1. Field of the Invention
The present invention relates to a surface-metallized polymers/resins that are suitable for printed wiring boards or the like, and processes for manufacturing 1) the surface-metallized polymers/resins, 2) a printed wiring boards using the surface-metallized resin, and 3) fine coaxial wirings which may suitably be incorporated in wiring boards mounted with electronic components or the like, and 4) wiring boards using the fine coaxial wirings.
2. Description of the Related Art
Conventionally, various methods have been proposed to metallize the surface of insulating resins such as polyimides or the like for the purpose of forming a wiring boards or the like Among them, dry processes using vacuum apparatus such as vapour deposition apparatus and sputtering apparatus or the like were found to be reliable methods capable of forming a metal tin layer with excellent uniformity and adhesion to resins. However, these dry process have some disadvantages that include high facility and maintenance cost, and difficulty in forming thin metal layers on irregularly shaped articles.
In electro-less plating, surface prtetreatment should be carried out in order to get a strong adhesion to metal. In the pretreatment process, dielectric surface is wet etched to create micro-cavities of about 1 to 3 μm, and through these cavities, the metal is anchored to the resin surface. However, in the case of polymers like polyimides, that are, susceptible to KmnO4 etching process, this process cannot be applied. The Alkaline KAnO4 etching would deteriorate the polymer/resin that would prevent micro cavity formation thus leading to poor adhesion to the metal.
Therefore, development of a new surface mattalization method for polyimide like polymers/resins, which gives good uniformity and adhesion like in dry processes as well as cost effectiveness like in wet process, is necessary. To satisfy this requirement a method of dipping a cured polyimide in a heated strong alkaline solution to open the imide ring in the cured polyimide, and making a noble metal ion adsorb into the carboxylic acid group, and then depositing the noble metal by the irradiation to form a desired pattern has been proposed (U.S. Pat. RE No. 29,015, Japanese Patent Application Laid-Open No. 11-246977).
Since cured polyimides undergo hydroxylation by strong alkali, the molecular weight of the polyimides on the surface decrease. This would lead to a poor adhesion to metals. However, the ring opening of the cured polyimides by a strong alkali depends on the molecular structure of the polyimide, particularly the crystallinity of the polymer. For instance, the ring opening reaction would not take place in polyimides having high crystalline structure. On the other hand, the ring opening reaction as well as depolymerization progress very rapidly in polyimides having amorphous surface, thus giving a poor adhesion to metal,
Recently, coaxial wiring systems have been proposed for high speed signal transmission systems wherein electro-magnetic induction due to the adjacent lines can be minimized, and characteristic impedance can be adjusted.
At present, a printed wiring boards free from disturbance of signal waveform caused by variable crosstalk noise, characteristic impedance and the like, and suitable for high-speed signal processing have been also proposed by introducing coaxial wirings to circuit structures in the wiring boards.
To fabricate fine coaxial wirings used in such wiring boards, for example, it is known to activate, the surface of the insulator covering the central conductor by creating micro-cavities trough which the external conductor is anchored to the insulator, thus giving strong adhesion to the metal layer deposited by electroless plating or electroplating (refer to Japanese Patent Application Laid-Open No, 6-187847); to etch the surface of a porous insulator to expose holes, thereby realizing cavities that give anchor effect (refer to Japanese Patent Application Laid-Open No. 2000-2150); to mix a catalyst for electroless plating to a resin insulator and developing an electroless plating film with the exposed catalyst as a nucleus (refer to Japanese Patent Application Laid-Open No. 5-81938); or the like.
These related arts are various contrivances based on that a fluorine resin such as polyimide, polyamide, fluorinated ethylene propane (FEP), ethylene-tetrafluoroethylene (ETFE), polytetrafluoroethylene (PTFE), etc. suitable as an insulator material for coaxial cables, but hardly forms a seed layer by electroless plating due to very poor adhesion of metals.
However, since a high frequency signal transmission in coaxial wiring concentrated to the outer surface of the internal conductor and the inner surface of the external conductor, irregularities present on the insulator surface may cause disturbances to the signal waveform. Further, the mixing of the plating catalyst into the insulator may also cause degradation of high frequency characteristics of the insulator because the catalyst is mainly comprised of metals such as Pt, Pd or the like. In addition to above problems, there is also difficulty in making the terminals of fine coaxial cables.