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Description  |
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The present invention relates to a multilayer board used as a printed
wiring board which consists of a base material on which a plurality of
circuit patterns are formed through an insulating layer, and a fabrication
method thereof.
BACKGROUND OF THE INVENTION
In order to increase the circuit density of a board including a base
material having formed thereon a wiring pattern, or make it
multifunctional or small-sized, there have been developed and proposed
various boards such as one having interstitial via holes, and one
including a base material on which a plurality of wiring patterns are
laminated through an insulating layer.
Typical examples of conventional multilayer boards include the following.
The term "double-sided board" as used herein refers to a board which has
provided thereon a wiring pattern on each side thereof.
(1) Printed Circuits Handbook, chapters 33.2-33.8 (3rd Ed.; 1988) describes
a multilayer board as shown in FIG. 2 in which double-sided boards 105 and
105' each having a wiring pattern 104 on each side thereof and a via hole
103 formed by plating a through hole, are laminated through a prepreg
consisting of an adhesive resin, with connection between the wiring
patterns of the laminated double-sided board being established by forming
a common via hole 102 penetrating all the layers.
(2) Denshi Zairyo (Apr., 1991) page 103-108 describes a multilayer board as
shown in FIG. 3. That is, a first layer wiring pattern 108 is formed on a
base material 106. A second wiring pattern 109 made of a plating layer is
formed on the first wiring pattern 108 through an insulating layer 107,
and a third wiring pattern 109' made of a plating layer is formed on the
second wiring pattern 109 through an insulating layer 107'. In addition, a
fourth or more wiring patterns are formed, and a through hole 102
penetrating all the layers is formed. Finally, an electric source layer
110 is connected as an outermost layer.
(3) Japanese Utility Model Application Laid-Open No. 16482/1988 describes a
board as shown in FIG. 4. That is, a base material 106 has provided
thereon a first layer wiring pattern 108 made of a plating layer, and
wiring patterns 109 and 109' are serially formed on the first layer wiring
pattern 108 through insulating layers 107 and 107', respectively. The
insulating layers between the wiring patterns are provided with
interstitial via holes 111 filled with a conductive paste so as to be
flush with the insulating layer to thereby establish electrical connection
between the wiring patterns.
Among the aforementioned, the multilayer board described in (1) above is a
popular one which is now put into practical use. In such a multilayer
board, a plurality of double-sided boards 105 and 105' separately
fabricated are laminated through a prepreg 101, which requires a very high
precision in the alignment between the boards. That is, it is necessary
that the through hole 102 provided after a plurality of boards are
laminated so as to penetrate all the layers should certainly penetrate at
predetermined places in each wiring pattern. If such alignment of the
laminate is missed, an immediate fault occurs. Therefore, it has been
necessary to ensure a penetration portion rather with some allowance in
each wiring pattern. This hinders increase in the circuit density. When
electroconduction is obtained between the through hole 102 penetrating all
the layers and intermediate wiring patterns by means of the through hole
102, the contact area between the two is small and a problem arises in the
reliability of electrical connection. In addition, through holes are
provided also in those wiring patterns which do not need
electroconduction, which limits freedom in wiring. This also hinders
increase in the circuit density.
Further, the aforementioned multilayer board is very complicated to
manufacture since it not only requires at least two plating operations in
the step of manufacturing a unit board but also requires steps of
laminating a unit board, producing an opening through the board, plating,
etc.
The multilayer board described in (2) above also has a through hole 102
penetrating all the layers, and like the multilayer board described in (1)
above, hinders increase in the circuit density. Wiring pattern is
laminated on one surface of the base material, and hence the degree of
increase in the number of electrical connection terminals between the
wiring patterns is greater than the increase in the number of pattern
layers. This also hinders increase in the circuit density.
Further, in the manufacture of the aforementioned multilayer boards,
formation of an insulating layer and plating must be performed once each
in order to form a single layer of wiring pattern. After all the wiring
patterns have been formed, production of openings through the board and
plating must be performed, and this leaves room for further improvement in
the manufacture.
