Circuit board

A circuit board includes a wiring board which includes an insulative board having a wiring pattern on one surface thereof, a connection hole being formed in the insulative board such that the connection hole reaches the wiring pattern, a conductive bump, which is formed by growing a plating on the wiring pattern, being embedded in the connection hole, and the conductive bump serves as a connection to a wiring pattern which is intimately attached to the other surface of the insulative board.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a circuit board which is advantageous for forming 
a multiple-layered wiring pattern. 
2. Brief Description of the Prior Art 
With electronic devices being progressively made much smaller and thinner, 
there are increasingly strong demands for a circuit board which is 
designed to have a much smaller and lighter construction so that the 
circuit board will be suited to be used for those electronic devices which 
are being made much smaller and thinner. 
One such representative example is a circuit board called a "multi-chip 
module (MCM)". In the MCM, a plurality of bare ICs are mounted on a 
multiple-layered circuit board and then mounted, as a single component 
part, on a circuit board acting as a mother board. 
It is particularly strongly demanded of such a MCM circuit board that 
wiring is designed very small and arranged at a high density. At the same 
time, it is strongly demanded that the multiple-layered circuit board be 
designed thinner and manufactured at a lower cost. 
One of the hindrances, which makes it difficult to meet such a demand, 
resides in the conventional techniques in which layers of the wiring 
patterns are inter-connected by means of a through-hole connection method. 
In this through-hole connection method, an insulative board is provided 
with very tiny connection holes which are formed all the way through the 
wiring patterns by drilling, laser or the like, and an inner wall surface 
of each connection hole is plated with a conductive layer (namely, the 
inner wall surface of each connection hole is subjected to a so-called 
through-hole plating). This conventional method has such shortcomings that 
when holes are bored in the insulative board by drilling or other means, 
burrs and metal powder tend to be produced, thus tending to degrade the 
quality and reliability of the through-hole plating. 
Particularly, with the wiring patterns being made much smaller in design, 
it became increasingly technically difficult to make an accurate drilling 
or the like with respect to the insulative board without accidentally 
cutting off the wiring patterns. 
Furthermore, the conventional techniques often encounter such problems that 
the areas for mounting the electronic parts and the areas for wiring are 
obliged to be limited because of a provision of the through-holes and due 
to installation of the wiring. 
In order to cope with such shortcomings and problems, one approach is made 
in which the through-holes formed in the wiring patterns are filled with 
resin, and then the wiring and electrodes for mounting parts are applied 
to the top thereof. These through-holes are called "blind barrier holes" 
or "inner via hole", and are already utilized in actual practice. 
However, they have problems not only in the area of techniques but also of 
cost. Accordingly, it is demanded to realize that wiring can be made at a 
very small pitch, and a circuit board, which is substantially of the blind 
barrier hole type or the inner via hole type, can be manufactured at a low 
cost. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention, to provide a circuit 
board in which multiple-layered patterns can be inter-connected reliably. 
Another object of the invention is to provide a circuit board in which 
multiple-layered patterns can be arranged at a high density. 
A further object of the invention is to provide a circuit board which can 
be manufactured at a low cost. 
To achieve the above objects, there is essentially provided a circuit board 
comprising a wiring board which comprises an insulative board having a 
wiring pattern on one surface thereof, a connection hole being formed in 
the insulative board such that the connection hole reaches the wiring 
pattern, a conductive bump, which is formed by growing a plating on the 
wiring pattern, being embedded in the connection hole, and the conductive 
bump being served as connection means with respect to a wiring pattern 
which is intimately attached to the other surface of the insulative board. 
Preferably the conductive bump is embedded in the insulative board such 
that the conductive bump does not reach the other surface of the 
insulative board, and thus leaves a vacant space in the connection hole, 
not occupied by the conductive bump, the vacant space serves as a hole for 
filling therein conductive paste for connecting the wiring pattern formed 
on the other surface of the insulative board and the conductive bump 
together. 
It may also be designed such that the conductive bump is embedded in the 
connection hole such that the conductive bump does not reach the other 
surface of the insulative board, and conductive paste is filled in the 
vacant space in the connection hole, which vacant space is not occupied by 
the conductive bump, thereby forming a fusible contact point, the contact 
point serving as connection means with respect to the wiring pattern 
formed on the other surface of the insulative board. 
