JP4290621B2 - Electroless copper plating method for multilayer flexible printed circuit board - Google Patents

Description

  The present invention relates to a method for manufacturing a flexible printed circuit board used in an electronic device, and more particularly to a method for conducting through holes.

  Interlayer connection in a multilayer flexible printed circuit board is often performed by forming a conductive film on the inner wall surface of the through hole. And as for the conductive treatment method for the inner wall of the through hole of the printed circuit board, electroless copper plating has been widely used so far, and conductive treatment using Pd—Sn colloid is performed. In this conductive treatment, a copper deposit layer of 0.1 to 1.0 μm is formed, and then a copper plating layer of 15 to 25 μm is formed by electrolytic plating.

  In a multilayer flexible printed board, the adhesive used for lamination is filled with various heat-resistant fillers. Mainly added substances include glass bodies and metal oxides such as aluminum oxide and magnesium oxide, but these filler substances have a hydrophobic particle interface and are difficult to adsorb Pd—Sn colloids. There is a point.

  The conditioner used in the conventional conditioning process is made of an amine surfactant such as triethanolamine, and has the effect of imparting hydrophilicity and the effect of charging the substrate in the through hole to the plus side. .

  However, it is not effective to make the particle interface of glass oxide or metal oxide such as aluminum oxide and magnesium oxide completely hydrophilic. For this reason, the method of performing a hydrophilic treatment more effective than the conventional conditioning process was desired.

  In addition, as a method for making the particle interface of a metal body such as a glass body or aluminum oxide / magnesium oxide hydrophilic, there is a method of performing a silane coupling treatment. As a silane coupling method, for example, a method of hydrophilizing the surface of a glass substrate as disclosed in Patent Document 1 is known.

  This method is an effective means for imparting a hydrophilic group to the surface and improving the hydrophilicity, but there is a process of dehydration bonding during the reaction, and generally it must be treated in a strong alkaline solution.

However, when a printed circuit board is immersed in a strong alkaline solution, there is a problem that polyimide as a main component of the insulating layer is dissolved.
Japanese Patent Publication No.59-52701

  The present invention has been made in view of the above points, and provides a method for manufacturing a printed circuit board having an electroless copper plating process in which the surface of a filler material is modified and voids are not generated on the wall surface of the through hole. Objective.

  It is a more specific object of the present invention to provide a method for making the particle interface between polyimide forming an insulating layer and a filler metal such as glass body or aluminum oxide / magnesium oxide hydrophilic. .

In order to achieve the above object, in the present invention,
In the method of manufacturing a printed circuit board in which through-holes of a multilayer flexible printed circuit board are subjected to electroless plating to form a metal conductor for interlayer connection, a conditioning process as a pretreatment is performed with an aqueous solution mainly composed of an amine-based surfactant. A method for producing a printed circuit board, characterized in that it is performed in two stages: a first conditioning step for immersing the object to be processed and a second conditioning step for immersing the object to be processed in an aqueous solution containing diols as a main component,
Is to provide.

  In the present invention, as described above, the object to be treated is conditioned in two stages, the first and second stages, so that when the object to be treated is conditioned only with a conventional amine surfactant, 2 to 3 ppm. Voids were generated with a frequency of approximately 5%, but no voids were generated at all. This confirms the superiority of the two-stage conditioning method according to the present invention.

Hereinafter, the background to the embodiments of the present invention and the embodiments will be described. The conductive treatment of the inner wall of the through hole of the printed circuit board that is the object to be processed is generally the following seven steps:
(1) Degreasing: Removal of water-insoluble substances such as uncured resin components.
(2) Conditioning: Since the Pd—Sn colloid as a catalyst has a negative charge, the substrate in the through hole is charged to the positive side.
(3) Soft etching: The removal of the oxide film on the surface of the copper foil and the provision of irregularities to suppress plating peeling.
(4) Pre-dip: Immerse in the same component as the solvent of the Pd—Sn colloid to give affinity to the Pd—Sn colloid.
(5) Activator: Pd-Sn colloid is adsorbed.
(6) Accelerator: Removes Sn from the Pd—Sn colloid and precipitates Pd alone.
(7) Electroless plating: Copper ions are reduced and deposited using Pd as a nucleus.
Is done by.