The multilayer board described in (3) above connects a plurality of wiring
patterns laminated through an insulating layer by means of an opening
provided in the insulating layer and filled with a conductive paste.
However, the opening differs from a hole in that the opening has a closed
bottom and hence the conductive paste filled and cured therein tends to
form a space between it and the surface of the wiring pattern which it
originally contacted since it shrinks upon curing. There is room for
improvement in reliability of electrical connection. Similarly to (2)
above, wiring patterns are laminated on one side of the base material, and
the degree of increase of the number of electrical connection terminals is
greater than the increase in the number of wiring pattern layers, which
hinders increase in the wiring density.
Further, in the fabrication of the aforementioned multilayer boards, curing
of the conductive paste filled in the insulating layer and the opening
must be performed for each layer. Therefore, the board is subjected to
thermal hysteresis twice for each connection between the wiring patterns,
which not only makes the production procedure complicated but also causes
a problem of thermal deterioration of the board.
In the multilayer boards described in (2) and (3) above, electronic
components are mounted on only one surface of the board, leading to
decrease in the mounting density.
Therefore, an object of the present invention is to solve the
aforementioned problems encountered in the conventional techniques, and to
provide a multilayer board which has a high reliability of electrical
connection between wiring patterns laminated thereon, enables increase in
circuit density, and also increase in the density of mounting components
thereon.
Another object of the present invention is to provide a method of
fabricating the aforementioned multilayer board.
SUMMARY OF THE INVENTION
The present inventors have made extensive investigation in order to achieve
the above-described objects, and as a result they have found that use of a
double-sided board having a smoothed surface, fabricated by forming a
first layer wiring pattern on both sides of a base material, and filling a
conductive material in a hole penetrating through the base material to
form a via hole portion, enables formation of an insulating layer on the
double-sided board and of wiring patterns from a plating layer with a high
precision. It has been found out that formation of a plating layer through
an insulating layer on both sides of the double-sided board, and formation
of a second layer wiring pattern from said plating layer, and,
furthermore, achieving electrical connection between said second layer
wiring pattern and the first layer wiring pattern, or between said second
layer wiring pattern and the via hole portion, make it possible to form
wiring patterns so as to electrically connect the wiring of each layer
with each other without separately providing via hole portions penetrating
all the layers as conventionally conducted, and thus make it possible to
design wiring patterns with high degree of freedom and to increase circuit
density, and that, in this case, coating said via hole portion and said
terminal portion of the first layer wiring pattern which connects with the
via hole portion with a plating layer gives a multilayer board with high
reliability. Further, it has been found that lamination of wiring patterns
on both surfaces of the double-sided board enables mounting of electronic
components on both surfaces of the board. In the fabrication procedure,
lamination of wiring patterns on both surfaces of the double-sided board
enables formation of insulating layers, plating layers, and wiring patters
from the plating layers to be performed simultaneously, which makes the
fabrication procedure more efficient.
Thus, the present invention provides a multilayer board comprising: a
smooth-surfaced double-sided board having: a base material provided with a
hole penetrating therethrough, a first layer wiring pattern provided on
both surfaces of the base material and having a surface, and a conductive
material filled in the hole, the filler material having end surfaces being
flush with the surface of the first layer wiring pattern, thus forming a
via hole portion having end surfaces; an insulating layer provided on each
surface of the base material, the insulating layer being formed with an
opening having an inner wall; and a second layer wiring pattern comprising
a plating layer provided on at least one surface of the double-sided board
through the insulating layer; wherein at least a portion of one of the end
surface of the via hole portion is exposed in the opening, and wherein the
inner wall of the opening and exposed portion of the end of the via hole
portion is continuously coated with a plating layer connecting to the
plating layer of the second layer wiring pattern, thus establishing
electrical connection between the second wiring pattern and the via hole
portion. Said first layer wiring pattern has a connecting terminal which
electrically connects with the via hole portion, and said via hole portion
and said terminal portion of the first layer wiring pattern which connects
with said via hole portion are coated with a continuous plating layer.
