Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-04-10
2002-10-08
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S674000, C428S675000, C428S457000, C428S473500, C438S584000, C438S652000, C174S11000P, C174S126100
Reexamination Certificate
active
06461745
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a copper foil to be used for the production of TAB (Tape Automated Bonding) tape carriers and a TAB carrier tape and a TAB tape carrier both using the copper foil. Particularly, it relates to a copper foil for TAB tape carriers, which effectively prevents the generation of Sn whiskers and Kirkendall voids, and a TAB carrier tape and a TAB tape carrier using the copper foil.
(b) Description of the Related Art
To automate and speed up the mounting of semiconductor elements, there have been extensively used the TAB technique wherein a semiconductor element, such as IC or LSI, is mounted by wireless-bonding on a tape carrier, which is a lengthy, flexible insulating film bearing copper lead patterns.
Being excellent in corrosion resistance and soldering capability, Sn-plating film has recently been formed on copper lead patterns in substitution for Au-plating film. The Sn-plating film, however, is known to generate with passage of time Sn whiskers, which are beardlike, needle-like crystals. The growth of Sn whiskers causes short circuit. To prevent this, after Sn-plating, heating treatment for 1 to 2 hours at 100 to 150° C. has been required. The heating, however, causes rapid counter diffusion between the Sn-plating film and Cu of copper foil, and, because Cu atoms diffuse faster than Sn atoms, voids, what are called Kirkendall voids, are made on the side of the copper foil contacting the Sn-plating film. This lowers the adhesion of the Sn-plating film, causing peeling on soldering.
As the pitch of copper lead patterns has recently been made finer, prevention of the generation of Sn whiskers and Kirkendall voids has become an important object to be achieved promptly.
There have been taken various measures to prevent the generation of Sn whiskers and Kirkendall voids. For example, a means proposed to prevent the generation of Sn whiskers is the use of a tin plating bath to which indium and a salt of a low-melting metal, such as lead, bismuth or antimony, are added in such an amount as to form a deposit film containing 0.1 to 3.0% by weight of the low-melting metal. It is also proposed to plate as a backing for the tin-plating film a metal other than tin, such as backing solder-plating or backing nickel-copper two-layer plating.
To prevent the generation of Kirkendall voids, it is proposed to inhibit the counter diffusion of Cu and Sn by forming a Zn layer or a copper alloy layer containing at least 10 % by weight of Zn at the parts for bonding with solder.
These methods, however, involve the problems that (1) a step is added prior to Sn-plating, thereby increasing the production cost and lowering productivity and (2) copper foil itself cannot be improved.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a highly reliable copper foil for TAB tape carriers, which effectively prevents the generation of Sn whiskers and Kirkendall voids without requiring any addition or change of steps after the formation of copper lead patterns, and to provide a TAB carrier tape and a TAB tape carrier both using the copper foil.
Another object of the present invention is to provide a copper foil for TAB tape carrier, which maintains good properties necessary for forming copper leads, including heat resistance and solder wettability.
As the result of study for achieving the objects, we have found that the generation of Sn whiskers on the Sn plating film formed on patterned copper leads and the generation of Kirkendall voids due to the counter diffusion of Cu and Sn can be outstandingly prevented by using as a copper foil for TAB tape carriers a copper foil having on its shiny surface (S-surface) an alloy layer comprising nickel, cobalt and molybdenum (nickel-cobalt-molybdenum layer), and have made the present invention based on the finding.
The present invention provides a copper foil for TAB tape carriers, comprising
(a) a copper foil having a shiny surface and a mat surface; and
(b) an alloy layer comprising nickel, cobalt and molybdenum (nickel-cobalt-molybdenum layer), which is formed at least on the shiny surface.
The present invention further provides a TAB carrier tape, which comprises a flexible insulating film and the above-described copper foil for TAB tape carriers applied onto the flexible insulating film, with a surface on the side of the mat surface contacting the flexible insulating film.
The present invention further provides a TAB tape carrier, which is produced from the above-described TAB carrier tape by etching the copper foil for TAB tape carriers to form a copper lead pattern.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The copper foil for TAB tape carriers of this invention is characterized in having on at least the shiny surface (S-surface) of a copper foil an alloy layer comprising nickel, cobalt and molybdenum (nickel-cobalt-molybdenum layer). The nickel-cobalt-molybdenum layer formed on the copper foil is a thin alloy layer comprising metallic nickel, cobalt and molybdenum as main components, and is firmly adhering to the surface of the underlying copper foil. Near the surface of the nickel-cobalt-molybdenum layer, nickel and oxides and hydroxides of cobalt and molybdenum may be mixed in. The copper foil having the nickel-cobalt-molybdenum layer generates less Sn whiskers and less Kirkendall voids after Sn-plating following to the formation of copper lead patterns, thereby preventing short circuit and solder peeling, and also maintains good properties necessary for forming the copper leads, including etching capability, heat resistance and solder wettability.
As to the coating amounts and ratios of nickel, cobalt and molybdenum constructing the nickel-cobalt-molybdenum layer as measured as metals, the coating amount of each of nickel, cobalt and molybdenum is preferably 30 to 1,000 &mgr;g/dm
2
. If they are less than 30 &mgr;g/dm
2
respectively, the layer may be too thin to prevent the generation of Sn whiskers and Kirkendall voids sufficiently, and if more than 1,000 &mgr;g/dm
2
, the generation of Sn whiskers and Kirkendall voids will be prevented sufficiently, but the etching capability for forming copper lead patterns may be adversely affected depending on the ratios among them. Each coating amount is particularly preferably 50 to 500 &mgr;g/dm
2
.
In the present invention, electrolytic copper foil having a shiny surface (S-surface) and a mat surface (M-surface) is generally used as the starting copper foil. The copper foil may have any thickness of copper foil commonly used for TAB tape carriers, preferably 3 to 70 &mgr;m, more preferably 5 to 18 &mgr;m.
The surface roughness Ra of the S-surface of the copper foil is preferably 0.1 to 0.5 &mgr;m, and that of the M-surface is preferably 0.2 to 0.7 &mgr;m. Of course, the M-surface is rougher than the S-surface. To form fine copper lead patterns, the copper foil used in the present invention preferably has an M-surface with a surface roughness smaller than that of common copper foil. Further preferred surface roughness Ra of the M-surface is 0.2 to 0.4 &mgr;m. To produce TAB tape carriers having good handling quality, the copper foil to be used in the present invention preferably has a large tensile strength, particularly a tensile strength of 400 to 700 N/mm
2
. To give high peeling strength after lamination between the M-surface of the copper foil and a flexible insulating film, such as a polyimide film or a polyester film, to be bonded to the M-surface, the copper foil to be used preferably has a surface roughening treatment layer formed by subjecting the M-surface to surface roughening treatment. The surface roughening treatment may be performed, for example, by carrying out cathodic electrolysis treatment using a plating bath containing copper ions and nickel ions to form a finely granular roughened surface.
The means for producing the copper foil of the present invention by forming a nickel-cobalt-molybdenum layer on a copper foil may be known one, such as electroplating, chemical plating, vacuum deposition o
Endo Ayumi
Noda Kojiro
Jones Deborah
LaVilla Michael
Nippon Denkai Ltd.
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