JP2001223443A - Flexible printed board and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flexible printed board (FPC) which is improved in elasticity and heat resistance without sacrificing a flexibility. SOLUTION: The FPC is made by laminating a base film 11, base film-side adhesive layer 12, metal foil layer 13 which constitutes a patterns circuit, coverlay-side adhesive layer 14, and coverlay film 15 in this order. A crossliking density on the side of the adhesive layers 12, 14 near and around the metal foil layer 13 is made larger than the other parts. As a result, without sacrificing a flexibility of the entire FPC, elasticity and heat resistance of the FPC can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible printed circuit board (FPC) having excellent bending resistance and flexibility, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Generally, an FPC is, as shown in FIG.
A pattern circuit of a metal foil layer 3 is formed on a base film side adhesive layer 2 of a base film 1, and a cover lay film 5 on which a cover lay side adhesive layer 4 is applied is attached thereon.

The base film 1 is usually made of a flexible film such as a polyimide resin or a polyethylene terephthalate (PET) resin. Usually, the base film 1 is coated on the entire surface of the film with a base film-side adhesive layer 2 in advance. A film (CCL = Copper Composite Laminate) with a metal (copper) foil to which the foil layer 3 is attached is used.

That is, a desired pattern circuit is formed on the metal foil layer portion by applying a technique such as a lithography technique to the CCL. More specifically, once dry film (DF) is laminated on CCL,
A pattern circuit is formed through development and etching steps, and unnecessary DF is removed.

Thereafter, the CCL on which the pattern circuit is formed is formed.
On top, a coverlay (CL = Cover Laye) also made of a flexible film such as polyimide resin or PET resin
r) The film 5 is attached by the cover lay side adhesive layer 4 applied to one side of the film 5 as described above,
Through a curing process, FPC is obtained.

[0006]

However, in the case of such a conventional FPC, when it is used, for example, as a substrate for a hard disk of personal computer equipment, a bending operation of several tens to several hundred million times is required. High flex resistance and flexibility are required, but in this case, there is a problem that fatigue deterioration occurs particularly in the metal foil layer 3 portion of the pattern circuit, cracks are generated, and the pattern circuit is disconnected. Such cracks tend to occur when the FPC is bent with a small radius of curvature and a high cycle.

[0007] For this reason, a method of using an adhesive having high elastic modulus and heat resistance has been proposed to suppress the generation and progress of cracks when used under severe bending conditions. In this case, however, there is a problem that the whole FPC becomes hard and flexibility is impaired.

The present invention has been made in view of such a conventional problem. Basically, the crosslinking density on the adhesive layer side near the periphery of a metal foil layer where cracks are easily generated is increased, An object of the present invention is to provide an excellent FPC that solves the above-mentioned conventional problems and a method of manufacturing the same by suppressing the occurrence of cracks.

[0009]

According to the present invention, a base film, an adhesive layer on the base film side, a metal foil layer forming a pattern circuit, an adhesive layer on the coverlay side, and a coverlay film are laminated in this order. In the flexible printed circuit board, the crosslink density on the side of the adhesive layer near the periphery of the metal foil layer is higher than that on portions other than the vicinity of the periphery.

According to a second aspect of the present invention, the crosslinking density on the side of the adhesive layer near the periphery of the metal foil layer is a gradient density distribution that maximizes a boundary portion with the metal foil layer. The flexible printed circuit board according to claim 1.

According to a third aspect of the present invention, there is provided a flexible printed circuit board in which a base film, a base film side adhesive layer, a metal foil layer forming a pattern circuit, a cover lay side adhesive layer, and a cover lay film are laminated in this order. In the flexible printing method, the adhesive layer side near the periphery of the metal foil layer is heated by energizing the metal foil layer portion to increase the crosslink density thereof compared to the portion other than the vicinity. There is a method of manufacturing a substrate.

[0012]

FIG. 1 shows an embodiment of an FPC according to the present invention. The basic structure of the FPC 10 is substantially the same as that of the FPC of FIG. 5, and a pattern circuit of a metal foil layer 13 is formed on a base film-side adhesive layer 12 of a base film 11, and a cover circuit is formed thereon. Coverlay film 1 coated with lay-side adhesive layer 14
5 is pasted.

In the present invention, of the base film side adhesive layer 12 and the cover lay side adhesive layer 14 positioned so as to sandwich the metal foil layer 13 forming the pattern circuit, in the vicinity of the periphery in contact with the metal foil layer 13 Adhesive layer 12 in
a, the cross-link density on the 14a side is reduced to the part 1
It is higher than 2,14. More specifically, it is a gradient density distribution that maximizes the boundary with the metal foil layer 13.

