JP2628741B2 - Flexible printed wiring board - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a flexible printed wiring board used by being embedded in a synthetic resin.

<Problems to be Solved by the Related Art and the Invention> In recent years, instead of ordinary cables, flexible printed wiring in which a flexible insulating substrate and a conductive circuit formed of a conductive thin film such as a metal foil are laminated. Board (flat flexible
printed circuit (FPC) has been increasingly used as a wiring material. Flexible printed wiring boards
As the name suggests, it has flexibility, so that it can be used in various wiring shapes as in the case of a conventional cable, and can simultaneously wire a large number of circuits in a small space.

However, flexible printed wiring boards are vulnerable to vibration,
There is a problem that the conductor circuit is easily disconnected. Therefore,
Attempts have been made to improve the vibration resistance by embedding this flexible printed wiring board in a synthetic resin.

However, flexible printed wiring boards embedded in synthetic resin, especially when the synthetic resin is a foamed resin,
It has been found that there is a new problem that the conductor circuit is liable to be disconnected when used under conditions where temperature changes are severe. According to the studies by the inventors, the cause of the disconnection is a large difference in the coefficient of thermal expansion between the material forming the flexible printed wiring board and the foamed resin material, and thermal stress is applied to the conductor circuit during a thermal shock. It was found that this was due to disconnection fatigue.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a flexible printed wiring board that is resistant to thermal stress and does not easily break.

<Means and Actions for Solving the Problems> In order to solve the above problems, the flexible printed wiring board of the present invention has a reinforcing material made of a polymer exhibiting thermotropic liquid crystallinity (hereinafter referred to as “liquid crystalline polymer”). It is characterized by:

In the flexible printed wiring board of the present invention having the above configuration, the linear expansion coefficient of the liquid crystalline polymer forming the reinforcing material is close to that of the metal material forming the conductive circuit, and the tensile modulus is absorbed by the ordinary resin material. In addition, a part of the stress applied to the conductor circuit is absorbed, the stress applied to the conductor circuit is reduced, the conductor circuit can be protected, and the disconnection of the conductor circuit is prevented.

Further, the above liquid crystalline polymer is brought into a liquid crystal state by heating. It exhibits so-called thermotropic liquid crystallinity and can flow at a temperature lower than the true melting temperature, so despite having high heat resistance, it has excellent moldability, and is formed into an appropriate shape such as a film. Is also easy.

As the liquid crystalline polymer forming the reinforcing material, those having a rigid aromatic ring in the main chain are preferably used, and in particular, a group consisting of compounds represented by the following general formulas [I] to [IV] At least one member selected from the following is more preferably used.

(Wherein, R represents a divalent aliphatic group or an aromatic group,
l, m, and n are each an integer of 1 or more, and are represented by the formula [I] [I
In the formula [I], m + n ≧ 100, and in the formulas [III] and [IV], it is necessary to satisfy l + m + n ≧ 100.) Examples of the divalent aliphatic group or aromatic group corresponding to R in the above formula include methylene , Ethylene, propylene, tetramethylene, 1,2-butylene, 1,3-butylene,
2,3-butylene, pentamethylene, α-amylene, β-
Amylene, α, δ-amylene, propenylene, 2-butenylene, o-phenylene, m-phenylene, p-phenylene, and compounds represented by the following formulas are preferred. Particularly, an ethylene group and a phenylene group are preferred. More preferably used.

(In the formula, X and Y represent a halogen or an alkyl group, and Z represents a hydrogen atom, a halogen or an alkyl group.) As currently available liquid crystal polymers, "Econol" (trade name, manufactured by Sumitomo Chemical Co., Ltd., DARTCO MFG, Inc.) "Xydar", a product name "Vectra" manufactured by Celanese, and a product name "LC-2000" manufactured by Unitika.

The reinforcing material comprising the liquid crystalline polymer is in the form of a film,
An appropriate shape such as a wire shape or a net shape depending on the shape or layer configuration of the flexible printed wiring board can be used.

The flexible printed wiring board of the present invention is not particularly limited in the configuration other than including the reinforcing material made of the liquid crystalline polymer and the conductive circuit, and is made of the same member as the conventional one, similarly to the conventional wiring board. Can be configured.

Along with the reinforcing material and the conductor circuit, the members constituting the flexible printed wiring board support the conductor circuit,
Examples include a flexible insulating substrate, an adhesive layer for bonding the insulating substrate, the reinforcing material, the conductor circuit, and the like to each other. Further, in the flexible printed wiring board of the present invention, the insulating substrate itself is formed of a liquid crystalline polymer,
It is also possible to use both the insulating substrate and the reinforcing agent, or to embed a reinforcing agent such as a wire made of a liquid crystalline polymer into the insulating substrate to integrate them.

In the flexible printed wiring board of the present invention composed of each of the above materials, an appropriate layer configuration can be adopted. For example, as described above, when the insulating substrate itself also serves as a reinforcing agent, or when the insulating substrate and the reinforcing agent are integrated, a single-sided type in which a conductor circuit is laminated on one surface of a single-layer insulating substrate, A double-sided type in which conductive circuits are stacked on both sides of a single-layer insulating substrate, a multilayer type in which a plurality of insulating substrates and a plurality of conductive circuits are alternately stacked, and a conductive circuit interposed between two-layered insulating substrates It can have a normal layer structure such as a flat cable type. Further, when the reinforcing material made of a liquid crystalline polymer is separate from the insulating substrate, the reinforcing material is arranged at an arbitrary position between the outermost layer and each layer of each of the above-described structures, so that the flexible printed wiring board is formed. Is done.

