JP2006108269A - Flexible printed wiring board and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flexible printed wiring board including a solder resist layer having enough flexibility without manifesting tackiness in a working process at an ordinary temperature. <P>SOLUTION: The solder resist layer comprises a first layer formed on the side of the printed wiring board, and a second layer formed outside the first layer. The first layer is formed of a flexible solder layer, and the second layer is formed of a solder resist not manifesting tackiness at the time of heating. The first layer is formed of a solder resist having a glass transition temperature lower by ≥30°C than that of the solder resist that forms the second layer, and the thickness of the second layer is thinner than the thickness of the first layer. The first layer is formed of a solder resist having a higher elongation rate by ≥50% than that of the solder resist that forms the second layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a flexible printed wiring board (flexible printed wiring board) and a method for manufacturing the same.

  A printed wiring board having a solder resist layer on its surface is produced as follows using, for example, a substrate material in which a copper foil having a desired thickness is provided on both surfaces of an insulating resin film.

(1) A through hole for a through hole for providing conduction on both sides of the substrate is provided, and a copper layer is provided on the inner wall surface of the through hole by a plating method.

(2) A photosensitive resist layer is provided on both sides. An etching mask is created by patterning the photosensitive resist layer by a projection method or a contact method using a mask having a desired wiring pattern.

(3) A portion of the copper foil exposed from the etching mask is removed by etching, and the etching mask is further removed to form a copper wiring.

(4) Apply a solder resist on both sides, expose using a mask having a desired solder resist pattern, develop, heat cure or UV cure, cure the solder resist, Form.

  In the printed wiring board having the solder resist layer on the surface manufactured as described above, flexibility may be required for use in a bent portion, for example. Such a flexible printed wiring board (flexible printed wiring board) requires flexibility for all of the insulating resin film, the copper foil, and the solder resist layer that constitute the printed wiring board.

  For insulating resin films, if the thickness is 50 μm or less, there is no problem in flexibility regardless of the material, and the copper foil has a problem in flexibility by setting the thickness to 16 μm or less. Can be prevented.

  However, because the solder resist layer is formed on the outermost layer of the flexible printed wiring board, the stress applied during bending is maximized, so that it has higher flexibility than an insulating resin film or copper foil. Needed. The reason for this is that when the flexible printed wiring board is bent, if the solder resist layer cracks, the copper wiring obtained by etching the copper foil cannot be protected, and the copper wiring may be broken. Is mentioned.

  As a flexible solder resist used for obtaining a solder resist layer of a flexible printed wiring board, for example, a photosensitive resist described in JP-A-11-065117, or, for example, JP-A-11-158252 There are described thermosetting solder resists. Generally, since a polymer has flexibility when the glass transition temperature (Tg) is low, a material having a low glass transition temperature is used as a solder resist for obtaining a solder resist layer that requires flexibility. The thermosetting solder resist described in JP-A-11-158252 is an example using a material having a low glass transition temperature of −60 ° C. to 40 ° C.

  However, when a solder resist having such a low glass transition temperature is used in order to increase the flexibility of the solder resist layer, the flexibility of the solder resist layer is increased while other characteristics are deteriorated. In particular, the tack (sticking) property at normal temperature is increased, and the solder resist adheres to the jig tool in the machining process, making it difficult to separate from the jig tool, resulting in a problem that machining cannot be performed. These solder resists improve tackiness at temperatures close to room temperature after curing, but when exposed to high temperatures above the glass transition temperature in the assembly process, tackiness develops again, and similar problems occur in the assembly process. May occur. For this reason, a solder resist for obtaining a solder resist layer having both sufficient flexibility and good tackiness is required. However, a solder resist that can sufficiently meet such requirements has not been proposed yet.

Japanese Patent Application Laid-Open No. 11-065117

JP-A-11-158252

  The present invention is a flexible having a solder resist layer that has sufficient flexibility, does not exhibit tackiness in a processing process at room temperature, and does not exhibit tackiness due to heating in an assembly process after the processing process. A printed wiring board and a manufacturing method thereof are provided.

  In one aspect of the flexible printed wiring board of the present invention, the solder resist layer is a flexible printed wiring board having a laminated structure, and the solder resist layer includes a first layer and a first layer provided on the printed wiring board side. The first layer is formed of a solder resist having a glass transition temperature lower by 30 ° C. or more than the glass transition temperature of the solder resist forming the second layer.

  In a different aspect of the flexible printed wiring board of the present invention, the solder resist layer is a flexible printed wiring board having a laminated structure, and the solder resist layer includes a first layer and a first layer provided on the printed wiring board side. The first layer is formed of a solder resist having flexibility, and the second layer is formed of a solder resist that does not exhibit tackiness when heated.

  Furthermore, it is desirable that the thickness of the second layer is made thinner than the thickness of the first layer.

  The first layer is preferably formed of a solder resist having an elongation percentage higher by 50% or more than the elongation percentage of the solder resist forming the second layer.