This cross-linking method is performed by the heat generated by energizing the metal foil layer 13 of the FPC obtained through a normal process, ie, the pattern circuit portion. At this time, it is preferable that the entire FPC is preferably cooled while being appropriately cooled by a method such as air cooling so that only the pattern circuit portion is effectively heated. The heating temperature is determined by monitoring the surface temperature of the FPC where the metal foil layer 13 is located with a thermocouple or the like, and adjusting the amount of current to be about 80 to 120 ° C., for example. At this time, the surface temperature of the metal foil layer 13 is about 120 to 180 ° C.

[0015] Even in the normal FPC manufacturing process, heating is performed, and in each adhesive layer portion to which a crosslinking agent is added in advance, crosslinking is performed to some extent, but in many cases, insufficient crosslinking is performed. The more complete shape of the crosslink density is obtained by electrical heating. More specifically, the cross-linking of the boundary portion of the metal foil layer 13 further progresses by the electric heating,
It is advisable to adjust the glass transition temperature of the adhesive portion to be equal to or higher than the operating temperature range (for example, 60 ° C. or higher).

In the case of such electric heating, the metal foil layer 13
Since the temperature of the boundary portion becomes the highest, and the temperature decreases as the distance from the boundary portion increases, the cross-linking density naturally has an inclined density distribution with the boundary portion with the metal foil layer 13 being maximized.

In the FPC, high bending resistance and flexibility are required in the metal foil layer 3 of the pattern circuit where fatigue deterioration is apt to occur, particularly at the boundary between the metal foil layer 3 and the metal foil layer 13. It can be said that a gradient density distribution with a maximum portion is an ideal shape.

That is, since only the boundary portion of the metal foil layer 3 requires a high elastic modulus and high heat resistance, by increasing the elastic modulus only in the vicinity thereof, the secondary moment of area of deformation is reduced. It can be suppressed to a low value, and the flexibility and heat resistance can be improved while maintaining the flexibility of the entire FPC. Also, due to the gradient distribution of crosslinking density,
Since the concentration of shear stress on the surface of the metal foil layer can be reduced, the flex life can be improved as compared with the case where the entire FPC is heat-treated.

As the adhesive used in the present invention, an adhesive in which the amount of the plastic component and the amount of the crosslinking agent are adjusted so that the change in elastic modulus and heat resistance before and after the heat treatment is sufficiently large is used. Further, in the above-described circuit for energizing and heating, it is possible to use only a portion that requires flexibility.

Example 1 A 20 μm-thick epoxy resin adhesive layer was provided on one side of a 25 μm-thick thermosetting polyimide film, and a 35 μm-thick metal foil layer made of copper foil was adhered thereto. Make a film with foil (CCL)
A predetermined pattern circuit is formed on the metal foil layer portion, and a coverlay (CL) on which a 20 μm-thick epoxy resin-based adhesive layer is provided on one side of a thermosetting polyimide film having a thickness of 25 μm. The film was attached to obtain a sample FPC. This is illustrated in FIG. 2. In this FPC 10, 11 is a base film,
13 is a pattern circuit of a metal foil layer, and 15 is a coverlay film.

Then, a current was applied to the pattern circuit portion of the metal foil layer of the FPC for one hour. At this time, the surface of the FPC was cooled by air cooling, and the heat generation temperature in the pattern circuit portion during energization was monitored with a thermocouple.
The temperature was adjusted so as to be kept at 0 ° C. (the metal foil layer portion was about 150 ° C.).

<Example 2> Electric power was applied to the pattern circuit portion of the metal foil layer of the FPC obtained in the same manner as in Example 1 for 5 hours. At this time, the surface of the FPC is cooled by air cooling, and the heat generation temperature in the pattern circuit portion during energization is monitored by a thermocouple.
(About 50 ° C.).

<Embodiment 3> An electric current was applied to the pattern circuit portion of the metal foil layer of the FPC obtained in the same manner as in Embodiment 1 for 10 hours. At this time, the surface of the FPC is cooled by air cooling,
The heat generation temperature in the pattern circuit portion during energization was monitored with a thermocouple and adjusted so that the FPC surface was maintained at 110 ° C. (the metal foil layer portion was maintained at approximately 150 ° C.).

Example 4 An electric current was applied to the pattern circuit portion of the metal foil layer of the FPC obtained in the same manner as in Example 1 for 5 hours. At this time, the surface of the FPC was cooled at room temperature without cooling, and the heat generation temperature in the pattern circuit portion during energization was monitored with a thermocouple, and adjusted so that the surface of the FPC was maintained at 110 ° C.

<Comparative Example 1> An FPC itself obtained in the same manner as in the above-mentioned Example 1, that is, one in which heating was not performed by supplying electricity to the pattern circuit portion.

The flex life, flexibility, and heat resistance of the FPCs of Examples 1 to 4 and Comparative Example 1 were determined, and the results are as shown in Table 1.