The above and other objects, characteristic features and advantages of the 
present invention will become more apparent to those skilled in the art by 
the following description of the preferred embodiments of the invention, 
as illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
A construction of a circuit board according to one preferred embodiment of 
the present invention, and the sequential steps of manufacturing the same 
will now be described with reference to the accompanying drawings. 
First, as shown in FIG. 1A, a plate having copper attached to one surface 
thereof (this plate is hereinafter referred to as the "single-surface 
copper-attached plate") is formed by intimately attaching a copper foil 2 
to an entire surface of an insulative board 1. 
The surface of the insulative board 1 is melted by hot pressing or the like 
and the copper foil 2 is fused thereto without a need of a provision of 
any adhesive layer between the insulative board 1 and the copper foil 2. 
The insulative board 1 and the copper foil 2 may be attached together by 
hot pressing or the like, with an adhesive layer interposed between the 
insulative board 1 and the copper foil 2. However, anticipating a 
difficulty which will be encountered in the step to follow, it is 
advantageous that no adhesive layer is provided. 
The insulative board 1 employed here refers to a synthetic resin plate 
having rigidity or a synthetic resin sheet having flexibility. The 
material of the insulative board 1 includes, for example, thermoplastic 
liquid crystal polymer resin, and polyimide resin such as, for example, 
reinforced-fiber contained polyimide resin. Also, the insulative board 1 
may be single formed in a or multiple layers. 
Next, as shown in FIG. 1B, a plurality of connection holes 3 are formed 
only in the insulative board 1 which is formed of the single-surface 
copper-attached plate, by laser, etching or the like, such that the holes 
3 extend all the way to the copper foil 2, thereby forming a drilled 
single-surface copper-attached plate. 
The connection holes 3 are through-holes piercing through only the 
insulative board 1 in its thickness direction. Accordingly, one end of 
each connection hole 3 reaches one surface of the insulative board 1, 
while the other end reaches the other surface of the insulative board 1. 
That end of each connection hole 3 which reaches the attachment surface of 
the copper foil 2, is closed by the copper foil 2. The steps of FIGS. 1A 
and 1B may be changed such that the connection holes 3 are preliminarily 
drilled in the insulative board 1 and then the copper foil 2 is attached 
to the second-mentioned surface of the insulative board 1, thereby forming 
a drilled single-surface copper-attached plate. 
Then, as shown in FIG. 1C, conductive bumps 4 for filling the connection 
holes 3 of the drilled single-surface copper-attached plate are formed on 
the copper foil 2 which closes the connection holes 3. The conductive 
bumps 4 are formed by enhancing the growth of plating within the 
connection holes 3 by way of electric plating, serving the copper foil 2 
as an electrode. 
As one example, each of the conductive bumps 4 is grown to a level of 
height which does not reach the other surface of the insulative board 1. 
In other words, each conductive bump 4 is grown to a level of height less 
than the full depth of each connection hole 3, so that there is a vacant 
space in the connection hole on the side of the other surface of the 
insulative board 1, which vacant space is occupied by no conductive bump 
4. This vacant space in each connection hole 3 serves as a hole 3a into 
which conductive paste is filled in the following step. 
Then, as shown in FIG. 1D, conductive paste 5 is applied and filled in the 
filling holes 3a from the other surface side of the single-surface 
copper-attached plate, which is provided with the connection holes 3 
having the conductive bumps 4 embedded therein, by means of screen 
printing or the like, such that a predetermined quantity of the conductive 
paste 5 slightly projects from the surface of the insulative board 1. 
Thereafter, the conductive paste 5 is dried and hardened, thereby forming 
fusible contact point portions. 
That is, each of the fusible contact point portions comprises conductive 
paste. Each contact point portion includes an embedded-portion which is 
embedded in the filling hole 3a and a projection portion projecting 
therefrom. The contact point portion is connected at its embedded portion 
to the conductive bump 4, thereby forming a bump as a whole. The 
conductive paste 5 is a soldering paste which is composed, for example, of 
kneading silver powders and synthetic resin adhesive or kneading metal 
powders of a lead series and synthetic resin. 
