Etching a substrate: processes – Forming or treating electrical conductor article
Reexamination Certificate
2000-06-30
2002-05-07
Stinson, Frankie L. (Department: 1746)
Etching a substrate: processes
Forming or treating electrical conductor article
43, C438S182000, C156S922000
Reexamination Certificate
active
06383401
ABSTRACT:
FIELD OF THE INVENTION
A process of manufacturing microelectronic circuit packages without the use of commoning bars is provided, specifically the manufacture of flexible wiring boards having a nickel/gold wirebonding surface on wirebond and ball grid array pads is described.
DEFINITIONS AND ABBREVIATIONS
To facilitate the understanding of this specification and the appended claims, definitions to various words and phrases are provided;
Component- a functional unit that is attached to a printed wiring board package either through a surface mounting or through hole,
Element- a functional unit integrally associated with the substrate of a printed wiring board (e.g., features and microcircuitry),
Features- Elements other than microcircuitry that are an integral part of the substrate of a printed wiring board (e.g., planar resistors, pads, lands, and commoning bar),
Lands- an SMT feature used for electrically interconnecting a component to a printed wiring board,
Microcircuitry (or fine-line circuitry)- electrically conductive lines that carry I/O signals, power, or create a path to ground,
Nascent- yet to be formed (e.g., in the process of manufacturing discrete microcircuitry, at early stages in the process the metallized microcircuits are formed but still are electrically attached to one another, at these stages the microcircuits are still in nascent form),
SMT-surface mount technology, and
VLSI-very large scale integrated circuit.
BACKGROUND OF THE INVENTION
In the past decade the density per unit area of electronic devices, such as VLSIs, has greatly increased. By some estimates this increase in density has been on the order of 10,000 times what it was in the earliest days of the technology. The space or area available outside of a VLSI in which to make the large number of necessary connections to and from it and to provide the necessary circuitry is becoming almost vanishingly small measured by previous standards.
Electronic circuits, and more particularly the more complex circuits found in computer logic systems, frequently employ one or more printed circuit or wiring boards on which various components or elements are mounted. Unlike the density increase of VLSIs, however, the density of passive circuits on printed wiring boards have increased by only a relatively small factor: less than about 4 to 1 as the components have not decreased in size significantly. This presents the difficult problem of providing circuitry on the printed wiring board which is small enough to fit the spaces available and which is also sufficiently reliable and manufacturable to be economically useful.
With this ever-continuing trend toward reduced size of electronic components and the resulting need for high density requirements in electronic packaging, there has been an increased demand to create a metallization process that will efficiently generate high quality, high density electronic packaging such as printed wiring board structures.
In order to electronically interconnect components and printed circuit boards for feeding electrical power and signals to the circuit elements and for extracting signals therefrom, various connector arrangements are utilized. Components of the external system can be integrated circuit chips, adapter cards, and insulating packages. The leads of these components are in the form of pads on the surface or other contacts extending therefrom in rows to form planar disposed arrays matched with conductive pads on circuits joined to circuit traces leading to and from the components.
Interconnection between the conductive pads or leads of a component to the conductive pads or traces of a circuit board is accomplished in a number of ways, including solder or, in instances where the removal and replacement of components is necessary during the life of a system, by some suitable electrical connector or disconnect. In the latter case, electrical connection between printed circuit boards and cards and external systems can be provided by gold contacts such as edge tabs, chip tabs, and lands. The gold contacts atop lands and tabs are typically provided by electrodeposition.
Electroplating is one method of depositing an adherent metal coating on a substrate for protection purposes. The substrate to be plated is connected to one terminal of d.c. or a pulsed plating voltage source and placed in an electrolyte. The metal to be deposited is connected to the other terminal and similarly immersed in the electrolyte. The transfer of the metal is accomplished via the ions contained in the current flowing between the electrodes.
Electroless plating is another method of depositing a metal. It involves the use of a plating bath without the imposition of any electric current. The substrate is plated by reduction of a plating metal from a solution of a salt of the plating metal. The plating solution contains controlled reducing agents which are generally catalyzed either by the surface of the substrate, or by some catalytic metal placed onto the surface both to initiate the reduction and to render good adherence. Since the plated-on surface is autocatalytic, an electroless process can be used to build up thicknesses.
Electrodeposition (electroplating) of gold has been the preferred method for plating gold, since the deposited gold has improved physical properties (e.g., less brittle) compared to electroless plated gold. This softness or plasticity is desirable in order to provide contact sites with high durability, especially where components can be replaced multiple times or wire bonding is required. The plasticity of electroplated gold also assists in ameliorating the brittleness of an electroless plated nickel or nickel/phosphorous layer that is optionally plated between the conductive copper features and the gold, or other precious metal overplating. However, a major disadvantage with electrolytic gold plating is the need for commoning bars to provide electrical connections to the features to be plated. These commoning bars require fairly large footprints on the printed wiring board, ultimately wasting valuable space that could otherwise be used for placement of additional circuitry or other features such as planar resistors, capacitors, inductors, diodes, or transistors.
As previously mentioned, numerous types and varieties of modern equipment and devices require sophisticated interconnection of electronic components. With the constant demand for reduced sizes in electronic components and the resulting high density of conductive interconnection surfaces on such equipment, there have been increased demands on the performance of contacts used to provide such interconnections.
A conventional pin and socket connector part, such as a 25 square metal wire-wrap post, has sufficient size and strength to permit it to be made and handled easily with conventional techniques. Typically parts of such “large” size are assembled into connector systems having “large” centers, such as one-tenth by one-tenth inch. But connectors this large and unwieldy are like the dinosaurs of a past age in the environment of the VLSIs of today. As interconnections are made smaller and smaller, the problems associated with manufacturing and assembling these miniature parts seem to grow exponentially.
Printed circuits are normally formed on boards or laminate sheets made of various epoxy compositions or fiberglass and relatively thin layers of copper which have been etched or deposited to define the desired circuit. The problem in each case is one of coupling interconnections from the relatively thin circuit conductor leads which are “printed” on the board to either a solder site or a mechanical connector which is generally three-dimensional. In the past, these boards or sheets were rigid substrates; but due to the desire to further reduce the size and weight of the passive circuitry and the added benefit of having the design of higher level packaging where planarity was no longer a restriction, the use of lightweight and flexible materials is advantageous. In turn, the trend toward flexible substrates requires the potent
Bajkowski David J.
Knoll Allan R.
Labzentis Daniel P.
Marconi Francesco F.
International Flex Technologies, Inc.
Salzman & Levy
Stinson Frankie L.
Winter Gentle E.
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