JP3897755B2 - Flexible printed wiring board - Google Patents

Description

  The present invention relates to a flexible printed wiring board used for electrical connection of electronic devices, for example, and in particular, an insulating protective layer made of a polyimide resin is formed on a flexible printed wiring board in which a polyimide resin is directly applied to a conductor such as a copper foil. It is about what you did.

  In so-called portable electrical products such as portable recording / reproducing devices, electrical circuit sections are connected using a flexible printed wiring board that is relatively inexpensive and can save space in order to reduce size and cost. There are many things.

  In the field of such flexible printed wiring boards, with the demand for miniaturization and multi-functionality of electronic devices, technologies such as circuit density increase, surface mounting by wire bonding, direct mounting of semiconductor chips and packaging, etc. Has been progressing day by day. For this reason, the flexible printed wiring board has a more complicated shape and more and more fine circuits necessary for higher density mounting.

  In a conventional flexible printed wiring board, for example, a circuit is formed by etching a flexible printed board as a base substrate, and an insulating protective layer (coverlay) for protecting the circuit is formed.

  A flexible printed circuit board, which is a base substrate used for such a flexible printed circuit board, often uses a polyimide film from the viewpoint of flexibility and heat resistance. For example, as this flexible printed circuit board, a three-layer flexible printed circuit board formed by bonding a polyimide film and a copper foil as a conductor to each other through a heat-resistant adhesive can be mentioned.

  However, this three-layer flexible printed circuit board has insufficient heat resistance of the adhesive, and the adhesive is likely to be deformed by heat. Therefore, when a thermal history such as thermocompression bonding is applied when bonding with the adhesive, the board As a result, there is a problem that surface mounting by forming fine circuit patterns or wire bonding becomes difficult.

  Therefore, as a flexible printed circuit board, in order to improve the drawbacks of such a three-layer flexible printed circuit board, as shown in FIG. 13, a polyimide precursor is directly formed on a metal foil 51 such as copper without using an adhesive. A two-layer flexible printed circuit board 53 is proposed and put into practical use, in which a polyamic acid is applied, the polyamic acid is imidized after drying, and a polyimide resin layer 52 is laminated. And as shown in FIG. 14, as the flexible printed wiring board 50 with a protective layer which uses this 2 layer flexible printed circuit board 53, while forming a circuit in the flexible printed circuit board 53, an insulation protective layer is formed on this circuit. 54 is formed. In FIG. 14, a portion of the metal foil 51 that is exposed without being covered with the insulating protective layer 54 is a portion that becomes a terminal.

JP-A-9-252169

  However, even in such a two-layer flexible printed circuit board 53, since the polyamic acid applied on the metal foil 51 is imidized at a high temperature, a polyimide resin having better heat resistance than the above-mentioned adhesive Although the layer 52 is used, the heat of the metal foil 51 and the polyimide resin layer 52 when returned to room temperature due to the difference in thermal expansion coefficient between the metal foil 51 and the polyimide resin layer 52. A difference occurs in the shrinkage rate, and curling occurs. As a result, if such a curl occurs in the flexible printed circuit board 53, the accuracy of the conductor spacing of the circuit after etching is lowered, and component mounting becomes very difficult.

  In view of this, a method has been proposed in which the chemical structure of the polyimide resin to be formed is specified for the purpose of removing the curl in such a two-layer flexible printed board. In addition, a method has been proposed in which the structure of the polyamic acid as a precursor is specified so that the thermal expansion coefficient of the polyimide resin is as small as possible.

  However, it is difficult to completely remove the curl in any of the methods proposed for removing the curl of the two-layer flexible printed circuit board as described above, and after the circuit is formed by etching the metal foil. It is impossible to correct curls.

  Moreover, in these conventional methods, the flexible printed circuit 53 itself, which is a base substrate in which the insulating protective layer 54 is not formed, is targeted, and the flexible printed circuit 53 and the circuit insulating protective layer 54 are integrated. The flexible printed wiring board 50 is not considered.

  In recent years, in the field of flexible printed circuit boards where advanced technological innovations such as higher density mounting are desired, considering the mounting of solder such as semiconductor chips, the performance of the flexible printed circuit board and the insulating protective layer of the circuit are integrated. Is fully achieved. Therefore, it is not sufficient that curling of the flexible printed circuit board 53 is suppressed, and it is necessary to consider the state of the flexible printed wiring board 50 itself in which the insulating protective layer 54 is integrated.

  Incidentally, since the insulating protective layer (coverlay) 54 used for the flexible printed wiring board 50 itself is also required to have heat resistance, the insulating protective layer 54 is often made of a layer made of a polyimide-based material. And as a formation method of such an insulation protective film 54, what is called a printing method, a film method, etc. are mentioned, for example.

  In the printing method, the resist ink is printed on the silk screen in the same way as the hard print plate, but the resist ink has the disadvantage that it is inferior in flexibility because it contains epoxy resin as the main component, and the thermal expansion coefficient of the resist. If the resist is not selected in consideration of the above, the flexible printed wiring board may be curled as a result. In addition, since the highly polar solvent contained in the polyimide resist ink absorbs moisture in the air, the printing workability is poor and the thickness of the film is difficult to manage.

  On the other hand, in the film method, after the holes corresponding to the lands and terminals are punched using a mold or a mold, the polyimide film with adhesive and the flexible printed circuit board on which the circuit is formed are heated. A flexible printed wiring board is obtained by bonding by crimping or the like.

