Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2000-09-28
2002-05-28
Cuneo, Kamand (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S255000, C174S256000, C174S258000, C174S259000, C361S749000, C361S750000, C361S751000, C361S767000, C361S779000, C361S792000, C439S067000
Reexamination Certificate
active
06395993
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of flexible wiring boards, particularly to the technique of laminating single-layer flexible wiring boards to form a multilayer flexible wiring board of laminated structure.
PRIOR ART
Recently, multilayer flexible wiring boards are widely used in the field of electronic apparatus.
A method for obtaining a multilayer flexible wiring board involves laminating a plurality of single-layer wiring boards.
In order to laminate single-layer wiring boards to form a multilayer flexible wiring board, we began by preparing first and second flexible wiring boards
110
,
120
as described below on trial.
Referring to FIG.
8
(
a
), first flexible wiring board
110
has a first base film
111
, on which a first conductive film
112
is formed by patterning a copper foil.
A gold coating
113
is formed on the surface of first conductive film
112
so that gold coating
113
and first conductive film
112
form a first wiring film
119
. A plurality of such first wiring films
119
are formed.
The reference
114
in FIG.
8
(
a
) represents an opening region including an opening or space located between first wiring films
119
, which are insulated from each other by said opening region
114
. First base film
111
is exposed at the bottom of opening region
114
.
The reference
120
in FIG.
8
(
b
) represents a second flexible wiring board to be laminated to said first flexible wiring board
110
.
This second flexible wiring board
120
has a second base film
121
, on which a second conductive film
122
is formed by patterning a copper foil. A resin film
123
including a thermoplastic resin having insulating properties is formed on the surface of second conductive film
122
, and a metal projection
125
is mounted on second conductive film
122
with the top projecting from the surface of resin film
123
.
A low-melting metal coating
126
including solder is formed on the surface of metal projection
125
so that low-melting metal coating
126
and metal projection
125
form a bump
124
.
For laminating said first and second flexible wiring boards
110
,
120
, these flexible wiring boards are first arranged in such a manner that first conductive film
112
of first flexible wiring board
110
and bump
124
on second flexible wiring board
120
face each other (FIG.
9
(
a
)).
Then, gold coating
113
on first conductive film
112
and low-melting metal coating
126
on bump
124
are contacted with each other (FIG.
9
(
b
)) and heated under pressure until low-melting metal coating
126
melts. Then, low-melting metal (solder in this example) is cooled to solidify to connect first conductive film
112
and bump
24
.
Thermoplastic resins soften to develop adhesiveness upon eating. Resin film
123
softens as low-melting metal coating
126
melts, and then it is cooled to bond two flexible wiring boards
110
,
120
via resin film
123
. As a result, first and second conductive films
112
,
122
are electrically connected via bump
124
into a multilayer flexible wiring board
130
(FIG.
9
(
c
)).
However, there is a demand for reducing the space between wiring films of flexible wiring boards to be heih-density.
In case of multilayer flexible wiring board
130
having the structure described above, low-melting metal coating
126
melts under pressure to sometimes fly about.
If the space between first wiring films
119
or between second conductive films
122
is narrow, the flying low-melting metal may form a bridge
127
to invite short circuit. In FIG.
9
(
c
), the resulting bridge
127
causes short circuit between first wiring films
119
.
An object of the present invention is to provide a technique for laminating flexible wiring boards at high yield to overcome the disadvantages of the prior art described above.
SUMMARY OF THE INVENTION
In order to attain the above object, the present invention provides a multilayer flexible wiring board comprising laminated first and second flexible wiring boards, said first flexible wiring board comprising a first base film, a first wiring film having a first conductive film applied on said first base film and patterned, a contact pad including the part of said first wiring film exposed at the surface, and a wall member provided around said contact pad and rising above the top of said contact pad, and said second flexible wiring board comprising a second base film, a second wiring film having a second conductive film applied on said second base film and patterned, and a plurality of bumps connected to said second wiring film and exposed at the top, wherein a low-melting metal coating is formed on either of the surface of the part of said first conductive film forming said contact pad or the surface of at least the top of said bump, and said low-melting metal coating melts under heat and then solidifies to connect said contact pad and said bump.
In the multilayer flexible wiring board of the present invention, said low-melting metal may be an alloy based on lead and tin.
In the multilayer flexible wiring board of the present invention, said low-melting metal may be an alloy based on tin and gold but not including lead.
In the multilayer flexible wiring board of the present invention, a highly wetting metal coating including a metal or alloy being wetter than copper with said low-melting metal coating, may be formed on the surface of the part of said metal film forming said contact pad or the surface of at least the top of said bump on which said low-melting metal coating is not formed.
In the multilayer flexible wiring board of the present invention, said highly wetting metal coating may be any of a gold coating based on gold, a platinum coating based on platinum, a silver coating based on silver, a nickel coating based on nickel, a copper-nickel alloy coating or a solder coating.
In the multilayer flexible wiring board of the present invention, the height of said wall member above the top of said contact pad may be 5 &mgr;m or more.
In the multilayer flexible wiring board of the present invention, a first resin film is provided on said first wiring film and an opening is provided in said first resin film at the top of said contact pad so that said wall member includes said first resin film forming the inner circumferential face of said opening.
In the multilayer flexible wiring board of the present invention, said first resin film has the property of developing adhesiveness upon heating so that said first flexible wiring board and said second flexible wiring board are bonded together via said first resin film.
Said opening of the multilayer flexible wiring board of the present invention has such a volume that said low-melting metal coating does not overflow outside said opening when said low-melting metal coating is molten while said bump is in contact with said contact pad.
Said opening of the multilayer flexible wiring board of the present invention has such shape and area that said bump can come into contact with said contact pad without touching said wall member.
The height of the projecting part of said bump of the multilayer flexible wiring board of the present invention is larger than the height of said wall member from the top of said contact pad.
Said second flexible wiring board of the multilayer flexible wiring board of the present invention has at least a second resin film provided on at least said second wiring film and the top of said bump projects from said second resin film.
Said second resin film of the multilayer flexible wiring board of the present invention has the property of developing adhesiveness upon heating so that said first flexible wiring board and said second flexible wiring board are bonded together via said second resin film.
The multilayer flexible wiring board of the present invention has the structure described above, comprising first and second wiring films wherein a low-melting metal coating is formed on at least one of the surface of the contact pad at the top of the first wiring film or the surface of the bump. A wall member rising abov
Fukuda Mitsuhiro
Nakamura Masayuki
Cuneo Kamand
Oliff & Berridg,e PLC
Patel I B
Sony Chemicals Corp.
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