Circuit board

A circuit board comprising a board body which includes an insulating substrate or a plurality of laminated insulating substrates. The circuit board includes a connection material for electrically connecting together circuit patterns that are formed on both surfaces of the board body and/or a connection material for electrically connecting together a circuit pattern formed on one surface of the board body and circuit patterns formed between the insulating substrates. The connection material is constituted by an electrically conducting substance filled in a through-hole formed in the board body, and at least one end surface of the electrically conducting substance is substantially flush with the circuit pattern formed on one surface of the board body. An electrically conducting pattern having a substantially uniform thickness is formed to cover a connection region between the one end surface of the electrically conducting substance and one of the circuit patterns, the electrically conducting pattern being composed of a cured product of a curable electrically conducting substance.

FIELD OF THE INVENTION 
The present invention relates to a circuit board comprising a board body 
which includes a single insulating substrate or a plurality of laminated 
insulating substrates, and a connection means for electrically connecting 
a circuit pattern formed on one surface of the board body to a circuit 
pattern formed on the other surface of the board body and/or to circuit 
pattern formed between the insulating substrates. 
DESCRIPTION OF THE PRIOR ART 
A typical circuit board includes a board body constituted by an insulating 
substrate, circuit patterns formed on both surfaces of the board body, and 
a connection means for electrically connecting together the circuit 
patterns formed on both surfaces of the board body. Such a circuit board 
is usually obtained (a) by forming a through-hole in the board body having 
electrically conducting layers on both surfaces thereof by drilling or 
punching, forming a connection means by chemically plating and 
electro-plating to the through-hole and, then, forming circuit patterns on 
the electrically conducting layers by etching, or (b) by forming a through 
hole in the board body having electrically conducting layers on both 
surfaces thereof by drilling or punching, forming circuit patterns on the 
electrically conducting layers by etching, filling the through-hole with a 
curable electrically conducting substance such as copper paste or silver 
paste by a screen-printing method or pin-insertion method, and curing the 
electrically conducting substance to form a connection means. 
According to the above-mentioned method (a) of forming the circuit board, 
however, plating must be effected more than two times to enhance 
reliability of the connection means, which is not necessarily advantageous 
from the standpoint of cost. Besides, since the electroplating is effected 
for the whole surfaces of the board body, the thickness of the 
electrically conducting layers becomes irregular, and etching for forming 
the circuit patterns is effected unevenly. Therefore, this method is not 
suited for obtaining circuit boards having fine circuit patterns. 
The above-mentioned method (b) of producing the circuit board features 
decreased number of production steps since no chemical plating or 
electroplating needs be effected on the through-hole for forming the 
connection means. Therefore, the above method (b) has been increasingly 
employed in recent years. In order to maintain reliable electric 
connection between the connection means formed by filling an electrically 
conducting substance and the wiring portions of circuit patterns, however, 
the electrically conducting substance is needed to cover even the 
periphery of the through-hole in a manner to protrude beyond the surfaces 
of the board body. As a result, uneven surfaces of the board body caused 
by the protruded electrically conducting substance hinder the operation 
for printing a solder paste for mounting the surface-mounting parts 
thereon, and prevent the operation for connecting the surface-mounting 
parts with good reliability. 
In order to improve the defect of the circuit board produced by the 
above-mentioned method (b), Japanese Patent Laid-Open Publication No. 
67874/1993 discloses a method according to which a through-hole is filled 
with a curable electrically conducting substance in a protruding manner 
and, after cured, the protruding portions of the electrically conducting 
substance are ground to be flush with the electrically conducting layers 
formed on both surfaces of the board body thereby to form a connection 
means and then, circuit patterns are formed by subjecting the electrically 
conducting layers to an etching treatment. This publication further 
discloses technology according to which a common plated layer is formed on 
the electrically conducting substance of the connection means and on the 
electrically conducting layers, and the electrically conducting layers are 
then etched to form circuit patterns in order to improve reliability in 
the electric connection between the circuit patterns and the connection 
means. 
The circuit board disclosed in the above Japanese Patent Laid-Open 
Publication No. 67874/1993 has surfaces which are little uneven, permits 
the solder paste to be well printed in mounting the surface-mounting parts 
thereon, and enables the surface-mounting parts to be reliably connected. 
However, a plating layer must be further formed on the electrically 
conducting layers still leaving some problem in forming fine circuit 
patterns though the problem is not so serious as that of the circuit board 
produced by the above-mentioned method (a). 
SUMMARY OF THE INVENTION 
The principal object of the present invention is to provide a novel and 
improved circuit board which features highly reliable electric connection 
between the connection means and the circuit patterns formed on the 
surfaces of the board body, is not adversely affected in etching treatment 
for forming circuit patterns by formation of a connection means, and 
hence, can be well used for forming highly fine circuit patterns. 
Another object of the present invention is to provide a novel and improved 
circuit board which makes it possible to print a solder paste highly 
precisely at the time of mounting the surface-mounting parts and hence, 
enables the surface-mounting parts to be reliably connected thereto. 
A further object of the present invention is to provide a novel and 
improved circuit board which can be produced with a high yield and offers 
a high mounting density and a high wiring density. 
Through keen study and experiment, the present inventors have discovered 
that the above-mentioned objects can be accomplished by filling a 
through-hole formed in a board body with an electrically conducting 
substance to form a connection means, forming at least one end surface of 
the electrically conducting substance so as to be substantially flush with 
a circuit pattern formed on one surface of the board body, and covering a 
connection region between the circuit pattern and the one end surface of 
the electrically conducting substance constituting the connection means 
with an electrically conducting pattern which is composed of a cured 
product of a curable electrically conducting substance and which has a 
substantially uniform thickness. 
According to one aspect of the present invention, there is provided a 
circuit board comprising a board body including at least one insulating 
substrate, circuit patterns formed on both surfaces of said board body, 
and a connection means for electrically connecting together the circuit 
patterns formed on both surfaces of the board body, wherein: 
The connection means is constituted by an electrically conducting substance 
filled in a through-hole formed in the board body, at least one end 
surface of the electrically conducting substance being substantially flush 
with one of the circuit patterns; and 
An electrically conducting pattern having a substantially uniform thickness 
is formed to cover a connection region between said one end surface of 
said electrically conducting substance and one of said circuit patterns, 
the electrically conducting pattern being composed of a cured product of a 
curable electrically conducting substance. 
According to another aspect of the present invention, there is provided a 
circuit board comprising a board body including a plurality of laminated 
insulating substrates, a surface-layer circuit pattern formed on at least 
one surface of the board body, inner-layer circuit pattern formed between 
the insulating substrates, and a connection means for electrically 
connecting together the surface-layer circuit pattern and the inner-layer 
circuit patterns, wherein: 
The connection means is constituted by an electrically conducting substance 
filled in a through-hole formed in the board body, one end surface of the 
electrically conducting substance being substantially flush with said 
surface-layer circuit pattern formed on one surface of said board body; 
and 
An electrically conducting pattern having a substantially uniform thickness 
is formed to cover a connection region between the one end surface of the 
electrically conducting substance and the surface-layer circuit pattern, 
the electrically conducting pattern being composed of a cured product of a 
curable electrically conducting substance. 