  However, in this method, it is difficult to form fine land portions and terminal portions even if the precision of a mold or the like is improved, and there is a drawback that a fine circuit is contaminated due to the protruding adhesive. Moreover, when mounting on a semiconductor chip, etc., the flatness of the flexible printed wiring board is necessary. However, when the insulating protective layer is formed, partial shrinkage or curling occurs due to variations in the thickness of the insulating protective layer. As a result, the flatness is impaired, and contraction occurs between the conductors. In addition, recently, there has been a demand for a circuit that completely separates adjacent conductors by embedding an insulating protective layer between conductors of a fine circuit at a terminal portion or the like, that is, between conductors. The dimensional stability is severely required, and conventional techniques cannot cope with it.

  As described above, in the flexible printed wiring board 50, when the insulating protective layer 54 is formed, curling caused by an error in selecting a resist, peeling of the resist, lowering of folding resistance, and protrusion of the adhesive. There are problems such as insufficient adhesive force and deterioration of dimensional accuracy.

  Therefore, the present invention has been proposed in view of such a situation, curling is suppressed as much as possible, excellent in dimensional stability, and high density required for downsizing and multi-functionalization of electronic devices. An object of the present invention is to provide a high-performance flexible printed wiring board that can handle fine circuits.

The flexible printed wiring board according to the present invention proposed to achieve the above-mentioned object is formed with a polyimide resin layer, and the second surface of the copper foil covers and protects the circuit. In the flexible printed wiring board formed with the polyimide resin layer, the difference between the coefficient of thermal expansion of the first polyimide resin layer and the coefficient of thermal expansion of the second polyimide resin layer is 3 × 10 −. 6 / K or less, the first polyimide-based resin layer and the second polyimide-based resin layer, rust inhibitor is added to the polyamic acid which is a precursor of polyimide resin, the first polyimide-based At least one of the resin layer and the second polyimide resin layer has a structure in which three polyimide resin layers are laminated, and the above three layers of polyimide resin The linear thermal expansion coefficient of the copper foil side and the outer side of the polyimide resin layer is larger than a linear thermal expansion coefficient of the polyimide resin layer positioned at the center, and linear thermal expansion coefficient of the copper foil, the first polyimide-based resin The difference between the thermal expansion coefficient of at least one of the resin layer and the second polyimide resin layer is in the range of 2 × 10 −6 / K to 10 × 10 −6 / K. And

As described above, the flexible printed wiring board according to the present invention has a difference in the thermal linear expansion coefficient between the first and second polyimide resin layers sandwiching the copper foil to an optimum value. The thermal expansion of the polyimide resin layer is made as equal as possible. Therefore, in the flexible printed wiring board of the present invention, the occurrence of curling is suppressed as much as possible even after a process such as heat treatment, and good flatness is obtained.

According to the present invention, curling is suppressed as much as possible, good flatness is obtained, dimensional stability is good, and sufficient for high-density fine circuits required for downsizing and multi-functionalization of electronic devices. High performance that can be handled is obtained.
Also, the conductor is rust-proof and can improve the adhesive strength at the interface.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following, a single-sided access type flexible printed wiring board is taken as an example, but the flexible printed wiring board of the present invention has a polyimide resin layer formed on both sides of a conductor on which a circuit is formed, That is, if it is a flexible printed circuit board with a protective layer, it will not be limited to this.

  FIG. 1 is a cross-sectional view of an example of a flexible printed wiring board 1 with a protective layer to which the present invention is applied. Moreover, FIG. 2 is sectional drawing of the other example of the flexible printed wiring board 1 with a protective layer to which this invention is applied.

  As shown in FIG. 1, a flexible printed wiring board 1 with a protective layer to which the present invention is applied has a first polyimide resin layer 3 formed on one surface of a copper foil 2 patterned according to a circuit. A flexible printed wiring board 4 and a second polyimide resin layer 5 as an insulating protective layer that covers and protects the circuit on the copper foil 2 of the flexible printed wiring board 4 are provided. In addition, the part 2a which the copper foil 2 exposes in FIG. 1 is a part used as a terminal part.

The copper foil 2 is patterned according to the circuit, and is supported by the first polyimide resin layer 3, and the circuit is covered and protected by the second polyimide resin layer 5. In addition, when this copper foil 2 heat-processes in the atmosphere of 250 to 400 degreeC which is the imidation temperature of the polyimide resin which comprises the 1st and 2nd polyimide resin layers 3 and 5, a thermal linear expansion coefficient will rise. There is a nature. For example, the thermal linear expansion coefficient of the copper foil 2 is 16.0 × 10 ^{−6 to} 18.0 × 10 ⁻⁶ / K before imidization, but 18.0 × 10 ^{−6 to} 20 after imidization. 0.0 × 10 ⁻⁶ / K.

  Moreover, as the copper foil 2, specifically, an electrolytic copper foil, a rolled copper foil, or the like can be used. And as thickness of this copper foil 2, 35 micrometers or less, Preferably 8 micrometers-18 micrometers are preferable when forming a fine circuit. If the thickness of the copper foil 2 is 18 μm or more, it is difficult to form a fine circuit. Further, when the thickness of the copper foil 2 is 8 μm or less, wrinkles and the like are likely to occur in the coating process, and the work is difficult.

  Furthermore, as the copper foil 2, a copper foil that is not subjected to surface treatment is optimal. However, when the surface is treated with zinc, chromium, oxidation, or the like, the center line average roughness Ra is 10 μm or less, preferably 7 μm or less. good.

In addition, the metal conductor used for the flexible printed wiring board of this invention can also use the copper foil which performed the mat process performed for the improvement of adhesive strength, nickel, galvanization, and an oxidation process among copper foils. is there. Further, chemical surface treatment such as aluminum alcoholate, aluminum chelate, silane coupling agent, imidazole treatment may be performed.

