U.S. patent application number 09/919319 was filed with the patent office on 2002-02-28 for printed circuit board and method for producing the same.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Echigo, Fumio, Ochi, Shozo, Ueda, Yoji.
Application Number | 20020023777 09/919319 |
Document ID | / |
Family ID | 18745748 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020023777 |
Kind Code |
A1 |
Ochi, Shozo ; et
al. |
February 28, 2002 |
Printed circuit board and method for producing the same
Abstract
A printed circuit board of the present invention is formed of an
electrical insulating base material with through holes that are
formed in a thickness direction of the electrical insulating base
material and are filled with an electrical conductor; the
electrical insulating base material including a core layer formed
by impregnating a holder with a resin and resin layers formed on
both sides of the core layer; and wiring layers that are formed on
both surfaces of the electrical insulating base material into a
predetermined pattern and are electrically connected to each other
by the electrical conductor. The wiring layer is embedded in at
least one of the resin layers. The resin layers on the both sides
have different thicknesses from each other, and a thinner layer out
of the resin layers has a thickness equal to or smaller than a mean
particle diameter of an electrically conductive filler contained in
the electrical conductor. By adjusting the thickness of the resin
layers formed on both sides of the resin holder such as a glass
cloth, it is possible to ensure high reliability when electrically
connecting circuit boards using an electrically conductive paste as
the electrical conductor.
Inventors: |
Ochi, Shozo; (Osaka, JP)
; Echigo, Fumio; (Osaka, JP) ; Ueda, Yoji;
(Osaka, JP) |
Correspondence
Address: |
Merchant & Gould P.C.
P.O. Box 2903
Minneapolis
MN
55402-0903
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
|
Family ID: |
18745748 |
Appl. No.: |
09/919319 |
Filed: |
July 31, 2001 |
Current U.S.
Class: |
174/256 |
Current CPC
Class: |
H05K 3/4655 20130101;
H05K 2201/0191 20130101; H05K 2203/0191 20130101; Y10T 428/12528
20150115; H05K 3/386 20130101; H05K 3/4069 20130101; H05K 3/4658
20130101; H05K 2201/0355 20130101; H05K 3/4626 20130101; Y10T
29/49165 20150115; Y10T 29/49155 20150115; H05K 3/4652 20130101;
H05K 2203/1461 20130101; H05K 2201/0195 20130101; H05K 3/20
20130101; Y10T 428/12028 20150115; H05K 3/4614 20130101 |
Class at
Publication: |
174/256 |
International
Class: |
H05K 001/03 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 28, 2000 |
JP |
2000-257260 |
Claims
What is claimed is:
1. A printed circuit board comprising: an electrical insulating
base material with through holes that are formed in a thickness
direction of the electrical insulating base material and are filled
with an electrical conductor; the electrical insulating base
material comprising a core layer formed by impregnating a holder
with a resin, and resin layers formed on both sides of the core
layer; and wiring layers that are formed on both surfaces of the
electrical insulating base material into a predetermined pattern
and are electrically connected to each other by the electrical
conductor; wherein the wiring layer is embedded in at least one of
the resin layers, the resin layers on the both sides have a
thickness different from each other, and a thinner layer out of the
resin layers has a thickness equal to or smaller than a mean
particle diameter of an electrically conductive filler contained in
the electrical conductor.
2. The printed circuit board according to claim 1, wherein the
electrical conductor is an electrically conductive paste containing
the electrically conductive filler and a resin.
3. The printed circuit board according to claim 1, wherein the
resin layers formed on both sides of the core layer have a
thickness equal to or smaller than that of the wiring layer
embedded in the resin layers.
4. The printed circuit board according to claim 1, wherein the
through holes are formed by a laser beam irradiation.
5. The printed circuit board according to claim 1, wherein one of
the resin layers on the both sides is more than one and less than
ten times as thick as the other.
6. The printed circuit board according to claim 5, wherein one of
the resin layers on the both sides is two to four times as thick as
the other.
7. The printed circuit board according to claim 1, wherein the
holder is a glass cloth.
8. The printed circuit board according to claim 1, wherein the
resin is a thermosetting epoxy resin.
9. The printed circuit board according to claim 1, wherein a
plurality of the printed circuit boards are laminated.
10. A method for producing a printed circuit board comprising:
laminating release films on both surfaces of an electrical
insulating base material, the electrical insulating base material
comprising a core layer including a prepreg formed by impregnating
a holder with a resin and resin layers that are formed on both
sides of the core layer and have different thicknesses from each
other; providing through holes in a thickness direction of the
electrical insulating base material; filling the through holes with
an electrically conductive paste; superposing a wiring base
material, on which a wiring layer is formed into a predetermined
pattern so as to correspond to a portion filled with the
electrically conductive paste, on at least one surface of the
electrical insulating base material; and embedding the wiring layer
in at least one of the resin layers on the electrical insulating
base material by heating and compressing the electrical insulating
base material on which the wiring base material has been
superposed.
