JP2009064872A - Method of forming metal thin film on front surface of polyimide resin surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of forming a metal thin film having a comparatively high close contact strength to a polyimide resin without giving damage to the polyimide resin and metal thin film. <P>SOLUTION: The method of forming the metal thin film on a polyimide resin surface includes the steps of; forming a reformed layer by opening an imide ring at the front surface of polyimide resin using alkali solution; adsorbing metal ion to the reformed layer using a solution including metal ion; reducing metal ion adsorbed to the reformed layer with reducing solution; replacing the metal ion remaining in the reformed layer after the reducing step with the alkali metal ion using solution including an alkali metal; replacing the alkali metal ion with the hydrogen ion using acidic solution; and obtaining imide again by closing the imide ring. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for forming a metal thin film on a polyimide resin surface.

  In recent years, as electronic devices have become smaller and more portable, flexible wiring boards have become thinner and denser. As a manufacturing method of a flexible wiring board, research on a polyimide metal laminate by a direct plating method has been actively conducted. The procedure shown in Patent Document 1 is shown below using a flowchart (FIG. 21).

(1) The polyimide resin 101 is treated with an alkaline solution (for example, 5M KOH), and the imide ring of the polyimide resin is opened to generate an ion exchange group (for example, a carboxyl group). The generated layer is the modified layer 102.
(2) Treating the polyimide resin into which the ion exchange group is introduced with a metal ion-containing liquid, and the metal ion is introduced by the group having the ion exchange ability introduced in (1) performing an ion exchange reaction with the metal ion. Is done. The modified layer into which metal ions have been introduced is the metal ion impregnated modified layer 211.
(3) The polyimide resin introduced with metal ions is immersed in a reducing agent solution to form a metal film 312 on the surface of the polyimide resin. At this time, in the modified layer 211, the amount of metal ions contained in the modified layer decreases (modified layer 212).

(4) The copper layer 411 for circuit formation is formed using an electrochemical method (electrolytic plating, electroless plating).
(5) An etching resist layer is formed on the surface of the copper layer 411, and the circuit shape is exposed, developed and etched. The etched copper layer is 412.
(6) The metal component is removed from the carboxyl metal salt remaining on the polyimide surface by acid treatment. The polyimide surface from which the metal component was removed is 413.
(7) A heat treatment is performed for 10 to 80 minutes in a high temperature atmosphere of 180 to 200 ° C. Since the polyimide surface 413 from which the metal component has been removed in the step (6) is in a state where the imide ring is opened, the ring is closed by heat treatment.

The procedure shown in Patent Document 2 is shown below using a flowchart (FIG. 22).
(1) Modification step to (5) Circuit pattern formation step is the same as that of Patent Document 1.
(6) The polyimide resin (part corresponding to 415 in FIG. 22) at the exposed part of the wiring board is dissolved and peeled off by alkali treatment.
(7) Treatment with acid is performed for the purpose of neutralization and removal of metal components remaining on the polyimide resin at the exposed portion of the wiring board. (Part corresponding to 416 in FIG. 22)
(8) A heat treatment is performed for 10 to 80 minutes in a high-temperature atmosphere of 180 to 200 ° C. Since the polyimide surface 416 from which the metal component has been removed in the step (7) is in a state where the imide ring is opened, the ring is closed by heat treatment. The closed ring is 417.
Japanese Patent Laid-Open No. 2004-6684 JP 2004-319918 A

  In the case of Espanex (Nippon Steel Chemical Co., Ltd.), for example, in the case of Espanex (Nippon Steel Chemical Co., Ltd.), the adhesion strength between the polyimide resin and the copper foil of the polyimide resin-based copper-clad laminate currently on the market is normally 14 to 15 N / cm 0.5 kgf / cm), and such adhesion strength must be targeted. However, in the processes described in Patent Documents 1 and 2, the adhesion strength between the metal layer and the polyimide resin layer is 0.97 kgf / cm in a normal state and 0.64 kgf / cm after heating. It was a low value with respect to the tension laminate.

  For example, after forming an electrolytic copper plating film having a thickness of about 20 μm on a polyimide resin substrate having a metal thin film obtained through steps (1) to (3) of Patent Document 1, the sample was simply air blown. In the state, the peeling test of the copper thin film was conducted. As a result, a strength of about 10 N / cm was obtained, but a sample prepared in the same manner was dried as it was at room temperature for 2 days and then subjected to a peeling test. As a result, the strength decreased to 3 N / cm. did. When the copper thin film of the sample was etched, it was found that the polyimide modified layer had numerous cracks. This is because when an electrolytic copper plating film was formed on the surface of the polyimide modified layer in a swollen state, a crack occurred between the copper thin film and the polyimide modified layer when the polyimide modified layer contracted with drying. It is thought that the adhesion between the copper thin film and the polyimide modified layer was weakened. Such cracks in the modified polyimide layer were particularly prominent when the modified layer was thick or when the amount of metal ions remaining in the modified layer was large.

  According to Patent Documents 1 and 2, residual metal on the surface of the polyimide resin exposed between the wirings after forming the wiring layer is removed, or the polyimide resin exposed between the wirings is removed. That is, after a metal layer (copper layer) is formed on a metal thin film on the surface in a state where metal ions are contained in the polyimide modified layer, a circuit pattern is formed and an exposed portion between wirings is processed. Therefore, metal ions remain in the modified layer immediately below the metal wiring layer. When metal ions remain in the modified layer, re-imidization hardly occurs even by heat treatment, and a modified layer having relatively low strength remains, so that the modified layer is more easily cracked by drying and has poor adhesion. It is considered to be.