Thereafter, as shown in FIG. 1E, another copper foil 6 is applied and 
attached, by hot pressing, intimately to the other surface of the 
single-surface copper-attached plate (the other surface of the insulative 
board) on which the fusible contact point portions are formed by the 
conductive paste 5. 
That is, the copper foil 6 is placed on the projected portions of the 
fusible contact point portions which are composed of the conductive paste 
5, in such a way as to cover the entire surface of the insulative board 1. 
Then the fusible contact point portions are softened or melted by hot 
pressing and at the same time, the surface of the insulative board 1 is 
melted, and thereafter, the copper foil 6 is intimately attached to the 
surface of the insulative board 1 and fused to the fusible contact point 
portions. Thus, the copper foil 6 is connected to the conductive bumps 4 
through the fusible contact point portions and further connected to the 
other copper foil 2 through the conductive bumps 4. 
The conductive paste 5, namely, the fusible contact point portions are 
softened or melted by heat of the hot pressing and at the same time, they 
are, while being compressed and stretched under pressure, pushed into the 
surface layer portion of the melted insulative board 1, thereby composing 
enlarged press-attachment portions 5a each having a wide connection area. 
In this way, there can be obtained a double-surface copper-attached plate 
in which the copper foils 2, 6 on the two surfaces are firmly joined and 
connected together at those places where the wiring patterns are desired 
to be connected together, by fusing or press-attaching through the 
conductive bumps 4 and the conductive paste 5. 
Then, as shown in FIG. 1F, the copper foils 2, 6 on the two surfaces are 
subjected to patterning by known techniques such as exposure, etching or 
the like. As a consequence, there can be obtained a double-surface circuit 
board in which the wiring patterns 2a, 6a are intimately attached to the 
two surfaces of the insulative board 1 and are connected together by the 
conductive bumps 4, which are formed by growing the plating within the 
connection holes 3, through the conductive paste 5. The double surface 
circuit board thus obtained can be used as a unit circuit board for 
forming a circuit board having three or more layers. 
As apparent from the above explanation, the conductive bumps 4 are 
positively grown, under the limitation of the connection holes 3, into and 
positively embedded in the connection holes from the surface of the copper 
foil 2 which closes the connection holes 3. Accordingly, the good or bad 
conditions of the walls of the connection holes 3 cannot be a big problem. 
Especially, since plating can fill such a very tiny hole having a dimension 
of a few dozen .mu.m, the connection areas of the copper foil 2 for 
growing the conductive bumps 4 by plating growth can be designed very 
small. 
In other words, the connection area of each wiring pattern 2a which is 
formed from the copper foil 2 can be designed very small. As a 
consequence, the wiring patterns 2a, 6a can be made very small without 
being limited by the connection hole 3. This makes it possible to realize 
a high density arrangement of the patterns. 
That is, FIGS. 1A-1F illustrate an idea which includes forming the 
connection holes 3 in the insulative board such that the connection holes 
3 extend to the wiring patterns 2a, in the wiring board 1 having the 
wiring pattern 2 which is intimately attached to one surface of the 
insulative board 1, then the conductive bumps 4 grown into the connection 
holes 3 from the wiring patterns 2a by means of plating growth, and the 
conductive bumps 4 are provided as connection means with respect to the 
wiring patterns 6a which are intimately attached to the other surface of 
the insulative board 1. 
Another example for forming the circuit board will now be described. 
First, as shown in FIG. 2A, a plate having copper attached to one surface 
thereof (this plate is hereinafter referred to as the "single-surface 
copper-attached plate") is formed by intimately attaching a copper foil 2 
to an entire surface of an insulative board 1. 
The insulative board 1 employed here refers to a synthetic resin plate 
having rigidity or a synthetic resin sheet having flexibility. The 
material of the insulative board 1 includes, for example, reinforced-fiber 
contained polyimide resin such as glass fiber or glass cloth. Also, the 
insulative board may be formed of a single layer or multiple layers made 
of the same or different materials. 
