Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
2000-11-15
2003-07-08
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S097100, C427S306000, C427S307000, C427S337000, C205S125000, C252S500000
Reexamination Certificate
active
06589593
ABSTRACT:
The invention pertains to a process for coating substrates having polymer surfaces with metals in the manufacturing of printed circuit boards, in particular in the manufacturing of printed circuit boards having microscopic holes and fine geometries by the application of an electroconductive polymer layer and a subsequent metallization, wherein the electroconductive polymer layer is preferably doped with a zinc-containing, colloidal palladium solution prior to the metallization step. In particular, printed circuit boards having microscopic holes and fine geometries are produced in the so-called build-up technology. Generally, in this technique circuits finished on both sides are used as a core and coated with polymers wherein microscopic holes are made in accordance with the state of the art, e.g., by photolithography or by lasers, said microscopic holes leading to the next conductor level as so-called blind vias.
Subsequently, a surface patterning of the polymer, a seeding with Pd, and a surface metallization of the complete circuit by electroless deposition of copper are performed according to the state of the art described in DE A 195 02 988. Optionally, the “chemical” copper layer is reinforced electrolytically.
In additional operating steps the conductor pattern having appropriate conductor runs is formed. Multilayers having a higher number of layers can be prepared by repeating this procedure several times. Therefore, this process is designated as a sequential build up.
From DE 38 06 884 a process for the formation of via holes in printed circuit boards (i.e., the metallization of bore walls) on the basis of intrinsically conducting polymers has been known. The substrate is pre-treated in an oxidizing solution, rinsed, and subsequently dipped in an aqueous monomer solution of pyrrole, furan, thiophene, and/or derivatives thereof, and an after-treatment in acidic solutions is performed. In this process an intrinsically conducting polymer film is selectively formed on electrically non-conducting surfaces (polymer, glass etc.) and subsequently galvanically metallized.
In the metallization of bore walls distances corresponding to the thickness of the printed circuit boards have to be bridged. Generally, distances exceeding 4 mm will have to be metallized only rarely. The conductivity of the intrinsically conducting polymers will generally be sufficient to complete the metallization within several minutes. The lateral growth of copper on said polymers will vary between 0.1 and 2.5 mm/min depending on the polymer type. Assuming such growth values, a surface metallization of complete printed circuit boards having surfaces of up to 0.2 m
2
—and optionally exceeding that value—can be achieved not at all or only after a long time with a very poor layer thickness distribution. Such printed circuit boards meet the technical demands not in the slightest. Therefore, it will be necessary to increase the conductivity of such intrinsically conducting polymers distinctly and to increase the lateral metal growth on said polymer films considerably.
In DE 195 02 988 there is described a process which is to solve the problems illustrated above. According to the process described in DE 38 06 884, initially the substrate polymer is applied with an electroconductive polymer. Prior to the galvanic metallization a treatment in an metal-ion containing aqueous solution, preferably a tin-containing colloidal palladium solution is performed. In this step the intrinsically conducting polymer is additionally doped. Thus, improved conductivity values and, above all, an increased lateral growth can be achieved. In the case of, e.g., poly-3,4-ethylenedioxythiophene metal growth values of up to 5 mm/min can be realized. However, also these values are still not satisfactory for a surface metallization. The adhesion of the conducting polymer to the substrate polymer is insufficient. Namely, the conductive polymer film has to be provided such that it strongly adheres to the substrate polymer. Hence, adhesive strength values of at least 5 N/cm, preferably, however, of 10 N/cm will be necessary.
Therefore, the objects of the present invention are to provide a sufficient adhesion of the conducting polymer film to the polymer substrate and, above all, to further increase the lateral growth of copper.
Said objects are achieved by a contacting with a copper(II) salt solution prior to the metallization. Preferably, the substrate surfaces are subjected to the following steps at least once prior to the application of the electroconductive polymer layer:
a) Swelling by aqueous leaches, organic solvents, or alkaline solvents;
b) treating with an alkaline permanganate solution; and
c) treating with a reducing agent.
Hence, the substrate surface is initially patterned. This is performed in step a) by the treatment with a swelling agent, a mixture of suitable solvents, and by a sodium or potassium hydroxide solution. Subsequently, the surface of the substrate polymer pre-treated this way is further patterned in an alkaline permanganate solution. In this step as much cavities as possible being distributed over the complete surface as evenly as possible and having diameters of some few &mgr;m or even smaller are to be formed in order to improve the adhesion of the conducting polymer. Such swelling agents and alkaline permanganate solutions have been known from, e.g., the so-called multilayer desmear process. It has been unknown that the adhesive strength of the conduction polymer layer can considerably be improved by these steps.
Depending on the substrate polymer it is advantageous to perform the swelling and permanganate treatment process several times in order to achieve an appropriate surface patterning. In any case, the last step of the process is a reducing process wherein the residues of the permanganate treatment, namely manganese dioxide, are reduced and hence the surface is made residue-free again. In particular, H
2
O
2
has been proved especially efficient as a reducing agent.
When carrying out the inventive process, the swelling of the substrate polymer is performed using a solvent or a solvent mixture to which optionally or even preferably an alkali hydroxide is added. Subsequently, the treatment with alkaline permanganate solution follows. These measures result in a uniform roughening and patterning of the substrate polymer and ensure a good adhesion of the conducting polymer layer which is to be applied subsequently.
It turned out that especially good adhesion strength values can be obtained if the polymer substrates are subjected to a mechanical pretreatment prior to the chemical treatment. For this brushing, sandblasting or also preferably a treatment with pumice powder is suited, the latter process being known as “pumice brushing” or “pumice blasting”. The adhesion strength can further be increased by about 30 to 40% by this process.
In principle, the formation of the conductive polymer layer takes place as described in DE 38 06 884.
The surface which is pretreated according to the invention and with that patterned is initially pretreated in an oxidizing bath, preferably a solution of potassium permanganate within a pH range from 1 to 14, preferably from about 5 to 8. In order to improve the adhesion of the polymer film to be formed, the oxidizing step may be preceded by an immersion in a so-called conditioner according to the description in DE-A-42 05 190.
Subsequently, a rinsing is performed and the substrate is immersed in a monomer solution of 3,4-ethylenedioxythiophene. Subsequently, the substrate covered with the monomer is inserted into an acidic solution without rinsing prior to the insertion, the formation of the conducting polymer films taking place within the acidic solution via an oxidative polymerization. In the case of potassium permanganate as the oxidant, a layer of manganese dioxide is formed in the first step as the reaction product of the substrate polymer and KMnO
4
, wherein said layer of manganese dioxide is polymer-impregnated and acts as an oxidant in the acidic solution described ab
Fix Sabine
Hupe Jürgen
Steinius Ortrud
Blasberg Oberflächentechnik GmbH
Jacobson & Holman PLLC
Talbot Brian K.
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