11. The method for producing the printed circuit board according to
claim 10, wherein a thinner layer out of the resin layers has a
thickness substantially equal to or smaller than a mean particle
diameter of the electrically conductive filler contained in the
electrically conductive paste.
12. The method for producing the printed circuit board according to
claim 10, wherein the resin layers formed on both sides of the core
layer have a thickness equal to or smaller than that of the wiring
layer embedded in the resin layers.
13. The method for producing the printed circuit board according to
claim 10, wherein the through holes are formed by a laser beam
irradiation.
14. The method for producing the printed circuit board according to
claim 10, wherein one of the resin layers on the both sides is more
than one and less than ten times as thick as the other.
15. The method for producing the printed circuit board according to
claim 14, wherein one of the resin layers on the both sides is two
to four times as thick as the other.
16. The method for producing the printed circuit board according to
claim 10, wherein the holder is a glass cloth, and the resin is a
thermosetting epoxy resin.
17. The method for producing the printed circuit board according to
claim 10, wherein a plurality of the printed circuit boards are
laminated.
18. The method for producing the printed circuit board according to
claim 10, wherein the wiring base material comprises a glass-epoxy
base material that has through holes filled with an electrically
conductive paste and at least one surface on which the wiring layer
is formed.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a printed circuit board and
a method for producing the same.
[0003] 2. Description of Related Art
[0004] Accompanying the reduction in size, thickness and weight of
electronic equipment and the improvement in performance thereof in
recent years, various technologies enabling a high-density
packaging have been developed actively not only in various types of
electronic components constituting the electronic equipment, but
also in printed circuit boards on which these electronic components
are mounted. In particular, along with the rapid advancement of the
packaging technology recently, there is an increasing demand for
low-cost multilayered circuit boards that can both achieve a
high-density packaging of semiconductor chips such as LSIs and
respond to high speed circuitry. In such printed circuit boards, it
is important to have high electrical connection reliability between
wiring patterns, which are formed on multiple layers with a fine
wiring pitch, and high frequency characteristics.
[0005] In response to this, a printed circuit board in which layers
are electrically connected using an electrically conductive paste
has been proposed recently in JP 2601128 B. FIGS. 6A to 6G show a
method for producing this printed circuit board. First, as shown in
FIG. 6A, release films 501 made of polyester or the like are
laminated on both surfaces of a porous base material 502 such as an
aramid-epoxy prepreg that is obtained by impregnating an aramid
nonwoven fabric with a thermosetting epoxy resin. Next, as shown in
FIG. 6B, through holes 503 are formed at predetermined positions in
the porous base material 502 by laser processing. Then, the through
holes 503 are filled with an electrically conductive paste 504 as
shown in FIG. 6C. As a filling method, the porous base material 502
with the through holes 503 is placed on a table of a screen
printing machine and the electrically conductive paste 504 is
printed directly over one of the release films 501. In this case,
the release film 501 on the printed side serves as a print mask and
to prevent the surface of the porous base material 502 from being
contaminated. Subsequently, the release films 501 are peeled off
from the both surfaces of the porous base material 502. Then, metal
foils 505 such as copper foils are laminated on the both surfaces
of the porous base material 502. With this state maintained,
heating and compression are carried out, so that the porous base
material 502 is compressed to be thinner as shown in FIG. 6D.
Simultaneously, the electrically conductive paste 504 within the
through holes 503 also is compressed, and a binder component
contained in the electrically conductive paste 504 is forced out,
thus strengthening the adhesion between electrically conductive
components and between the electrically conductive component and
the metal foils 505. As a result, the electrically conductive
substance contained in the electrically conductive paste 504
becomes dense, thus achieving an electrical connection between
layers. Thereafter, the thermosetting resin constituents of the
porous base material 502 and the electrically conductive paste 504
are cured. Then, as shown in FIG. 6E, the metal foils 505 are
etched selectively into a predetermined pattern, thus completing a
double-sided circuit board. Furthermore, as shown in FIG. 6F,
porous base materials 506 on which an electrically conductive paste
508 is printed and metal foils 507 are attached on both sides of
the double-sided circuit board, followed by heating and
compression. Subsequently, as shown in FIG. 6G, the metal foils 507
are etched selectively into a predetermined pattern, thus
completing a multilayered circuit board.
