Wiring board and a process of producing a wiring board

Details Wiring board and a process of producing a wiring board Wiring board and a process of producing a wiring board

2001-10-25

2004-08-24

Lam, Cathy (Department: 1775)

Stock material or miscellaneous articles

Structurally defined web or sheet

Discontinuous or differential coating, impregnation or bond

C428S901000, C174S258000, C174S259000, C174S264000

Reexamination Certificate

active

06780493

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wiring board having a through hole and a production process of such a wiring board.
2. Description of the Related Art
Recently, under pursuing compaction, thinning, lightning, and multi-functionality of electronic equipment, various technical developments of a wiring board for highly densely mounting of electronic parts which constitute the electronic equipment have been intensively carried out together with compaction and thinning of the electronic parts.
Especially nowadays, it is highly desired with rapid developments of mounting techniques to provide an inexpensive multi-layer printed wiring board on which semiconductor chips such as LSI chips can be mounted highly densely and which can adapt to a high speed circuit.
In order to meet such a desire, a substrate for circuit formation is disclosed (in Japanese Patent Kokai Publication No. 6-268345), in which wiring patterns which are so laminated on an insulation substrate that they sandwich it, are electrically connected by an electrically conductive material filled in through holes or via holes.
FIGS.
9
(
a
) to (
f
) schematically show steps for the production of such a substrate for the circuit formation. It is to be noted that
FIG. 9
shows cross sectional views which are perpendicular to main surfaces of the substrate (also in the other drawings, the same cross sectional views are shown). The substrate for the circuit formation is produced as follows:
As shown in FIG.
9
(
a
), through holes
2
are first formed at predetermined positions through an insulation substrate
1
made of a resin impregnated fibrous sheet
1
as a prepreg such as an aramid fiber/epoxy resin composite of which each side is coated with a non-tacky release film
17
(which may be referred to as a parting film).
Then, the through holes
2
are filled with an electrically conductive paste
3
by for example printing, followed by removing the release films
17
to have a state as shown in FIG.
9
(
b
). The release paste comprising electrically conductive particles (for example metal fine particles) and a thermosetting resin component as a binder protrudes above each surface of the insulation substrate
1
in an amount corresponding to a thickness of the release film
17
. It is to be noted that the protrusion is shown exaggeratedly for ease of understanding (the same is applicable to the other drawings).
Next, a copper foil
4
is placed on the each side of the resin impregnated fibrous sheet
1
, and by pressing and heating the sheet, the resin impregnated fibrous sheet
1
in a prepreg state and the conductive paste
3
are compressed, the resin contained in the sheet and the paste is cured, and the copper foils
4
are bonded to the sheet
1
as shown in FIG.
9
(
c
). Since the resin impregnated fibrous sheet
1
includes at least some voids therein, it is compressed when it is heated and pressed. Simultaneously, the resin of the conductive paste penetrates into the inside of the resin impregnated fibrous sheet
1
when heated and pressed, so that a concentration of the conductive fine particles in the paste is increased (that is, the conductive fine particles are dandified, and therefore an electrical resistance of the conductive fine particles is reduced, which increases reliability of the electrical connection with the paste). Thus, simultaneously with bonding the copper foils
4
to the both sides of the resin impregnated fibrous sheet
1
, the copper foils
4
are electrically connected with the through holes which are filled with the conductive paste
3
.
Then, the copper foils
4
on both sides of the resin impregnated fibrous sheet
1
are etched with the conventional photolithography method, and wiring patterns
5
a
and
5
b
are formed, so that a wiring board
6
having the wiring pattern on each side is produced as shown in FIG.
9
(
d
).
Further, the produced two-sided wiring board
6
is located between other insulation substrates
1
a
and
1
b
which function as intermediate connectors as shown in FIG.
9
(
e
). The intermediate connectors are produced by the steps of FIGS.
9
(
a
) and (
b
) so that they have the through holes
2
a
and
2
b
at predetermined positions which are filled with electrically conductive materials
3
a
and
3
b
, respectively. Then, copper foils
7
a
and
7
b
are placed on the outsides of the insulation substrates
1
a
and
1
b
(namely, the sides remote from the insulation substrate
1
).
Thereafter, the wiring board
6
, the insulation substrates
1
a
and
1
b
, and the copper foils
7
a
and
7
b
are heated and pressed so that a multi layer integrated body is formed, of which copper foils
7
a
and
7
b
as the outermost surfaces are then etched by the conventional photolithography method. Thus, a multi layer wiring board
9
having the four wiring layers as shown in FIG.
9
(
f
) can be obtained in which the wiring patterns
5
a
and
5
b
on the resin impregnated fibrous substrate
1
are electrically connected with the wiring patterns
8
a
and
8
b
by the conductive paste
3
a
and
3
b
filled in the through holes
2
a
and
2
b.
With the resin impregnated fibrous sheet such as an aramid fiber/epoxy resin prepreg in which a non-woven fabric of aramid fibers is impregnated with an epoxy resin, the aramid fibers contained therein cause irregularities on surfaces of the resin impregnated fibrous sheet, and for example, the resin impregnated fibrous sheet
1
has surfaces having irregularities which directly correspond to forms of the aramid fibers.
When the wiring patterns
5
a
and
5
b
are formed by etching the copper foils
4
bonded with heat and pressure to such a resin impregnated fibrous sheet
1
, the copper foils
4
also have irregularities on their surfaces due to the irregularities of the resin impregnated fibrous sheet
1
, which is likely to cause voids or gaps between the copper foils
4
and etching resist layers (not shown). Upon etching, an etching solution penetrates into the gaps, so that it becomes difficult to form wiring pattern
5
a
and
5
b
as predetermined. Especially, a problem is caused in that it is difficult to form fine wiring patterns, which accommodate the highly densely mounting of microelectronic parts such as semiconductor bare chips.
Further, the resin impregnated fibrous sheet is in such a state that a resin material is impregnated into a substrate made of the fibers, so that a structure of the resin impregnated fibrous sheet is unlikely to be uniform as a whole. For example, for the resin impregnated fibrous sheet such as an aramid fiber/epoxy resin prepreg in which the non-woven fabric of the aramid fibers is impregnated with an epoxy resin, since the aramid fibers are present discontinuously or exposed partly on surface skin layers of the sheet, the following two problems are likely to occur:
1) When the resin impregnated fibrous sheet is bonded with heat and pressure to a copper foil, the resin impregnated in the resin impregnated fibrous sheet contributes to the bond between them. An amount of the resin is small in portions under the copper foil where the aramid fibers are present or exposed partly on the surface skin layer of the insulation substrate. As a result of this, since the bond between the copper foil and the resin impregnated fibrous sheet is insufficient, no sufficient bonding strength is available between them. Thus, when electronic parts are mounted on a wiring pattern on such a wiring board, no large mounting strength is available; and
2) When the surfaces of the resin impregnated fibrous sheet have the irregularities, adhesion strength between the release film
17
and the resin impregnated fibrous sheet
1
is insufficient during the production steps of the wiring board as shown in FIGS.
9
(
a
) to (
f
), so that the conductive paste
3
is likely to penetrate into gaps between the release film
17
and the resin impregnated fibrous sheet
1
upon filling the conductive paste
3
into the through holes. As a result of this, the conduct

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