Desirably, the through-hole formed in the board body should have a circular 
shape in cross section with a diameter of from 0.1 to 2 mm. The 
electrically conducting substance constituting the connection means is in 
the form of a paste obtained by mixing an electrically conducting powder 
and a crosslinking thermosetting resin, and is cured after it has been 
filled in the through-hole. The electrically conducting powder may be at 
least any one of gold, silver, copper, nickel, lead or carbon, and the 
thermosetting resin may be an epoxy resin. The curable electrically 
conducting substance constituting the electrically conducting patterns is 
preferably in the form of a paste obtained by mixing an electrically 
conducting powder and a crosslinking thermosetting resin. The electrically 
conducting powder may be at least any one of gold, silver, copper, nickel, 
lead or carbon, and the thermosetting resin may be a resol-type phenol 
resin. Preferably, the electrically conducting substance constituting the 
connection means is curable, and has an electric resistance after cured of 
not larger than 1.times.10.sup.-2 .OMEGA..multidot.cm. Desirably, the 
curable electrically conducting substance constituting the electrically 
conducting patterns can be subjected to a soldering or nickel-gold 
plating, after cured. In a preferred embodiment, the electrically 
conducting pattern covers substantially the whole one end surface of the 
connection means and a connection edge portion of the circuit pattern, the 
connection edge portion being connected to the one end surface of the 
connection means of the circuit patterns. It is desired that the 
connection edge portion of the circuit patterns covered with the 
electrically conducting pattern has a width of not smaller than 0.05 mm. 
The circuit pattern has a land portion surrounding the one end surface of 
the connection means, and the electrically conducting pattern desirably 
covers substantially the whole one end surface of the connection means and 
the land portion of the circuit pattern. The electrically conducting 
pattern may have a tear-drop shape. In another preferred embodiment, the 
circuit pattern has a land portion surrounding the one end surface of the 
connection means, and the electrically conducting pattern has a doughnut 
shape covering the one end surface of the connection means except its 
central portion and covering the land portion of the circuit pattern. The 
electrically conducting pattern may have a thickness of from 5 to 100 
.mu.m.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will be described in further 
detail with reference to the accompanying drawings. 
Referring to FIG. 1, a circuit board generally designated at 2 includes a 
board body 4 constituted by an insulating substrate 6. The insulating 
substrate 6 may be in any form such as paper base-phenolic resin laminated 
substrate, paper base-epoxy resin laminated substrate, paper 
base-polyester resin laminated substrate, glass base-epoxy resin laminated 
substrate, paper base-teflon resin laminated substrate, glass 
base-polyimide resin laminated substrate, glass base-BT 
(bismaleimide-triazine) resin laminated substrate, or a synthetic resin 
substrate such as composite resin board, or a flexible substrate such as 
of polyimide resin or polyester resin, or a metallic insulating substrate 
of aluminum, steel, stainless steel and the like coated with an epoxy 
resin to have electrically insulating property, or a ceramic substrate. 
Circuit patterns 12 and 14 are respectively formed on both surfaces of the 
board body 4, i.e., on an upper surface 8 and on a lower surface 10. As 
will be easily understood with reference to FIG. 1 as well as FIGS. 2 and 
3, the circuit patterns 12 and 14 usually include wiring portions 16 and 
17, land portions 18 and 19, and pad portions 21. The circuit patterns 12 
and 14 can be formed by using a suitable electrically conducting material. 
Here, however, typical examples of the material forming the circuit 
patterns 12 and 14 may be copper and nickel. Desirably, the circuit 
patterns 12 and 14 have a thickness which is usually from about 5 to about 
70 .mu.m. 
The circuit board 2 shown in FIG. 1 has, at a required position, a 
connection means 20 for electrically connecting together the circuit 
pattern formed on the upper surface 8 of the board body 4 and the circuit 
pattern 14 formed on the lower surface 10. A through-hole 22 is formed in 
the board body 4 at a required position penetrating therethrough in the 
direction of thickness, and the connection means 20 is constituted by an 
electrically conducting substance filled in the through-hole 22. It is 
desired that the through-hole 22 has a circular shape in cross section 
with a diameter which is sufficiently small but is large enough for being 
filled with the electrically conducting substance, i.e., which is from 
about 0.1 to about 2.0 mm and, particularly, from about 0.2 to about 2.0 
mm. As desired, the through-hole may be formed in the board body 4 in a 
rectangular shape, polygonal shape, oval shape or in any other suitable 
shape in cross section. It is desired that the electrically conducting 
substance constituting the connection means 20 is a curable electrically 
conducting substance that becomes, after cured, a cured product having 
electrically conducting property from the standpoint of easily filling the 
through-hole 22 with the electrically conducting substance in a step of 
forming the connection means 20 (which will be described later). 
As the curable electrically conducting substance, there can be used a known 
curable electrically conducting substance in the form of a paste which is 
obtained by mixing an electrically conducting powder of an electrically 
conducting material such as gold, silver, copper, nickel, lead or carbon 
and a crosslinking thermosetting resin such as phenol resin and, as 
required, an organic solvent. It is desired to select from these curable 
electrically conducting substances the one that forms a cured product 
which is not substantially dissolved by an etching solution that will be 
used for the etching treatment, such as ferric chloride etching solution, 
cupric chloride etching solution, ammonium persulfate etching solution, 
sodium persulfate etching solution, potassium persulfate etching solution, 
hydrogen peroxide/sulfuric acid etching solution, or alkaline etching 
solution composed chiefly of ammonium sulfate complex ions. 
Preferably, the connection means 20 has an electric resistance of not 
larger than 1.times.10.sup.-2 .OMEGA..multidot.cm and hence, it is desired 
to select the material and amount of the electrically conducting powder so 
as to obtain the above-mentioned electric resistance. 
It is desired that the electrically conducting powder is composed of a 
metal that hardly causes migration from the standpoint of maintaining 
reliable insulation from the neighboring connection means 20. Copper is 
particularly desired from the standpoint of electrically conducting 
property, operation for preventing oxidation and cost. A particularly 
preferred example of the thermosetting resin is an epoxy resin since it 
forms by-products in less amounts in the curing. 
With reference to FIG. 1, both end portions of the connection means 20 
constituted by the electrically conducting substance filled in the 
through-hole 22 protrude beyond both surfaces of the board body 4. In the 
circuit board constituted according to the present invention, it is 
important that at least one end surface of the connection means 20 is 
substantially flush with the circuit pattern 12 or 14 formed on the 
surface of the board body 4 from the standpoint of favorably sufficiently 
excellently effecting the solder paste printing at the time of mounting 
the surface-mounting parts, thereby to enable reliably connecting the 
surface-mounting parts and forming the electrically conducting patterns 
that will be later described with high precision. In the embodiment shown 
in FIG. 1, both ends of the connection means 20, i.e., an upper end 
surface 24 and a lower end surface 26 are substantially flush with the 
circuit patterns 12 and 14 formed on the upper surface 8 and on the lower 
surface 10 of the board body 4. 
With further reference to FIGS. 1 to 3, both end portions of the connection 
means 20 protrude beyond both surfaces of the board body 4 as described 
above, and are surrounded by the land portions 18 and 19 of the circuit 
patterns 12 and 14. The land portions 18 and 19 may be of an annular shape 
concentric with the connection means 20. It is desired that the land 
portions 18 and 19 surrounding both end portions of the connection means 
20 have a width w1 which is not smaller than 0.05 mm. When the width w1 of 
the land portions 18 and 19 is too great, wiring densities of the circuit 
patterns 12 and 14 decrease. It is therefore desired that the land 
portions 18 and 19 have a width w1 which is not larger than 2 mm. 