  In the flexible printed wiring board of the present invention, the first solder resist is applied to the surface of the substrate material on which the copper wiring is formed, preliminarily dried to form the first layer, and then the second solder resist is applied. It is manufactured by applying and pre-drying to form a second layer, and then exposing, developing, and performing thermal curing or UV curing using a mask having a solder resist pattern.

  According to the present invention, there is provided a flexible printed wiring board having a solder resist layer that has high flexibility and does not exhibit tackiness in a processing step and an assembly step.

  Generally, the glass transition temperature of the solder resist used for the flexible printed wiring board on which the solder resist layer is laminated is 30 ° C. to 170 ° C. depending on the material selected, and depending on the characteristic conditions required for the printed wiring board, The material is appropriately selected.

  As described above, the solder resist has a good flexibility when the glass transition temperature is relatively low, whereas the tackiness is poor, and when the glass transition temperature is relatively high, the solder resist is not flexible. However, tackiness is improved.

  The inventor pays attention to this point, and the solder resist layer has a two-layer structure including a first layer and a second layer, and the substrate material side has a relatively low glass transition temperature and has flexibility. A first layer is formed from a solder resist having poor tackiness, and a second layer is formed on the first layer from a solder resist having a relatively high glass transition temperature and lacking flexibility, but having good tackiness. Therefore, what is used for the solder resist for forming the first layer and what is used for the solder resist for forming the second layer ensures the flexibility and tackiness required for a flexible printed circuit board. Thus, it is determined by selecting a combination of two different types of solder resists from various types of solder resists on the market.

  More specifically, the material is selected so that the glass transition temperature (Tg1) of the solder resist forming the first layer is 30 ° C. lower than the glass transition temperature (Tg2) of the solder resist forming the second layer. To do. If the difference between the glass transition temperature (Tg1) of the first layer and the glass transition temperature (Tg2) of the second layer is less than 30 ° C., the solder resist is 2 in terms of preventing the development of tackiness while ensuring flexibility. It is not preferable because an effect sufficient to increase the processing cost due to the coating is not obtained.

  With such a configuration, it is possible to prevent the solder resist from adhering to the tool in the processing step by selecting a material that does not exhibit tackiness at room temperature for the second layer solder resist on the surface of the substrate. Even when the heating temperature of the assembly process is higher than the temperature at which the solder resist of one layer develops tackiness, the tackiness at the time of processing does not develop if the solder resist of the second layer does not develop. The problem can be solved.

  Moreover, when a material inferior in flexibility is selected for the second layer, if the thickness of the second layer is made thicker than the thickness of the first layer, the flexibility of the substrate is impaired, and the solder resist is applied twice. The effect which is commensurate with the processing cost increase by the thing cannot be obtained. Therefore, it is preferable to form the thickness (A2) of the second layer thinner than the thickness (A1) of the first layer.

  Furthermore, it is preferable to select a material so that the elongation rate (T1) of the solder resist forming the first layer is 50% or more higher than the elongation rate (T2) of the solder resist of the second layer. The elongation rate is defined as the elongation rate until the film-like solder resist is pulled to break. If the elongation rate (T2) of the second layer is not sufficiently higher than the elongation rate (T1) of the first layer, the processing cost is increased by applying the solder resist twice for the flexibility of the solder resist layer. It is not preferable because an effect corresponding to the price cannot be obtained.

  The method for producing a flexible printed wiring board according to the present invention is basically the same as the conventional method for producing a flexible printed wiring board except that a solder resist having two different characteristics is applied twice. However, the first solder resist is applied and preliminarily dried to form the first layer, and then the second solder resist is applied and preliminarily dried to form the second layer. This prevents the solder resist layer from sticking to the jig during subsequent exposure.

Example 1
Copper wiring was formed on both sides of a double-sided copper-clad substrate (made by Shin-Kobe Electric Machinery Co., Ltd., CEL541) having a resin thickness of 50 μm and a copper foil thickness of 12 μm using a photosensitive dry film according to a photometric method. Subsequently, a first liquid photosensitive resist (manufactured by Tamura Kaken, DSR2200-FOX12) having a glass transition temperature (Tg1) of 50 ° C. and an elongation rate (T1) of 100% is preliminarily dried. Was applied to both sides by a roll coating method so as to be 25 μm, and pre-dried. Subsequently, a second liquid photosensitive resist having a glass transition temperature (Tg2) of 85 ° C. and an elongation rate (T2) of 4% (manufactured by Tamura Kaken Co., Ltd., DSR2200-FOX5-11) is subjected to a total thickness ( It was applied on both sides by a roll coating method so that A2 + A1) was 35 μm and pre-dried.

  Next, a mask having a predetermined solder resist pattern is brought into close contact with the surface, exposed, developed, thermally cured, UV cured, and a two-layer structure comprising a first layer and a second layer. A flexible printed wiring board having a solder resist layer was obtained.