[0006] However, in the structure and the producing method described
above, when the aramid-epoxy prepreg is used, there is a slight
deterioration in characteristics observed under a severe
environment where electronic equipment causing a sharp temperature
change is used. Accordingly, resin circuit boards with still higher
reliability have been desired.
[0007] In order to solve these problems, it has been considered
that a glass-epoxy prepreg obtained by impregnating a glass cloth
with a thermosetting epoxy resin is used as an electrical
insulating base material. However, in the glass-epoxy prepreg,
resin layers having the same thickness are formed on both sides of
the glass cloth. Accordingly, when the copper foil is laminated on
one surface and the substrate on which a certain pattern is formed
is laminated on the other surface, an excessively large amount of
the resin causes a resin flow on the copper foil side, making it
difficult to obtain connection reliability, while an excessively
small amount thereof makes it difficult to obtain a sufficient
adhesion on the patterned layer side. Alternatively, when the
patterned substrates having a different thickness are laminated on
the respective surfaces, an excessively large amount of the resin
causes a resin flow on the side of the thinner patterned layer,
making it difficult to obtain connection reliability, while an
excessively small amount thereof makes it difficult to obtain a
sufficient adhesion on the side of the thicker patterned layer.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to solve the
conventional problems described above and to provide a printed
circuit board and a method for producing the same, in which, by
adjusting the thickness of resin layers formed on both sides of a
resin holder such as a glass cloth, high reliability can be
achieved when layers are electrically connected by an electrical
conductor such as an electrically conductive paste.
[0009] In order to achieve the above-mentioned object, a printed
circuit board of the present invention includes an electrical
insulating base material with through holes that are formed in a
thickness direction of the electrical insulating base material and
are filled with an electrical conductor; the electrical insulating
base material including a core layer formed by impregnating a
holder with a resin and resin layers formed on both sides of the
core layer; and wiring layers that are formed on both surfaces of
the electrical insulating base material into a predetermined
pattern and are electrically connected to each other by the
electrical conductor. The wiring layer is embedded in at least one
of the resin layers. The resin layers on the both sides have
different thicknesses from each other, and a thinner layer out of
the resin layers has a thickness equal to or smaller than a mean
particle diameter of an electrically conductive filler contained in
the electrical conductor.
[0010] Furthermore, a method for producing a printed circuit board
of the present invention includes laminating release films on both
surfaces of an electrical insulating base material, the electrical
insulating base material including a core layer including a prepreg
formed by impregnating a holder with a resin and resin layers that
are formed on both sides of the core layer and have different
thicknesses from each other, providing through holes in a thickness
direction of the electrical insulating base material, filling the
through holes with an electrically conductive paste, superposing a
wiring base material, on which a wiring layer is formed into a
predetermined pattern so as to correspond to a portion filled with
the electrically conductive paste, on at least one surface of the
electrical insulating base material, and embedding the wiring layer
in at least one of the resin layers on the electrical insulating
base material by heating and compressing the electrical insulating
base material on which the wiring base material has been
superposed.
[0011] The present invention can form a via hole and a wiring layer
with high reliability. In other words, at least one of the wiring
layers is embedded in the resin layer, thereby compressing the
electrical conductor within the through holes sufficiently. As a
result, a conductor component of the electrical conductor becomes
dense, thus allowing a via-hole connection with high reliability.
In addition, the resin layers on both sides have a different
thickness. Therefore, when the copper foil and the substrate on
which different patterns are formed are laminated, or when the
patterned substrates having a different thickness are laminated, it
is possible to design the thickness of those resin layers so as to
achieve high connection reliability and adhesive strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1A to 1H are sectional views illustrating processes in
a method for producing a double-sided circuit board according to a
first embodiment of the present invention.
[0013] FIGS. 2A to 2E are sectional views illustrating processes in
a method for producing a multilayered circuit board according to a
second embodiment of the present invention.
[0014] FIGS. 3A to 3E are sectional views illustrating processes in
a method for producing a multilayered circuit board according to a
third embodiment of the present invention.
[0015] FIGS. 4A to 4C are sectional views illustrating processes in
a method for producing a multilayered circuit board according to a
fourth embodiment of the present invention.
[0016] FIG. 5 is a drawing for describing a method for producing a
glass-epoxy prepreg in one embodiment of the present invention.
[0017] FIGS. 6A to 6G are sectional views illustrating processes in
a conventional method for producing a multilayered circuit
board.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] In a printed circuit board of the present invention, a
thinner layer out of the resin layers has a thickness substantially
equal to or smaller than a mean particle diameter of the
electrically conductive filler contained in the electrically
conductive paste. With this structure, even when the resin layers
are heated and melted, the electrically conductive filler can be
prevented from flowing out. As a result, a via-hole connection with
conduction reliability can be achieved.