Here, as shown in FIG. 3, the flex life test (fatigue test) is performed on the lower jigs 20 arranged in parallel with each other.
Then, the sample FPC 10 is attached to the upper jig 30 so that the bending radius R is 2 mm, and the lower jig 20 is fixed, and the upper jig 30 is moved at a stroke of 20 mm at a speed of 1500 times / minute. To reciprocate in the horizontal direction,
The measurement was performed by measuring the number of times of bending until the FPC 10 was broken.

In the flexibility test, as shown in FIG.
Lower jig 20 and upper jig 30 arranged parallel to each other
A sample FPC 10 is attached so that its bending radius R is 2 mm, and the lower jig 20 is fixed, and the upper jig 30 is centered on the shaft 31 on the left end side in FIG. The FPC 10 was pressed down to the stopper pin 32, thereby applying a bending force to the FPC 12 for 12 hours. After that, the FPC 10 was set in a free state, and the repulsive force at this time was measured by a sensor (load cell) 33. The unit of the measured value is × 10-
In 4N · m, N · m represents (1Kgf · m) /9.8.

On the other hand, the heat resistance test was performed by disassembling the FPC of the sample, collecting an adhesive material in a portion of the metal foil layer near the pattern circuit of 2 μm, and measuring the glass transition temperature. In other words, in order to obtain good flexibility, the glass transition temperature of the adhesive material around the metal foil layer is required to be 60 ° C. or higher. ), And those having a glass transition temperature of 60 ° C. or higher were evaluated as acceptable (○).

[0030]

[Table 1]

From Table 1, it can be seen that Examples 1 to 4 of the FPC according to the present invention, which were subjected to electric heating, had a long flexing life, had excellent flexibility, and had good heat resistance. On the other hand, in Comparative Example 1 in which the electric heating was not performed, the bending life was short, the flexibility was poor, and further,
It turns out that it is poor even in heat resistance.

In the above embodiment, the single-sided FP is used.
C, but the present invention can be similarly applied to a double-sided type FPC. .

[0033]

First, according to the FPC according to the present invention, only the crosslink density on the adhesive layer side near the periphery of the metal foil layer is made higher than that on the portion other than the periphery. As compared with the case where the crosslink density of the portion is increased or the case where an adhesive having high elastic modulus and heat resistance is used from the beginning, excellent bending resistance can be obtained while maintaining the flexibility of the entire FPC. That is, excellent flexibility is obtained in the metal foil layer, that is, the pattern circuit portion, without sacrificing the flexibility of the entire FPC, and it is possible to supply the FPC with extremely high durability.

Next, according to the method of manufacturing an FPC according to the present invention, it is only necessary to apply a current to the metal foil layer, that is, the pattern circuit portion. Moreover, it is possible to easily obtain a gradient density distribution in which the crosslink density at the boundary between the metal foil layers is maximized without performing any special operation. In addition, this energization can be easily performed only on the metal foil layer in a portion where high flexibility is required depending on the pattern circuit.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing one embodiment of an FPC according to the present invention.

FIG. 2 is a schematic plan view of a sample FPC.

FIG. 3 is a schematic view showing a method of a flex life test of an FPC.

FIG. 4 is a schematic view showing a method of an FPC flexibility test.

FIG. 5 is a longitudinal sectional view showing a conventional FPC.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 FPC 11 Base film 12 Base film side adhesive layer 13 Metal foil layer (pattern circuit) 14 Coverlay side adhesive layer 15 Coverlay film

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Jumoji Sadamitsu 1-5-1, Kiba, Koto-ku, Tokyo Inside Fujikura Co., Ltd. (72) Inventor Masahiko Arai 1-1-1, Kiba, Koto-ku, Tokyo Stock Company F term in Fujikura 4F100 AB01B AB33B AK53 AR00C AT00A BA05 BA07 BA10A BA10C GB43 JA13D JA13E JB12D JB12E JL11D JL11E 5E338 AA01 AA12 AA16 BB51 BB63 BB72 BB75 CC01 CD13 EE26 EE27

Claims (3)

[Claims] 1. A flexible printed circuit board comprising a base film, a base film-side adhesive layer, a metal foil layer forming a pattern circuit, a coverlay-side adhesive layer, and a coverlay film laminated in this order, in the vicinity of the periphery of the metal foil layer. 3. The flexible printed circuit board according to claim 1, wherein the crosslink density on the side of the adhesive layer is higher than that of the portion other than the vicinity. 2. The method according to claim 1, wherein the crosslinking density on the side of the adhesive layer near the periphery of the metal foil layer is a gradient density distribution that maximizes a boundary portion with the metal foil layer. Flexible printed circuit board. 3. A method of manufacturing a flexible printed board in which a base film, a base film side adhesive layer, a metal foil layer forming a pattern circuit, a coverlay side adhesive layer and a coverlay film are laminated in this order. A method of manufacturing a flexible printed circuit board, characterized in that the adhesive layer side near the periphery of the metal foil layer is heated by energizing the layer portion to increase the crosslink density of the adhesive layer compared with the portion other than the vicinity.