  In the direct plating method, it is necessary to reduce the metal ions remaining in the polyimide modified layer after the reduction treatment as much as possible, or to make it “0 (zero)”. In the reduction treatment, if all Cu ions are deposited as metal Cu, Cu ions do not remain in the polyimide resin. In order to reduce all Cu ions, there are methods such as increasing the reducing agent's reducing power, that is, increasing the concentration of the reducing agent, raising the temperature of the reducing agent, and extending the reduction time. However, since the polyimide modified layer is actually damaged, it is difficult to select these methods. Therefore, when the reduced polyimide resin with a metal thin film is immersed in an acid solution and the operation of substituting metal ions and hydrogen ions is performed, the residual metal ions cannot be sufficiently exchanged with hydrogen ions only by the acid treatment. It was. If the acid treatment time was increased, residual metal ions could be removed, but the metal thin film deposited in the reduction process was dissolved.

  The present invention has been made in view of the above problems, and provides a method for forming a metal thin film having relatively high adhesion strength to a polyimide resin without damaging the polyimide resin or the metal thin film. Objective.

The present invention
Treating the polyimide resin surface with an alkaline solution to open the imide ring and forming a modified layer;
Treating the modified layer with a metal ion-containing solution;
Treating the modified layer with a reducing bath;
Treating the modified layer with an alkali metal-containing solution;
Treating the modified layer with an acid solution; and a re-imidation step for closing an imide ring;
The present invention relates to a method for forming a metal thin film on the surface of a polyimide resin.

The effects of the metal thin film forming method on the polyimide resin surface of the present invention are as follows.
(1) A relatively high value, for example, 14 N / cm, can be achieved as the adhesion strength between the metal thin film and the polyimide resin, and a metal thin film with high adhesion can be formed on the polyimide resin surface. For example, when manufacturing a flexible wiring board, the adhesive strength of the metal thin film as an electrolytic Cu plating base and polyimide resin is as high as 12 N / cm, and a highly reliable flexible wiring board can be obtained.
(2) Since the modified layer effectively returns to the original polyimide molecular structure, the mechanical strength, heat resistance, and chemical resistance of the substrate itself are also improved.
As a result, when the method for forming a metal thin film on the polyimide resin surface of the present invention is applied to a method for manufacturing a polyimide wiring board, particularly a flexible wiring board, a highly reliable wiring board can be manufactured at low cost. It can greatly contribute to the generalization of polyimide wiring boards in the field.

Hereinafter, the best mode for carrying out the present invention will be described.
The metal thin film forming method on the polyimide resin surface of the present invention uses a so-called direct plating method (direct metallization).

The method for forming a metal thin film on the polyimide resin surface of the present invention is detailed below.
(1) A step of treating the surface of the polyimide resin with an alkaline solution to open the imide ring to form a modified layer;
(2) treating the modified layer with a metal ion-containing solution to adsorb the metal ions to the modified layer;
(3) treating the modified layer with a reducing bath to reduce metal ions adsorbed on the modified layer;
(4) A step of treating the modified layer with an alkali metal-containing solution to replace metal ions remaining in the modified layer after the reduction step with the alkali metal ions;
(5) treating the modified layer with an acid solution to replace the alkali metal ions with hydrogen ions; and (6) a re-imidization step for closing the imide ring;
It is characterized by including.

  Hereinafter, the present invention will be described with reference to FIGS. FIG. 1 is an example of a flow diagram showing a manufacturing process of a method for forming a metal thin film on a polyimide resin surface according to the present invention, and FIGS. 2 to 8 are schematic views of a polyimide resin (substrate) for explaining the manufacturing process. It is an example of sectional drawing.

(1) Modification step In the present invention, when the direct plating method is used, the polyimide resin surface needs to be subjected to a ring-opening treatment in order to form a metal thin film. Specifically, by immersing a polyimide resin 1 as shown in FIG. 2 in an alkaline solution, a modified layer 2 is formed on both sides as shown in FIG. 3 and washed with water. By this step, the imide ring of the polyimide molecule on the polyimide resin surface is opened by hydrolysis, and a carboxyl group is generated. At the same time, hydrogen ions of the carboxyl group are replaced with metal ions in the alkaline solution, thereby forming a modified layer. The For example, when a polyimide represented by the chemical formula (1) is treated with an aqueous KOH solution, potassium ions are adsorbed on the carboxyl group generated by ring opening by hydrolysis, and have a structure represented by the chemical formula (2). Become.

  The structure of the polyimide resin is not particularly limited. For example, a polyimide resin produced by reacting pyromellitic dianhydride and an aromatic diamine as represented by the above chemical formula (1) can be used. It is. Moreover, for example, a polyimide resin produced by reacting pyromellitic dianhydride or biphenyltetracarboxylic dianhydride with an aromatic diamine can be used. In any polyimide resin, the aromatic diamine is not particularly limited, and for example, those containing 4,4'-diaminodiphenyl ether and paraphenylenediamine are preferable.

  The form of the polyimide resin is not particularly limited, and any polyimide resin can be used from a flexible film-like one to a rigid board-like one, but from the viewpoint of manufacturing a flexible wiring board, the thickness A film having a flexibility of 10 to 200 μm is preferably used. When using a polyimide substrate, a commercially available product can be used. For example, Apical (R) (manufactured by Kaneka), Kapton (R) (manufactured by Toray DuPont), Upilex (R) (manufactured by Ube Industries), etc. are available. It is. In particular, as a polyimide resin produced by reacting pyromellitic dianhydride and biphenyltetracarboxylic dianhydride with 4,4′-diaminodiphenyl ether and paraphenylenediamine, Kapton EN (R) (manufactured by Toray DuPont) ) Is available.

