Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-02-28
2003-11-11
Chang, Richard (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S842000, C029S830000, C174S267000, C361S760000, C361S774000, C257S737000, C257S782000
Reexamination Certificate
active
06643923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of flexible wiring boards, particularly to a process for manufacturing a flexible wiring board capable of forming fine metal bumps and the flexible wiring board manufactured thereby.
2. Description of Related Art
Recently, there is an increasing demand for miniaturized semiconductor devices and a great importance is placed on flexible wiring boards on which a bare-chip semiconductor can be mounted.
FIGS.
4
(
a
)-(
d
) is a processing diagram showing a process for manufacturing a flexible wiring board of the related art. Referring to the processing diagram, the process is explained in order. At first, a copper foil is applied on a polyimide film
111
and then the copper foil is patterned into a copper wiring
112
(FIG.
4
(
a
)).
Then, the surface of polyimide film
111
is irradiated with laser beam
114
(FIG.
4
(
b
)) to form openings
115
having a predetermined diameter (FIG.
4
(
c
)). At this stage, the top surface of copper wiring
112
is exposed at the bottoms of openings
115
, and then copper wiring
112
is plated with copper while the bottom surface is protected with a resin film
117
so that copper grows in openings
115
to form metal bumps
116
.
When a bare-chip semiconductor device is to be mounted on such a flexible wiring board
110
, an anisotropic conductive film is applied on metal bumps
116
and bonding pads of the semiconductor device are brought into contact with metal bumps
116
via the anisotropic conductive film and pressure is applied. Then, circuits within the semiconductor device contact copper wiring
112
via the anisotropic conductive film and metal bumps
116
.
Flexible wiring boards of this type
110
are recently much used because they are thin and light and freely foldable to provide a high mounting flexibility.
However, residues of polyimide film
111
remain on the top surface of metal wiring
112
exposed at the bottoms of openings
115
when openings
115
are formed with laser beam
114
as described above. Residues are removed by immersing the assembly in a chemical solution after openings
115
have been formed. However, it becomes more difficult for the chemical solution to penetrate into openings
115
as openings
115
become finer, and therefore more difficult to remove residues.
If residues cannot be removed, copper deposition speed varies from opening
115
to opening
115
, whereby homogeneous metal bumps
116
cannot be formed.
Another problem is variation in the diameter of fine openings
115
(about 40 &mgr;m to 50 &mgr;m) formed by irradiating a rigid polyimide film
111
with laser beam
114
, resulting in variation in the diameter and height of metal bumps
116
which causes failure of connection with semiconductor chips.
Still another problem is that it is difficult to reduce the spot diameter of high power laser beam
114
, which makes it impossible to form openings
115
having a diameter smaller than 40 &mgr;m, contrary to the recent demand for finer openings
115
.
SUMMARY OF THE INVENTION
An object of the invention is to provide a technique capable of forming fine metal bumps with good precision to overcome the above disadvantages of the related art.
To attain the above object, the invention provides a process comprising the steps of forming a mask film, patterned by exposure and development, on a metal foil and growing metal bumps on the metal foil exposed at the bottoms of openings in the mask film.
In the invention, the step of growing metal bumps is followed by the steps of removing the mask film, applying a liquid resin material to form a resin material coating on the surface of the metal foil on which the metal bumps have been formed, and then curing the resin material coating into a resin film.
In the invention, the resin material coating may consist of a plurality of layered coatings.
When the resin material coating consists of a plurality of layered coatings, at least the uppermost coating may be a thermoplastic coating.
In the invention, the surface of the resin material coating on the metal foil may be located below the height of the metal bumps.
In the invention, the height of said metal bumps from the surface of the resin film may be 35 &mgr;m or less.
In the invention, the curing step may be preceded by the step of etching surface portions of the resin material coating.
In the invention, the resin material may be a liquid containing a polyimide precursor to form the resin film from a polyimide.
In the invention, the step of forming a resin film may be followed by the step of partially etching the metal foil from the bottom surface to form a patterned metal wiring.
In this case, a support film may be formed on the bottom surface of the metal wiring.
In the invention, the support film may be partially etched to expose desired regions of the metal wiring.
Said process may further comprise the steps of bringing bonding lands of a semiconductor chip into contact with the metal bumps and applying heat and pressure to allow the resin film to develop adhesiveness, whereby the semiconductor chip is bonded to flexible wiring board.
The invention also provides a flexible wiring board manufactured by the process as defined above.
Flexible wiring boards of the invention include those having a semiconductor device connected to the metal bumps.
As defined above, the invention relates to a process for manufacturing a flexible wiring board having metal bumps and the flexible wiring board manufactured thereby.
In the invention, an exposable dry film or resist film is applied or deposited on a metal foil and patterned by exposure and development to form a mask film.
The metal foil is exposed at the bottoms of openings in the mask film, so that metal bumps grow at exposed regions of the metal foil when the metal foil is immersed in a plating solution while its bottom surface is protected.
The openings in the mask film can be formed in a fine size with high precision by photolithography. Therefore, the metal bumps can also be homogeneously grown both in width and height.
Then, a liquid resin material is applied and dried or otherwise treated to form a resin material coating on the surface of the metal foil on which the metal bumps have been formed, after which the resin material coating is heated or otherwise cured into a resin film, whereby the surface of the metal foil on which the fine metal bumps have been formed can be covered with the resin film. If the resin material coating has a thickness smaller than the height of metal bumps, the tops of the metal bumps may project from the surface of the resin film without post-treatment.
If the resin material cover the tops of the metal bumps, the resin film is also formed by curing on the surfaces of the metal bumps, which can be, however, exposed by polishing or etching.
If etching is used, an uncured resin material coating can be etched to form a resin film with the tops of the metal bumps being exposed.
The resin film may be a thermosetting or thermoplastic film or a laminate of such films as far as it is flexible. From the viewpoint of durability or reliability, it is preferable that the resin material is a polyimide precursor to be cured into a polyimide film.
After the resin film has been formed, the bottom surface of the metal foil can be exposed and etched using a dry film or photoresist as a mask to give a copper wiring. Then, a support film can be formed on the bottom to protect the copper wiring, whereby a flexible wiring board having reliable insulating properties is obtained.
The resin film can be formed to have a multilayer structure by layering resin material coatings. If the uppermost layer of the resin film consists of a thermoplastic resin, the thermoplastic resin film develops adhesiveness upon heating to ensure bonding to a semiconductor device or the like without using anisotropic conductive film.
The support film may be formed by applying a sheet-like film or coating a resin material solution as defined above and curing it. The support film may be patterne
Hishinuma Hiroyuki
Ito Ryo
Kurita Hideyuki
Nakamura Masayuki
Chang Richard
Oliff & Berridg,e PLC
Sony Chemicals Corp.
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