  In the present invention, the first polyimide resin layer 3 is formed on a copper foil 2 with a solution containing, as a main component, a polyamic acid that is a condensation compound of an acid dianhydride that is a precursor of a polyimide resin and an aromatic diamine. It is formed by coating and then drying and imidizing this polyamic acid solution.

  Moreover, the 2nd polyimide resin layer 5 in this invention plays the role of the insulation protective layer for protecting the circuit patterned by the copper foil 2 by the etching. As with the first polyimide resin layer 5, the second polyimide resin layer 5 is mainly composed of polyamic acid, which is a condensate of acid dianhydride and aromatic diamine, which is a precursor of polyimide resin. The solution is applied on the flexible printed wiring board 4, and after drying, a terminal part is formed by etching, and then imidized to form.

In particular, in the flexible printed wiring board 1 with a protective layer of the present invention, the thermal expansion coefficient of the polyimide resin constituting the first polyimide resin layer 3 and the polyimide resin constituting the second polyimide resin layer 5 are described. The difference from the thermal linear expansion coefficient is 3 × 10 ⁻⁶ / K or less. Here, the difference in the coefficient of thermal expansion between the polyimide resin constituting the first polyimide resin layer 3 and the polyimide resin constituting the second polyimide resin layer 5 is larger than 3 × 10 ⁻⁶ / K. Then, curling is likely to occur.

  Thus, in the flexible printed wiring board 1 with a protective layer of the present invention, the difference in the coefficient of thermal expansion between the first and second polyimide resin layers 3 and 5 sandwiching the copper foil 2 is limited to an optimum value. Therefore, the thermal expansion properties of both polyimide resin layers 3 and 5 sandwiching the copper foil 2 are made as equal as possible. Therefore, in the flexible printed wiring board 1 with a protective layer of the present invention, the occurrence of curling is suppressed as much as possible even after a heat treatment step such as imidization, and good flatness is obtained, which is excellent in heat resistance and fine. The circuit can be formed with high quality so that the circuit can be formed with high accuracy.

  In addition, in the flexible printed wiring board with a protective layer of the present invention, it is considered that the first and second polyimide resin layers 3 and 5 are preferably made of the same material when only curling is considered. In consideration of mounting of a semiconductor chip by solder or anisotropic conductive film, the roles of the first polyimide resin layer 3 and the second polyimide resin layer 5 may be different from each other. In addition, the second imide resin layer 5 needs to be etched with an alkaline solution, as will be described later. For this reason, the first polyimide resin layer 3 and the second polyimide resin layer 5 may be sufficiently formed using different materials. Therefore, considering only curl reduction, it is considered best to use exactly the same polyamic acid as the material for the first and second polyimide resin layers. It may be necessary to specify the polyimide resin constituting the second polyimide resin layer 5 based on the difference in the coefficient of thermal expansion from the polyimide resin constituting the first polyimide resin layer 3.

  Further, as described above, since the second polyimide resin layer 5 is formed by applying polyamic acid, the selection of the resist can be performed as in the conventional manufacturing method using an adhesive or a resist. There are no problems such as curling caused by errors, peeling of resist or lowering of folding resistance, adhesive sticking out, insufficient adhesive force, deterioration of dimensional accuracy, etc. Flatness is obtained.

Moreover, in the flexible printed wiring board 1 with a protective layer of this invention, the polyimide resin which comprises at least any one of the said 1st polyimide resin layer 3 and the 2nd polyimide resin layer 5, and an imide preferably the difference in linear thermal expansion coefficient between the copper foil 2 after the heat treatment of equalization is ^{2.0 × 10 -6 /K~10.0×10 -6 / K} , more preferably 2.0 × 10 ⁻⁶ / K to 5.0 × 10 ⁻⁶ / K is preferable.

This is because the thermal expansion coefficient of the copper foil 2 increases by about 2.0 × 10 ⁻⁶ / K as described above after the heat treatment for imidization. Further, when the difference in the coefficient of thermal expansion between the polyimide resin constituting the polyimide resin layers 3 and 5 and the copper foil 2 is 10.0 × 10 ⁻⁶ / K or more, the curl and flatness of the flexible printed wiring board Is inferior.

Moreover, in order to make the difference in thermal linear expansion coefficient of the polyimide resin from that of the copper foil 2 2 × 10 ⁻⁶ to 10 × 10 ⁻⁶ / K, the polyimide resin layer 3 of the present invention, 3 linear thermal expansion coefficient of each polyimide resin constituting the 5, preferably ^{20 × 10 -6 / K~30 × 10} -6 / K, more preferably ^{21 × 10 -6 / K~24 × 10} -6 / K Is good.

  Thus, in the flexible printed wiring board 1 with a protective layer of the present invention, the thermal expansion coefficients of the polyimide resin constituting the copper foil 2 and the polyimide resin layers 3 and 5 are made as equal as possible, Since the amount of thermal expansion and the amount of thermal contraction between the conductor portions and between the conductor portions are substantially the same, the pattern accuracy of the fine circuit and the planarity of the fine circuit can be improved. Moreover, it becomes possible to suppress the occurrence of curling more effectively, and the heat resistance can be further improved.

  Moreover, the polyimide resin material which comprises the polyimide resin layers 3 and 5 which satisfy the above thermal expansion coefficients is a polyamic acid which is a condensation compound of an acid dianhydride and an aromatic diamine as shown below. It is formed by imidization.