  In a multilayer flexible printed board, the adhesive used for lamination is filled with various heat-resistant fillers. Substances that are mainly added as fillers are glass oxides and metal oxides such as aluminum oxide and magnesium oxide. These filler substances have hydrophobic particle interfaces and are difficult to adsorb Pd—Sn colloids. There is a characteristic. However, since these filler substances are buried in the adhesive resin at the time of through-hole drilling or only a part is exposed on the hole wall surface (FIG. 3), there is no particular problem.

  However, problems arise after desmear processing, which is essential for multilayer substrates. By this desmear treatment, more adhesive resin is removed, and the filler material is exposed to the through-hole hole wall, so that the hydrophobicity becomes strong (FIGS. 4A and 4B). For this reason, a portion called “void (FIGS. 5A and 5B)” that is not attached with electroless copper plating tends to occur.

  By the desmear treatment, more polyimide as a resin component is removed and the filler material is exposed to the through-hole hole wall. However, when the silane coupling treatment is performed, the hole wall is further etched to cause an egress (FIG. 6). Since this aggression causes migration, a hydrophilic treatment such as silane coupling is inappropriate.

  On the other hand, as a method for avoiding the problem of the strong alkali solution as described above, there is a method of performing a hydrophilization treatment under a mild condition such as a weak alkali solution, and a side chain alkyl group, a hydroxyl group, a carboxyl group, an amino group, etc. There is known a method of coupling the modified polyvinyl alcohol having a surface of a crude water substance.

  However, usually polyvinyl alcohol is also used as a thickener, and even when it is made into an aqueous solution, since there are many generally high viscosity, the degree of crosslinking of the modified polyvinyl alcohol used, the through-hole inner diameter of the printed circuit board, Depending on the ratio between the inner diameter of the through hole and the thickness of the substrate, the treatment solution may not penetrate into the through hole when applied to a printed circuit board.

  As a result of a number of conditioner / hydrophilization treatment methods, instead of using modified polyvinyl alcohol as a method to make the particle interface of fillers such as glass bodies and metal oxides such as aluminum oxide and magnesium oxide hydrophilic. It has been found that the effect of imparting hydrophilicity can be obtained by a method of coupling diols such as propylene glycol and trimethylene glycol with a filler substance.

  These diols have the property of reducing the surface tension in a state dissolved in water and easily penetrating into the through holes of the printed circuit board. Therefore, it is effective as a means for making the particle interface of a glass body or metal oxide such as aluminum oxide / magnesium oxide, which is a filler material exposed in the through hole of the printed circuit board, hydrophilic.

  As a coupling agent, phosphorus compound salts such as Ca and Mg or boron compound salts are added. However, phosphorus-containing compounds have difficulty in wastewater treatment, and boron compound salts are preferably used. For example, sodium borate, sodium borofluoride, magnesium borofluoride and the like can be mentioned.

  However, the above method can only obtain the effect of making only the particle interface of glass oxide or metal oxide such as aluminum oxide / magnesium oxide hydrophilic, and this only makes the resin such as polyimide hydrophilic and charges it to the plus side. I can't.

  Thus, when an amine surfactant such as triethanolamine, which is a conventional conditioner, is used in combination, all the composition substances in the through hole can be brought into a surface state on which the Pd—Sn colloid can be adsorbed.

That is, by combining the conditioning step mainly containing an amine surfactant with the conditioning step mainly containing a diol, a treatment that easily adsorbs the Pd—Sn colloid can be performed.
From this,
1) First, in a first conditioning process using an amine surfactant, which is a conventional conditioner, a resin such as polyimide is made hydrophilic and charged positively.

2) Next, after washing with water, the filler material is hydrophilized in a second conditioning step with an aqueous solution of diols.

  3) After the two-stage conditioning process, the process up to electroless plating is performed according to the conventional method.

  In addition, although it is not impossible to mix amine surfactant and diol and perform conditioning at once, the pH for amine surfactant to react efficiently is 12-13. In addition, the effective pH range for hydrophilization with diols is 9 to 11, which is not preferable. In addition, if the diols are hydrophilized first, the diols are removed by the amine-based surfactant.

  As a result of the above findings, it has been found that it is optimal to perform the first and second conditioning steps as follows.

