Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1998-03-31
2001-08-07
Ball, Michael W. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S252000, C156S273300, C029S846000, C174S257000, C174S264000
Reexamination Certificate
active
06270607
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multilayer printed wiring board (hereinafter referred to PWB) extensively used in various electronics products for consumer-use as well as industrial-use.
BACKGROUND OF THE INVENTION
Personal computers, word-processors, video movie cameras, portable telephones, and the like are widely used, all of which boosts the demand for the multilayer PWB. Those electronics products require a compact size, light weight, and multi-function, and thus a more capacious accommodation for wiring as well as a higher density of surface mounting is required for the multilayer PWB.
In order to satisfy this request, a multilayer PWB employing IVH (interstitial via hole) coupling method was developed. This method uses a via hole (non-through hole) to make electric contact. Typical examples of IVH coupling method are as follows: (1) a resin-made multilayer PWB in which an IVH filled with conductive paste couples all conductive pattern layers, (2) a resin-made multilayer PWB in which photosensitive resin is used as an external layer, and a metal plated IVH is used in a provided hole on the external layer so that an internal conductive pattern layer makes contact with an external conductive pattern layer. A method of manufacturing a conventional multilayer PWB is described hereafter by referring to the attached drawings.
FIGS.
2
(
a
),
2
(
b
) and
2
(
c
) depict a method of manufacturing the multilayer PWB in which the IVH filled with the conductive paste couples all conductive pattern layers. First, as shown in FIG.
2
(
a
), a hole
13
c
is punched on a laminated board made of aramid nonwoven fabric which is impregnated with epoxy resin. Second, the hole is filled with conductive paste
13
d
, whereby an insulating substrate
13
b
is completed. Next, copper foil for copper-clad laminate is pasted on both faces of the insulating substrate
13
b
, and then the copper foil is processed into an internal conductive pattern layer
13
a
by using a printing method or a photographic method. An inside layer
13
, in which the conductive pattern layer
13
a
on both faces thereof is formed, is thus produced.
A copper foil
11
normally used for copper-clad laminate and a pre-preg
12
are prepared. The pre-preg
12
is made of aramid nonwoven fabric with a punched hole in which conductive paste
12
b
is filled. Then, as shown in FIG.
2
(
b
), the pre-preg
12
is laminated on both sides of the inside layer
13
, and the copper foil
11
is laminated on top of the pre-preg
12
. Heat and press is applied to the laminated body thus produced with a heating press machine so that the resin is melted, cooled, and solidified. The copper foil
11
is processed to an external conductive pattern layer
14
a
through the printing method or photographic method. The multilayer PWB
14
shown in FIG.
2
(
c
) is thus manufactured.
FIGS.
3
(
a
),
3
(
b
) and
3
(
c
) depict a method of manufacturing the multilayer PWB in which the metal plated IVH is used. First, as shown in FIG.
3
(
a
), by using the printing method or a photographic method, an inside layer
23
is produced having an internal conductive pattern layer
23
a
which comprises a copper foil normally used for copper-clad laminate on both sides of an insulating substrate
23
b.
Then, as shown in FIG.
3
(
b
), is a photosensitive resin film
22
is formed on both sides of the insulating substrate
23
b
. Next, a hole is formed communicating to the internal conductive pattern layer
23
a
on the photosensitive resin film
22
by exposure and development. Further, a through hole is drilled where conductivity between the internal conductive pattern layers
23
a
on both sides of the inside layer
23
is required. Then, an inside wall of the through hole and non-through hole with metal. A metal plated non-through hole is plated
24
a
(called photo via) and a metal plated through hole
24
b
are thus formed.
Finally, as shown in FIG.
3
(
c
), a copper foil
21
is provided on the surface of photosensitive resin film
22
, and the copper foil
21
is processed into an external conductive pattern layer
24
c
. The internal conductive pattern layer
23
a
thus makes electrical contact with the external conductive pattern layer
24
c
through the photo via
24
a
. Therefore, not only between the internal conductive pattern layers
23
a
, but also between the external conductive pattern layer
24
c
and the internal conductive pattern layer
23
a
, the electrical conductivity is assured, whereby the multilayer PWB
24
is obtained.
The conventional multilayer PWB described above, however; has increased wiring density on the external conductive pattern layer for obtaining the parts more densely mounted.
In the case of using the IVH filled with conductive paste shown in
FIG. 2
, the smaller diameter of the hole
12
a
is required in order to increase the wiring density of the external conductive pattern layer of the pre-preg
12
. When the diameter of the hole
12
a
is narrowed, the small hole must be fully filled with conductive paste in order to avoid losing conductivity. For this purpose, the viscosity of the conductive paste must be lowered by increasing a quantity of solvent to be added. However, when increasing the solvent quantity, solvent vapors due to heating in the mounting process are increased cause delamination. Because the hole
12
a
filled with the conductive paste is shielded, it is not possible to increase the viscosity of the conductive paste by vaporizing the solvent of the first filled conductive paste. Thus, the conductive paste of rather higher viscosity with a little solvent must be used from the first. Due to the above problem, it has been difficult to narrow the diameter of the hole
12
a
to less than 200 &mgr;m. In addition, due to the rather large diameter, the land formed around the hole
12
a
takes space, which restricts the wiring density on the external conductive pattern layer
14
a
from being increased.
In the case of using the photo via
24
a
shown in
FIG. 3
, since the metal plated film is used instead of the conductive paste, a photo via having a smaller hole diameter ranging 50-100 &mgr;m can be formed. However, on the multilayer PWB
24
, a number of through holes must be formed where conductivity is required between the internal conductive pattern layers
23
a
on both sides of the inside layer
23
. The numerous through holes
24
b
hamper the design of a desirable external conductive pattern layer
24
c
, and thus restrict the wiring density from being increased.
In order to make electrical contact between the internal conductive pattern layers
23
a
on both sides of the inside layer, the through holes may be punched beforehand on, inside layer
23
. However, when applying a photosensitive resin to the inside layer, the through holes must be filled with the photosensitive resin. It is almost impossible to fill the through holes with resin, thus this solution is not practical.
SUMMARY OF THE INVENTION
The present invention aims to provide a method of manufacturing a multilayer PWB having the higher wiring density.
The method of manufacturing the multilayer PWB comprises the following steps:
(a) forming an insulating substrate which has a hole filled with conductive paste and an internal conductive pattern layer on a surface of the insulating substrate, where the internal conductive pattern layer has a conductivity to the conductive paste,
(b) forming photosensitive resin film on the surface where the internal conductive pattern layer is formed, and forming a hole communicating to the internal conductive pattern layer on the photosensitive resin film,
(c) forming a conductive member in the hole on the photosensitive resin film, where the conductive member has conductivity with the internal conductive pattern layer, and
(d) forming an external conductive pattern layer on the photosensitive resin film, where this conductive pattern layer has conductivity with the conductive member.
According to the present invention, the conductive paste filled into the hole o
Ball Michael W.
Matsushita Electric - Industrial Co., Ltd.
Parkhurst & Wendel LLP
Tolin Michael A.
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