Next, as shown in FIG. 2B, a plurality of connection holes 3 are formed 
only in the insulative board 1 which is formed of the single-surface 
copper-attached plate, by laser, etching or the like, such that the holes 
3 extend all the way through an adhesive layer 7 to the copper foil 2, 
thereby forming a drilled single-surface copper-attached plate. 
The steps of FIGS. 1A and 1B may be changed such that the connection holes 
3 are preliminarily drilled in the insulative board 1 and then the copper 
foil 2 is attached to the second-mentioned surface of the insulative board 
1, thereby forming a drilled single-surface copper-attached plate. 
The connection holes 3 are through-holes piercing through only the 
insulative board 1 in its thickness direction. Accordingly, one end of 
each connection hole 3 extends through the adhesive layer 7 and reaches 
one surface side of the insulative board 1, while the other end reaches 
the other surface of the insulative board 1. That end of each connection 
hole 3, which reaches the attachment surface of the copper foil 2, is 
closed by the copper foil 2. 
Then, as shown in FIG. 2C, conductive bumps 4 for filling the connection 
holes 3 of the drilled single-surface copper-attached plate are formed on 
the copper foil 2 which closes the connection holes 3. The conductive 
bumps 4 are formed by enhancing the growth of plating within the 
connection holes 3 by way of electric plating, serving the copper foil 2 
as an electrode. 
As one example, each of the conductive bumps 4 is grown to a level of 
height which does not reach the other surface of the insulative board 1. 
In other words, each conductive bump 4 is grown to a level of height less 
than the fill depth of each connection hole 3, so that there is a vacant 
space in the connection hole on the side of the other surface of the 
insulative board 1, which vacant space is occupied by no conductive bump 
4. This vacant space in each connection hole 3 serves as a hole 3a into 
which conductive paste is filled in the following step. 
Then, as shown in FIG. 2D, conductive paste 5 is applied and filled in the 
filling holes 3a from the other surface side of the single-surface 
copper-attached plate, which is provided with the connection holes 3 
having the conductive bumps 4 embedded therein, by means of screen 
printing or the like, such that a predetermined quantity of the conductive 
paste 5 slightly projects from the surface of the insulative board 1. 
Thereafter, the conductive paste 5 is dried and hardened, thereby forming 
fusible contact point portions. 
That is, each of the fusible contact point portions comprises conductive 
paste. Each contact point portion includes an embedded-portion which is 
embedded in the filling hole 3a and a projection portion projecting 
therefrom. The contact point portion is connected at its embedded portion 
to the conductive bump 4, thereby forming a bump as a whole. 
The projection portions are sharp, as illustrated, at their distal ends so 
that they can pierce an adhesive layer 8. The distal end of the conductive 
paste 5 may be flat or sharp at its distal end. The same is true for the 
embodiment of FIGS. 1A-1F. The conductive paste 5 is one which is 
composed, for example, of kneading silver powders and synthetic resin 
adhesive. 
Thereafter, as shown in FIG. 2E, another copper foil 6 is applied through 
the adhesive layer 8 and attached, by hot pressing, intimately to the 
other surface of the single-surface copper-attached plate (the other 
surface of the insulative board) on which the fusible contact point 
portions are formed by the conductive paste 5. 
That is, the copper foil 6 is placed on the projection portions of the 
fusible contact point portions which are composed of the conductive paste 
5, in such a way to cover the entire surface of the insulative board 1, 
and then the fusible contact point portions are softened or melted all the 
way through the adhesive layer 8 by hot pressing, so as to be intimately 
attached (fused) to the copper foil 6. Thus, the copper foil 6 is 
connected to the conductive bumps 4 through the fusible contact point 
portions and further connected to the copper foil 2 through the conductive 
bumps 4. 
The conductive paste 5, namely, the fusible contact point portions are 
softened or melted by heat of the hot pressing and at the same time, they 
are, while being compressed and stretched under pressure in their state 
piercing the adhesive layer 8 under pressure, pushed into the surface 
layer portion of the adhesive layer 8, thereby composing enlarged 
press-attachment portions 5a each having a wide connection area. 