<To be deleted> (Method claims are to be deleted.)
Achievement of the aforementioned objects and other objects will become
apparent by the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view showing an example of a multilayer board
of this invention;
FIG. 2 is a cross sectional view showing an example of a conventional
multilayer board;
FIG. 3 is a cross sectional view showing another example of a conventional
multilayer board;
FIG. 4 is a cross sectional view showing still another example of a
conventional multilayer board;
FIG. 5 is a cross sectional view showing another example of a multilayer
board of this invention;
FIG. 6 is a series of cross sectional views showing an example of the
fabrication procedure of a multilayer board of this invention;
FIG. 7 is a series of cross sectional views showing another example of the
fabrication procedure of a multilayer board of this invention;
FIG. 8 is a series of cross sectional views showing the fabrication
procedure of a double-sided board used in the fabrication procedure shown
in FIG. 6;
FIG. 9 is a series of cross sectional views showing the fabrication
procedure of a double-sided board used in the fabrication procedure shown
in FIG. 7;
FIG. 10 is a series of cross sectional views showing another example of the
fabrication of the multilayer board of this invention.
FIG. 11 is a series of cross sectional views showing another example of the
fabrication of the multilayer board of this invention.
FIG. 12 is a cross sectional view showing an example of the multilayer
board of this invention which has a three-layer wiring pattern.
FIG. 13 is a cross sectional view showing another example of the multilayer
board of this invention which has a three-layer wiring pattern.
DESCRIPTION OF PREFERRED EMBODIMENTS
Representative embodiments of the multilayer board of this invention are
shown in cross section in FIGS. 1 and 5.
As shown in FIGS. 1 and 5, the multilayer board of this invention includes
a double-sided board 5 having a smooth surface, which has first layer
wiring patterns 2,2' on a base material 1, and a via hole portion A. The
via hole portion A is formed by filling a conductive material 4 in a hole
3 penetrating the base material, and processing so that the ends of the
material are substantially flush with the surfaces of the first layer
wiring patterns 2,2'. On both surfaces of the double-sided board 5 are
formed second layer wiring patterns 7,7' made from a plating layer through
an insulating layer 6. The insulating layer 6 is provided with an opening
8 so that a portion of the ends of the via hole portion A can be exposed
therein. Also, the insulating layer 6 is provided with another opening 8'
so that a portion of a terminal portion 9 of the first layer wiring
patterns 2 can be exposed therein. A continuous plating layer is coated
over the outer surface of the insulating layer 6, the inner walls of the
openings 8,8', the exposed surface of the via hole portion, and the
exposed surface of the terminal portion 9, and required places of the
plating layer on the insulating layer 6 are etched to form the second
layer wiring patterns 7,7'. The continuous plating layer electrically
connects the second wiring patterns 7,7' to the via hole portion A, and
the first layer wiring patterns to the second layer wiring patterns.
The electrical connection between the end portion of said via hole portion
A and the terminal of said first layer wiring pattern which connects with
said end portion is surely achieved by forming a continuous plating layer
on the surfaces thereof.
For example, the mode shown in FIG. 1 exhibits a case where the above
connected portion is coated with plating layer 12. The mode shown in FIG.
5(1) exhibits a case where the above connected portion is coated with
plating layer 12', in place of said plating layer 12, which is connected
to the second layer wiring pattern 7'. In this case, as shown in FIG.
5(2), the end portion of said first layer wiring pattern preferably takes
a configuration wherein a land portion 11 is formed on the periphery of
the end portion of said via hole portion A. Of course, such a
configuration without a land portion as shown in (1) may be employed.