[0019] Furthermore, it is preferable that the resin layers formed
on both sides of the holder have a thickness equal to or smaller
than that of the wiring layer embedded in the resin layers. With
this structure, the wiring layers can be embedded substantially to
the glass cloth, making it possible to minimize an escape of
pressure exerted on the electrical conductor owing to the spread of
the resin layer in a horizontal direction.
[0020] Moreover, it is preferable that the electrical conductor is
an electrically conductive paste. With this structure, when the
electrically conductive paste within the through holes is
compressed, a resin component in the electrically conductive paste
is pressed out from the through holes, so that a conductor
component contained in the electrically conductive paste becomes
dense, achieving a via-hole connection with high reliability.
[0021] In addition, it is preferable that the holder is a glass
cloth and the resin is a thermosetting epoxy resin. This improves
an adhesive strength between a substrate material and metal foils
such as copper foils. Thus, when electronic components or the like
are mounted on a wiring pattern formed on this substrate, it
becomes possible to obtain high packaging strength. Also, since the
moisture resistance improves, layers are prevented from being
separated, causing no change in connection resistance during
reliability tests such as a temperature cycling test or a pressure
cooker test.
[0022] It is preferable that the through holes are formed with a
laser beam. This makes it possible to form the through holes having
a fine diameter easily and rapidly, corresponding to ever finer
wiring patterns.
[0023] Also, it may be possible to form a multilayered printed
circuit board in which a plurality of the printed circuit boards
are laminated. In this manner, a multilayered printed circuit board
having a via-hole connection with high reliability can be
provided.
[0024] Next, in accordance with a method for producing a printed
circuit board of the present invention, a new printed circuit board
that has a still higher reliability than a conventional printed
circuit board and that can form a minute wiring pattern allowing a
high-density packaging of super-miniature electronic components can
be obtained at low cost.
[0025] In addition, it is preferable that the wiring base material
includes a glass-epoxy base material that has through holes filled
with an electrically conductive paste and at least one surface on
which the wiring layer is formed. With this structure, it becomes
possible to laminate the wiring base materials while applying a
sufficient compression force to the electrical conductor within the
through holes in the glass-epoxy prepreg. Therefore, it is possible
to achieve a multilayered printed circuit board of the glass-epoxy
base material having a via-hole connection with high
reliability.
[0026] The following is a description of embodiments of the present
invention, with reference to the accompanying drawings.
[0027] Method for Producing Glass-Epoxy Prepreg
[0028] A glass-epoxy prepreg in the following embodiments was
produced by a method for producing the glass-epoxy prepreg in one
embodiment of the present invention shown in FIG. 5.
[0029] First, a glass cloth 2 was drawn out from a feed roller 1,
put into an impregnation tank 3 of a liquid epoxy resin so as to be
impregnated thoroughly with the resin, and lifted up to be passed
between thickness-adjusting rollers 5 and 6. At this time, the
thickness of epoxy resin layers on both sides of the glass cloth
was adjusted. More specifically, when the gap between the surface
of the glass cloth and the thickness-adjusting roller 5 was large,
a thick epoxy resin layer was obtained. On the other hand, when the
gap was small, a thin epoxy resin layer was obtained. Thus, by
passing the glass cloth not at the center between the
thickness-adjusting rollers 5 and 6 but closer to one of the
rollers, it was possible to form the resin layers to be thicker on
one surface than on the other surface. The thickness adjustment
also may be made by other methods, such as with a doctor knife.
[0030] The glass cloth whose thickness had been adjusted as above
then passed through a drying zone 7, in which a liquid component
such as a solvent was removed. An initial curing also may be
started.
[0031] Next, the glass cloth passed through a heat treatment zone
8, in which the epoxy resin was semi-cured or partially cured.
Subsequently, a cured glass-epoxy prepreg 9 was pulled out and cut
into a predetermined length.