  The alkaline solution is not particularly limited as long as the imide ring of the polyimide can be opened. For example, an aqueous solution containing an alkali metal hydroxide such as K or Na, or an alkaline earth metal hydroxide such as Mg or Ca. An aqueous solution containing the product can be used. The metal ion contained in the alkaline solution and substituted for hydrogen ion in this step is hereinafter referred to as metal ion A.

  The alkaline solution used in this step usually has a concentration of 3 to 10 M (mol / L), a solution temperature of 20 to 70 ° C., and the treatment time is obtained by reacting pyromellitic dianhydride and an aromatic diamine. It is 1 to 10 minutes for the manufactured polyimide resin. Preferably, the solution temperature is 30-50 ° C. and the treatment time is 3-7 minutes. It is 10 to 100 minutes for a polyimide resin produced by reacting pyromellitic dianhydride or biphenyltetracarboxylic dianhydride with an aromatic diamine. Preferably, the solution temperature is 30 to 50 ° C. and the treatment time is 30 to 70 minutes. If the treatment temperature is too high, there is a possibility of destroying the molecular structure other than the imide ring opening, and if the treatment time is too long, the thickness of the modified layer becomes thick and the strength as a polyimide wiring board may be reduced. .

  Washing with water is performed to remove the alkaline solution adhering to the polyimide resin surface. Therefore, washing with running water is desirable. Usually, the water is washed for 5 minutes or more at a water amount of 1 to 5 L / min.

(2) Metal ion adsorption step In this step, the modified layer 2 is treated with a metal ion-containing solution, and metal ions contained in the solution are adsorbed on the modified layer as shown in FIG. Specifically, the polyimide resin having the modified layer 2 is immersed in a metal ion-containing solution, and the metal ions A in the modified layer are replaced with the metal ions in the metal ion-containing solution, followed by washing with water. In FIG. 4, reference numeral 21 denotes a modified layer that adsorbs metal ions. For example, when the polyimide represented by the chemical formula (2) is treated with a copper ion-containing solution, it has a structure represented by the chemical formula (3).

The metal ions contained in the metal ion-containing solution are reduced in a process described later and deposited as a metal thin film, and are not particularly limited as long as they have a smaller ionization tendency than the metal ions A substituted in the modification process. For example, Ni ion, Cu ion, Co ion, Pd ion, Ag ion, Pt ion, Au ion, or a combination thereof can be used. As a solution containing such metal ions, specifically, for example, NiSO ₄ solution, CuSO ₄ solution, CoSO ₄ solution, PdSO ₄ aqueous solution, AgNO _{3 solution, H 2 [PtCl 6] ·} 6H 2 O ( chloride A platinum acid) aqueous solution, a KAu (CN) ₂ (potassium gold cyanide) aqueous solution, a NiCl ₂ aqueous solution, a mixture thereof, or the like can be used. The metal ions that are contained in the metal ion-containing solution and replace the metal ions A in this step are hereinafter referred to as metal ions B.

  The metal ion-containing solution used in this step usually has a concentration of 0.01 to 0.1 M (mol / L), a processing temperature of 20 to 30 ° C., and a processing time of 5 minutes or more. In particular, the treatment time in this step is appropriately set depending on the conditions for the reforming treatment in step (1). For example, when the thickness of the polyimide modified layer is about 5 μm, it is sufficient to perform the adsorption treatment for at least 5 minutes.

  Washing with water is performed to remove the metal ion-containing solution adhering to the polyimide resin surface. Since the metal ion-containing solution is usually acidic, if it is brought into the next step as it is, it causes a change in pH of the reducing agent. Further, the metal salt adhering to the polyimide resin surface is reduced in the reducing bath, and the reducing agent is decomposed and exhausted. Washing with water is usually performed at a water amount of 1 to 5 L / min under conditions of 5 minutes or more.

(3) Reduction step In this step, the modified layer 21 is treated with a reducing bath to reduce metal ions adsorbed on the modified layer 21. Specifically, by immersing a polyimide resin containing metal ions B in the modified layer 21 in a reduction bath, the metal ions B are reduced to deposit metal particles as shown in FIG. To do. In this step, not all the metal ions B in the modified layer are reduced, so that the metal ions B remain in the modified layer. The modified layer in which the metal ions B remain after the metal thin film is formed is shown as 22 in FIG.

  The reducing bath is not particularly limited as long as it is a liquid that can reduce the metal ion B by contact with the metal ion B, and an aqueous solution of a reducing agent used in general electroless plating can be used. Specifically, for example, an aqueous solution of a boron compound such as sodium borohydride or dimethylamine borane, sodium hypophosphite, or the like can be used. Although the temperature at the time of reduction varies depending on the reducing agent used, it is generally preferably 20 ° C to 50 ° C. If the temperature is too low, the nucleation reaction becomes more dominant than the metal nucleation reaction in the reduction reaction, and it precipitates near the outermost surface of the polyimide resin, and the anchor effect cannot be obtained. Further, if the temperature is too high, the modified layer is likely to be peeled off. The concentration of the reducing agent varies depending on the reducing agent used, but in the case of sodium borohydride, it is 0.0005 mol / L to 0.05 mol / L, desirably 0.001 mol / L to 0.01 mol / L. The immersion time is not particularly limited, and is usually 5 to 60 minutes, particularly 7 to 30 minutes. The reducing agent may be used not only as a single type but also as a mixture of a plurality of types. Moreover, you may add a pH adjuster, a complexing agent, etc. to a reducing bath as needed.

  As the metal ions are reduced, the modified layer in which the metal ions are adsorbed is in a state in which H ions and Na ions derived from the reducing bath are adsorbed on the carboxyl groups.

