Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2002-08-29
2004-05-04
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
83, 83
Reexamination Certificate
active
06729022
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of flexible wiring boards, particularly the field of multilayer flexible wiring boards.
PRIOR ART
Double-sided flexible wiring boards having patterned metal films on both sides of a resin film are widely used because of the high degree of freedom of interconnection.
The metal films on both sides of the resin film are electrically connected to each other. Conventional methods for connecting these metal films are explained below.
First, the through hole method is explained. Referring to FIG.
7
(
a
), the reference number
110
represents a base material for flexible wiring boards having metal films
112
,
113
consisting of a copper foil adhered to the top surface and the bottom surface of a polyimide film
111
.
This base material
110
is punched with a drill or the like to form a through hole
118
as shown in FIG.
7
(
b
). Then, the assembly is carbonized and then electroplated, so that a copper plating layer
115
grows within the through hole
118
and on the surfaces of the metal films
112
,
113
to connect the two metal films
112
,
113
via the copper plating layer
115
within the through hole
118
, as shown in FIG.
7
(
c
).
Secondly, the via hole method is explained. Referring to FIG.
8
(
a
), a base material
120
having a polyimide film
121
adhered on a metal film
122
consisting of a copper foil is prepared and an opening
128
is formed in the polyimide film
121
by photolithography (FIG.
8
(
b
)).
Thus, the metal film
122
is exposed at the bottom of the opening
128
, and a copper thin film is formed by sputtering on the surface of the metal film
122
exposed at the bottom of the opening
128
and on the surface of the polyimide film
121
in this state followed by electroplating to form a copper plating layer
123
on the top surface of the polyimide film
121
and the inner face of the opening
128
and the top of the metal film
122
exposed at the bottom of the opening
128
. This copper plating layer
123
is connected to the metal film
122
at the bottom of the opening
128
.
However, the through hole connection method requires a long time to form many through holes
118
because through holes
118
are drilled one by one. Moreover, it cannot satisfy the demand for fine patterns because the size of through holes
118
is limited to about 0.2 mm &phgr;.
The metal film
123
formed by the via hole method is easily separated because of the weak adhesive power between the copper plating layer
123
and the polyimide film
121
. Furthermore, defects such as pinholes are more liable to occur in the copper plating layer
123
to make inferior the reliability.
An object of the present invention is to overcome the disadvantages of the prior art described above and to provide a technique that can connect metal films without forming any opening.
SUMMARY OF THE INVENTION
In order to attain the above object, the present invention provides a process for manufacturing a flexible wiring board, comprising the steps of forming an uncured first resin film including a solvent on a first metal film, pressing bumps on a second metal film against said first resin film to force said bumps into said first resin film until the tops of said bumps come into contact with said first metal film, then patterning at least one of said first or second metal films, and heat-treating said first resin film while the top surface of said first resin film is at least partially exposed to cure said first resin film.
In one embodiment of the process for manufacturing a flexible wiring board according to the present invention, said uncured first resin film is semicured by heating it before said bumps are pressed against said first resin film.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said semicuring step takes place at a temperature lower than the boiling point of said solvent included in said uncured first resin film.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said semicuring step takes place at a temperature ranged from 80° C. to 300° C.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said first resin film is softened by heating it when said bumps are forced into said first resin film.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said curing step is followed by applying ultrasonic wave to either one or both of said bumps and said first metal film to connect said bumps to said first metal film.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said step of curing said first resin film is preceded by patterning either one of said first or second metal film and ultrasonic treating the unpatterned metal film and then patterning it.
Another embodiment of the process for manufacturing a flexible wiring board according to the present invention further comprises the steps of forming a second resin film on the top surface of said patterned first or second metal film, then pressing bumps on a third metal film against said second resin film to force said bumps into said second resin film until they come into contact with said first or second metal film, then patterning said third metal film and then curing said second resin film.
In this embodiment, said uncured first resin film may be semicured by heating it before said bumps are pressed against said first resin film.
Another embodiment of the process for manufacturing a flexible wiring board according to the present invention further comprises the steps of forming a second resin film on the top surface of said patterned first or second metal film, then pressing bumps on a third metal film against said second resin film to force said bumps into said second resin film until they come into contact with said first or second metal film, then patterning said third metal film, then curing said second resin film and then applying ultrasonic wave to said bumps on said third metal film to connect said bumps to said first or second metal film.
In this embodiment, said curing step maybe followed by applying ultrasonic wave to said bumps on said third metal film to connect said bumps to said first or second metal film. It is possible to apply ultra sonicwave indirectly to said bumps by applying ultrasonic wave to said first or second metal film to connect said bumps to said first or second metal film. It is also possible to apply ultrasonic wave to both said bumps and said first or second metal film.
In this embodiment, said uncured first resin film may also be semicured by heating it before said bumps are pressed against said first resin film.
The present invention also provides a flexible wiring board comprising a plurality of patterned metal films with a resin film being interposed there between among which adjacent two metal films are electrically connected to each other via bumps, wherein said resin film is cured after said bumps are pressed against the top surface of said resin film and forced into said resin film to electrically connect said two metal films via said bumps.
In one embodiment of the flexible wiring board according to the present invention, said resin film is cured by heat-treating it while the surface of said resin film is at least partially exposed between said patterned metal films.
In another embodiment of the flexible wiring board according to the present invention, one of said two adjacent metal films connected via said bumps is ultrasonically bonded to said bumps.
According to the present invention as defined above, bumps are pressed against a first resin film and forced into the first resin film. Thus, the bumps can be contacted with the metal film underlying the first resin film without forming any opening in the first resin film. The first resin film is preferably softened by heating it when the bumps are forced int
Kurita Hideyuki
Watanabe Masanao
LandOfFree
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