[0032] First Embodiment
[0033] FIGS. 1A to 1H are sectional views illustrating processes in
a method for producing a double-sided circuit board according to
the first embodiment of the present invention. First, as shown in
FIG. 1A, an electrical insulating base material 100 formed of a
core layer 102 and resin layers 101a and 101b on both sides of the
core layer 102 was prepared. As the core layer 102, a glass-epoxy
prepreg obtained by impregnating a glass cloth with a thermosetting
epoxy resin was used. A cloth for electrical insulation,
manufactured by NITTO BOSEKI Co., Ltd., (product number: WE116E104,
thickness: 100 .mu.m, weight per unit area: 105 g/m.sup.2, the
number of warps: 60/25 mm and the number of wefts: 60/25 mm, single
yarn, plain weave) was used for the glass cloth. The amount of
impregnated resin was 54 wt %. Such glass-epoxy base material was
characterized by having an excellent stiffness, a low water
absorption and high adhesive properties, thus having high
reliability as a material of the printed circuit board. The resin
layers 101a and 101b were thermosetting epoxy resin formed on both
sides of the glass cloth. The epoxy resin can be phenol novolac
type, cresol novolac type or bisphenol type epoxy resin. More
specifically, 2:1 mixture by weight of "EPIKOTE 6090" and "EPICURE
YLH129," manufactured by JAPAN EPOXY RESINS CO., LTD., was
used.
[0034] The resin layers 101a and 101b had a thickness different
from each other, with the former being 10 .mu.m, and the latter
being 5 .mu.m. The thermosetting epoxy resin was in a semi-cured
state for ensuring that a wiring layer can be embedded therein.
[0035] Next, as shown in FIG. 1B, release films 103 made of a
material such as polyester were laminated on both surfaces of the
electrical insulating base material 100 at about 120.degree. C.
Consequently, the surfaces of the resin layers 101a and 101b were
melted slightly, thus permitting the release films 103 to adhere
onto the resin layers 101a and 101b. In the present embodiment,
polyethylene terephthalate (PET) films having a thickness of 16
.mu.m were used as the release films 103.
[0036] Then, as shown in FIG. 1C, through holes 104 were formed in
the electrical insulating base material 100 provided with the
release films 103 with a laser beam. The laser beam was irradiated
under the condition of pulse width: 200 .mu.s and attenuator: 270
pulse. The through holes 104 formed by the above-mentioned laser
beam machine had a diameter of about 100 .mu.m.
[0037] As shown in FIG. 1D, the through holes 104 were filled with
an electrically conductive paste 105 by printing the electrically
conductive paste 105 directly over the release film 103 using a
screen printing machine. In this case, by a vacuum adsorption
applied from the side opposite to the printed surface via a porous
sheet such as Japan paper, a resin component contained in the
electrically conductive paste 105 within the through holes 104 was
absorbed, so as to increase the proportion of a conductor
component. As a result, the through holes 104 were filled with the
electrically conductive paste containing the denser conductor
component. In addition, the release film 103 served as a print mask
and a contamination preventer for the surfaces of the resin layers
101a and 101b.
[0038] As shown in FIG. 1E, the release films 103 were peeled off
from the both surfaces. In this case, owing to the minute through
holes 104 with a diameter of 100 .mu.m, the influence at the ends
of the through holes of the release films 103 could not be ignored.
Consequently, a part or a whole of the electrically conductive
paste 105 within the through holes in the release films 103
sometimes was removed together with the release films 103. Although
the electrically conductive paste 105 remained in various states,
the remaining electrically conductive paste 105 was not scooped
below the surfaces of the resin layers 101a and 101b. Even in the
worst case, the surfaces of the electrically conductive paste 105
were flush with those of the resin layers 101a and 101b. Such
removal of the electrically conductive paste by the release films
103 was found significantly when the diameter of the through holes
104 was reduced to 100 .mu.m or less.
[0039] As shown in FIG. 1F, a base material (a sacrificial layer)
106 provided with a wiring layer 107 formed into a predetermined
shape was superposed from the resin layer 101a side of the
electrical insulating base material 102 such that at least the
wiring layer 107 was positioned immediately above the portion
filled with the electrically conductive paste 105. A metal foil 108
was superposed on the side of the other wiring layer 101b. They
were then heated and compressed by a vacuum press under the
condition of heat-up speed: 5.degree. C./min, pressure: 30
kgf/cm.sup.2, held at the maximum temperature of 180.degree. C. for
one hour, and degree of vacuum: 2.66.times.10.sup.3 Pa (20 Torr) or
less.
[0040] This heating and compression allowed the resin layer 101a to
flow, so that the wiring layer 107 was embedded into the resin
layer 101a as shown in FIG. 1G. By embedding the wiring layer 107
into the resin layer 101a as above, the electrically conductive
paste 105 within the through holes 104 was compressed, and thus the
resin component contained in the electrically conductive paste 105
flowed out into the resin layer 101a. Accordingly, the conductor
component contained in the electrically conductive paste 105 became
dense. Furthermore, since the resin layer 101b was thin with a
thickness of 5 .mu.m, it became possible to minimize an escape of
pressure exerted on the electrically conductive paste owing to a
resin flow in the resin layer. Thereafter, the electrically
conductive paste 105 and the electrical insulating base material
100 including the resin layers 101a and 101b were cured.