  After the reduction treatment, washing with water is usually performed to remove the reduction bath attached to the polyimide resin surface. Washing with water is performed at a water amount of 1 to 5 L / min for 5 minutes or more.

(4) Alkali metal-containing solution treatment step In this step, the modified layer 22 is treated with an alkali metal-containing solution to replace the metal ions B remaining in the modified layer 22 with the alkali metal ions. Specifically, the polyimide resin having the modified layer 22 is immersed in an alkali metal-containing solution, the metal ions B in the modified layer 22 are replaced with the alkali metal ions in the alkali metal-containing solution, and washed with water. When metal ions are contained in the modified layer 22 such as when metal ions are contained in the reducing bath used in the reduction step (3), in this step, such metals are used. Ions can also be substituted for alkali metal ions. In FIG. 6, reference numeral 23 denotes a modified layer containing alkali metal ions after the alkali metal-containing solution treatment.

In the present invention, the metal ions B (for example, Cu ions) are once replaced with alkali metal ions in this step, so that the replacement with hydrogen ions in the next step can be performed quickly. When the polyimide resin is R, the modified layer is considered to dissociate as follows under neutral to alkaline conditions.
R—COOH → R—COO ⁻ + H ⁺
The selectivity of cation adsorption to the carboxyl group in this neutral to alkaline region is that a large amount of cation ions present in the solution are adsorbed. The metal ion B adsorbed on the modified layer in the metal ion adsorption step is about 2.2 × 10 ⁻⁶ mol per 1 cm ² of the polyimide resin. In the alkali metal-containing solution treatment step, most of the adsorbed metal ions are reduced and deposited as metal, so the amount of metal ions B remaining in the modified layer is 2.2 × 10 ^−6. Much less than mol. Even if the area of the polyimide resin is 1 m ² , it is about 10 ⁻² mol. The concentration of the solution used in the alkali metal-containing solution treatment step is, for example, in the range described below, and contains much more alkali metal ions than the concentration of metal ions B remaining in the modified layer. Therefore, the metal ion B and the alkali metal ion are exchanged,
R-COO ^- M ⁺
(Wherein M ⁺ represents an alkali metal ion).

  As the alkali metal-containing solution, an aqueous solution containing alkali metals such as sodium, potassium and lithium, particularly sodium and potassium is used. Such an aqueous solution preferably has alkalinity, and specific examples include an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, and an aqueous lithium hydroxide solution.

  The concentration of the alkali metal-containing solution only needs to contain a sufficient amount of alkali metal ions relative to the amount of metal ions B in the modified layer remaining in step (3). 2 mol / L, desirably 0.5 mol / L to 1 mol / L. The treatment time is 1 minute to 15 minutes, preferably 3 minutes to 10 minutes. The treatment temperature is 10 ° C to 40 ° C, desirably 20 ° C to 30 ° C. If the temperature is too high, the polyimide modified layer may be further modified, leading to a decrease in the adhesion strength between the polyimide resin and the metal thin film. If the temperature is too low, it becomes difficult to stabilize the temperature, which increases costs. Moreover, you may add antioxidant, for example, sodium sulfite, sodium nitrite, etc. to the solution used at this process.

  Washing with water is performed to remove the alkali metal-containing solution adhering to the polyimide resin surface. Washing with water is usually performed at a water amount of 1 to 5 L / min under conditions of 5 minutes or more.

(5) Acid Solution Treatment Step In this step, the modified layer 23 is treated with an acid solution to replace the alkali metal ions contained in the modified layer 23 with hydrogen ions. Specifically, a polyimide resin having the modified layer 23 is immersed in an acid solution, and alkali metal ions in the modified layer 23 are replaced with hydrogen ions in the acid solution. In FIG. 7, reference numeral 24 denotes a modified layer containing hydrogen ions after the acid solution treatment.

In this step, substitution of alkali metal ions with hydrogen ions occurs quickly. As a result, metal ions can be prevented from remaining in the modified layer, so that the modified layer can be re-imidized effectively, and the adhesion between the metal thin film and the polyimide resin can be improved. Replacement of alkali metal ions with hydrogen ions occurs much more smoothly than replacement of metal ions B such as copper ions with hydrogen ions. This is presumably because the difference in ion exchange sequence between alkali metal ions and hydrogen ions is larger than the difference between metal ions B and hydrogen ions under acidic conditions. The order of ion exchange under acidic conditions is
Alkali metal ion <metal ion B <H ⁺
Can be considered. In other words, hydrogen ions are more easily ion-exchanged with respect to alkali metal ions than with metal ions B. Therefore, it is considered that substitution from alkali metal ions to hydrogen ions occurs relatively quickly.

  The acid solution is not particularly limited as long as hydrogen ions are present in the solution, and usually an acid stronger than the aromatic carboxylic acid is used. Specifically, for example, sulfuric acid aqueous solution, hydrochloric acid aqueous solution, nitric acid aqueous solution, citric acid aqueous solution and the like can be used. If possible, it is desirable to use an acid solution that does not dissolve or hardly dissolves the metal thin film deposited by the reduction treatment in step (3).

  For example, when a copper thin film is deposited, a citric acid aqueous solution is preferable, and when a nickel thin film is deposited, a sulfuric acid aqueous solution is preferably used.

  The concentration of the acid solution only needs to contain a sufficiently large amount of hydrogen ions with respect to the amount of alkali metal ions in the modified layer replaced in step (4), for example, 0.1 mol / L to 1.0 mol. / L, desirably 0.2 mol / L to 0.5 mol / L. The treatment time is 1 minute to 15 minutes, preferably 2 minutes to 10 minutes. The treatment temperature is 10 ° C to 40 ° C, desirably 20 ° C to 30 ° C.