As the base material 1, any known ones made of a known material and having
a known configuration may be used without restriction. Representative
examples include synthetic resin laminate such as phenolic paper laminate,
epoxy-paper laminate, polyester paper laminate, epoxy-glass laminate, PTFE
resin-paper laminate, polyimide-glass laminate, BT (bismaleimide
triazine)-glass laminate, and composite laminate; flexible board such as
polyimide board or polyester board, metallic base material obtained by
coating with an epoxy resin a perforated plate of metal such as aluminum,
iron, or stainless steel, or ceramic board.
Material of the first layer wiring patterns 2,2' formed on both surfaces of
the base material 1 is not limited particularly and any known conductive
material may be used freely. Representative materials are, for example,
copper, nickel, aluminum, etc. Among them, cooper is used most preferably.
The thickness of the wiring patterns is not limited particularly.
Generally, a thickness of 5 to 70 .mu.m is suitable.
In the aforementioned double-sided board, the via hole portion A is formed
by filling the material 4 having conductivity in the hole 3 provided
through the base material in such a manner that the end surfaces of the
filler material are smooth and substantially flush with both surfaces of
the double-sided board. As such a material having a conductivity, any
known material may be used without limitation. For example, it is
typically a cured material derived from a curable conductive substance
which gives rise to a cured material having conductivity. As the curable
conductive substance, there can be used known curable conductive
substances obtained by mixing a conductive material such as gold, silver,
copper, nickel, lead, or carbon with a known crosslinking thermosetting
resin, together with an organic solvent as necessary to form a paste.
In order to form a via hole portion having a good conductivity, it is
preferred to select conductive material and adjust the amount of each
component when the aforementioned curable conductive substance is prepared
so that the electric resistance after curing can be 1.times.10.sup.-2
.OMEGA.cm or less.
Further, the diameter of the via hole portion is not limited particularly,
and may be set freely. Generally, the diameter of the via hole portion may
be one which enables filling of the aforementioned curable conductive
substance, specifically, at least 0.2 mm, preferably within the range of
0.3 to 2 mm.
In order to establish reliability of electrical connection between the via
hole portion and the first layer wiring pattern, it is necessary to
achieve said electrical connection between said via hole portion A and the
connecting terminal portion of said first layer wiring pattern which
connects therewith by means of forming a continuous plating layer on the
surface thereof.
In this case, as mentioned above, with a view to further increasing the
electrical connection between said via hole portion A and the first layer
wiring pattern 2,2' of said double-sided board, the reliability of
electrical connection attained by coating with said plating layer can be
further increased by giving the configuration of a land portion, which
encircles said via hole portion, to the connecting terminal portion of
said first layer wiring pattern which connects with said via hole portion.
In the present invention, on at least one surface of the aforementioned
double-sided board 5 is formed at least one wiring pattern made of a
plating layer formed on the board through the insulating layer 6.
As the insulating layer 6, there can be used any known material without
limitation. For example, suitable ones are conventional curable insulating
resins known as photosensitive insulating resist, for example PROBIMER 52
(trade name, produced by CIBA GEIGY), PROBICOTE 5000 (trade name, produced
by NIPPON PAINT), etc. Any thickness of the insulating layer may be used
so far as the insulation between the wiring patterns present on both
surfaces thereof can be maintained, and generally a thickness of 20 to 100
.mu.m is suitable.
The surface of the insulating layer is preferably roughened in order to
increase adhesion of the plating layer formed on the surface thereof.
Method of surface roughening is not limited particularly and there can be
used any known methods such as physical scrubbing using buff, brush, etc.,
chemical roughening by immersing in an alkaline potassium permanganate
solution, chromic acid solution, etc.
Any known material may be used without limitation for the plating layer
constituting the wiring pattern. For example, there can be cited, copper,
nickel, etc. Among them, copper is used most advantageously. Any thickness
of the plating layer may be used so far as a thickness sufficient for
exhibiting conductivity is ensured. Usually, the thickness is suitably 50
.mu.m or less, preferably 5 to 35 .mu.m.