It is important that the connection region between the upper end 24 of the 
connection means 20 and the land portion 18 of the circuit pattern 12 is 
covered with an electrically conducting pattern 28, thereby to maintain a 
required electric connection between the connection means 20 and the 
circuit pattern 12. Similarly, it is important that the connection region 
between the lower end surface 26 of the connection means 20 and the land 
portion 19 of the circuit pattern 14 is covered with an electrically 
conducting pattern 30, thereby to maintain a required electric connection 
between the connection means 20 and the circuit pattern 14. It is 
important that the electrically conducting patterns 28 and 30 are composed 
of a cured product of a curable electrically conducting substance and 
having substantially a uniform thickness (method of forming such 
electrically conducting patterns 28 and 30 will be described later). In 
the embodiment shown in FIGS. 1 to 3, the electrically conducting patterns 
28 and 30 are of a circular shape, and cover substantially the whole end 
surfaces (24 and 26) of the connection means 20 and the inner peripheral 
edge portion of the land portions 18 and 19 of the circuit patterns 12 and 
14. It is desired that the inner peripheral edge portion of the land 
portions 18 and 19 covered with the electrically conducting patterns 28 
and 30 have a width w2 which is not smaller than 0.05 mm from the 
standpoint of achieving the reliable electric connection. 
As will be easily understood with reference to FIGS. 1 and 2, a pad portion 
21 of the circuit pattern 12 can be also covered with an electrically 
conducting pattern 32. When the pad portion 21 has a rectangular shape, 
the electrically conducting pattern 32 has a rectangular shape, too, and 
can cover substantially the whole pad portion 21. 
The electrically conducting patterns 28 and 30 that reliably and 
electrically connect the connection means 20 to the circuit patterns 12 
and 14, are not limited to those of circular ones as shown in FIGS. 2 and 
3, and may be formed in a variety of shapes. In an embodiment shown in 
FIGS. 4 and 5, the circuit pattern 12 has the land portion 18 surrounding 
an end portion of the connection means 20 and a wiring 16 that extends 
from the land portion 18. A connection portion where the wiring 16 is 
connected to the land portion 18 is gradually broadened toward the land 
portion 18. The electrically conducting pattern 28 that connects the 
connection means 20 to the circuit pattern 12 has a shape of a tear-drop 
as a whole, and covers substantially the whole one end surface 24 of the 
connection means 20, substantially the whole land portion 18 of the 
circuit pattern 12, and part of the gradually broadening portion of the 
wiring 16. The electrically conducting pattern 28 having such a tear-drop 
shape makes it possible to sufficiently improve reliability in the 
electric connection between the connection means 20 and the circuit 
pattern 12. In the embodiment shown in FIG. 6, the circuit pattern 12 has 
the concentrically annular land portion 18 which surrounds one end surface 
24 of the connection means 20, and a wiring 16 that extends from the land 
portion 18. The electrically conducting pattern 28 has the shape of a 
doughnut and surrounds the annular portion of one end surface 24 of the 
connection means 20 excluding the central portion thereof and an annular 
portion of the land portion 18 of circuit pattern 12 excluding the outer 
peripheral edge portion of the land portion 18 of circuit pattern 12. In 
the electrically conducting pattern 28 of such a shape, the material 
amount requiring for forming the conducting pattern can become smaller 
than that of the material used for forming the electrically conducting 
pattern 28 of the shape shown in FIGS. 1 to 3, 4 and 5. 
In order to reliably connect the connection means 20 to the circuit 
patterns 12 and 14 as in the embodiment shown in FIGS. 1 to 3, it is 
desired that the end portions of the connection means 20 are surrounded by 
the land portions 18 and 19 of circuit patterns 12 and 14, and that the 
electrically conducting patterns 28 and 30 are formed spanning across the 
end surfaces 24 and 26 of the connection means 20 and the land portions 18 
and 19 of the circuit patterns 12 and 14. As desired, however, land 
portions may not be formed and, instead, the wirings of the circuit 
patterns may be directly extended from the end portion of the connection 
means in order to form electrically conducting patterns spanning across 
the end surfaces of the connection means and the wirings of the circuit 
patterns. In an embodiment shown in FIGS. 7 and 8, an end portion of the 
connection means 20 is not surrounded by the land portion of the circuit 
pattern 12 and, instead, the wiring 16 of the circuit pattern 12 directly 
extends from one end portion of the connection means 20. A connecting 
portion where the wiring 16 of the circuit pattern 12 is connected to one 
end portion of the connection means 20 is gradually broadened toward the 
one end portion of the connection means 20. The electrically conducting 
pattern 28 for connecting the connection means 20 to the circuit pattern 
12 has a tear-drop shape as a whole, and covers substantially the whole 
one end surface 24 of the connection means 20 and part of the gradually 
broadening portion of the wiring 16 of the circuit pattern 12. It is 
desired that the wiring 16 covered with the electrically conducting 
pattern 28 has a width w2 which is not smaller than 0.05 mm from the 
standpoint of achieving reliable electric connection. 
In an embodiment shown in FIGS. 9 and 10, one end portion of the connection 
means 20 is surrounded by a pad portion 21 of the circuit pattern 12. The 
pad portion 21 has a rectangular shape, and there is provided a 
rectangular electrically conducting pattern 32 covering substantially the 
whole one end surface 24 of the connection means 20 and substantially the 
whole pad portion 21 of the circuit pattern 12. 
The electrically conducting patterns 28, 30 and 32 will be described in 
further detail. It is important that the electrically conducting patterns 
28, 30 and 32 have very uniform thicknesses and have substantially flat 
surfaces. This makes it possible to precisely print the solder paste for 
mounting parts and to precisely form an overcoat layer, contributing to 
enhancing reliability in the connection of surface-mounting parts. 
Besides, the surface-mounting parts are stably mounted on the electrically 
conducting patterns 28, 30 and 32. It is thus made possible to directly 
connect the surface-mounting parts to the electrically conducting patterns 
28, 30 and 32 and hence to increase the part-mounting density. 
It is desired that the electrically conducting patterns 28, 30 and 32 have 
thicknesses of from 5 to 100 .mu.m from the standpoint of reliability of 
the connection means 20 and printability of the solder paste and, 
particularly preferably, have thicknesses of from 5 to 50 .mu.m from the 
standpoint of very easily mounting the parts. It is desired that 
fluctuation in the thicknesses of the electrically conducting patterns 28, 
30 and 32 is suppressed to be .+-.30% or less of the average thicknesses 
though it may vary depending upon the method of forming electrically 
conducting patterns 28, 30 and 32 and the sizes of the portions connecting 
the surface-mounting parts. 
The electrically conducting patterns 28, 30 and 32 may be composed of the 
same material as the curable electrically conducting substance that is 
used for forming the connection means 20. It is, however, desired to use a 
curable electrically conducting substance capable of forming a cured 
product having superior moistureproof property to that of the electrically 
conducting substance that forms the connection means 20. By forming 
electrically conducting patterns 28, 30 and 32 having excellent 
moistureproof property, it is allowed to obtain a circuit board having 
excellent reliability and, particularly, excellent moistureproof property. 