After the acid solution treatment, it is usually washed with water and dried to remove the acid solution adhering to the polyimide resin surface.
Washing with water is usually performed at a water amount of 1 to 5 L / min under conditions of 5 minutes or more.
The drying conditions are not particularly limited, and the temperature is usually 80 to 140 ° C, preferably 100 to 120 ° C, and the time is 30 to 60 minutes. In order to prevent oxidation of the obtained metal thin film, it is desirable to perform heat treatment in a nitrogen gas atmosphere, an inert gas atmosphere, or a vacuum atmosphere. When drying is performed in a vacuum atmosphere, heating is not particularly necessary, and it may be performed at room temperature. In that case, it is desirable to lengthen the time, for example, the drying time is 120 minutes or more.

(6) Re-imidization process In this process, an imide ring is closed. Specifically, the modified layer 24 is re-imidized by heating the polyimide resin obtained in the previous step (re-imidization bake treatment). Thereby, as shown in FIG. 8, the modified layer inferior in mechanical strength, heat resistance, and chemical resistance can be returned to the original polyimide molecular structure. In FIG. 8, 25 is the original modified layer imidized by the re-imidation treatment.

  The treatment conditions are not particularly limited as long as the re-imidation of the modified layer can be achieved. For example, the temperature is 250 ° C. or higher and the time is 1 hour or longer. In order to prevent oxidation of the obtained metal thin film, it is desirable to perform heat treatment in a nitrogen gas atmosphere, an inert gas atmosphere, or a vacuum atmosphere.

The method for forming a metal thin film on the surface of the polyimide resin of the present invention described above can be applied to a method for manufacturing a polyimide wiring board, particularly a flexible wiring board.
For example, a wiring pattern may be formed by a so-called subtractive method or additive method on a polyimide resin having a metal thin film obtained by the method for forming a metal thin film on the polyimide resin surface of the present invention (wiring forming step).

  Specifically, in the subtractive method, for example, a polyimide film having a metal thin film obtained in the present invention is further formed with a metal film by electrolysis / electroless plating if desired, and a resist pattern is formed in the wiring region. Thereafter, the exposed portion (non-wiring region) of the metal film may be removed by etching to remove the resist pattern.

  In the additive method, for example, a resist pattern is formed in a non-wiring region with respect to the polyimide resin having a metal thin film obtained in the present invention, and the exposed portion (wiring region) of the metal thin film is electrolyzed / electrolessly plated. After the film is formed, the resist pattern is removed, and the exposed portion of the metal thin film may be removed by etching using the metal film as a mask.

  As described above, in the present invention, since the residual metal ions in the polyimide resin-modified layer can be removed before the metal thin film is thickened and before the drying process, the adhesion strength between the polyimide resin and the metal layer can be removed. There is no fear of decline. In addition, the polyimide reforming layer returns to the original polyimide molecular structure by the re-imidization process after removing the residual metal ions in the polyimide resin modifying layer, so it is possible to obtain a wiring board with excellent heat resistance. It becomes.

  The method for forming a metal thin film on the polyimide resin surface according to the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

<Experimental example 1>
The carboxyl group functions as an ion exchange group, and the ease of ion exchange is generally Na ⁺ <K ⁺ <Mg ²⁺ <Ca ²⁺ <H ⁺ under acidic conditions.
It is said. H ⁺ is most easily attached to a carboxyl group.
In addition, since typical metal ions such as Cu ions, Ni ions, Ag ions, and Pd ions used in the direct plating method are exchanged with K ⁺ ions in the ion exchange treatment, the order of ion exchange under acidic conditions Is
Na ⁺ <K ⁺ <(Cu ²⁺ , Ni ²⁺ , Ag ⁺ , Pd ²⁺ ) <H ⁺
Can be considered.

  Regarding the exchange of metal ions adsorbed on the carboxyl group with hydrogen ions, it is considered that the metal ions having a positional relationship with the hydrogen ions are more easily exchanged in the above order. Therefore, taking Na ions as an example, it was verified whether Cu ions could be replaced with Na ions and then replaced with H ions.

When an alkali modification treatment, for example, a modification treatment using an aqueous potassium hydroxide solution is performed, the position where the imide ring is opened is in a state in which potassium ions (K ⁺ ) are coordinated to the carboxyl group (COO-). Yes. FIG. 9 shows an EDX analysis result of a polyimide resin that was subjected to modification treatment by being immersed in an aqueous potassium hydroxide solution at a temperature of 50 ° C. and a concentration of 5 mol / L for 3 minutes. It can be seen that potassium is detected from the modified layer. When this is immersed in, for example, an aqueous copper sulfate solution for 5 minutes, the potassium ions coordinated to the carboxyl groups are replaced with copper ions (Cu ²⁺ ). FIG. 10 shows an EDX analysis result of a polyimide resin treated with a potassium sulfate aqueous solution at a temperature of 25 ° C. and a concentration of 0.05 mol / L for 5 minutes after the modification treatment with a potassium hydroxide aqueous solution. It can be seen that copper is detected from the modified layer and potassium is no longer detected.

  FIG. 11 shows an EDX analysis result after treating the polyimide resin adsorbed with Cu ions for 3 minutes with a citric acid solution having a temperature of 25 ° C. and a concentration of 0.2 mol / L. The purpose of this treatment is to exchange Cu ions in the polyimide resin and hydrogen ions in the citric acid solution. A Cu peak is detected, and the Cu ions are not completely exchanged for hydrogen ions.