  Examples of acid dianhydrides include pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, bis (3,4-dicarboxyphenyl) sulfonic dianhydride, and bis (3,4-dicarboxyphenyl). Ether dianhydride, benzophenone tetracarboxylic dianhydride and the like can be used.

  Examples of the diamine include p-phenylenediamine, 4,4 diaminodiphenyl ether, 2,2-bis [4- (4-aminophenoxy) phenyl] propane, and 2,2-bis [4- (4-aminophenoxy) phenyl. Sulfone, 4,4-diaminobenzanilide, 1,4-bis (4-aminophenoxy) benzene and the like can be preferably used.

  In particular, the polyimide resin constituting the polyimide resin layers 3 and 5 of the present invention is not limited to the above, but the polyimide resin used as the insulating protective layer is a strong alkali in the land portion, that is, the terminal portion 2a. Since it forms by etching using aqueous solution and warm water, what melt | dissolves in a strong alkali aqueous solution easily in the state of the polyamic acid of a precursor is good. In this respect, a combination using pyromellitic dianhydride that is easily dissolved at a relatively low temperature is preferable.

  In addition, since it is thought that the carboxylic acid group of polyamic acid which is a condensation compound of said acid dianhydride and diamine corrodes copper foil 2 which is a conductor, a rust inhibitor is added to this polyamic acid. You can also. Moreover, the antirust agent can be expected to improve the adhesive strength at the interface of the copper foil 2 in addition to the antirust function.

  Examples of such a rust inhibitor include triazole compounds such as 3- (N-salicyloyl) amino-1,2,3-triazole, and imidazole compounds such as 2-methylimidazole and salts thereof. And it is preferable to set it as 1-10 weight part with respect to 100 weight part of polyamic acids as addition amount of such a rust preventive agent.

  Here, the polyimide resins constituting the polyimide resin layers 3 and 5 are disclosed in JP-A-60-157286, JP-A-60-243120, JP-A-63-239998, JP-A-1- No. 245586, Japanese Patent Laid-Open No. 3-123093, Japanese Patent Laid-Open No. 5-1390527, and the like, combinations of existing acid dianhydrides and aromatic diamines, their respective chemical structures, and blending ratios thereof By changing the linear expansion coefficient, the linear expansion coefficient can be freely adjusted and synthesized.

  In addition, as a flexible printed wiring board with a protective layer of this invention, the 1st and 2nd polyimide resin layers 3 and 5 as shown in FIG. 1 are not limited to those having a single-layer structure, but are shown in FIG. As described above, the polyimide resin layer may have a multilayer structure. At this time, each polyimide resin layer preferably has a multilayer structure of three or less layers. This is because if it is more than 4 layers, cost increases and it is not economical. On the other hand, if one polyimide resin layer is used, curling may not be 100%, but it can be used sufficiently depending on the application of the circuit board. Therefore, it is preferable that the polyimide layer has a structure of three layers or less.

Specifically, as the flexible wiring board 10 with a protective layer of the present invention, as shown in FIG. 2, a flexible printed wiring board 6 is formed by laminating three polyimide resin layers 3 a, 3 b, 3 c on a copper foil 2. The polyimide resin layer that is formed and that serves as the insulating protective layer 7 on the copper foil 2 may be formed as three polyimide resin layers 5a, 5b, and 5c. At this time, the polyimide resin layers 3a, 3b, 3c are intermediate layers of the polyimide resin layers 3a, 3c in that the thermal expansion coefficients of the polyimide resin layer 3a on the copper foil 2 side and the outer polyimide resin layer 3c are intermediate layers. It is preferable that the coefficient of thermal expansion of the polyimide resin layer 3b is higher. That is, the polyimide resin layer 3a on the copper foil 2 side and the outer polyimide resin layer 3c on the copper foil 2 side become a high thermal linear expansion polyimide resin layer, and the intermediate polyimide resin layer 3b has a low thermal linear expansion property. It becomes a polyimide layer. However, the difference between the thermal linear expansion coefficient of the polyimide resin layers 3a and 5a on the copper foil 2 side and the outer polyimide resin layers 3c and 5c and the thermal linear expansion coefficient of the polyimide resin layers 3b and 5b located in the center is It is preferable that it is not larger than 3 × 10 ⁻⁶ / K. By setting it as such a structure, while being able to improve the adhesive strength with the copper foil 2, the curling of the flexible printed wiring board 6 can be suppressed.

  In particular, it is more preferable that the thermal linear expansion coefficient of the polyimide resin layers 3a and 5a on the copper foil 2 side is slightly larger than the thermal linear expansion coefficient of the outer polyimide resin layers 3c and 5c. This is because the surface roughness of the copper foil 2 affects the curl.

  As described above, by forming the polyimide resin layer in a multilayer structure, the thermal linear expansion coefficient can be adjusted to a value closer to the thermal linear expansion coefficient of the copper foil, and curling can be controlled effectively. In particular, the curl can be controlled by the thickness of the polyimide resin in the outer layer with respect to the copper foil 2.

  In addition, in order to improve the adhesive force so as not to cause delamination between individual polyimide resins forming the multilayer structure, or to improve the adhesive force with the copper foil 2, an epoxy resin may be added. As this epoxy resin, for example, general-purpose epoxy resins such as bisphenol A type and novolak phenol type can be used. Although an epoxy resin curing agent is not necessarily required, it may be added. In that case, a hardening | curing agent is mix | blended with a polyamic acid solution.