In this way, there can be obtained a double-surface copper-attached plate 
in which the copper foils 2, 6 on the two surfaces are firmly joined and 
connected together at those places where the wiring patterns are desired 
to be connected together, by metal welding through the conductive bumps 4 
and the conductive paste 5. 
Then, as shown in FIG. 2F, the copper foils 2, 6 on the two surfaces are 
subjected to patterning by known techniques such as exposure, etching or 
the like. As a consequence, there can be obtained a double surface circuit 
board in which the wiring patterns 2a, 6a are intimately attached to the 
two surfaces of the insulative board 1 and are connected together by the 
conductive bumps 4, which are formed by growing the plating within the 
connection holes 3, through the conductive paste 5. The double surface 
circuit board thus obtained can be used as a unit circuit board for 
forming a circuit board having three or more layers. 
As apparent from the above explanation, the conductive bumps 4 are 
positively grown, under the limitation of the connection holes 3, into and 
positively embedded in the connection holes from the surface of the copper 
foil 2 which closes the connection holes 3. Accordingly, good or bad 
conditions of the walls of the connection holes 3 cannot be a big problem. 
Especially, since plating can fill such a very tiny hole having a dimension 
of a few dozen .mu.m, the connection areas of the copper foil 2 for 
growing the conductive bumps 4 by plating growth can be designed very 
small. 
In other words, the connection area of each wiring pattern 2a which is 
formed from the copper foil 2 can be designed very small. As a 
consequence, the wiring patterns 2a, 6a can be made very small without 
being limited by the connection hole 3. This makes it possible to realize 
a high density arrangement of the patterns. 
Irregularity of the height of the conductive bumps 4 can favorably be 
absorbed by interposing the fusible contact point portions composed of the 
conductive paste 5 between the grown conductive bumps 4 and the copper 
foil 6 (wiring pattern 6a). 
FIG. 3 shows a construction of a four-layer circuit board which is 
manufactured utilizing the techniques of forming the circuit board as 
disclosed in FIGS. 1A-1F and 2A-2F, and a method of manufacturing such a 
construction. 
Only one surface (only the copper foil 6) of the double-surface 
copper-attached plate manufactured by FIGS. 1A through 1E and FIGS. 2A 
through 2E is subjected to patterning by etching, and as a result, a pair 
of circuit boards 1A and 1B are obtained as shown in FIGS. 1A and 1B. 
That is, a pair of circuit boards 1A and 1B, which include, as one set, an 
insulative board 1, a copper foil 2, a wiring pattern 6a, and a conductive 
bump 4 grown from the copper foil within a connection hole 3, are 
manufactured. Then, conductive paste 9 is applied to a predetermined place 
on the outer surface of the wiring pattern 6a of the circuit board 1A or 
1B by printing or by a dispenser, and hardened to form the conductive 
bump. The conductive paste in the form of the conductive bump is 
appropriately sharpened at its distal end. 
As shown in FIG. 3A, the circuit boards 1A and 1B are disposed to face each 
other at those surfaces on which the wiring pattern 6a and the conductive 
paste 9 are formed. A heat fusible adhesive sheet 10 is interposed between 
the circuit boards 1A and 1B. Then, those three component members, namely, 
the circuit boards 1A and 1B and the adhesive sheet 10, are thermo-bonded 
together at a temperature equal to or higher than the thermo-softening 
temperature of the adhesive sheet 10. 
As shown in FIG. 3B, by this thermo-bonding, the bump consisting of the 
conductive paste 9 is allowed to pierce through the adhesive sheet 10 and 
become pressure-bonded to the wiring pattern 6a of the circuit board 1A 
while being thermally deformed. As a consequence, the mutually 
corresponding patterns 6a are connected together through the bump which 
consists of the conductive paste 9. As a consequence, the circuit boards 
1A and 1B are intimately attached together by thermally fusing the 
adhesive sheet 10. 
Then, both the circuit boards 1A and 1B are subjected to patterning by 
exposing/etching the copper foils 2 thereof. As a consequence, the wiring 
patterns 2a are formed on the circuit boards 1A and 1B, respectively. In 
this way, there can be obtained a four-layers circuit board having the 
four-layers of the wiring patterns 2a, 6a, 6a 2a which are connected 
together. 