Copper, tin, silver,
Nickel, aluminum, and alloys of these metals are exemplified. As a method of forming a conductor circuit, a so-called subtractive method in which an unnecessary portion of a thin film made of the above metal material laminated on the surface of an insulating substrate is removed by etching,
It is possible to apply a conventionally known method of manufacturing a conductor circuit, such as an additive method of depositing a metal material only on a portion required as a conductor circuit on an insulating substrate surface, a semi-additive method combining a subtractive method and an additive method, or the like. it can. The thickness of the conductor circuit can be selected in an optimal range depending on the conditions such as the material of the conductor circuit and the manufacturing method, but it is usually preferably about 18 to 70 μm.

As the insulating substrate, polyester, polyimide,
Conventionally known flexible insulating films for flexible wiring boards such as glass epoxy type, glass Teflon type, polyamide imide type, polyvinyl chloride type and polyethylene type can be used. Further, as described above, an insulating substrate also serving as a reinforcing material can be formed of a liquid crystalline polymer. The thickness of the insulating substrate can be the same as that of the conventional one, or can be made thinner than that of the conventional one because of the lamination of the reinforcing material, but is usually preferably in the range of 25 to 125 μm.

As an adhesive that forms the adhesive layer, it has excellent heat resistance,
Not prevent the flexibility of the flexible printed wiring board, and
An urethane-based, polyester-based, or epoxy-based adhesive that is resistant to thermal stress is used.

<Example> Hereinafter, the present invention will be described based on examples.

(Example 1) A long strip-shaped polyethylene terephthalate (PET) film having a thickness of 50 µm, a width of 2 cm, and a length of 18 cm
After laminating and bonding an electrolytic copper foil having a thickness of μm via a urethane-based adhesive (trade name: A-540, manufactured by Takeda Pharmaceutical Co., Ltd.), the copper foil is etched to form a wire parallel to the longitudinal direction of the strip. width
Two 0.5 mm conductor circuits and one 0.1 mm line width conductor circuit were formed. Next, the same PET film as described above was laminated and bonded on this conductive circuit using the same urethane-based adhesive to obtain a laminate corresponding to a normal flat cable type printed wiring board. The total thickness of the laminate was 195 μm. Then, both sides of this laminate, that is, two layers of PE
On the outside of the T film, a reinforcing material film having a thickness of about 200 μm made of a liquid crystalline polymer (trade name: Vectra manufactured by Celanese) formed by extrusion molding, and a 30 μm thick adhesive made of the urethane-based adhesive, respectively. By laminating and bonding through the layers, a flexible printed wiring board (A) having a layer configuration shown in FIG. 1 was produced. In the figure,
(1) is a conductor circuit, (2) and (2) are PET films,
(3) (3) indicates a reinforcing material film, and (4)... Indicate an adhesive layer made of a urethane-based adhesive.

(Example 2) A flexible printed wiring board was produced in the same manner as in Example 1 except that a rolled copper foil having a thickness of 70 µm was used instead of the electrolytic copper foil.

(Comparative Example 1) In Example 1, the laminated body at the stage before laminating a reinforcing material film made of a liquid crystalline polymer on both surfaces was used as it was as a flexible printed wiring board.

Comparative Example 2 A flexible printed wiring board was produced in the same manner as in Comparative Example 1 except that a rolled copper foil having a thickness of 70 μm was used instead of the electrolytic copper foil.

(Cold and thermal shock resistance test) Each of the flexible printed wiring boards of Examples 1 and 2 and Comparative Examples 1 and 2 was loaded into a mold and subjected to polyurethane foam molding. A flexible printed wiring board embedded molded product having a urethane foam layer having a thickness of 20 mm was produced. Then, each molded product was subjected to a thermal shock of one cycle of −40 ° C. × 2 hours and + 90 ° C. × 2 hours, and the disconnection of the conductor circuit was observed. The flexible printed wiring board of Comparative Example 1 was embedded. 198 cycles for molded products, and 411 for molded products in which the flexible printed wiring board of Comparative Example 2 is embedded.
In each cycle, the conductor circuit was broken.

On the other hand, in each of the molded products in which the flexible printed wiring boards of Examples 1 and 2 were embedded, the conductor circuit was not broken even by a thermal shock of 1000 cycles or more. It has been found that a part of the thermal stress during the thermal shock is absorbed to reduce the stress applied to the conductor circuit.

<Effect of the Invention> In the flexible printed wiring board of the present invention, a part of the thermal stress generated at the time of a thermal shock is absorbed by a reinforcing material made of a liquid crystalline polymer to protect the conductor circuit from the thermal stress.
The conductor circuit is not easily broken, has a long life, has high reliability, and is suitable for use in an environment where temperature changes are drastic.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a layer configuration of a flexible printed wiring board according to an embodiment. (A) ... flexible printed wiring board (1) ... conductor circuit, (2) (2) ... PET film, (3) (3) ... reinforcement film, (4) ... adhesive layer.

Claims (3)

(57) [Claims] 1. A flexible printed wiring board used by being embedded in a synthetic resin, comprising a reinforcing material made of a polymer exhibiting thermotropic liquid crystallinity. 2. The flexible printed wiring board according to claim 1, wherein the polymer has an aromatic ring in a main chain. 3. The polymer of the following general formulas [I] to [I]
3. The flexible printed wiring board according to claim 2, wherein the flexible printed wiring board is at least one selected from the group consisting of compounds represented by the following formulas: (Wherein, R represents a divalent aliphatic group or an aromatic group,
l, m, and n are each an integer of 1 or more, and are represented by the formula [I] [I
M + n ≧ 100 in I] and l + m + n ≧ 100 in formulas [III] and [IV])