In the present invention, in the insulating layer 6, there is provided an
opening 8 for connecting the wiring pattern and the via hole portion. The
opening is provided such that the terminal portion of the wiring pattern
and at least a portion of the end surface of the via hole portion is
exposed. In case the wiring patterns are connected to each other, the
opening 8 is formed so that the terminal portion of the wiring pattern
closer to the double-sided board is exposed. The area of the exposed
portion may be such that electrical connection can be established by
coating a plating layer 10 as described below. Generally, the area of the
exposed portion exposed from the insulating layer may be an area having a
corresponding diameter of 50 .mu.m or more. Furthermore, the shape of the
opening is not limited particularly, and any shape suitable for designing
wiring patterns, such as circle, ellipse, rectangle, square, etc., may be
adopted properly.
When neither said via hole portion A nor the connecting terminal portion of
said first layer wiring pattern which connects therewith is coated with
the plating layer shown in FIG. 1, a mode may be employed wherein the end
portion of said via hole portion A and the connecting terminal portion of
said first layer wiring pattern which connects with the end surface
thereof are exposed by means of forming an opening in the above insulating
layer and wherein, as shown in FIG. 5, the inner wall of said opening and
the above exposed portion are coated with a plating layer which forms the
second layer wiring pattern, so that the connection between said second
layer wiring pattern and said via hole portion and the one between said
via hole portion A and the terminal of said first layer wiring pattern
which connects therewith may be respectively established more surely.
As other configuration of the multilayer board of this invention, any known
multilayer board structure may be used without limitation. For example,
though not shown in the drawings, it is preferred to provide a layer
having a good resistance using a solder resist at required or desired
places in the outermost layer.
The multilayer boards according to the embodiments shown in FIGS. 1 and 5
are examples of 4-layer board having formed one layer of wiring pattern,
through an insulating layer, on the first layer wiring pattern present on
the both surfaces of a double-sided board. However, the present invention
is not limited thereto, and may be configured as one in which a further
wiring board is laminated through an insulating layer on the second wiring
pattern. Such an example is shown in FIG. 12 (1) to (4). That is, FIG. 12
shows some embodiments in which a third layer wiring patterns 19,19' made
of a plating layer, are formed on the surfaces of the second layer wiring
patterns 7',7 through an insulating layer 18. FIG. 12(1) shows an
embodiment in which a third layer wiring pattern is formed on the second
layer wiring pattern of the multilayer board according to the embodiment
shown in FIG. 5. FIG. 12(2) shows an embodiment in which a third layer
wiring pattern is formed on the second layer wiring pattern of the
multilayer board according to the embodiment shown in FIG. 1. Further,
FIGS. 12(3) and 12(4) show an embodiment in which in the position of the
via hole portion the second layer wiring pattern and the third layer
wiring pattern are connected to the via hole portion in order, or in which
the third layer wiring pattern is directly connected to the via hole
portion without intervention of the second layer wiring pattern.
In these embodiments, formation of the insulating layer 18 and the third
layer wiring pattern, and electrical connection between the third layer
wiring pattern and other layer or the via hole portion are performed in a
manner similar to the method of forming the insulating layer 6 and the
second layer wiring pattern.
For example, the formation of the third layer wiring pattern may be carried
out by forming an insulating layer having an opening at predetermined
places, forming a plating layer, and etching it off at predetermined
places. The electrical connection between the third layer wiring pattern
and the other wiring patterns or the via hole portion may be performed as
follows. That is, the insulating layer 18 is provided with an opening 21
such that at least a portion of the terminal portion 9 of the first layer
wiring pattern, the terminal portion 20 of the second layer wiring pattern
or the end surface of the via hole portion can be exposed therein, and the
inner wall of the opening and the exposed portion are coated with a
plating layer which is continuous with the plating forming the third layer
wiring pattern 19,19' to thereby connect the third layer wiring pattern to
the second layer wiring pattern, the first layer wiring pattern or the via
hole portion.
In the aforementioned connection, in case the third layer wiring pattern is
directly connected to the first layer wiring pattern or the via hole
portion, it is sufficient to provide an opening in the insulating layers 6
and 8 serially and then perform coating with the aforementioned plating
layer, as shown in FIG. 12(4).