Desirably, the curable electrically conducting substance for forming the 
electrically conducting patterns 28, 30 and 32 is in the form of a paste 
that is obtained by mixing an electrically conducting powder, a 
crosslinking thermosetting resin and, as required, an organic solvent. 
As the electrically conducting powder, there can be preferably used a metal 
such as gold, silver, copper, nickel or lead that is hardly oxidized and 
has a small resistivity, or carbon which is not at all oxidized though the 
resistivity may be slightly large. Among the above-mentioned electrically 
conducting materials, furthermore, it is desired to select a metal that 
causes little migration from the standpoint of maintaining reliable 
electric insulation from the neighboring circuit patterns 12 and 14. 
Copper is particularly preferably used from the standpoint of electrically 
conducting property, operation for preventing oxidation and cost. 
As the thermosetting resin, it is desired to use a resol-type phenol resin 
that works as a binder and produces a reducing atmosphere at the time of 
thermosetting, so that a metal such as copper or the like is prevented 
from being oxidized. 
When the resol-type phenol resin is used as a thermosetting resin for the 
curable electrically conducting substance for forming the connection means 
20, it is a posibility that voids occur in the through-hole 22 due to 
moisture and formalin produced as by-products at the time of heat-setting 
the resin and as a result, the cured product of the curable electrically 
conducting substance filled in the through-hole 22 loses electrically 
conducting property, whereby circuit patterns 12 and 14 near the 
through-hole 22 lose reliability. Despite of these facts, however, the 
electrically conducting patterns 28, 30 and 32 can be favorably formed by 
using the resol-type phenol resin. 
As described above, furthermore, it is desired that the curable 
electrically conducting substance for forming the connection means 20 in 
the through-hole 22 employs, as a binder component, an epoxy resin binder 
comprising a curing agent and an epoxy resin that little produces 
by-products when the binder undergoes the curing. However, the curable 
electrically conducting substance using such a binder exhibits inferior 
moistureproof property to the curable electrically conducting substance 
that chiefly uses the above-mentioned resol-type phenol resin as a binder. 
According to the above-mentioned embodiment of the present invention, 
however, it is allowed to protect the curable electrically conducting 
substance having poor moistureproof property for forming the connection 
means 20 by using high moistureproof property of the electrically 
conducting patterns 28, 30 and 32. 
It is further desired that the cured product forming the connection means 
20 has a thermal expansion coefficient which is close to that of the cured 
product forming the electrically conducting patterns 28, 30 and 32. When 
the two cured products have thermal expansion coefficients that are close 
to each other, thermal stress produced by the reflow at the time of 
mounting the parts is small and hence, improved reliability is obtained. 
According to an embodiment of the present invention, it is advisable that 
the through-hole 22 is filled with a curable electrically conducting 
substance using an epoxy resin-type binder that little produces 
by-products when the binder is being cured to form the connection means 
20, and that the electrically conducting patterns 28, 30 and 32 covering 
the connection regions between the connection means 20 and the circuit 
patterns 12 and 14 are formed by using a paste which uses a resol-type 
phenol resin as a main binder component and copper that hardly causes 
migration as a metal component. This makes it possible to obtain a circuit 
board that is free from defect in the connection means 20 and has 
excellent moistureproof property. 
Various known technologies can be applied without any particular limitation 
to the constitution of the circuit board 2 of the present invention except 
the connection means 20 and the electrically conducting patterns 28 and 
30. For instance, the circuit patterns 12 and 14, except the connection 
terminal portions, may be protected by forming an overcoat layer by using 
a known insulating resin (resist). Moreover, an insulating layer may be 
formed on the surfaces of the circuit patterns 12 and 14 except the 
connection terminal portions, and circuit patterns of a plated layer such 
as of copper may be formed on the insulating layer. In a circuit board in 
which the surface-mounting parts are mounted on one surface only, the 
electrically conducting substance is allowed to exist around the periphery 
of the through-hole on the surface on which no part is mounted, and the 
electrically conducting substance is allowed to protrude beyond the 
surface of the circuit board in a customary manner. 
Moreover, the surface-mounting parts can be directly connected onto the 
electrically conducting patterns 28, 30 and 32 and, hence, the 
part-mounting density can be increased when the electrically conducting 
patterns 28, 30 and 32 that are so formed as to cover the connection 
regions between the connection means 20 and the circuit patterns 12 and 
14, are formed of a curable electrically conducting substance that forms a 
cured product enabling a solder to be applied thereon or that forms a 
cured product enabling nickel-gold plating that is used for forming 
ordinary terminals to be plated thereon. 
FIGS. 11 and 12 illustrate a modified embodiment of the circuit board 
constituted according to the present invention. In the circuit board 2 
shown in FIG. 11, the upper end surface 24 of the connection means 20 is 
substantially flush with the circuit pattern 12 on the upper surface 8 
only of the board body 4 on which the surface-mounting parts are mounted, 
and an electrically conducting pattern 28 is formed to cover the upper end 
surface 24 of the connection means 20 and the land portion 18 of the 
circuit pattern 12, and an electrically conducting pattern 32 is formed to 
cover the pad portion 21 of the circuit pattern 12. On the lower surface 
10 of the board body 4 on which no surface-mounting part is mounted, the 
electrically conducting substance forming the connection means 20 
overflows from the through-hole 22 onto the land portion 19 of the circuit 
pattern 14. Therefore, the lower surface 10 of the board body 4 is not 
flat. In the circuit board 2 shown in FIG. 12, a nickel-gold plated layer 
34 is plated on the circuit patterns 12 and 14 and on the electrically 
conducting patterns 28, 30 and 32 in both surfaces 8 and 10 of the board 
body 4. In the circuit board 2 of this embodiment, the surface-mounting 
parts can be electrically connected to the nickel-gold plated layer 34 and 
hence, the electrically conducting patterns 28, 30 and 32 can be made of a 
material having poor solder-wetting property. Except the above-mentioned 
points, the circuit boards 2 shown in FIGS. 11 and 12 have substantially 
the same constitution as that of the circuit board 2 shown in FIGS. 1 to 
3. 
FIG. 13 illustrates a circuit board constituted according to a further 
modified embodiment of the present invention. In a circuit board 102 shown 
in FIG. 13, a board body 104 is constituted by three laminated insulating 
substrates 106a, 106b and 106c. The number of the insulating substrates to 
be laminated can be suitably selected depending upon the requirement. In 
the circuit board 102, a circuit pattern 113 is formed between the 
insulating substrate 106a and the insulating substrate 106b, and a circuit 
pattern 115 is formed between the insulating substrate 106b and the 
insulating substrate 106c, in addition to the circuit patterns 112 and 114 
that are formed on the upper surface 108 and on the lower surface 110 of 
the board body 104. This board body 104 can be formed by a customary 
manner such as a pin-laminate method or a mass-laminate method in which 
the insulating substrates 106a, 106b and 106c on which the circuit 
patterns 113 and 115 (and, as required, circuit patterns 112 and 114) have 
been formed in advance, are laminated one upon the other with pre-pregs 
sandwiched between them. The circuit board 102 has four kinds of 
connection means 120a, 120b, 120c and 120d. The connection means 120a 
electrically connects the circuit pattern 112 formed on the upper surface 
108 of the board body 104 to the circuit pattern 114 formed on the lower 
surface of the board body 104. The connection means 120b electrically 
connects the circuit pattern 113 formed between the insulating substrate 
106a and the insulating substrate 106b to the circuit pattern 115 formed 
between the insulating substrate 106b and the insulating substrate 106c. 
