Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating predominantly nonmetal substrate
Reexamination Certificate
2000-03-24
2003-10-14
Wong, Edna (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating predominantly nonmetal substrate
C205S220000, C205S164000, C205S125000
Reexamination Certificate
active
06632344
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Introduction
This invention relates to new metal plating processes for a variety of uses including fabrication of printed circuit boards. In one embodiment, the invention relates to direct electroplating of copper over a substrate while avoiding the use of an electroleess metal plating solution or the use of an extraneous conductive layer. The process is selective to non-conducting portions of a printed wiring board base material and therefore, copper clad surfaces of a circuit board substrate remain essentially unaffected by the procedure.
2. Description of the Prior Art
For purposes of discussion only, the major portion of the description that follows is directed to the fabrication of printed circuit boards (PCBs) though the process is suitable for formation of any metal coated substrate. Accordingly, the process is useful for decorative plating, formation of mirrors, formation of circuits inclusive of multilayer circuits, double sided circuits and flex circuits.
A conventional process for fabricating a multilayer circuit board starts with a copper clad epoxy innerlayer substrate. A circuit is formed on the innerlayer, multiple innerlayer circuits are stacked, and the stack laminated using heat and pressure. To provide electrical connections between circuit layers, conductive pathways are formed within and through the stack at desired locations. A first step in the process of forming conductive pathways is to drill an array of holes into the stack at desired locations.
Following the drilling of holes, a pretreatment sequence is required prior to metallizing the hole walls. The process involves subjecting the hole walls to a step of smear removal. Smear refers to the drill smear on the walls of holes formed by frictional heating and melting of epoxy during the drilling step. The elimination of smear is practiced in all processes for circuit fabrication to achieve metal adhesion to the hole wall and full electrical continuity between layers of the board through plated holes or vias. Etchback is a term used in connection with smear removal. The term refers to smear removal and is used to remove epoxy to a level deeper than the thickness of the smear, typically to expose about 0.5 mils on the top and bottom surfaces of the innerlayer copper circuitry. Desirably, innerlayer copper protrudes into the drill hole to provide three point connection for copper bonding as required for some military spec boards.
Four methods are used for smear removal and etchback. All use oxidation, neutralization-reduction and glass etching. For many years, sulfuric acid has been used. Major disadvantages include lack of control which leads to hole-wall pullaway and rough holes, and the corrosive nature of sulfuric acid which makes it hazardous to use and difficult to waste treat. Chromic acid provides more control and a longer bath life but problems are encountered with copper voids due to Cr
+6
poisoning, waste water pollution and contamination of plating processes. Plasma etching is a dry-chemical method to achieve dielectric oxidation. In this process, boards are exposed to oxygen and fluorocarbon gases. Persistent problems with the process are non-uniform treatment of holes and high cost of equipment. Moreover, the process is a batch process and not subject to automation, controls must be provided to prevent air pollution and the cost of the process has limited its use. Permanganate is the current method of choice due to improved copper adhesion to the hole wall resulting in less pullaway, smoother plated through-holes and better control. Permanganate desmear is described by C. A. Deckert, E. C. Couble and W. F. Bonetti, “Improved Post-Desmear Process for the Multilayer Boards”, IPC
Technical Review
, January 1985, pp. 12-19 incorporated herein by reference. Moreover, permanganate is non-toxic and easily waste treated.
It is known that permanganate desmear leaves a residue on the hole wall that is a mixture of oxides of manganese, unreacted permanganate and organo manganese compounds. The art has long considered this residue to be undesirable and therefore, has adopted procedures to remove this residue.
A conventional permanganate desmear and etchback process involves the following steps:
Solvent Pretreatment
90° F.
5 minutes
Alkaline Permanganate
170° F.
10 minutes
Neutralizer
120° F.
5 minutes
Glass Etch
RT
4 minutes
Solvent pretreatment softens the surfaces of the epoxy. The permanganate solution activates the surface of the epoxy by oxidation and etching. The epoxy substrate is composed of organic micelles. Contact with the alkaline permanganate solution is believed to etch or oxidize micelles from the surface of the epoxy resulting in a roughened porous surface into which a subsequently applied metal deposits. This mechanically bonds the metal deposit to the surface of the epoxy. Contact of the permanganate solution with the hole wall reduces permanganate leaving the above described dielectric oxidation/permanganate residues on the surface of the epoxy. Neutralizer (acid solution) is then used to dissolve the dielectric oxidation residues from the surface of the non-conductor. This is believed to be a critical part of the process as the presence of permanganate residues is thought to interfere with bond strength between the substrate and a subsequently applied metallic coating. Finally, a glass etchant is used to remove glass fibers protruding into the hole due to removal of the epoxy surface during the permanganate etch step.
Metallization is a series of chemical steps following desmear. The process is used to make panel side-to-side and innerlayer connections by plating non-conducting epoxy surfaces with copper. Metallization may be conducted using either electroless techniques or by direct electroplating of copper, a process the industry has termed the direct metallization technique (DMT).
The electroless process includes the above described steps of solvent pretreatment, desmear, treatment with neutralizer, racking the PCB substrates, copper microetching, hole and surface catalyzing with a palladium colloid and electroless copper deposition. A typical sequence following desmear and omitting rinse steps, follows:
Step 1: Cleaner-Conditioner. Alkaline cleaning to remove soil and condition holes.
Step 2: Microetch. Acid etching to remove copper surface contaminants.
Step 3: Sulfuric Acid. Used to remove microetch residues.
Step 4: Pre-dip. Used to maintain chemical balance for the next treatment step.
Step 5: Catalysis. Use of an acid solution of palladium and tin to deposit a thin layer of surface active palladium on the holes walls.
Step 6: Acceleration. Used to remove colloidal tin from the palladium colloid.
Step 7: Electroless Copper. Alkaline chelated copper reducing solution that deposits a thin copper deposit on the surfaces of the holes and other surfaces.
Difficulties are encountered using electroless copper solutions. For example, it is difficult to obtain uniform deposits because the solution composition changes with use. Voids may occur within the holes due to improper hole wall preparation. Plated copper on the hole-walls may pull away and form blisters. Electroless copper solutions may decompose resulting in a rapid plate out of the copper onto the circuit board substrates and onto all surfaces exposed to the copper solution such as tanks and racks. Copper to copper bond failure often occurs if copper cladding is inadequately prepared since electroless copper is deposited onto the copper cladding of the printed circuit board. Waste treatment of electroless copper solutions is costly since the spent solutions contain complexing agents and copper ions, both of which must be removed prior to discharge to the environment. The reducing agent used in electroless copper solutions is conventionally formaldehyde, a suspected carcinogen.
The industry is currently adopting direct metallization techniques (DMT) as an alternative to electroless plating. The basic concept of using a palladium system for direct metallization is disclosed in
Goldberg Robert L.
Gulla Michael
Goldberg Robert L.
Rosenthal Robert G.
Wong Edna
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