Etching a substrate: processes – Nongaseous phase etching of substrate – Etching inorganic substrate
Reexamination Certificate
2000-07-27
2003-01-14
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Nongaseous phase etching of substrate
Etching inorganic substrate
C216S002000, C216S083000, C428S174000, C428S446000, C428S607000, C427S098300, C427S096400
Reexamination Certificate
active
06506314
ABSTRACT:
FIELD OF THE INVENTION
In one embodiment, this invention relates to a process for improving adhesion of polymeric materials to metal substrates including the steps of intergranular etching a surface of the metal substrate and applying to the intergranular etched metal surface an immersion plated metal, and in another embodiment, the invention relates to a foil having a surface with improved adhesion to polymeric materials. The intergranular etching produces a highly irregular surface profile including deep intergranular crevices. The immersion plated metal is applied from an immersion plating solution which includes one or more plating metals selected from tin, silver, bismuth, copper, nickel, lead, zinc, indium, palladium, platinum, gold, cadmium, ruthenium, cobalt, gallium and germanium and mixtures or alloys thereof. The surface obtained by this process provides enhanced adhesion between the metal substrate and polymeric materials applied to the surface of the metal substrate.
BACKGROUND OF THE INVENTION
Many highly varied processes of chemically or physically treating the surface of a metal substrate, such as copper, to improve the bonding of the metal to a polymeric material, such as epoxy or polyimide, are used in industries such as printed circuit board (PCB) fabrication. PCBs generally comprise non-conducting or dielectric layers such as a fiberglass/epoxy sheets which are clad with a metal conductive layer such as copper or a copper alloy on one or both surfaces. The metal layer of the PCB, before processing, typically is a continuous layer of copper which may be interrupted by a pattern of plated through-holes linking both surfaces of the board. During processing, selected portions of the copper layer are removed to form a raised copper circuit image pattern, i.e., circuitry. Multilayer PCB's are typically constructed by inter-leaving such circuit-bearing conductive layers with dielectric adhesive layers into a multilayer sandwich which is then bonded together by applying heat and pressure. The dielectric adhesive layer is often a partially cured B-stage resin, referred to as a prepreg. Production of these types of PCB's is described in “Printed Circuits Handbook,” Fourth Edition, Edited by C. F. Coombs, Jr., McGraw-Hill, 1996, and in “Printed Circuit Board Basics”, Second Edition, by Michael Flatt, Miller-Freeman, 1992, the teachings relating to PCB manufacture of both are incorporated herein by reference. Since the conductive layer with an untreated surface does not bond well to the prepreg, various surface treatments have been developed to increase the bond strength between the layers of the multilayer PCB sandwich.
Originally such treatments consisted of oxidizing the metal surface with treatments such as alkaline chlorite solutions (e.g., U.S. Pat. Nos. 2,364,993, 2460,896, and 2,460,898). Over time the treatment evolved (e.g., Slominski and Landau, “Adhesion Promoting Copper Oxide for Plastic on Printed Circuit Boards”
Plating
, June 1982 pp. 96-99) and the oxide became a ‘reduced oxide’ (e.g., U.S. Pat. No. 4,642,161). By utilization of a reduced oxide, multilayer printed wiring boards were less prone to a problem known as ‘pink ring’ where oxide is dissolved adjacent to drilled holes during subsequent through hole plating processes. These methods suffer from several drawbacks. The resulting oxide and reduced oxide treatment surfaces are fragile. Thus, processing was typically done by dipping individual parts into solutions. This method of manufacture is not conducive to high volume manufacturing which generally prefers continuous processing such as horizontal conveyorized treatment. Further, due to possible re-oxidation by atmospheric oxygen, reduced oxide treatment requires that layers be bonded (or be otherwise treated or stored to protect from atmospheric oxygen) within a relatively short time (typically less than 48 hours), which also adversely impacts manufacturing time and costs.