The connection means 120c electrically connects the circuit pattern 112 
formed on the upper surface 108 of the board body 104 to the circuit 
pattern 113 formed between the insulating substrate 106a and the 
insulating substrate 106b. The connection means 120d electrically connects 
together the circuit pattern 112 formed on the upper surface 108 of the 
board body 104, the circuit pattern 113 formed between the insulating 
substrate 106a and the insulating substrate 106b, and the circuit pattern 
115 formed on the lower surface 110 of the board body 104. Upper end 
surfaces 124a, 124b, 124c and 124d of the connection means 120a, 120b, 
120c and 120d are substantially flush with the circuit pattern 112 formed 
on the upper surface 108 of the board body 104. Furthermore, lower end 
surfaces 126a, 126b, 126c and 126d of the connection means 120a, 120b, 
120c and 120d are substantially flush with the circuit pattern 114 formed 
on the lower surface 110 of the board body 104. As for the connection 
means 120a, there are formed an electrically conducting pattern 128a 
covering the connection region between the upper end surface 124a and the 
circuit pattern 112, and an electrically conducting pattern 130a covering 
the connection region between the lower end surface 126a and the circuit 
pattern 114. No electrically conducting pattern is formed for the 
connection means 120b that is connected neither to the circuit pattern 112 
nor to the circuit pattern 114. As for the connection means 120c, an 
electrically conducting pattern 128c is formed to cover the connection 
region between the upper end surface 124c and the circuit pattern 112. No 
electrically conducting pattern is formed for the lower end surface 126c 
of the connection means 120c that is not connected to the circuit pattern 
114. As for the connection means 120d, there are formed an electrically 
conducting pattern 128d covering the connection region between the upper 
end surface 124d and the circuit pattern 112, and an electrically 
conducting pattern 130d covering the connection region between the lower 
end surface 126d and the circuit pattern 114. The connection means 120a, 
120b, 120c, 120d and the electrically conducting patterns 128a, 128c, 
128d, 130a, 130b have substantially the same constitutions as those of the 
connection means 20 and electrically conducting patterns 28 and 30 of the 
circuit board 2 described with reference to FIGS. 1 to 3. 
FIGS. 14 and 15 illustrate circuit boards 102 according to modified 
embodiments in which the board bodies 104 are each made up of three 
insulating substrates 106a, 106b and 106c. In the circuit board 102 shown 
in FIG. 14, no circuit pattern is formed on the lower surface of the board 
body 104. In FIG. 14, there are provided three connection means 120e, 120f 
and 120g. The connection means 120e is substantially the same as the 
connection means 120b in the circuit board 102 shown in FIG. 13, and the 
connection means 120f and 120g are substantially the same as the 
connection means 120c in the circuit board 102 shown in FIG. 13. In other 
respects, the circuit board 102 shown in FIG. 14 has substantially the 
same constitution as that of the circuit board 102 shown in FIG. 13. The 
circuit board 102 shown in FIG. 15 has three connection means 120h, 120i 
and 120j. On the lower surface 110 of the circuit board 102 on which no 
surface-mounting part is mounted, the electrically conducting substance 
forming the connection means 120h, 120i and 120j is overflows onto the 
lower surface of the board body 104 and, hence, the lower surface of the 
circuit board 102 is not flat. In other respects, the circuit board 102 
shown in FIG. 15 has substantially the same constitution as that of the 
circuit board 102 shown in FIG. 14. 
FIGS. 16-A to 16-D, FIGS. 17-A to 17-E, and FIGS. 18-A to 18-E illustrate 
typical methods of producing the circuit board 2 of the form explained 
with reference to FIGS. 1 to 3. 
According to the production method illustrated in FIGS. 16-A to 16-D: 
(A) the through-hole 22 is perforated in the insulating substrate 6 at a 
portion where it is necessary to electrically connect the circuit pattern 
12 formed on the upper surface 8 of the insulating substrate 6 to the 
circuit pattern 14 formed on the lower surface 10 of the insulating 
substrate 6; 
(B) the through-hole 22 is filled with a curable electrically conducting 
substance which is then cured to form the connection means 20; 
(C) the upper end surface 24 of the connection means 20 is ground so as to 
become substantially flush with the circuit pattern 12, and the lower end 
surface 26 of the connection means 20 is ground so as to become 
substantially flush with the circuit pattern 14; and 
(D) a curable electrically conducting substance is applied to required 
portions on both surfaces of the insulating substrate 6 in order to form 
electrically conducting patterns 28, 30 and 32 having substantially 
uniform thicknesses. 
According to the production method illustrated in FIGS. 17-A to 17-E: 
(A) the through-hole 22 is perforated at a required portion in the 
insulating substrate 6 having electrically conducting layers 9 and 11 
formed on both surfaces thereof; 
(B) the through-hole 22 is filled with a curable electrically conducting 
substance which is then cured to form the connection means 20; 
(C) the upper end surface 24 of the connection means 20 is ground so as to 
become substantially flush with the electrically conducting layer 9, and 
the lower end surface 26 of the connection means 20 is ground so as to 
become substantially flush with the electrically conducting layer 11; 
(D) circuit patterns 12 and 14 are formed by subjecting the electrically 
conducting layers 9 and 11 to the etching treatment; and 
(E) a curable electrically conducting substance is applied to required 
portions on both surfaces of the insulating substrate 6 in order to form 
electrically conducting patterns 28, 30 and 32 having substantially 
uniform thicknesses. 
According to the production method illustrated in FIGS. 18-A to 18-E: 
(A) the through-hole 22 is perforated at a required portion in the 
insulating substrate 6 having electrically conducting layers 9 and 11 
formed on both surfaces thereof; 
(B) the through-hole 22 is filled with a curable electrically conducting 
substance which is then cured to form the connection means 20; 
(C) the upper end surface 24 of the connection means 20 is ground so as to 
become substantially flush with the electrically conducting layer 9, and 
the lower end surface 26 of the connection means 20 is ground so as to 
become substantially flush with the electrically conducting layer 11; 
(D) a curable electrically conducting substance is applied to required 
portions on both surfaces of the insulating substrate 6 in order to form 
electrically conducting patterns 28, 30 and 32 having substantially 
uniform thicknesses; and 
(E) circuit patterns 12 and 14 are formed by subjecting the electrically 
conducting layers 9 and 11 to the etching treatment. 
The circuit boards 102 of the forms shown in FIGS. 13 to 15 in which the 
board bodies 104 are each constituted by a plurality of insulating 
substrates 106a, 106b and 106c, can be produced by the same production 
methods as those mentioned above. As required, a smear removal or etchback 
treatment is effected onto, the inner surface of the through-hole 22 
formed in the board body 104, in order to secure a reliable electrical 
connection of the connection means 120 to the circuit patterns 113 and 115 
formed among the insulating substrates 106a, 106b and 106c. 
In the above-mentioned production methods, the through-holes 22 and 122 can 
be perforated by any suitable method such as drilling, punching or laser 
working. 