The material of the thermo-fusible adhesive sheet 10 is appropriately 
selected from those which can be thermo-bonded at a temperature which is 
as much lower than the softening temperature (Tg temperature) of the 
insulative board 1 as possible. 
That is, when the adhesive sheet 10 is thermo-bonded at a temperature near 
the Tg temperature, the softening of the insulative boards 1 causes the 
bump consisting of the conductive paste 9 to locally compress and deform 
the wiring patterns 6a. When the wiring patterns 6a are overly deformed, 
the wiring patterns 6a form a short circuit with respect to the copper 
foils 2. This results in an inferior product as shown in FIG. 4. 
Next, FIG. 5 shows still another example of a method of manufacturing the 
four-layer circuit board and a construction thereof. As shown in FIG. 5A, 
a double surface circuit board 1C provided on both surfaces thereof with 
patterns 2a and 6a which are manufactured in accordance with the teachings 
of FIGS. 1A-1F and 2A-2F, is prepared. On the other hand, two copper foils 
provided with bumps having sharpened distal ends are formed by applying 
conductive paste 12 to the copper foils 11. Then, the double surface 
circuit board 1C is interposed between the bump-attached copper foils 11. 
Then, an adhesive sheet 10 composed of a thermo-fusible synthetic resin 
sheet is interposed between one side of the double surface circuit board 
1C and the bump-attached copper foil 11, and another such adhesive sheet 
10 is likewise interposed between the other side of the double surface 
circuit board 1C and the other bump-attached copper foil 11. Thereafter, 
those five component members 11, 10, 1C, 10 and 11 are superimposed in 
this order, and they are thermo-bonded at a temperature which is equal to 
or higher than the thermo-softening temperature of the adhesive sheet 10 
but which is equal to or lower than the softening temperature of the 
double surface circuit board 1C. 
This thermo-bonding causes the bumps composed of the conductive pastes 12 
to pierce through the adhesive sheets 10 so as to be pressure attached to 
the wiring patterns 2a and 6a, respectively, while being softened. At the 
same time, the bump-attached copper foils 11 are fused to the two surfaces 
of the double surface circuit board 1C, respectively, through the adhesive 
sheets 10. 
Subsequently, the copper foils 11 are subjected to patterning by applying 
an etching process or the like thereto. As a consequence, a wiring pattern 
11a is formed on each surface of the double surface circuit board 1C. 
Although FIGS. 4 and 5 illustrate the four-layer circuit boards, there can 
be obtained a multiple-layered circuit board having five or more layers by 
arranging additional bump-attached copper foils 11 and additional adhesive 
sheets 10 on one and the other sides of the four-layer circuit board and 
performing the same steps as previously mentioned. 
According to the present invention, the conductive bumps can be positively 
grown, under the limitation of the connection holes, into and positively 
embedded in the connection holes from the surface of the copper foil which 
closes the connection holes. Accordingly, reliable conductive bumps can be 
formed irrespective of the good or bad conditions of the walls of the 
connection holes. 
Especially, since plating can fill such a very tiny hole as having a 
dimension of a few dozen .mu.m, the connection areas of the copper foils 
for growing the conductive bumps by plating growth, namely, the connection 
areas between the wiring patterns which are formed from the copper foil, 
can be designed very small. As a consequence, the wiring-patterns can be 
made very small without being limited by the connection hole. This makes 
it possible to realize a high density arrangement of the patterns. 
Thus, the problems inherent in the prior art in which the wiring patterns 
are drilled and connected together by means of through-hole plating, can 
effectively be obviated. 
Furthermore, irregularity of the height of the conductive bumps can 
favorably be absorbed by interposing the fusible contact point portions 
composed of the conductive paste between the grown conductive bumps and 
the copper foil (wiring pattern). Moreover, irregularity of the thickness 
of the plating can also be absorbed when the conductive bumps are 
thermo-fusible solder plating or the like. 
While there has been described what is at present considered to be the 
preferred embodiments of the invention, it will be understood that various 
modifications may be made thereto, and it is intended that the appended 
claims cover such modifications which fall within the true spirit and 
scope of the invention.