  In FIG. 12, a polyimide resin modified by immersion in an aqueous potassium hydroxide solution at a temperature of 50 ° C. and a concentration of 5 mol / L for 3 minutes was treated with a citric acid solution at a temperature of 25 ° C. and a concentration of 0.2 mol / L for 1 minute. The following EDX analysis result is shown. The potassium peak was not detected, indicating that the potassium ion was replaced with a hydrogen ion. Here, we focused on the speed of substitution from potassium ions to hydrogen ions. It is assumed that the rate of substitution from hydrogen ions to alkali metal ions (potassium ions in the above example) is higher than substitution from heavy metal ions (Cu ions in the above example) to hydrogen ions.

  Next, it was verified whether or not metal ions could be replaced with alkali metal ions. FIG. 13 shows an EDX analysis result after the polyimide resin that has been subjected to the copper sulfate aqueous treatment after the alkali modification is immersed in an aqueous solution of sodium hydroxide (NaOH) having a temperature of 25 ° C. and a concentration of 1 mol / L for 2 minutes. It can be seen that the peak of Cu is not detected and the peak of Na is detected. From this, it can be seen that substitution of Cu ions and Na ions is possible.

Since the exchange of alkali metal ions and hydrogen ions is easy as described above, it is estimated that the exchange of Na ions and hydrogen ions is also easy. FIG. 14 shows an EDX analysis result after treating the Na-substituted polyimide resin with a citric acid solution having a temperature of 25 ° C. and a concentration of 0.2 mol / L for 1 minute. No Na peak was detected, and it is considered that Na ions were replaced with hydrogen ions.
Thereafter, by performing drying and re-imidization baking, the imide ring that has been opened by the alkali modification treatment is closed, and the polyimide-modified layer returns to the original polyimide molecular structure.

  FIG. 15 and FIG. 16 show the degree of return (imidation rate) to the polyimide molecular structure after the re-imidization bake treatment depending on the presence or absence of the alkali metal-containing solution treatment step, and the ATR (FT-IR (Fourier transform infrared spectroscopy) ATR ( The result measured by the total reflection measurement) method is shown. FIG. 15 shows the case where the hydrochloric acid solution treatment was performed without performing the alkali metal-containing solution treatment after the reduction step, and FIG. 16 shows the case where the alkali metal-containing solution treatment was conducted after the reduction step, and further the acid solution treatment was conducted. is there. The FT-IR results of untreated polyimide resin are shown for comparison. The method for evaluating the ratio of returning to the molecular structure of polyimide was performed as follows.

The absorption observed at 1500 cm ⁻¹ in the FT-IR spectrum is an absorption band derived from the skeleton vibration of the benzene ring, and the number of benzene rings does not change even when the polyimide resin is subjected to alkali modification treatment. It can be used as an internal standard in determining rates. The absorption spectrum observed at 1780 cm ⁻¹ is an absorption band derived from the target stretching vibration of imide carbonyl, and is an absorption band specific to imide. That is, it is a peak that appears when the imide ring is closed after the re-imidization bake treatment. Re imidization ratio was calculated by the reflection light intensity ratio of 1780 cm ^-1 relative to _{^{1500cm -1 (T 1780 / T 1500}} ).

When the reflected light intensity ratio (T ₁₇₈₀ / T ₁₅₀₀ ) is calculated from FIG. 15, the untreated polyimide resin is 0.78, and the reflected intensity ratio after re-imidization baking of the sample not subjected to the alkali metal-containing solution treatment Was 0.65. Further, when the reflected light intensity ratio (T ₁₇₈₀ / T ₁₅₀₀ ) is calculated from FIG. 16, the untreated polyimide resin is 0.20, and the reflected intensity after re-imidation baking of the sample subjected to the alkali metal-containing solution treatment. The ratio was 0.22. Note that the values in FIG. 15 and the values in FIG. 16 are different because the measurement devices are different. As described above, the reflection intensity ratio of the sample subjected to the alkali metal-containing solution treatment of FIG. 16 shows a value close to the reflection intensity ratio of the untreated polyimide resin, and the imidation ratio is considered to be very high. . On the other hand, in the case of the sample not subjected to the alkali metal-containing solution treatment of FIG. 15, the reflection intensity ratio is 0.13 different from the reflection intensity ratio of the untreated polyimide resin, and re-imidization is insufficient. It shows that there is. Therefore, it can be said that performing the alkali metal-containing solution treatment after the reduction step is an indispensable step in the process of manufacturing the polyimide wiring board by the direct plating method.

<Experimental example 2>
[Example 1]
A Cu thin film was formed on the polyimide resin surface by the steps described below.
As the target polyimide resin, a 125 μm thick polyimide film was used. As the polyimide film, Kapton H manufactured by Toray DuPont was used (FIG. 2).

  First, the polyimide resin was immersed in a 50 ° C. aqueous KOH solution for 3 minutes to carry out a modification step (FIG. 3). At this time, the KOH aqueous solution was set to a concentration of 5M (mol / L). Through this step, the imide ring in the polyimide molecule was hydrolyzed on both sides of the polyimide resin, and in the modified layer 2 on the polyimide surface, a carboxylic acid potassium salt in which potassium ions were coordinated to the carboxyl group was formed. The thickness of the modified layer 2 was found to be about 3 μm by observing the cross section of the polyimide resin with an SEM (scanning electron microscope). Thereafter, the polyimide resin was washed with running water of 2 liters / minute for 5 minutes.

  Next, Cu ion adsorption treatment was performed on the polyimide resin (FIG. 4). A copper sulfate aqueous solution was used for Cu ion adsorption. The treatment conditions are a concentration of 0.05 M (mol / L), a temperature of 25 ° C., and a treatment time of 10 minutes. The solution is agitated. Thereafter, the polyimide resin was washed with water. Washing with water was performed for 5 minutes with running water of 2 liters / minute.