It is desired that the through holes 22 and 122 formed in the board bodies 
4 and 104 are filled with a curable electrically conducting substance in 
such amounts that the curable electrically conducting substance fills the 
whole space of the through-holes 22 and 122 and that the curable 
electrically conducting substance slightly protrudes beyond the surfaces 
of the circuit patterns 12, 14, 112 and 114 or of the electrically 
conducting layers 9 and 11, or specifically speaking, protrudes by not 
smaller than 0.1 mm and, preferably, by about 0.1 mm to 2 mm. Typical 
methods of filling the curable electrically conducting substance include a 
method in which the application of the curable electrically conducting 
substance is effected one time or plural times by a printing method, a 
method in which the curable electrically conducting substance is forcibly 
introduced from the sides of both front and back surfaces of the 
insulating substrates using a pair of front and back squeezes, and a 
method in which the curable electrically conducting substance is filled by 
using a roll coater or a curtain coater. The curable electrically 
conducting substance exhibits electrically conducting property when the 
electrically conducting material contained in the curable electrically 
conducting substance comes in contact with each other by the 
cure-contraction at the time of curing the binder. It is therefore desired 
that the through-holes 22 and 122 are filled with the curable electrically 
conducting substance by taking the contraction coefficient into 
consideration so as not to allow the surfaces of the curable electrically 
conducting substance after cured to become lower than the surfaces of the 
circuit patterns 12, 14, 112 and 114 or of the electrically conducting 
layers 9 and 11. The curable electrically conducting substance filled in 
the through-holes 22 and 122 can be cured by any known method using a hot 
air furnace, an infrared-ray furnace, a far infrared-ray furnace, an 
ultraviolet-ray curing furnace or an electron-beam curing furnace. The 
curable electrically conducting substance filled in the through holes 22 
and 122 is usually cured immediately after it is filled, but may be cured 
simultaneously with the formation of the electrically conducting patterns 
28, 30, 32, 128 and 130 at a subsequent step. 
Concretely described below is a method of making both end surfaces 24, 26, 
124 and 126 of the connection means 20 and 120 constituted by the 
electrically conducting substance which has been cured after filled in the 
through-holes 22 and 122, to be substantially flush with the circuit 
patterns 12, 14, 112 and 114 or the conducting layers 9 and 11 of the 
board bodies 4 and 104. That is, the curable electrically conducting 
substance is filled into the through holes 22 and 122, and is cured. Then, 
those portions of the cured electrically conducting substance that are 
protruding beyond the circuit patterns 12, 14, 112 and 114 or the 
electrically conducting layers 9 and 11, are ground and smoothed. The 
portions of the electrically conducting substance protruding beyond the 
circuit patterns 12, 14, 112 and 114 or the electrically conducting layers 
9 and 11, can be smoothly ground by a method that is usually used for 
abrading the circuit boards, such as slurry abrasion, buff abrasion, scrub 
abrasion or belt abrasion. As required, at the time of smoothly grinding 
the electrically conducting substance that constitutes the connection 
means 20 at the step of FIG. 16-C, the circuit patterns 12 and 14 may be 
protected by forming an overcoat layer composed of an insulating resin 
(resist) on the circuit patterns 12 and 14. 
The electrically conducting layers 9 and 11 of the board bodies 4 and 104 
can be formed into circuit patterns 12, 14, 112 and 114 by any widely 
known method without any limitation. For instance, according to a widely 
employed method, an etching pattern is formed on the surfaces of the 
electrically conducting layers 9 and 11 using an etching resist, followed 
by etching. The etching resist employed here may be a dry film, a resist 
ink or the like, and is suitably selected and is used depending upon the 
fineness of the pattern. Furthermore, the etching resist pattern may be a 
positive pattern or a negative pattern depending upon the etching method. 
For example, an etching method as represented by the tenting process will 
use a positive pattern, and an etching method as represented by the solder 
plate stripping process or the SES process will use a negative pattern. 
When the circuit patterns 12 and 14 are to be formed after the surfaces 
defined by both end surfaces 24 and 26 of the connection means 20 and by 
the electrically conducting layers 9 and 11, are smoothly ground, as shown 
in FIGS. 17-D and 18-E, it is recommended to employ the ED process by 
using an electrodeposited photoresist since it electrically forms the 
resist and makes it possible to obtain circuit patterns with high 
precision and reliability without adversely affected by dust and dirt. In 
particular, use of the negative electrodeposited photoresist eliminates 
the need of exposing the through-hole 22 and 122 to light since the 
through-hole 22 and 122 has been filled with the electrically conducting 
substance, and makes it possible to reliably form the circuit boards 2 and 
102 having connection means 20 and 120 in a diameter of as small as 0.3 mm 
or less. 
As a method of forming electrically conducting patterns 28, 30, 32, 128 and 
130 having uniform thicknesses by applying a curable electrically 
conducting substance, there can be desirably employed a conventional 
method of forming electrically conducting patterns by printing. Concretely 
speaking, there can be exemplified a method in which the curable 
electrically conducting substance is applied to required portions by using 
a dispenser and is cured, and a method in which the curable electrically 
conducting substance is applied by using a screen printer and is cured. 
The curable electrically conducting substance can be cured by a widely 
known method using a hot air furnace, an infrared-ray furnace, a far 
infrared-ray furnace, an ultraviolet-beam curing furnace or an 
electron-ray curing furnace like curing the curable electrically 
conducting substance that is filled into the through-holes 22 and 122. 
According to the present invention, the connection means 20 and 120 
constituted by an electrically conducting substance filled and cured in 
the through-holes 22 and 122, are formed substantially in flush with the 
circuit patterns 12, 14, 112 and 114 or the electrically conducting layers 
9 and 11. Therefore, the curable electrically conducting substance can be 
applied onto the connection portions by using, for example, a screen 
printer without causing spreading. That is, the curable electrically 
conducting substance is applied excellently and uniformly. The 
electrically conducting patterns 28, 30, 32, 128 and 130 have uniform and 
small thicknesses. In the conventional circuit boards in which the 
electrically conducting substance was filled in the through-hole in a 
manner to overflow onto the periphery thereof to form the connection 
means, a holder resist layer had to be formed twice being divided into the 
circuit pattern and the connection means. In the circuit boards 2 and 102 
of the present invention, however, the solder resist layer can be formed 
in one time with good precision. 
Referring to FIG. 18-E, the surfaces defined by both end surfaces 24 and 26 
of the connection means 20 and by the electrically conducting layers 9 and 
11 are smoothly ground, the electrically conducting patterns 28, 30 and 32 
are formed and, then, the circuit patterns 12 and 14 are formed. In this 
case, the surfaces of the electrically conducting substance filled in the 
through-hole 22 are not exposed to an etching resist, an alkaline solution 
such as a solution for stripping the etching resist or an acidic solution 
for treating the surfaces. Therefore, the surfaces of the electrically 
conducting substance and the interfaces between the electrically 
conducting substance and the circuit patterns are not contaminated, making 
it possible to greatly improve reliability of the connection means 20. 