  During the exposure, there was no problem that the solder resist stuck to the exposure table and could not be removed.

Next, in order to investigate the effect of improving the tackiness in the assembly process, the tackiness during heating was evaluated. In the evaluation method, an aluminum foil is placed on a hot plate, a test material is placed on the aluminum foil, and a weight is placed thereon so that a pressure of 9.8 kPa (100 g / cm ² ) is obtained. Pressed for a minute. Thereafter, when the test material was lifted, it was examined whether the aluminum foil adhered to the test material and lifted.

  Even when the temperature was 150 ° C., 170 ° C., or even 190 ° C. by heating the hot plate, sticking did not occur.

  Next, in order to examine the flexibility, the degree of occurrence of disconnection of the copper wiring when the flexible printed wiring board was bent was evaluated. In the evaluation method, 32 copper wirings having a wiring width of 100 μm at a pitch of 200 μm are formed as an evaluation pattern in advance, and a flexible printed wiring board on which a solder resist layer is formed as described above is attached to a round bar having a diameter of 0.6 mmφ. Then, the 180 degree bending was performed a plurality of times, and the number of bendings until the evaluation pattern was disconnected was measured. It means that the more the number of bendings that lead to disconnection is, the more the solder resist layer is not cracked and the copper wiring is less likely to be disconnected.

  The measurement results are shown in Table 1. In the table, the valley fold indicates that the copper wiring is located inside the fold, and the mountain fold indicates that the copper wiring is located outside the fold.

(Comparative Example 1)
A single-layer structure was used in the same manner as in Example 1 except that only the second liquid photosensitive resist was used without using the first liquid photosensitive resist, and the total thickness after preliminary drying was 35 μm. A flexible printed wiring board having a solder resist layer was obtained.

  During the exposure, there was no problem that the solder resist stuck to the exposure table and could not be removed.

  Further, in the same manner as in Example 1, the tackiness during heating was evaluated.

  Even when the temperature was 150 ° C., 170 ° C., or even 190 ° C. by heating the hot plate, sticking did not occur.

  Furthermore, the flexibility was examined in the same manner as in Example 1. The measurement results are shown in Table 1.

(Comparative Example 2)
A single-layer structure was used in the same manner as in Example 1 except that the first liquid photosensitive resist alone was not used and the total thickness after preliminary drying was 35 μm. A flexible printed wiring board having a solder resist layer was obtained.

  At the time of exposure, there was a problem that the solder resist stuck to the exposure table and could not be removed.

  Further, in the same manner as in Example 1, the tackiness during heating was evaluated.

  Adhesion did not occur at a temperature of 150 ° C. due to heating of the hot plate, but adhesion occurred at 170 ° C. or higher, particularly 190 ° C.

  Furthermore, the flexibility was examined in the same manner as in Example 1. The measurement results are shown in Table 1.

  In Example 1 according to the present invention, which is equivalent to Comparative Example 1, the tackiness is less likely to appear in the solder resist layer than Comparative Example 2, and as shown in Table 1, the flexibility is higher than Comparative Example 1, It can be seen that the folding performance is almost the same as in Example 2.

  The effect obtained by the present invention is not limited to the first embodiment, judging from its principle. If the solder resist has a two-layer structure and there is a difference in characteristics between the first layer and the second layer, It is considered that there is an effect corresponding to the difference.

Claims (5)

  The solder resist layer is a flexible printed wiring board having a laminated structure, and the solder resist layer is composed of a first layer provided on the printed wiring board side and a second layer provided outside the first layer. The flexible printed wiring board is characterized in that the first layer is formed of a solder resist having a glass transition temperature lower by 30 ° C. or more than the glass transition temperature of the solder resist forming the second layer.   The solder resist layer is a flexible printed wiring board having a laminated structure, and the solder resist layer is composed of a first layer provided on the printed wiring board side and a second layer provided outside the first layer. The flexible printed wiring board is characterized in that the first layer is formed of a solder resist having flexibility, and the second layer is formed of a solder resist that does not exhibit tackiness when heated.   The flexible printed wiring board according to claim 1, wherein a thickness of the second layer is formed thinner than a thickness of the first layer.   The said 1st layer is formed with the soldering resist which has 50% or more of elongation rate higher than the elongation rate of the soldering resist which forms the said 2nd layer, The any one of Claims 1-3 characterized by the above-mentioned. Flexible printed wiring board.   A method for producing a flexible printed wiring board according to any one of claims 1 to 4, wherein a first solder resist is applied to a surface of a substrate material on which copper wiring is formed, and preliminarily dried to form a first layer. Subsequently, a second solder resist is applied, preliminarily dried to form a second layer, and subsequently exposed using a mask having a solder resist pattern, developed, and thermally cured. Or the manufacturing method of the flexible printed wiring board characterized by performing UV cure.