  Next, a reduction treatment was performed on the polyimide resin (FIG. 5). The reducing solution was an aqueous sodium borohydride solution with a concentration of 0.001 M (mol / L), a pH of 9.2, a temperature of 25 ° C., and a treatment time of 30 minutes. The reducing solution is agitated. 30 minutes after the start of immersion, the polyimide resin was taken out of the sodium borohydride reducing solution and washed with water. Washing with water was performed for 5 minutes with running water of 2 liters / minute.

  Next, the alkali resin containing solution process was performed about the polyimide resin (FIG. 6). The alkali metal-containing solution was an aqueous sodium hydroxide solution, the concentration was 1 mol / L, the temperature was 25 ° C., and the treatment time was 3 minutes. Then, it washed with water. Washing with water was performed for 30 minutes with running water of 2 liters / minute.

  Next, an acid solution treatment was performed on the polyimide resin (FIG. 7). The acid solution was a citric acid aqueous solution, the concentration was 0.2 mol / L, the temperature was 25 ° C., and the treatment time was 7 minutes. Thereafter, the polyimide resin was washed with water. Washing with water was performed for 5 minutes with running water of 2 liters / minute.

  Next, the polyimide resin was dried. In the drying process, first, water adhering to the polyimide resin surface was removed by nitrogen blowing. Next, drying treatment was performed in a vacuum (10 hPa) atmosphere at a temperature of 25 ° C. and for a time of 3 hours.

  Next, the reimidation process was performed about the polyimide resin (FIG. 8). The re-imidation treatment was performed at 300 ° C. for 1 hour in a nitrogen gas atmosphere. A schematic view of the obtained Cu thin film-forming polyimide resin is as shown in FIG. It shows that the Cu metal thin film 3 is formed on the surface of the polyimide resin. FIG. 17 shows an image obtained by observing a cross section of an actual polyimide resin with an SEM. The deposited Cu thin film forms a layer near the polyimide surface. FIG. 18 shows a chart obtained by performing EDX (energy dispersive X-ray analysis) analysis on the point A in FIG. Cu and Na peaks are not detected, and it can be seen that Cu ions and Na ions do not remain in the polyimide resin.

  Furthermore, a peel test was performed to measure the adhesion strength between the Cu thin film and the polyimide resin. The peel strength was 10-12 N / cm. Furthermore, the decrease in adhesion strength after high temperature treatment (150 ° C., maintained for 196 hours) was about 10%. The fracture surface after the peel test was the interface between the Cu thin film and the polyimide resin. Since the polyimide resin portion that has been modified has not been broken, re-imidization treatment can increase the adhesion strength between the polyimide resin and the Cu thin film layer while maintaining the mechanical strength of the polyimide resin. It was.

  A method for measuring peel strength (adhesion strength) will be described. The peel strength was measured by a 90 ° peeling test method defined in JISC6471. A general tensile tester was used as the tester. After the film thickness of the metal film is increased to 18 μm by electrolytic plating on the polyimide resin on which the metal thin film is formed by the process described above, the back surface of the polyimide resin is attached to an appropriate metal plate. A commercially available epoxy adhesive was used for pasting. After the adhesive is cured, a metal plate to which the polyimide resin is attached is attached to a peel test jig, and one of the test pieces is connected to a load cell of a tensile tester. Thereafter, a tensile test was performed to determine the peel strength, which was defined as peel strength.

[Example 2]
In this example, a Cu thin film was formed on the polyimide resin surface by the same method as in Example 1 except that the reduction process was performed under the following processing conditions.
Dimethylamine borane was used as the reducing agent, the concentration was 0.5 mol / L, the pH was 8.9, the temperature was 50 ° C., and the treatment time was 12 minutes.
The obtained polyimide resin was the same precipitation form as Example 1 as a result of cross-sectional observation. Further, as a result of EDX analysis of the portion corresponding to the point A in FIG. 17, the peaks of Cu and Na were not detected as in Example 1. Furthermore, the adhesion strength between the polyimide resin and the Cu thin film layer was 12 to 14 N / cm. The decrease in adhesion strength after high temperature treatment (150 ° C., maintained for 196 hours) was about 10%.

[Example 3]
In this example, a Cu thin film was formed on the polyimide resin surface by the same method as in Example 1 except that the alkali metal-containing solution treatment step was performed under the following treatment conditions.
The alkali metal-containing solution was an aqueous potassium hydroxide solution, the concentration was 1 mol / L, the temperature was 25 ° C., and the treatment time was 3 minutes. Then, it washed with water. Washing with water was performed for 30 minutes with running water of 2 liters / minute.
The obtained polyimide resin was the same precipitation form as Example 1 as a result of cross-sectional observation. Further, as a result of the EDX analysis of the portion corresponding to the point A in FIG. 17, the Cu and K peaks were not detected as in Example 1. Furthermore, the adhesion strength between the polyimide resin and the Cu thin film layer was 12 to 14 N / cm. The decrease in adhesion strength after high temperature treatment (150 ° C., maintained for 196 hours) was about 10%.

[Example 4]
In this example, the polyimide resin was subjected to the same method as in Example 1 except that Kapton EN manufactured by Toray DuPont was used as the target polyimide resin and the KOH treatment time in the modification step was 40 minutes. A Cu thin film was formed on the surface.
The obtained polyimide resin was the same precipitation form as Example 1 as a result of cross-sectional observation. Further, as a result of EDX analysis of the portion corresponding to the point A in FIG. 17, the peaks of Cu and Na were not detected as in Example 1. Furthermore, the adhesion strength between the polyimide resin and the Cu thin film layer was 9 to 10 N / cm. The decrease in adhesion strength after high temperature treatment (150 ° C., maintained for 196 hours) was about 10%.