Further, the thermal linear expansion coefficient of the polyimide resin layer forming the multilayer structure is 2 × 10 ^{−6 in} the difference from the thermal linear expansion coefficient of the copper foil 2 as in the case of the polyimide resin layers 3 and 5 having a single layer structure. / K～10 is preferably × ^{10 -6} / K, and more preferably a good ^{2 × 10 -6 / K~5 × 10} -6 / K.

The linear thermal expansion coefficient of each polyimide resin constituting the polyimide resin layer of the multilayer structure may be a ^{20 × 10 -6 / K~55 × 10} -6 / K, preferably 20 × ^{10 -6} / K~30 × ¹⁰ -6 / K, it is better and more preferably from ^{21 × 10 -6 / K~24 × 10} -6 / K. If the thermal expansion coefficient of the single layer polyimide resin constituting the layer is less than 20 × 10 ⁻⁶ / K, the adhesive strength to the copper foil 2 becomes insufficient when the resin is applied to the copper foil 2. Moreover, when the thermal linear expansion coefficient of the polyimide-based resin constituting one layer exceeds 55 × 10 ⁻⁶ / K, the difference in thermal linear expansion coefficient from the copper foil 2 is 2.0 × 10 ⁻⁶ / K to 10.0 ×. A polyimide resin layer satisfying 10 ⁻⁶ / K cannot be formed.

However, the second polyimide resin layer 5 serving as an insulating protective layer has a difference of 2.0 × 10 ⁻ after the imidization in the case where the thickness of the second polyimide resin layer 5 may be thin. ^{6 /K~10.0×10 -6} / K, preferably, a polyamic acid having a linear thermal expansion coefficient of ^{2.0 × 10 -6 /K~5.0×10 -6 / K} , the inter-layer withstand voltage A single layer may be applied and formed so as to satisfy the product requirements. Usually, the second polyimide resin layer 5 has a film thickness of 3 μm to 10 μm. When the thickness is 3 μm or less, the withstand voltage is inferior, and when the thickness is 10 μm or more, curling occurs in the circuit. Therefore, when coating with a film thickness of 10 μm or more, two or more layers are coated, and curling is controlled by the thickness of the outer layer.

  Next, details of the method for producing the flexible printed wiring board of the present invention having the above-described configuration will be described with reference to the drawings. In the following, a flexible printed wiring board with a protective layer having a single-layered polyimide resin layer shown in FIG. 1 is taken as an example, but a protective layer having a polyimide-based resin layer with a multilayer structure as shown in FIG. The attached flexible printed wiring board can be similarly manufactured by laminating a polyimide resin layer.

  First, in order to manufacture the flexible printed circuit board 5, the copper foil 2 which is a conductor as shown in FIG. 3 is prepared.

  Next, a polyamic acid solution which is a precursor of the polyimide resin layer formed on the copper foil 2 is synthesized and adjusted as follows.

  As a method for synthesizing the polyamic acid solution, the acid dianhydride and the aromatic diamine as described above are reacted in a polar solvent. Since this reaction is an exothermic reaction, the reaction is controlled while cooling as necessary. Usually, the reaction is carried out at about 0 ° C to 90 ° C, preferably about 5 ° C to 50 ° C. When the viscosity of the solution is high, the viscosity can be lowered by heat treatment at a temperature close to 90 ° C.

  At this time, the acid dianhydride and the aromatic diamine may be added at the same time, or one of them may be dissolved or suspended in a polar solvent first, and the other may be reacted while gradually added. good. The molar ratio between the acid dianhydride and the aromatic diamine is preferably equimolar, but either one of both components may be used in an excess amount within the range of about 10: 9 to 9:10. As the polar solvent, pyrrolidone solvents such as N-methyl-2-pyrrolidone, acetamide solvents such as N, N′-dimethylacetamide, and phenol solvents such as cresol can be used. In view of the above, use of N-methyl-2-pyrrolidone is particularly preferable. Also, xylene, toluene, ethylene glycol monoethyl ether, etc. can be mixed and used.

  As described above, a polyamic acid solution that is a precursor of the polyimide resin layer is synthesized.

Next, the polyamic acid solution synthesized in this way is applied onto the copper foil 2 as shown in FIG. 4 and then dried to form the polyamic acid layer 13. In this case, the polyamic acid solution is coated on the copper foil 2, linear thermal expansion coefficient after imidization ^{20 × 10 -6 / K~30 × 10} -6 / K, more preferably 21 × ¹⁰ -6 / K It is preferable to be ˜24 × 10 ⁻⁶ / K.

  Here, as a method of applying the polyamic acid solution, a conventionally used method such as a knife coater, a comma coater, a gravure coater, or a wire coater having a blade can be used. Moreover, it does not specifically limit as drying temperature in the temperature which does not produce the foam accompanying scattering of a solvent.

  In order to fabricate a multilayer resin layer 3a, 3b, 3c having a multilayer structure as shown in FIG. 2, a polyamic acid solution is further applied on the layer coated with the polyamic acid solution. What is necessary is just to repeat the process of drying sequentially.

  Specifically, first, the layer made of the applied polyamic acid solution is dried so that the residual solvent amount in this layer is 50 wt% to 80 wt%. Here, when the amount of the remaining solvent is 50% by weight or less, delamination occurs from the interface where the layers are applied. Therefore, for example, when N-methyl-2-pyrrolidone is used as the polar solvent of the polyamic acid solution, the boiling point of N-methyl-2-pyrrolidone is 204 ° C., so that the residual solvent amount is 80% by weight or less. Needs to have a maximum temperature of 170 ° C.

