexible printed circuit board according to the present invention in the order of steps, and FIG. 4 is a double-sided flexible printed circuit board obtained by the method according to the present invention. FIG. 6 is a cross-sectional view schematically showing an example in which a required circuit pattern is formed on the substrate. 11 …… Single-sided flexible printed circuit board 12 …… Plastic film 13 …… Adhesive 14 …… Metal conductor foil 15 …… Metal conductor film, 16 …… Metal conductor layer 17 …… Double-sided flexible printed circuit board

Claims (1)

(57) [Claims] 1. A single-sided flexible printed circuit board, in which a metal foil conductor having a thickness of 18 μm or more is adhered to one side of a plastic film via an adhesive agent, and the one-sided flexible printed circuit board has the remaining one side of the plastic film, Thickness 0.1 via physical metal deposition means
A metal conductor film having a thickness of 0.5 μm is formed, and a metal conductor film having a thickness of 1 to 1 is formed on the metal conductor film through an electrochemical metal coating means.
A method for manufacturing a double-sided flexible printed circuit board, which comprises forming a metal conductor layer having a thickness of 10 μm.