[Comparative Example 1]
A Cu thin film was formed on the polyimide resin surface in the same manner as in Example 1 except that the alkaline aqueous solution treatment was not performed in the treatment steps of Example 1 and that the acid solution treatment time was 15 minutes.
As a result of cross-sectional observation, the obtained Cu thin film-forming polyimide resin had the same structure as Example 1. In FIG. 19, the image which observed the cross section of the actual polyimide resin by SEM is shown. FIG. 20 shows a chart obtained by EDX (energy dispersive X-ray analysis) analysis of point B in FIG. A Cu peak is detected, and it can be seen that Cu ions remain in the polyimide resin. Furthermore, a peel test was performed to measure the adhesion strength between the Cu thin film and the polyimide resin. The peel strength was 4-5 N / cm. The portion of the fracture surface that was the original modified layer of the polyimide resin was mostly. It is considered that the re-imidization bake treatment was carried out with Cu ions remaining in the modified layer, so that it did not sufficiently return to the original polyimide molecular structure and the bulk strength of the polyimide resin was lowered. .

  INDUSTRIAL APPLICABILITY The present invention can be used in the manufacture of flexible wiring boards. Rigid-flexible wiring in which a single-sided flexible wiring board, a multilayer flexible wiring board in which a plurality of double-sided flexible wiring boards are laminated, and a hard board such as a glass epoxy board are laminated together. It can be widely used for the manufacture of plates.

The process flow figure of the metal thin film formation method to the polyimide resin surface by this invention. The schematic cross-sectional schematic diagram of the polyimide resin used for this invention. The schematic cross-sectional schematic diagram after the polyimide resin surface modification by this invention. The schematic cross-sectional schematic diagram after metal ion adsorption | suction to the polyimide resin by this invention. The schematic cross-sectional schematic diagram after the metal thin film formation to the polyimide resin by this invention. The schematic cross-sectional schematic diagram after the alkali metal containing solution process to the polyimide resin by this invention. The schematic cross-sectional schematic diagram after the acid solution process to the polyimide resin by this invention. The schematic cross-sectional schematic diagram after the re-imidation baking process by this invention. The EDX analysis result of the polyimide resin modified layer after alkali modification. The EDX analysis result of the polyimide resin modified layer processed with the copper sulfate aqueous solution after alkali modification. The EDX analysis result of the polyimide modified layer which processed the copper sulfate aqueous solution after alkali modification, and also processed with the citric acid aqueous solution. The EDX analysis result of the polyimide modified layer processed with the citric acid aqueous solution after alkali modification. The EDX analysis result of the polyimide modified layer which processed the copper sulfate aqueous solution after alkali modification, and also processed with the sodium hydroxide aqueous solution. The EDX analysis result of the polyimide modification layer which performed the copper sulfate aqueous solution process after alkali modification, then the sodium hydroxide aqueous solution process, and also processed with the citric acid aqueous solution. The FT-IR analysis result after a re-imidation baking process (no alkali metal containing solution process). FT-IR analysis results after re-imidization bake treatment (with alkali metal-containing solution treatment). The cross-sectional SEM image after Cu thin film formation to the polyimide resin surface by Example 1 was shown. The cross-sectional EDX analysis result after Cu thin film formation to the polyimide resin surface by Example 1 was shown. The cross-sectional SEM image after Cu thin film formation to the polyimide resin surface by the comparative example 1. FIG. The cross-sectional EDX analysis result after Cu thin film formation to the polyimide resin surface by the comparative example 1 was shown. The process flowchart of an example of a prior art. The process flowchart of an example of a prior art.

Explanation of symbols

  1: polyimide resin, 2: modified layer, 3: metal thin film deposited in polyimide resin, 21: modified layer adsorbing metal ions, 22: modified layer containing residual metal ions after depositing metal thin film , 23: Modified layer after treatment with alkali metal-containing solution, 24: Modified layer after treatment with acid solution, 25: Original modified layer after re-imidization baking treatment, 101: Polyimide resin, 211: Adsorption of metal ions 212: Modified layer after depositing metal thin film, 312: Metal thin film layer deposited by reduction treatment, 411: Metal layer increased for circuit formation, 412: Between wirings formed by etching 413: Modified layer in which residual metal ions have been reduced by acid treatment, 414: Polyimide resin in inter-wiring space returned to original polyimide structure by re-ringing process, 415: By alkaline solution treatment Inter-wiring space from which polyimide resin has been etched away, 416: Modified layer in which residual metal ions have been reduced by acid treatment, 417: Polyimide resin in inter-wiring space that has returned to the original polyimide structure by re-ringing process, A: EDX analysis Location, B: EDX analysis location.

Claims (4)

Treating the polyimide resin surface with an alkaline solution to open the imide ring and forming a modified layer;
Treating the modified layer with a metal ion-containing solution;
Treating the modified layer with a reducing bath;
Treating the modified layer with an alkali metal-containing solution;
Treating the modified layer with an acid solution; and a re-imidation step for closing an imide ring;
A method for forming a metal thin film on a polyimide resin surface, comprising:   The method for forming a metal thin film on a polyimide resin surface according to claim 1, wherein the polyimide resin is a polyimide resin produced by reacting pyromellitic dianhydride and an aromatic diamine.   2. The polyimide resin surface according to claim 1, wherein the polyimide resin is a polyimide resin produced by reacting pyromellitic dianhydride and biphenyltetracarboxylic dianhydride with an aromatic diamine. Metal thin film forming method.   The method for forming a metal thin film on a polyimide resin surface according to claim 2 or 3, wherein the aromatic diamine contains 4,4'-diaminodiphenyl ether and paraphenylenediamine.