In the first conditioning step, treatment is performed using an amine surfactant. As the amine surfactant used in the first conditioning step, one or two of triethanolamine, 2-aminoethanol and the like are used, and the amine surfactant aqueous solution is used as the amine surfactant aqueous solution. The concentration is controlled at 0.04 to 0.16 mol / liter . The liquid temperature is preferably 50 to 60 ° C., and the object to be treated is immersed in this solution for 60 to 300 seconds.

After the water washing, the second conditioning process using diols is performed. As the diol used in the second conditioning step, one or more of propylene glycol, trimethylene glycol, dimethylene glycol and the like are used, and the concentration as an aqueous solution is 0.05. Managed at ~ 0.20 mol / liter . Furthermore, sodium borate is added to this so that it may become 0.01-0.04 g / liter. The liquid temperature is preferably 40 to 50 ° C., and the object to be treated is immersed in this solution for 60 to 720 seconds.

  After washing with water, through-hole conduction of the printed circuit board can be obtained through a conventional electroless plating process. At that time, palladium (Pd) and tin (Sn) colloidal catalyst can be adsorbed more efficiently.

  No void was generated during the monitoring period of 3000 sheets per day for 2 years or more after the two-stage conditioning process.

  The wall surface 11 of the through hole in FIG. 1 and the wall surface 12 of the through hole in FIG. 2 are both uniformly smooth and free of voids, as can be seen from the comparison with that in FIG. As described above, the conditioning method according to the present invention is effective for the wall treatment of the through hole.

The photograph which shows the cross section of the through hole of a printed circuit board, and shows the wall surface state after the conditioning process by this invention. The photograph which shows the cross section of the through hole of a printed circuit board, and shows the wall surface state after the conditioning process by this invention. The microscope picture which shows the cross section of the through hole which the void in the circuit board generate | occur | produced by the conventional process. A photomicrograph of the entire cross section immediately after the through hole of the printed circuit board has been drilled. A micrograph taken by enlarging the cross section immediately after the through hole of the printed circuit board was drilled. A photomicrograph of the entire cross-section of the through-hole after desmearing. A micrograph taken by enlarging the cross-section of the through-hole after desmearing. A photomicrograph of a cross-section of a through-hole where an egress has occurred.

Explanation of symbols

1 Void generation place 2 Polyimide resin part 3 Glass body
4 Filler substance, 5 Aegle occurrence point,
11, 12 Through hole wall surface.

Claims (7)

In the method for manufacturing a printed circuit board, in which a metal conductor for interlayer connection is formed by performing electroless plating on the through hole of the multilayer flexible printed circuit board,
The conditioning process for the pre-treatment object
A first conditioning step of immersing the object to be treated in an aqueous solution containing an amine-based surfactant as a main component;
A method for producing a printed circuit board, comprising: a second conditioning step in which an object to be treated is immersed in an aqueous solution containing diol as a main component. In the manufacturing method of the printed circuit board of Claim 1,
1 or 2 types of triethanolamine and 2-aminoethanol are used for the said amine type surfactant, The manufacturing method of the printed circuit board characterized by the above-mentioned. In the manufacturing method of the printed circuit board of Claim 2,
The method for producing a printed circuit board, wherein the concentration of the amine surfactant as an aqueous solution is 0.04 to 0.16 mol / liter . In the manufacturing method of the printed circuit board of Claim 2,
The method for producing a printed circuit board, wherein the temperature of the aqueous solution of the amine surfactant is 50 to 60 ° C., and the aqueous solution is immersed in the aqueous solution for 60 to 300 seconds. In the manufacturing method of the printed circuit board of Claim 1,
A method for producing a printed circuit board, wherein at least one of propylene glycol, trimethylene glycol, and dimethylene glycol is used as the diol, and sodium borate is added to the aqueous diol solution . In the manufacturing method of the printed circuit board of Claim 5,
The concentration of the diol as an aqueous solution is 0.05 to 0.20 mol / liter , and the concentration of sodium borate is 0.01 to 0.04 g / liter. . In the manufacturing method of the printed circuit board of Claim 5,
The method for producing a printed circuit board, wherein the temperature of the aqueous solution of the diol is 40 to 50 ° C., and the solution is immersed in the aqueous solution for 60 to 720 seconds.