Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1994-02-02
2001-06-12
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S675000, C428S680000, C428S929000, C427S125000, C257S677000
Reexamination Certificate
active
06245448
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to integrated circuits, and more particularly to integrated circuit devices having reduced galvanic migration of corrosive products.
BACKGROUND OF THE INVENTION
In the construction of integrated circuit devices, lead frames are used to provide electrical interconnection to a semiconductor circuit. Typically, the base metal of the lead frame is copper because of its high thermal conductivity. Other base metals include stainless steel and Alloy 42, which is an alloy of 42% nickel and 58% iron. In some instances, a nickel layer on the order of 100 microinches is formed over the base metal to prevent temperature driven diffusion of the copper to the surface of the lead frame. Corrosion products formed by copper diffusion, such as copper sulfides and oxides, will degrade the solderability of the lead frame and will reduce the shelf life of the final product.
The nickel layer, however, contains pores through which the corrosion products may migrate. A nickel layer thickness of at least 400 microinches would be needed to reasonably assure that no continuous paths through the nickel layer would be available for copper migration. Unfortunately, a thickness of this magnitude will crack when the leads are eventually bent to form the dual inline package (DIP) or surface mount integrated circuit (SMIC).
A layer of palladium (Pd) may be formed over the nickel layer. The palladium top surface may be thermosonically bonded to gold wire, providing a stronger bond than available with a silver lead end. Furthermore, palladium will not tarnish or oxidize in air, thus retaining a clean bonding surface indefinitely. The palladium layer, however, will produce a galvanic potential between the palladium layer and the copper base, drawing copper ions to the surface. This galvanic couple accelerates pore corrosion in the palladium plated lead frame, which results in oxides and sulfides and other reaction products of copper appearing on the lead frame surface. The oxides and sulfides and other corrosion products discolor the surface of the lead frame and degrade its solderability.
Therefore, a need has arisen to provide an integrated circuit lead frame with reduced galvanic potential for preventing surface corrosion.
SUMMARY OF THE INVENTION
In accordance with the present invention, a lead frame is provided which substantially eliminates or prevents the disadvantages and problems associated with prior lead frames.
The lead frame of the present invention provides a palladium plated lead frame which may be used with a copper base metal, or other base metal, without contamination of the top surface due to a galvanic potential between the palladium top surface and the base metal. A palladium
ickel alloy layer separates the base metal from a nickel intermediate layer which is used to prevent thermal diffusion of the base metal atoms to the upper surface. The palladium
ickel alloy layer minimizes the galvanic potential across the nickel intermediate layer, thereby preventing migration of the base metal ions to the top surface. Corrosion products formed in the palladium
ickel alloy layer due to the potential between the palladium
ickel alloy layer and the base metal will be neutral and insoluble; therefore, they will not migrate through the nickel intermediate to the top surface.
This aspect of the present invention provides the technical advantage of a palladium plated lead frame which is not subject to corrosion due to galvanic potential forces aiding the migration of the base metal ions to the top surface where they will form corrosion products.
In another aspect of the present invention, a nickel strike layer is formed between the base metal and the palladium
ickel alloy layer in order to prevent contamination of the palladium
ickel bath used to form the palladium
ickel alloy layer. The nickel strike in conjunction with the palladium
ickel layer also provides the technical advantage of reducing the porosity of the material between the base metal and the palladium top layer, thereby mininizing the paths through which a base metal ion could migrate.
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Brady W. James
Courtney Mark
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Zimmerman John J.
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