  Then, after the polyamic acid solution is applied in multiple layers, finally, it may be dried at 230 ° C. in order to make the residual solvent amount 0% in the entire layer made of the multilayer polyamic acid solution. At this time, a part of the polyamic acid solution is imidized, but in order to finally obtain a polyimide-based resin layer in which the whole is imidized to a desired level, imidization is performed by a heating step described later. .

  Next, this polyamic acid layer 13 was imidized by heat-treating the polyamic acid layer 13 at 280 ° C. to 350 ° C., and as shown in FIG. 5, the imidization rate was 80% or more. The first polyimide resin layer 3 is formed, and the flexible printed circuit board 11 is obtained.

  The surface of the flexible printed board 11 obtained in this way may not be flat as long as it does not affect the circuit formation. In particular, when the surface of the copper foil 2 is faced upward, the slightly convex curl absorbs the formation of the second polyimide-based resin layer 5 that is a subsequent insulating protective layer and the shrinkage during imidization. And the planarity of the flexible printed wiring board with a protective layer finally obtained can be obtained. However, as the degree of the convex curl described above, the radius of curvature of a sample having a size of 100 mm × 100 mm is preferably 100 mm or more.

  Next, a predetermined circuit pattern is formed on the copper foil 2 of the flexible printed board 11 formed by such a method.

  Specifically, a liquid resist is applied and dried on the copper foil 2 of the flexible printed circuit board 11 by an ordinary substruct method, and a desired pattern is exposed to the resist using ultraviolet rays. The development process is performed. As a result, the resist portion exposed to the ultraviolet rays is removed by the development process, and a resist 12 having a desired pattern is formed on the copper foil 2 as a mask as shown in FIG.

  Then, the copper foil 2 is etched with a normal etching solution such as a cupric chloride aqueous solution to form a desired uneven pattern on the copper foil 2. Thereafter, by removing the mask made of the resist 12, a circuit pattern is formed on the copper foil 2 as shown in FIG. 7, and the flexible printed wiring board 4 is obtained.

  Next, in order to protect the circuit pattern formed on the copper foil 2, a polyamic acid is applied on the flexible printed wiring board 4 in the same process as the first polyimide resin layer forming process, By drying, a polyamic acid layer 14 is formed as shown in FIG.

The polyamic acid layer 14 is imidized to become a second polyimide resin layer. That is, the polyamic acid layer 14 is a precursor of a polyimide resin, and the polyimide resin preferably has a thermal linear expansion coefficient of 21 × 10 ^{−6 to} 24 × 10 ⁻⁶ / K. The difference between the thermal expansion coefficient of the polyimide resin forming the first polyimide resin layer 3 is preferably 3 × 10 ⁻⁶ / K or less. Therefore, the polyamic acid layer 14 is formed using a polyamic acid such that the polyimide resin after imidization satisfies the condition of the thermal linear expansion coefficient.

  Next, the polyamic acid layer 14 formed on the copper foil 2 is etched with an alkaline aqueous solution conventionally used in a semiconductor manufacturing process or the like to form land portions and terminal portions.

  Specifically, a photoresist that can be developed with a neutral or weakly acidic aqueous solution and is excellent in alkali aqueous solution resistance is applied on the polyamic acid layer 14 so that the thickness after solvent drying is about 10 μm. Examples of this photoresist include NR-41 (nylon-oligoester, manufactured by Sony Chemical Corporation). Then, a predetermined pattern is exposed to the photoresist using a laser beam, and then a development process is performed on the photoresist. As a result, the photoresist portion exposed to the laser beam is removed by the developing process, and the photoresist 15 is formed as a mask as shown in FIG.

  Then, the polyamic acid layer 14 is etched by using a 10% caustic potash aqueous solution and warm water in combination, and then the photoresist 15 is peeled off with a strong acid aqueous solution to form the terminal portion 2a on the polyamic acid layer 14 as shown in FIG. Form.

  Finally, the polyamic acid layer 14 is subjected to heat treatment at 280 ° C. to 350 ° C. for about 30 minutes to 120 minutes to imidize the polyamic acid layer 14, and as shown in FIG. The polyimide resin layer 5 of 2 is formed, and the flexible printed wiring board 1 with a protective layer of this invention as shown in FIG.1 and FIG.12 is obtained. Here, FIG. 1 is a cut surface taken along the broken line A0-A1 in FIG.

  In addition, although imidation at the time of forming the 1st polyimide resin layer 3 is performed at 250 to 350 degreeC, imidation does not necessarily need to be achieved 100%. This is because the first polyimide resin layer 3 is also heated in imidization when forming the second polyimide resin layer 5 as an insulating protective layer, and therefore excessive thermal energy is This is because the adhesive strength may be reduced. Therefore, the imidation rate in the first polyimide resin layer 3 in the previous stage of forming the second polyimide resin layer 5 is a conductor circuit that does not affect shrinkage caused by insufficient imidization. Is preferably about 80%. Then, after the polyamic acid layer 14 which is the precursor of the second imide layer is applied, as shown in FIG. 11, the polyamic acid layer 14 and the first polyimide resin layer 3 are still imidized. 100% imidization is performed at 250 ° C to 350 ° C.

  Examples of the present invention will be described below based on specific experimental results.

  First, polyamic acid was synthesized as follows. And in order to measure the thermal linear expansion coefficient of the polyimide resin formed by imidizing the synthesized polyamic acid solution, the following polyimide film was prepared.