[0041] Finally, the sacrificial base material 106 was removed while
leaving the wiring layer 107 embedded in the resin layer 101a as
shown in FIG. 1H. Then, the metal foil 108 was patterned by etching
so as to form a wiring layer 108a, thus completing a double-sided
circuit board.
[0042] In the present embodiment, an aluminum foil was used for the
sacrificial base material 106, and a copper foil was used for the
wiring layers 107 and 108a. The sacrificial base material 106 was
removed by melting the aluminum foil by selective etching of the
aluminum foil and the copper foil. Since the sacrificial base
material 106 was removed by melting the aluminum foil, the
double-sided circuit board was not stressed so as to be broken. In
addition, the sacrificial base material 106 was removed in a single
step, thus improving the productivity. Ammonium persulfate or the
like can be used as an etchant for the selective etching. The same
method was applied to form the wiring layers 107 and 108a into a
predetermined pattern. A composite material of the aluminum foil
and the copper foil used for the sacrificial base material 106 and
the wiring layer 107 was, for example, a copper foil with an
aluminum carrier, UTC-Foil, manufactured by Mitsui Mining &
Smelting Co., Ltd. The composite material allowed a fine pattern
formation, since the copper foil was thin with a thickness of 5 to
12 .mu.m. A similar composite material also may be obtained by
preforming a resist pattern on an aluminum foil, treating it with
acidic zincate, followed by copper electroplating. In the
electroplating method, one having a thick copper foil and a fine
pattern was obtained. In this method, one having a linewidth of 10
.mu.m, a space of 10 .mu.n, and a copper foil with a thickness of
15 .mu.m was produced experimentally. In the present embodiment,
the copper foil had a thickness of 12 .mu.m and the aluminum foil
had a thickness of 40 .mu.m. Since the wiring layer 107 was
designed to be thicker than the resin layer 101a, it became
possible to minimize the escape of pressure exerted on the
electrically conductive paste owing to spread of the resin layer in
the horizontal direction.
[0043] Second Embodiment
[0044] In the following, a method for producing a multilayered
circuit board according to the second embodiment of the present
invention will be described with reference to FIGS. 2A to 2E.
[0045] First, as shown in FIG. 2A, a double-sided substrate with a
sacrificial base material 206 attached thereto was produced in a
similar manner to the first embodiment. Resin layers 201a and 201b
had different thicknesses from each other, with the former being 10
.mu.m, and the latter being 5 .mu.m. A core layer 202 had a
thickness of 100 .mu.m. Numeral 200 denotes an electrical
insulating base material, and numeral 205 denotes an electrically
conductive paste filled in through holes provided in the electrical
insulating base material 200. The electrically conductive paste 205
was compressed from one side by a wiring layer 207. Numeral 208
denotes a metal foil.
[0046] On one surface of the double-sided circuit board, which had
been formed as above, the metal foil 208 was patterned to form a
wiring layer 208a. Thereafter, as shown in FIG. 2B, the electrical
insulating base material 200 in which resin layers 211a and 211b
were formed and an electrically conductive paste 215 was filled at
a predetermined position was placed on the side of the wiring layer
208a. On an outer side thereof, a metal foil 218 further was
superposed, followed by heating and compressing similar to the
first embodiment. At this time, the wiring layer 208a and the
portion of the electrically conductive paste 215 were positioned to
match each other. The resin layers 211a and 211b had different
thicknesses from each other, with the former being 10 .mu.m, and
the latter being 5 .mu.m. They were heated and compressed by a
vacuum press under the condition of heat-up speed: 5.degree.
C./min, pressure: 30 kgf/cm.sup.2, held at the maximum temperature
of 180.degree. C. for one hour, and degree of vacuum:
2.66.times.10.sup.3 Pa (20 Torr) or less.
[0047] This heating and pressurization allowed the resin layer 211a
to flow, so that the wiring layer 208a was embedded into the resin
layer 211a as shown in FIG. 2C. By embedding the wiring layer 208a
into the resin layer 211a as above, the electrically conductive
paste 215 was compressed, and thus the resin component contained in
the electrically conductive paste 215 flowed out into the resin
layer 211a. Accordingly, the conductor component contained in the
electrically conductive paste 215 became dense. Furthermore, since
the resin layer 211b was thin with a thickness of 5 .mu.m, it
became possible to minimize an escape of pressure exerted on the
electrically conductive paste owing to a resin flow in the resin
layer 211b. Thereafter, the electrically conductive paste 215 and
the electrical insulating base material 200 including the resin
layers 211a and 211b were cured, similar to the first
embodiment.