In producing the circuit boards 2 and 102 of the present invention, the 
surface defined by the connection means 20 and 120, circuit patterns 12, 
14, 112 and 114 or electrically conducting layers 9 and 11 can be smoothly 
ground and, then, the electrically conducting patterns 28, 30, 32, 128 and 
130 can be formed on the surfaces on one side of the board bodies 4 and 
104. Thereafter, the same steps may be executed onto the surfaces on the 
other side of the board bodies 4 and 104. 
Described below are Examples and Comparative Examples for concretely 
explaining the present invention which, however, is in no way limited to 
these Examples only. 
EXAMPLE 1 
A copper paste of the following composition was used as a curable 
electrically conducting substance to be filled in the through-hole. That 
is, a copper paste was prepared by kneading a mixture of a bisphenol A 
diglycidyl ether having an epoxy equivalent of 173 g/equivalent as a 
binder component, 35 parts by weight of a decylglycidyl ether 39 parts by 
weight of a novolak-type phenol resin as a curing agent per 100 parts by 
weight of the bisphenol A diglycidyl ether, a tree twig-like copper powder 
having an average particle diameter of 10.5 .mu.m as a copper powder in an 
amount of 360 parts by weight per 100 parts by weight of the binder, and a 
2-ethyl-4-methylimidazole in an amount of 2.8 parts by weight per 100 
parts by weight of the binder, using a triple roll for 45 minutes. 
Hereinafter, this copper paste is referred to as "copper paste A". 
As a curable electrically conducting substance for forming electrically 
conducting patterns, furthermore, use was made of a paste NF-2000 in which 
the main component of the binder was a resol-type phenol resin, produced 
by Tatsuta Densen Co. This copper paste is hereinafter referred to as 
"copper paste B". 
A circuit board was produced according to the steps shown in FIGS. 16-A to 
16-D. That is, (A) 100 through-holes having a diameter of 0.6 mm were 
formed, by drilling, in an insulating substrate which was a glass-epoxy 
board having a thickness of 1.2 mm and having electrically conducting 
layers made up of a copper foil formed on both surfaces thereof, circuit 
patterns including wirings having a width of 50 .mu.m at a gap of 50 .mu.m 
and further including land portions were formed on both surfaces of the 
insulating substrate, (B) the copper paste A was filled as a curable 
electrically conducting substance into the through-holes by the 
screen-printing method so as to protrude by 0.25 mm beyond the circuit 
patterns, and was cured using an air oven under the conditions of 
50.degree. C. for 30 minutes and 180.degree. C. for 60minutes to form 
connection means, (C) the surfaces defined by the circuit patterns and the 
connection means were smoothly ground by using a buff of #200 and then a 
buff of #360, and (D) the copper paste B was applied as a curable 
electrically conducting substance by the screen-printing method to cover 
substantially the whole end surfaces of the connection means and inner 
peripheral edge portions of a width of 0.1 mm of the land portions in the 
circuit patterns that connect to the connection means, and was cured by 
using the air oven under a condition of 160.degree. C. for 30 minutes to 
form electrically conducting patterns having an average thickness of 30 
.mu.m (fluctuation of about .+-.10%). One hundred pieces of the circuit 
boards were thus obtained. 
The thus obtained circuit boards did not undergo short circuit or open 
circuit in the circuit patterns, and their yield was 100%. The electric 
resistance of the connection means connecting together the electrically 
conducting patterns formed on both surfaces of the circuit board was 
measured to be 15 m.OMEGA./connection means in average. After the circuit 
board was subjected to high-temperature/high-humidity conditions of 
60.degree. C.-90% RH for 1000 hours, the resistance of the connection 
means was again measured and was found to be 17 m.OMEGA./connection means 
in average. Furthermore, the surfaces of the circuit board were little 
uneven and on which the solder paste could be favorably printed at the 
time of mounting the surface-mounting parts, enabling the surface-mounting 
parts to be reliably connected. 
EXAMPLE 2 
A circuit board was produced according to the steps shown in FIGS. 17-A to 
17-E. That is, (A) 100 through-holes having a diameter of 0.5 mm were 
formed by drilling in an insulating substrate which was a glass-epoxy 
board of a thickness of 1.2 mm having electrically conducting layers made 
up of a copper foil formed on both surfaces thereof, (B) the copper paste 
A was filled as a curable electrically conducting substance into the 
through-holes by the screen-printing method, and was cured using an air 
oven under the conditions of 50.degree. C. for 30 minutes and 180.degree. 
C. for 60 minutes to form connection means, (C) the surfaces defined by 
the electrically conducting layers and the connection means were smoothly 
ground by using a buff of #200 and then a buff of #360, (D) circuit 
patterns including wirings having a width of 50 .mu.m at a gap of 50 .mu.m 
and further including land portions were formed in the electrically 
conducting layers by using an etching resist on both surfaces, and (E) the 
copper paste B was applied as a curable electrically conducting substance 
by the screen-printing method to cover substantially the whole end 
surfaces of the connection means and inner peripheral edge portions of a 
width of 0.1 mm of the land portions in the circuit patterns that connect 
to the connection means, and was cured by using the air oven under a 
condition of 160.degree. C. for 30 minutes to form electrically conducting 
patterns having an average thickness of 25 .mu.m (fluctuation of about 
.+-.10%). One hundred pieces of the circuit boards were thus obtained. 
The thus obtained circuit boards did not undergo short circuit or open 
circuit in the circuit patterns, and their yield was 100%. The electric 
resistance of the connection means connecting together the electrically 
conducting patterns formed on both surfaces of the circuit board was 
measured to be 19 m.OMEGA./connection means in average. After the circuit 
board was subjected to high-temperature/high-humidity conditions of 
60.degree. C.-90% RH for 1000 hours, the resistance of the connection 
means was again measured and was found to be 22 m.OMEGA./connection means 
in average. Furthermore, the surfaces of the circuit board were little 
uneven and on which the solder paste could be favorably printed at the 
time of mounting the surface-mounting parts, enabling the surface-mounting 
parts to be reliably connected. 
EXAMPLE 3 
A circuit board was produced according to the steps shown in FIGS. 18-A to 
18-E. That is, (A) 100 through-holes having a diameter of 0.5 mm were 
formed by drilling in an insulating substrate which was a glass-epoxy 
board having a thickness of 1.2 mm and having electrically conducting 
layers made up of a copper foil formed on both surfaces thereof, (B) the 
copper paste A was filled as a curable electrically conducting substance 
into the through-holes by the screen-printing method, and was cured using 
an air oven under the conditions of 50.degree. C. for 30 minutes and 
180.degree. C. for 60 minutes to form connection means, (C) the surfaces 
defined by the electrically conducting layers and the connection means 
were smoothly ground by using a buff of #200 and then a buff of #360, (D) 
the copper paste B was applied as a curable electrically conducting 
substance by the screen-printing method to cover substantially the whole 
end surfaces of the connection means and inner peripheral edge portions of 
a width of 0.1 mm of the electrically conducting layers that connect to 
the connection means, and was cured by using the air oven under a 
condition of 160.degree. C. for 30 minutes to form electrically conducting 
patterns having an average thickness of 25 .mu.m (fluctuation of about 
.+-.10%), and (E) circuit patterns including wirings having a width of 50 
.mu.m at a gap of 50 .mu.m and further including land portions were formed 
in the electrically conducting layers by using an etching resist on both 
surfaces. One hundred pieces of the circuit boards were thus obtained. 