Experimental example 1
<Synthesis of polyamic acid solution>
First, as shown in Table 1, 0.866 kg (8.00 mol) of paraphenylenediamine (PDA, manufactured by Mitsui Chemicals) and 4,4′-diamino were added to a 60 liter reaction kettle with a temperature-controllable jacket. Diphenyl ether (DPE, manufactured by Wakayama Seika) 1.603 kg (8.00 mol) was dissolved in about 44 kg of a solvent N-methyl-pyrrolidone (NMP, manufactured by Mitsubishi Chemical) under a nitrogen gas atmosphere. Then, it was made to react for 3 hours, adding 3.523 kg (16.14 mol) of pyromellitic dianhydride (PMDA, Mitsubishi Gas Chemical Co., Inc.) gradually at 50 degreeC. In this manner, a polyamic acid solution having a solid content of about 12% and a viscosity of 20 Pa · S at 25 ° C. was obtained. Here, this polyamic acid solution is referred to as a synthesis sample 1.

  Next, this polyamic acid solution was applied onto a copper foil so that the thickness of the film after imidization was 25 μm, and after the solvent was scattered in a continuous furnace at 80 ° C. to 170 ° C., 230 ° C. to 350 ° C. The mixture was heated to 350 ° C. for 30 minutes and imidized.

Thereafter, a part of the copper foil was etched with a cupric chloride solution to produce a polyimide film. And when the coefficient of thermal expansion of the obtained polyimide film was measured, it was 20 × 10 ⁻⁶ / K.

Experimental Example 2 to Experimental Example 12
Next, a polyamic acid solution was synthesized in the same manner as described above by changing the kind of acid dianhydride, the kind of diamine, and the ratio of diamine to those shown in Table 1. At this time, the synthesized polyamic acid solution is referred to as synthesis sample 2 to synthesis sample 12.

  And the polyimide film was produced like the said process using the polyamic acid solution of these synthetic samples 2-12 and the thermal expansion coefficient of each produced polyimide film was measured. Table 1 shows the measurement results of the thermal linear expansion coefficient.

  Next, a flexible printed wiring board was produced using the polyamic acid solution obtained as described above.

Example 1
<Production of flexible printed circuit board and flexible printed wiring board>
First, an electrolytic copper foil (trade name: CF-T9-LP, manufactured by Fukuda Metal Co., Ltd.) having a thickness of 18 μm as shown in Table 2 was prepared, and a synthetic sample 2 was prepared on the copper foil as shown in Table 3. The polyamic acid solution was applied to a thickness of 3 μm and the solvent was scattered to form a polyamic acid layer. The residual solvent content was 45%.

  Next, similarly, as shown in Table 3, the polyamic acid solution of Synthesis Sample 1 was applied so that the thickness after imidization was 18 μm. The residual solvent content of the first layer and the second layer was 75%.

Next, similarly, as shown in Table 3, the polyamic acid solution of Synthesis Sample 2 was applied on the second polyamic acid layer so that the thickness after imidization was 4 μm. The residual solvent content of these three layers was 70%.

  Thereafter, heat treatment was performed at 230 ° C. for 5 minutes. The residual solvent content was 1% or less. Furthermore, the temperature was raised to 350 ° C. over 1 hour in an atmosphere of nitrogen gas, and the resultant was baked at a temperature of 350 ° C. for 15 minutes to produce a flexible printed board.

  Next, a liquid resist (trade name: RX-20, manufactured by Tokyo Ohka Kogyo Co., Ltd.) is applied onto the copper foil of the flexible printed circuit board manufactured by the above method, followed by drying, exposure, and development steps. The copper foil was etched with a copper aqueous solution to form a round land having a diameter of 5 mm and a parallel circuit having a pitch of 100 μm and a width of 50 μm, thereby producing a flexible printed wiring board. In the current etching technique, the pitch of the above size is the minimum pitch. That is, a circuit having the smallest pitch possible with the current etching technique was formed.

  The following evaluation test was performed on the flexible printed wiring board obtained as described above, and the measurement results are shown in Table 4.

Method of evaluation test (1) Measurement of coefficient of thermal expansion The thermal mechanical analyzer (TMA / SCC150CU, manufactured by S11) was used and a tensile method with a load of 2.5 to 5.0 gr was applied. The thermal expansion coefficient was measured according to the measurement data of the range.

(2) Measurement of imidization rate The amount of absorbance at an absorption wavelength of 1780 cm-1 of the imide group by infrared absorption analysis was calculated from the percentage with respect to the amount of absorbance of the imide group when the sample was imidized 100%.

(3) Measurement of curl A flexible printed wiring board was cut into a size of 100 mm × 100 mm, and the radius of curvature was calculated from the average of the heights of the four corners when placed on a horizontal plate in a convex state.

<Preparation of a flexible printed wiring board with a protective layer by forming an insulating protective layer>
Next, a second polyimide resin layer was formed as an insulating protective layer between conductors of parallel conductors with a pitch of 100 μm in the flexible printed wiring board as described above. Specifically, in the formation of the second polyimide resin layer, a three-layer polyamic acid layer having a thickness as shown in Table 3 was formed using a synthetic sample as shown in Table 3, respectively.

  Next, the three-layer polyamic acid layer formed on the copper foil was etched by using a 10% caustic potash aqueous solution and warm water in combination to form land portions and terminal portions.

  Finally, the polyamic acid layer is subjected to a heat treatment at 280 ° C. to 350 ° C. for about 30 minutes to 120 minutes, so that the polyamic acid layer is imidized to form a second polyimide resin layer as an insulating protective layer. The formed flexible printed wiring board with a protective layer of the present Example was obtained.