[0048] In the multilayered circuit board formed as above, the metal
foil 218 was patterned so as to form a wiring layer 218a.
Subsequently, as shown in FIG. 2D, an electrical insulating base
material 220 provided with resin layers 221a and 221b on both sides
of a core layer 222 formed of a glass cloth with a thickness of 100
.mu.m impregnated with epoxy resin and filled with an electrically
conductive paste 225 at a predetermined position was placed at the
center. Then, on each side thereof, the multilayered circuit board
described above was superposed such that the wiring layer 218a side
contacts the electrical insulating base material 220, followed by
heating and compression under the condition of heat-up speed:
5.degree. C./min, pressure: 30 kgf/cm.sup.2, held at the maximum
temperature of 180.degree. C. for one hour, and degree of vacuum:
2.66.times.10.sup.3 Pa (20 Torr) or less. The thickness of the
resin layers 221 was 10 .mu.m on both sides.
[0049] This heating and compression allowed the resin layer 221 to
flow, so that the wiring layer 218a was embedded into the resin
layer 221 as shown in FIG. 2E. By embedding the wiring layer 218a
into the resin layer 221 as above, the electrically conductive
paste 225 was compressed, and thus the resin component contained in
the electrically conductive paste 225 flowed out into the resin
layer 221. Accordingly, the conductor component contained in the
electrically conductive paste 225 became dense. Thereafter, the
electrically conductive paste 225 and the electrical insulating
base material 220 including the resin layers 221 were cured.
Finally, the sacrificial base material 206 was removed, thus
completing a multilayered circuit board.
[0050] In the present embodiment, an aluminum foil was used for the
sacrificial base material 206, and a copper foil was used for the
wiring layers 207, 208a and 218a. The copper foil had a thickness
of 12 .mu.m, and the aluminum foil had a thickness of 40 .mu.m.
[0051] The multilayered circuit board of the present embodiment had
a smooth surface. Therefore, when mounting semiconductor bare
chips, the excellent flatness of the surface under the chips
brought about high mounting yield, thus improving mounting
reliability.
[0052] Third Embodiment
[0053] In the following, a method for producing a multilayered
circuit board according to the third embodiment of the present
invention will be described with reference to FIGS. 3A to 3E.
[0054] First, as shown in FIG. 3A, an electrical insulating base
material 302 that was filled with an electrically conductive paste
304 at a predetermined position was prepared. As the electrical
insulating base material 302, an aramid-epoxy prepreg (thickness:
130 .mu.m, weight per unit area: 140 g/m.sup.2) that was obtained
by impregnating an aramid nonwoven fabric with a thermosetting
epoxy resin was used. Metal foils 308 with a thickness of 12 .mu.m
were superposed on both surfaces of the electrical insulating base
material 302.
[0055] Next, as shown in FIG. 3B, they were heated and compressed
by a vacuum press under the condition of heat-up speed: 5.degree.
C./min, pressure: 30 kgf/cm.sup.2, held at the maximum temperature
of 180.degree. C. for one hour, and degree of vacuum:
2.66.times.10.sup.3 Pa (20 Torr) or less, so as to cure the
electrical insulating base material 302 and the electrically
conductive paste 304. Subsequently, wiring layers 308a were formed
by patterning.
[0056] Next, two electrical insulating base materials 310, similar
to the one used in the first embodiment, in which resin layers 311a
and 311b were formed on both sides of a core layer 312 and an
electrically conductive paste 315 was filled at a predetermined
position, were prepared. Then, as shown in FIG. 3C, they were
placed on both surfaces of the double-sided circuit board produced
in FIG. 3B. In this case, the electrical insulating base materials
310 were superposed from the side of the resin layers 311a such
that at least the wiring layers 308a were positioned immediately
above the portion filled with the electrically conductive paste
315. Metal foils 318 were superposed on the side of the other
wiring layers 311b, followed by heating and compressing under the
condition of heat-up speed: 5.degree. C./min, pressure: 30
kgf/cm.sup.2, held at the maximum temperature of 180.degree. C. for
one hour, and degree of vacuum: 2.66.times.10.sup.3 Pa (20 Torr) or
less. The resin layers 311a and 311b had different thicknesses from
each other, with the former being 10 .mu.m, and the latter being 5
.mu.m.
[0057] This heating and compression allowed the resin layers 311a
to flow, so that the wiring layers 308a were embedded into the
resin layers 311a as shown in FIG. 3D. By embedding the wiring
layers 308a into the resin layers 311a as above, the electrically
conductive paste 315 was compressed, and thus the resin component
contained in the electrically conductive paste 315 flowed out into
the resin layers 311a. Accordingly, the conductor component
contained in the electrically conductive paste 315 became dense.