The thus obtained circuit boards did not undergo short circuit or open 
circuit in the circuit patterns, and their yield was 100%. The electric 
resistance of the connection means connecting together the electrically 
conducting patterns formed on both surfaces of the circuit board was 
measured to be 18 m.OMEGA./connection means in average. After the circuit 
board was subjected to high-temperature/high-humidity conditions of 
60.degree. C.-90% RH for 1000 hours, the resistance of the connection 
means was again measured and was found to be 19 m.OMEGA./connection means 
in average. Furthermore, the surfaces of the circuit board were little 
uneven and on which the solder paste could be favorably printed at the 
time of mounting the surface-mounting parts, enabling the surface-mounting 
parts to be reliably connected. 
EXAMPLE 4 
A circuit board was produced according to the steps shown in FIGS. 17-A to 
17-E and, finally, nickel and gold were plated thereon. That is, (A) 100 
through-holes having a diameter of 0.5 mm were formed by drilling in an 
insulating substrate which was a glass epoxy board of a thickness of 1.2 
mm having electrically conducting layers made up of a copper foil formed 
on both surfaces thereof, (B) the copper paste A was filled as a curable 
electrically conducting substance into the through-holes by the 
screen-printing method, and was cured using an air oven under the 
conditions of 50.degree. C. for 30 minutes and 180.degree. C. for 60 
minutes to form connection means, (C) the surfaces defined by the 
electrically conducting layers and the connection means were smoothly 
ground by using a buff of #200 and then a buff of #360, (D) circuit 
patterns including wirings having a width of 50 .mu.m at a gap of 50 
.mu.m, land portions and pad portions were formed in the electrically 
conducting layers by using an etching resist on both surfaces, (E) the 
copper paste B was applied as a curable electrically conducting substance 
by the screen-printing method to cover substantially the whole end 
surfaces of the connection means, inner peripheral edge portions of a 
width of 0.1 mm of the land portions in the circuit patterns that connect 
to the connection means, and to cover pad portions on which the 
surface-mounting parts will be mounted, and was cured by using the air 
oven under a condition of 160.degree. C. for 30 minutes to form 
electrically conducting patterns having an average thickness of 25 .mu.m 
(fluctuation of about .+-.10%), and (F) nickel-plating having a thickness 
of 4 .mu.m and gold-plating having a thickness of 0.2 .mu.m were effected 
on the circuit patterns and on the electrically conducting patterns. One 
hundred pieces of the circuit boards were thus obtained. 
The thus obtained circuit boards did not undergo short circuit or open 
circuit in the circuit patterns, and their yield was 100%. The electric 
resistance of the connection means connecting together the electrically 
conducting patterns formed on both surfaces of the circuit board was 
measured to be 19 m.OMEGA./connection means in average. After the circuit 
board was subjected to high-temperature/high-humidity conditions of 
60.degree. C.-90% RH for 1000 hours, the resistance of the connection 
means was again measured and was found to be 20 m.OMEGA./connection means 
in average. Furthermore, the surfaces of the circuit board were little 
uneven and on which the solder paste could be favorably printed at the 
time of mounting the surface-mounting parts, enabling the surface-mounting 
parts to be reliably connected. 
EXAMPLE 5 
By using a board body consisting of three laminated insulating substrates, 
a circuit board was produced according to the steps shown in FIGS. 17-A to 
17-E and, finally, nickel-gold plating was effected thereon. That is, (A) 
100 through-holes having a diameter of 0.5 mm were formed by drilling in 
the board body consisting of three laminated insulating substrates each 
being a glass-epoxy board with a thickness of 1.2 mm and having circuit 
patterns made up of a copper foil formed between the insulating substrates 
and further having electrically conducting layers made up of a copper foil 
formed on both surfaces of the board body, (B) the copper paste A was 
filled as a curable electrically conducting substance into the 
through-holes by the screen-printing method, and was cured using an air 
oven under the conditions of 50.degree. C. for 30 minutes and 180.degree. 
C. for 60 minutes to form connection means, (C) the surfaces defined by 
the electrically conducting layers and the connection means were smoothly 
ground by using a buff of #200 and then a buff of #360, (D) circuit 
patterns including wirings having a width of 50 .mu.m at a gap of 50 
.mu.m, and portions and pad portions were formed in the electrically 
conducting layers by using an etching resist on both surfaces, (E) the 
copper paste B was applied as a curable electrically conducting substance 
by the screen-printing method to cover substantially the whole end 
surfaces of the connection means, inner peripheral edge portions of a 
width of 0.1 mm of the land portions in the circuit patterns that connect 
to the connection means, and to cover pad portions on which the 
surface-mounting parts will be mounted, and was cured by using the air 
oven under a condition of 160.degree. C. for 30 minutes to form 
electrically conducting patterns having an average thickness of 25 .mu.m 
(fluctuation of about .+-.10%), and (F) nickel-plating having a thickness 
of 4 .mu.m and gold-plating having a thickness of 0.2 .mu.m were effected 
on the circuit patterns of both surfaces and on the electrically 
conducting patterns. One hundred pieces of the circuit boards were thus 
obtained. 
The thus obtained circuit boards did not undergo short-circuiting or 
breakage in the circuit patterns, and their yield was 100%. The electric 
resistance of the connection means connecting together the electrically 
conducting patterns formed on both surfaces of the circuit board was 
measured to be 19 m.OMEGA./connection means in average. The electric 
resistance of the connection means connecting together the circuit 
patterns formed between the insulating substrates was measured to be 22 
m.OMEGA./connection means. After the circuit board was subjected to 
high-temperature/high-humidity conditions of 60.degree. C.-90% RH for 1000 
hours, the resistance of the connection means connecting together the 
electrically conducting patterns formed on both surfaces of the circuit 
board was again measured and was found to be 20 m.OMEGA./connection means 
in average, and the resistance of the connection means connecting together 
the circuit patterns formed between the insulating substrates was measured 
to be 22 m.OMEGA./connection. Furthermore, the surfaces of the circuit 
board were little uneven and on which the solder paste could be favorably 
printed at the time of mounting the surface-mounting parts, enabling the 
surface-mounting parts to be reliably connected. 
Comparative Example 1 
A circuit board was produced according to the steps (A) and (B) of Example 
1. In order to mount the parts on the circuit board, a solder paste was 
applied onto the circuit board by the screen-printing method. However, the 
solder paste was not almost printed onto the circuit board and the parts 
could not be mounted. The electric resistance of the connection means 
connecting both surfaces of the circuit board was measured to be 17 
m.OMEGA./connection means in average. After the circuit board was 
subjected to high-temperature/high-humidity conditions of 60.degree. 
C.-90% RH for 1000 hours, the resistance of the connection means was again 
measured to be 98 m.OMEGA./connection means, which was a conspicuous 
increase in the resistance. 
Comparative Example 2 
A connection means was formed according to the steps (A) to (C) of Example 
3, a common plating layer was formed on the connection means and on the 
electrically conducting layers, and circuit patterns including wirings 
having a width of 50 .mu.m at a gap of 50 .mu.m were formed in the plating 
layer and in the electrically conducting layer by using an etching resist 
on both surfaces. One hundred pieces of the circuit boards were thus 
obtained. The obtained circuit boards had short-circuiting and breakage in 
the circuit patterns, and their yield was 27%.