  Here, the thermal expansion coefficient of the insulating protective layer was also prepared in the same manner as the method for measuring the thermal linear expansion coefficient of the flexible printed wiring board before forming the insulating protective layer, and a polyimide film corresponding to the insulating protective layer was prepared. The thermal expansion coefficient of the film was measured. The results are shown in Table 4.

  The following evaluation test was conducted on the flexible printed wiring board with a protective layer obtained as described above, and the measurement results are shown in Table 4.

Method of evaluation test (4) Presence / absence of curling and flatness The entire flexible printed wiring board with protective layer was visually observed to evaluate the presence or absence of curling. In addition, the presence or absence of flatness was evaluated by visually observing the presence or absence of shrinkage or swelling occurring locally between the conductor portions of the flexible printed wiring board and the conductors.

(5) Measurement of circuit pitch The conductor width and the conductor interval were measured using a three-dimensional measuring machine.

(6) Electrical reliability evaluation An anisotropic conductive film (trade name: CP7131, manufactured by Sony Chemical Corporation) was used to connect the conductor part of the produced flexible printed wiring board with protective layer and the conductor part of ITO glass. And pasted together. And the electrical resistance value when this was left to stand in 85 degreeC 85% atmosphere was measured. In the measurement results, 0.5Ω or less was accepted.

Examples 2 to 18 and Comparative Examples 1 to 5
Flexible as in Example 1 except that the polyimide resin layer supporting the copper foil on the flexible printed wiring board side and the polyimide resin layer as the insulating protective layer were formed to have the composition and thickness shown in Table 3. A printed wiring board and a flexible printed wiring board with a protective layer using the printed wiring board were produced, and the same evaluation test as in Example 1 was performed.

<Results of evaluation test>
As shown in the results of Table 4, the difference in the coefficient of thermal expansion between the polyimide resin layer supporting the copper foil of the flexible printed wiring board and the polyimide resin layer as the insulating protective film is 3 × 10 ⁻⁶ / K or less. It can be seen that Examples 1 to 16 are free from curling, have good flatness without causing shrinkage or swelling locally between conductor portions and conductors, and have high electrical reliability.

On the other hand, Comparative Example 4 and Comparative Example 5 in which the difference in the coefficient of thermal expansion between the polyimide resin layer supporting the copper foil of the flexible printed wiring board and the polyimide resin layer as the insulating protective film is 3 × 10 ⁻⁶ / K or more. Has curls and is inferior in flatness and electrical reliability.

Therefore, when the difference in the coefficient of thermal expansion between the polyimide resin layer supporting the copper foil of the flexible printed wiring board and the polyimide resin layer as the insulating protective film is 3 × 10 ⁻⁶ / K or less, the curl is reduced. It was found that the flatness was good and electrical reliability could be secured.

In Example 17 and Example 18, as shown in Table 1, the difference in thermal expansion coefficient between polyimide resin layers is 3 × 10 ⁻⁶ / K, but the flatness and electrical reliability are insufficient. It is. This is because the thermal expansion coefficient of the polyimide resin constituting the polyimide resin layer is as very low as about 10 × 10 ⁻⁶ / K, and therefore the adhesive strength to the copper foil is insufficient. Therefore, from this, in the flexible printed wiring board having a copper foil, the thermal expansion coefficient of each polyimide resin constituting the polyimide resin layer is 20 × 10 ^{−6 to} 55 × 10 ⁻⁶ / K, preferably 20 × it is found to be a ^{10 -6 ~30 × 10 -6 / K} .

  Furthermore, in Comparative Examples 1 to 3 in which the insulating protective layer was not formed, the curl was large. In particular, in Comparative Example 3 in which the polyimide resin layer is formed using a polyimide resin having a small coefficient of thermal expansion, curling cannot be completely removed because there is no insulating protective layer. From this, it can be seen that the curl can be removed more effectively by forming the insulating protective layer in which the difference in the coefficient of thermal expansion is defined as described above.

It is sectional drawing of an example of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing of the other example of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows 1 process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows the other process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows the other process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows the other process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows the other process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows the other process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows the other process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows the other process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows the other process of the manufacturing process of the flexible printed wiring board with a protective layer to which this invention is applied. It is a perspective view of the flexible printed wiring board with a protective layer to which this invention is applied. It is sectional drawing which shows an example of the conventional 2 layer flexible printed circuit board. It is sectional drawing which shows an example of the conventional flexible printed wiring board with a protective layer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Flexible printed wiring board with a protective layer, 2 Copper foil, 3 1st polyimide resin layer, 5 2nd polyimide resin layer

Claims (1)

With the first polyimide resin layer for supporting the copper foil on one surface of the patterned copper foil is formed in accordance with the circuit, to protect and cover the circuit on the other surface of the copper foil In the flexible printed wiring board in which the second polyimide resin layer is formed,
The difference between the thermal linear expansion coefficient of the first polyimide resin layer and the thermal expansion coefficient of the second polyimide resin layer is 3 × 10 −6 / K or less, and the first polyimide resin layer and In the second polyimide resin layer, a rust inhibitor is added to polyamic acid which is a precursor of polyimide resin,
At least one of the first polyimide resin layer and the second polyimide resin layer has a structure in which three polyimide resin layers are laminated, and the three polyimide resin layers The layer is made larger than the thermal linear expansion coefficient of the polyimide resin layer located in the center of the thermal expansion coefficient of the polyimide resin layer on the copper foil side and the outside,
The difference between the thermal linear expansion coefficient of the copper foil and the thermal linear expansion coefficient of at least one of the first polyimide resin layer and the second polyimide resin layer is 2 × 10 −6 / A flexible printed wiring board having a range of K to 10 × 10 −6 / K.

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