Furthermore, since the resin layers 311b were thin with a thickness
of 5 .mu.m, it became possible to minimize an escape of pressure
exerted on the electrically conductive paste owing to a resin flow
in the resin layers 311b. Thereafter, the electrically conductive
paste 315 and the electrical insulating base material 310 including
the resin layers 311a and 311b were cured. Then, the metal foils
318 were patterned so as to form wiring layers 318a, thus
completing a four-layered circuit board.
[0058] Finally, with further laminations by repeating the processes
shown in FIGS. 3C and 3D, a six-layered circuit board was completed
as shown in FIG. 3E.
[0059] In the present embodiment, copper foils having a thickness
of 12 .mu.m were used for the wiring layers 308a and 318a.
[0060] In the multilayered circuit board of the present embodiment,
an aramid-epoxy base material was used for a core substrate in
which the electrically conductive paste was not compressed very
much, while a glass-epoxy base material was used for outer
substrates in which the electrically conductive paste was
compressed. Consequently, it was possible to achieve
characteristics such as an excellent stiffness, low water
absorption and high adhesive properties, thus obtaining still
higher reliability.
[0061] Fourth Embodiment
[0062] In the following, a method for producing a multilayered
circuit board according to the fourth embodiment of the present
invention will be described with reference to FIGS. 4A to 4C.
First, a fine circuit board 410 having a predetermined number of
insulating layers and wiring patterns and a core substrate 411,
produced similarly to the process of FIG. 2C in the second
embodiment, were prepared. In the present embodiment, a polyimide
film having both surfaces provided with epoxy-based adhesive layers
was used as an electrical insulating base material of the fine
circuit board 410. Through holes filled with an electrically
conductive paste had a diameter of 50 .mu.m. A copper foil with a
thickness of 5 .mu.m was used for the wiring layer on the fine
circuit board, and that with a thickness of 12 .mu.m was used for
the wiring layer on the core substrate.
[0063] Next, as shown in FIG. 4A, the fine circuit board 410 and
the core substrate 411 were superposed on both sides of an
electrical insulating base material 400 provided with resin layers
401a and 401b, having different thicknesses, on both sides of a
core layer 402 and filled with an electrically conductive paste 405
at a predetermined position. The resin layers 401a and 401b had
different thicknesses from each other, with the former being 10
.mu.m, and the latter being 5 .mu.m. The core layer 402 had a
thickness of 100 .mu.m. In this case, the core substrate 411 was
arranged on the side of the resin layer 401a, and the fine circuit
board 410 was arranged on the side of the resin layer 401b. In
addition, wiring layers 407 and 417 and the portion of the
electrically conductive paste 405 were positioned to match each
other. Numerals 406 denote sacrificial substrates.
[0064] Thereafter, as shown in FIG. 4B, the assembly was heated and
compressed by a vacuum press, so as to embed the wiring layer 407
on the surface of the fine wiring board 410 and the wiring layer
417 on the surface of the core substrate 411 into the resin layers
401b and 401a respectively, thereby compressing the electrically
conductive paste 405. In this manner, the fine wiring board 410 and
the core substrate 411 were electrically connected to each
other.
[0065] Finally, as shown in FIG. 4C, the sacrificial substrates 406
on the surfaces of the fine wiring board 410 and the core substrate
411 were removed, thus completing a multilayered circuit board. By
providing the core substrate below the fine wiring board, which was
a substrate having a large capacity of wiring, it became possible
to produce a high-density circuit board with a high stiffness.
[0066] The multilayered circuit board of the present embodiment had
the electrical insulating base material in which the resin layers
formed on both sides of the glass cloth had a thickness equal to or
smaller than that of the wiring layers embedded in these resin
layers. Therefore, the wiring layers can be embedded substantially
to the glass cloth, making it possible to minimize an escape of
pressure exerted on the electrical conductor owing to spread of the
resin layer in a horizontal direction and to achieve a via-hole
connection with high reliability.
[0067] According to the method for producing the multilayered
circuit board of the present embodiment, the multilayered circuit
board on the surface and the core substrate can be produced and
inspected individually, thereby improving yield as a whole.
[0068] As described above, by adjusting the thickness of the resin
layers formed on both sides of the resin holder such as the glass
cloth, the via-hole connection with high reliability can be
provided when layers are electrically connected by the electrical
conductor such as the electrically conductive paste.
[0069] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The embodiments disclosed in this application are to be
considered in all respects as illustrative and not restrictive, the
scope of the invention being indicated by the appended claims
rather than by the foregoing description, all changes that come
within the meaning and range of equivalency of the claims are
intended to be embraced therein.
* * * * *