FIG. 13 (1)-(4) show a mode of FIG. 12 (1)-(4) wherein no land portion is
formed at the connecting terminal portion of said first layer wiring
pattern.
Representative fabrication method of the multilayer board of this invention
is exemplified as follows.
That is, FIG. 6 is a cross sectional view showing a fabrication procedure
for fabricating a multilayer board. As shown in FIG. 6, (1) on at least
one surface of a smooth-surfaced double-sided board 5 having a base
material 1 provided with a hole 3 penetrating therethrough, first layer
wiring patterns 2,2' are provided on both surfaces of the base material,
and a conductive filler material 4 is filled in the hole, thus forming a
via hole portion A; (2) an insulating layer 6 is coated such that an
opening 8 is formed in which a terminal portion 13 of the first layer
wiring pattern and the via hole portion A which need be connected to a
second layer wiring pattern formed on the first layer wiring pattern are
exposed; (3) thereafter, the exposed portion, the inner wall of the
opening 8, and the surface of the insulating layer are coated with a
plating layer 14; (4) predetermined places of the plating layer are etched
to form wiring patterns to thereby form second layer or more wiring
patterns 7,7' and a plating layer 10 for conduction which is continuous
with the wiring patterns.
The method of forming the insulating layer 6 is not limited particularly
and any conventional method may be used freely. Generally, there can be
used curable insulating resins of various forms which cure by light or
heat, such as photosensitive dry film, liquid solder resist, combination
of photosensitive dry film and liquid solder resist, etc. As the method of
forming the insulating layer, printing method, photographic method, etc.
may be selected properly depending on fineness using the aforementioned
curable insulating resin. For example, liquid solder resist which cures
upon irradiation of light, or curable insulating resin layer such as
photosensitive dry film is laminated all over the surface of the
double-sided board, and after irradiating light except where formation of
an opening is needed, uncured portion is removed by development to form an
insulating layer.
In the aforementioned method, use of photosensitive dry film which cures
with light for forming an insulating layer results in high precision of
thickness of the insulating resin layer and allows simultaneous formation
on both surfaces, thus enabling formation of insulating layers efficiently
and with high precision.
In the embodiment shown in FIG. 6, the dimension of the opening 8 in the
insulating layer 6 is preferably within the aforementioned range which can
establish electrical connection with a plating layer.
FIG. 7 is a variation of the embodiment shown in FIG. 6, i.e., an
embodiment in which a double-sided board is used of which the plating
layer 12 is omitted. In this case, in order to ensure conduction to the
via hole portion A which needs electrical connection to the first layer
wiring pattern 2', the opening 8 provided in the insulating layer 6 is
formed so that the total area of the end surface of the via hole portion A
and the land portion 11 of the wiring pattern therearound are exposed.
This configuration enables simultaneous plating of the exposed portions by
subsequent formation of the plating layer 14, ensuring electrical
connection of these portions.
By the procedure shown in FIG. 7, it is possible to omit one plating step
from the procedure shown in FIG. 6.
In the aforementioned method, while the method of forming the plating layer
14 is not limited particularly, generally electroless plating of a metal
is preferred.
Also, in order to form the wiring pattern from the plating layer, there can
be used a similar method to the formation of the aforementioned first
layer wiring pattern. Generally, etching methods are preferred.
In the present invention, similarly, an insulating layer and a wiring
pattern can be laminated in order on the second layer wiring pattern.
The aforementioned laminated wiring pattern can be used in various
applications without limitation, such as signal conductor, power line,
ground plane, electromagnetic interference shielding layer, etc.
While FIG. 6 shows the embodiment in which end surfaces of the via hole
portion A of the double-sided board are coated with the plating layer 12
in advance, such a plating layer is not indispensable.
