Seed plating to insulating parts of printed circuit boards has conventionally been conducted primarily by electroless copper plating processes. A representative process is to apply electroless copper plating by adopting, as a pretreatment process, a process that uses a Pd—Sn alloy colloid as a catalyst or an alkaline Pd ion solution as a catalyst and conducts metallization of Pd in a solution of a reducing agent in a subsequent step. By this plating process, insulating parts of most printed circuit boards can be rendered conductive. There is, however, an increasing difficulty with flexible substrates or rigid flex substrates employed in cellular phones, digital cameras, HD or DVD systems, and the like. As a reason for this increasing difficulty, it can be mentioned that many electroless copper plating solutions are highly alkaline and that in these alkaline solutions, polyimide materials generate functional groups such as amino groups, hydroxyl groups, carbonyl groups and/or carboxyl groups to have hydrophilicity and are hence provided with higher hygroscopicity. Due to this property, long-term treatment with an electroless copper plating solution of high alkalinity results in the penetration of the plating solution into the polyimide substrate, and subsequent to the plating treatment, the penetrated plating solution remains between the resulting plating film and the polyimide substrate and oxidizes the copper, thereby developing a problem that an adhesion failure is induced. In addition, an adhesive layer employed in such a substrate is prone to dissolution in the alkaline solution and the dissolved matter causes a reduction in the deposition rate of electroless copper plating, leading to shortening of the solution life.
With a view to resolving such problems, there are now many processes making use of direct plating that does not use an electroless copper plating solution of high alkalinity and conducts electroplating to workpieces without applying electroless copper plating.
Japanese Patent No. 2660002 (Patent Document 1) describes a process that can apply electroplating by converting a Pd—Sn colloid catalyst into a metal chalcogenide compound film by sulfuration treatment.
Japanese Patent No. 2799076 (Patent Document 2) describes a process that subsequent to treatment with a colloidal acidic solution of a noble metal stabilized with an organic polymer, conducts sulfuration treatment to effect metal coating under galvanic action.
Japanese Patent No. 3117216 (Patent Document 3) describes a process that, after a thin oxide film layer is formed in an aqueous solution of potassium permanganate which has been adjusted to pH 0 to 6 with a sulfonic acid or the like, a conductive polymer layer of a pyrrole derivative is formed, followed by electrolytic plating.
Each of Japanese Patent No. 3284489 (Patent Document 4) and Japanese Patent No. 3261569 (Patent Document 5) describes a process that causes a carbon layer to deposit on a surface, conducts treatment in an acidic solution to remove carbon from a copper surface, and applies electrolytic plating.
Practically without exception, however, many of these pretreatment processes require a copper etching step after the step that forms a conductive layer, because in the case of a workpiece composed of insulating parts and copper parts as in a printed circuit board having through-holes and/or via-holes, a component employed to form a conductive layer on the copper substitutes or adsorbs, leading to a potential problem of a reduction in the reliability of connection between the copper existing on the substrate and the copper plating film unless a step is included to remove the component. Further, the copper etching treatment is technically considered to be more difficult than an ordinary dissolution step for copper, because the component employed for the formation of the conductive layers still remains in adhesion on the copper. Furthermore, the conductive layers applied to the insulating parts are accompanied by a potential problem that they may be dissolved or caused to fall off to some extent by the above-described copper etching treatment and also by acid cleaning as pretreatment for copper sulfate plating.
Other patent documents and general publications also include those which describe neutral electroless copper plating solutions. It is, however, the current situation that they have not arrived yet on the market, since they require the use of costly reducing agents and also difficult efforts for the maintenance of solution stability.
Patent Document 1: Japanese Patent No. 2660002
Patent Document 2: Japanese Patent No. 2799076
Patent Document 3: Japanese Patent No. 3117216
Patent Document 4: Japanese Patent No. 3284489
Patent Document 5: Japanese Patent No. 3261569