Horizontal conveyorized treatment methods eliminating pink ring (Americus C. Vitale, “DuraBOND Process Eliminates Pink Ring and Wedge Void Defects.”
IPC
32
nd
Annual Meeting
, (April 1989), and allowing extended storage time for the layers (e.g., U.S. Pat. No. 5,073,456) have been developed. Such methods are widely used, employing, e.g., a process sequence comprising immersion tin plating followed by an organosilane coupling agent treatment from, e.g., an aqueous solution. The '456 patent teaches that the immersion tin coatings alone are insufficient to form a direct bond between the electrically conductive layer (e.g., copper) surface and a dielectric material. The organosilane coupling agent is provided by the '456 patent as a solution to this problem.
The process of the '456 patent includes a step referred to as microetching, in which the surface of the metal is briefly treated to form a clean, uniform, microetched surface. In the present specification, the term “microetched” includes cleaning and/or pretreating methods in which an amount of metal equivalent to less than 40 microinches, usually less than 20 microinches, is removed from the surface of the treated metal substrate and in which, after the microetching, the surface is quite regular, being substantially free of deep intergranular crevices and steep-sided ridges and valleys.
The method for calculating the amount of metal removed in an etching process is based on the weight loss of a coupon of metal of a given area, which provides an average of the actual etch depth. This method is more fully described below. Such microetching results in a uniform, lightly microetched metal surface which facilitates application of a tin oxide/hydroxide layer by immersion plating, and which is relatively smooth. Such metal surfaces are free of deep, intergranular crevices, although some relatively isolated, angular-sided copper grains may be exposed on the microetched surface.
Recently a ‘new generation’ of continuous (e.g., horizontal conveyorized) processing methods which use neither immersion tin nor silane treatments to promote adhesion of a metal surface to a polymeric material has been taught (e.g., WO 96/17975, U.S. Pat. No. 5,869,130, WO 99/40764). These methods rely on the resulting roughness and high irregularity of the surface for mechanical enhancement of adhesion between the metal substrate and a polymeric material, such as a prepreg. These methods typically employ a strong acid/oxidizing agent intergranular etching solution, such as a sulfuric acid/hydrogen peroxide solution, modified by the addition of an inhibitor such as benzotriazole (or related compounds) and other additives, such as quaternary ammonium chloride surfactants, sodium chloride, or triphenyl-sulfonium chloride. These methods produce an intergranular etch of the metal surface resulting in a surface morphology characterized by the presence of steep-sided ridges and valleys and/or deep intergranular crevices, and may include a light coating of an oxide over the entire surface. In some cases the solutions include an additional inhibitor such as sodium meta-nitrobenzene sulfonate (U.S. Pat. No. 6,036,758).
Another ‘new generation’ continuous processing method (U.S. Pat. No. 5,807,493) also produces an intergranular etch, including deep intergranular crevices as described above, but does not result in an oxide uniformly over the surface. This method employs an etching chemistry based upon an organic acid having a pK
a
of at least 5, such as formic acid, with copper ion and chloride ion, to produce the intergranular etch. The etch step is followed by a desmutting step (e.g. hydrochloric acid ‘pickling’).
Methods for improving adhesion of metallic substrates to non-conductive, polymeric surfaces of PCBs have included application of additional metal layers by electrodeposition, e.g., Luce et al, U.S. Pat. No. 4,260,449 and Sadey et al, U.S. Pat. No. 6,042,711, and by immersion tin plating, e.g., Holtzman et al, U.S. Pat. No. 4,882,202 and Bokisa, U.S. Pat. No. 5,928,790. The electrodeposition processes provide improved peel streng
Bishop Craig V.
Bokisa George S.
Vitale Americus C.
Whitney, Jr. Dickson L.
Atotech Deutschland GmbH
Gulakowski Randy
Renner , Otto, Boisselle & Sklar, LLP
Winter Gentle E.
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