In case the plating layer is absent, a plating layer may be provided so as
to be continuous with the plating layer of the second layer wiring pattern
when it is formed, as shown in FIG. 7. That is, FIG. 7 shows the
embodiment in which the insulating layer is formed so that the end
surfaces of the via hole portion A which needs to be connected to the
first layer wiring pattern and the layer portion 11 formed therearound can
be exposed, and the exposed portions can be simultaneously plated.
In the aforementioned fabrication method, specific method of fabricating
the double-sided board is not limited particularly, and may be decided
properly depending on the structure of the multilayer board. To exemplify
such a fabrication method for the double-sided board, there can cited, for
example, the fabrication method shown in FIG. 8 as a representative method
for the double-sided board used in the fabrication procedure shown in FIG.
6, and the method shown in FIG. 9 as a representative method for the
double-sided board used in the fabrication procedure shown in FIG. 7.
That is, FIG. 8 shows the embodiment in which the hole 3 for the via hole
portion is provided in the base material 1 having on each side thereof a
conductive layer 15, curable conductive substance which gives rise to the
cured material 4 having conductivity is filled in the hole and cured in
such a manner that after curing the cured material projects out of the
hole, and the surfaces of the conductive layer and the cured material are
scrubbed smooth so that the surfaces of the conductive layer and the cured
material are substantially flush with each other to form the via hole
portion A between the aforementioned conductive layers, and then the
plating layer 16 is provided on the smoothed surface, followed by etching
predetermined places of the conductive layers composed of the conductive
layer and the plating layer to form the first layer wiring patterns 2,2'.
On the other hand, FIG. 9 shows the embodiment in which the base material
1 having on each surface thereof the conductive layer 15 is provided with
the hole 3 for the via hole portion, a curable conductive substance which
gives rise to the cured material 4 having conductivity is filled in the
hole and cured in such a manner that after curing the cured material
projects out of the hole, and the surfaces of the conductive layer and the
cured material are scrubbed smooth so that the surfaces of the conductive
layer and the cured material are substantially flush with each other to
form the via hole portion A between the aforementioned conductive layers,
and then predetermined places of the conductive layer are etched to form
the first layer wiring b patterns 2,2'.
Hereinafter, the aforementioned procedures will be described in more
detail.
The base material provided with a conductive layer on each surface thereof
is formed with the hole 3 for forming a via hole portion. The diameter of
the hole may be selected properly so as to correspond to the diameter of
the objective via hole portion. As the method of forming the hole 3, there
can be used any known means for fabricating ordinary boards such as
drilling, punching, laser processing, etc. without particular limitation.
In this case, in order to increase the reliability of electrical
connection between the cured material of a curable conductive substance
filled in the hole 3 and the conductive layer, an inclined surface is
provided on the conductive layer positioned around a peripheral portion of
the hole so that the contact area with the cured material can increase.
There is no limitation on the method of forming an inclined surface on at
least a portion of the hole surrounding the conductive layer, and there
can be used a method in which after a base material having a conductive
layer on each surface thereof is formed with a hole for a via hole
portion, mild etching is performed with a mild etchant to form the desired
inclined surface, a method in which after the hole is formed, the
conductive layer and the insulating layer are tapered by scrubbing using a
drill having a diameter slightly larger than that of the hole, a method in
which etching is performed with an etching resist having a diameter
slightly larger than that of the hole formed around the hole, and so on.
It is preferred that the conductive filling material present in the via
hole portion A of the double-sided board is formed by filling the curable
conductive substance in the hole 3 in the base material and curing it.
To fill the curable conductive substance in the hole, it is recommended
that the curable conductive substance is filled so that it occupies all
the space of the hole and protrudes slightly out of both surfaces thereof,
specifically by 0.05 mm or more, preferably 0.1 to 2 mm. As representative
method for filling the curable conductive substance, there can be used
advantageous various means such as a method in which one or more coatings
are performed by printing, a method in which the substance is pressed in
using a pair of squeegees on both surfaces of the base material, a method
in which the substance is filled by a roll coater or a curtain coater, and
excessive coating composition is removed using a squeegees, and so on.
Curing method for the curable conductive substance filled in the hole may
be selected properly from known curing methods that are suitable for
curing of the curable conductive substance, such as curing oven, infrared
oven, far infrared oven, UV curing oven, electron beam oven, etc.
It is important that after curing the curable conductive substance, the
surface of the conductive layer and the surface of the cured material of
the curable conductive substance are scrubbed so that both surfaces are
substantially flush with each other. That is, such scrubbing enables
pattern formation using an etching resist and etching with high precision
in the subsequent wiring pattern formation, and also enables formation of
an insulating layer on the wiring pattern with high reliability.
As the method of scrubbing the surfaces of the conductive layer and the
cured material of the curable conductive substance smooth as described
above, there can be used generally conventional methods such as slurry
scrubbing, buff scrubbing, etc.
FIG. 8 shows the embodiment in which the plating layer 16 is formed on the
smoothed surface of the conductive layer 15 containing the via hole
portion A. By forming such a plating layer and etching it to leave the
conductive layer so that the land portion 11 is formed around the
periphery of the via hole portion, the via hole portion and the land
portion of the wiring pattern can be coated with a common plating layer.
Formation of the common plating layer increases the reliability of
electrical connection by the via hole portion. Of course, the coating of
the via hole portion and the land portion of the wiring pattern by the
plating layer may be performed through an insulating layer at the time of
laminating the second layer wiring pattern.
The method of forming the plating layer 16 may be either by an electroless
plating process or an electroplating process. As the material of the
plating layer, while any conventional conductive metal may be used without
limitation, generally it is preferred to select the same material as the
conductive metal, such as copper, used as the curable conductive substance
which gives conductive cured material. The thickness of the plating layer
is not limited particularly, and it may be on the order of usually 50
.mu.m, preferably 5 to 35 .mu.m.
As described above, the first layer wiring pattern is formed on the via
hole portion, the conductive layer or the smoothed surfaces of conductive
layer and the plating layer as shown in the steps shown in FIG. 8 and FIG.
9.
That is, the first layer wiring pattern is formed generally by forming an
etching pattern using an etching resist 17 and subsequently performing
etching. The etching resist used here is not limited particularly, and
photosensitive dry film, liquid resist, etc. may be used that may be
selected properly depending on the fineness of the pattern. The etching
pattern may be selected properly from positive pattern and negative
pattern depending on the type of the etching process. For example,
positive patterns are used in the etching process represented by tenting,
and negative patterns are used in a pattern plating method, and an SES
method.
The above-explained production process relates to the method to produce a
multilayer board a wherein a land portion is formed on the periphery of
the via hole portion. However, the multilayer board of this invention
includes also a mode wherein such a land portion is not formed. FIG. 10
and 11 each show a flow sheet to produce a multilayer board without
forming a land portion 11 in the production process exhibited by FIG. 6
and 7.
In FIG. 10, the end surface of the via hole portion A and the connecting
terminal portion 22 of said first layer wiring pattern are coated with
common plating layer 12 so that the connection between said via hole
portion A and the first layer wiring pattern may be established more
surely.
In FIG. 11, the end surface of the via hole portion A and the surface of
the connecting terminal portion 22 of said first layer wiring pattern are
exposed by means of forming an opening 8 in the insulating layer 6, and
said exposed portion and the inner wall of the opening are coated with the
plating layer 7' which forms the second layer wiring pattern, so that the
connection between said via hole portion A and the first layer wiring
pattern may be established more surely.
As will be understood from the above, according to the present invention, a
double-sided board having a smoothed surface, fabricated by forming a
first layer wiring pattern on both sides of a base material, and filling a
conductive material in a hole penetrating through the base material to
form a via hole portion, is used in the formation of an insulating layer
and a second layer wiring pattern on the double-sided board, and this
enables formation of the laminated second layer wiring pattern with a high
precision. Formation of an insulating layer on both sides of the
double-sided board, and subsequent forma | | |
