Process for forming a sintered conductor circuit board

A circuit board that is made compact or highly dense and can be prepared using no chemical etching is obtained by patternwise printing a sinterable conductor on a releasing substrate, sintering the conductor by heating at a given temperature, and transferring the sintered conductor to a resin support. A plurality of sintered conductors each prepared by the above process may be laminated to give a multi-layered circuit board.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a sintered conductor circuit board on which LSIs 
or chip components are mounted and through which they are mutually 
connected. 
2. Description of the Prior Art 
Recently, a reasonably large number of functions can be achieved using an 
electric circuit, and accordingly the electric circuit has become 
complicated, resulting in a considerable increase in the number of 
electronic parts. For this reason, circuit boards have also become 
complicated and have become required to be made compact. In order to 
settle these subjects, the wiring or circuit density has become 
increasingly higher and at the same time a tendency toward a multi-layered 
circuit board has come to be seen. 
A conventional process of fabricating multi-layered circuit boards 
comprises chemically etching (or photoetching) a copper foil on a resin 
support to form a pattern, laminating a plurality of the patterned foil, 
making a hole through the laminate in its thickness direction, and plating 
the inner wall of the hole (through-hole) so that the layers of the 
laminate can be mutually connected. 
Since the through-hole is made using a drill, it can have a diameter of 
about 300 .mu.m at best. Since also the layers are connected by plating, a 
severe precision is required for the positional precision between layers 
and the shape of the inner wall of the through-hole, where an increase in 
the number of the layer results in a poor yield. Increasing the number of 
the layers for the purpose of a higher circuit density may also result in 
an increase in the number of the through-hole that connects such layers, 
rather making the wiring space smaller. This brings about a difficulty for 
achieving a higher circuit density. In addition, industrial waste of an 
etchant or the like used during fabrication has been problematic. The 
fabrication also has required a lengthy process, resulting in a high 
fabrication cost. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a circuit board that can 
be made compact or highly dense, may require no chemical etching and can 
promise a low fabrication cost. 
Another object of the present invention is to provide a multi-layered 
circuit board that can achieve a high-density wiring not relying on the 
through-hole but making use of a perfect inner via hole. 
To achieve the above objects, the circuit board of the present invention is 
a circuit board obtained by patternwise printing a sinterable conductor on 
a releasing substrate, sintering the conductor by heating at a given 
temperature, and transferring the sintered conductor to a resin support. 
Thus a sintered conductor circuit board comprising a resin support 
patternwise provided thereon with a sintered conductor can be obtained. 
A double-sided sintered conductor circuit board can also be prepared using 
a resin support having a hole made therein, and patternwise providing a 
sintered conductor on each side of the resin support, which hole is filled 
with a conductive connecting material to electrically connect the sintered 
conductor on each side. 
A plurality of sintered conductors may be laminated and the layers of the 
laminate may be connected with a conductive connecting material through a 
hole to give a multi-layered circuit board. 
The manufacturing process can be simple because no photoetching is used, 
and the positional precision between layers may be loosely managed, 
compared with fabrication of conventional circuit boards, to make it 
possible to readily obtain a multi-layered circuit board. The present 
invention not only can provide a high-density circuit board with a high 
precision, but also requires only a simple fabrication process, can be 
free from the step that produces any industrial waste, and can make 
fabrication equipment compact, thus achieving a low cost.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The sintered conductor circuit board of the present invention and a process 
for its fabrication will be described below with reference to the 
accompanying drawings. 
FIG. 1 cross-sectionally illustrates an embodiment of the sintered 
conductor circuit board according to the present invention. In FIG. 1, 
reference numeral 101 denotes a resin substrate or support. Reference 
numeral 102 denotes patterned wiring formed of a sintered conductor, and 
is adhered onto the resin support 101 through an adhesive layer 103. 
The resin support 101 may ,be made of polyimide or PET (polyethylene 
terephthalate). The conductor that constitutes the sintered conductor may 
include copper, gold and silver. The sintered conductor may be formed of 
any one of these conductors, or may be formed of two or more of these to 
give a multi-layered sintered conductor. The adhesive layer 103 may be 
made of epoxy resin, phenol resin, polystyrene resin or the like. 
The sintered conductor circuit board can be fabricated, for example, in the 
following way: 
To a releasing substrate made of, for example, boron nitride of a hexagonal 
system, a sinterable conductor paste is applied by screen printing in 
stripe lines with a stripe width of from 50 to 500 .mu.m. After the 
conductor paste applied has been dried, the releasing substrate with the 
conductor pattern is heated in a furnace at a temperature of from 
600.degree. to 1,000.degree. C. for 2 to 60 minutes in an atmosphere of an 
inert gas such as nitrogen. As a result, a pattern of the sintered 
conductor is formed without separation of the pattern from the substrate. 
The sintered conductor may be formed in a thickness of from 10 to 35 
.mu.m. The sinterable conductor paste used herein can be prepared, for 
example, by adding 0.5 to 5 parts by weight of a binder such as ethyl 
cellulose or polymethacrylate of layer alcohol to 100 parts by weight of 
an inorganic composition containing the above conductor, and dissolving 
the mixture in 5 to 20 parts by weight of a solvent such as terpinenol, an 
aliphatic alcohol or an aliphatic alcohol ester, to give a vehicle 
composition, followed by kneading using an kneader such as a three-roll 
mill. 
Next, the resin support with the adhesive is superposed on the above 
releasing substrate with the sintered conductor pattern, which are then 
hot-pressed at a temperature of from 5.degree. to 200.degree. C. After 
cooling, the resin support is peeled from the releasing substrate, so that 
the former can be separated from the latter in the state that the sintered 
conductor pattern has adhered to the resin support, in other words, the 
sintered conductor pattern is transferred to the resin support. 
FIG. 2 cross-sectionally illustrates another embodiment of the sintered 
conductor circuit board in which part of the sintered conductor protrudes 
from a resin support. 
The sintered conductor circuit board shown in FIG. 2 has the same basic 
structure as the one shown in FIG. 1, except that part of the sintered 
conductor protrudes from the resin support. Those having this shape are 
commonly called TAB (tape automated bonding). Reference numeral 201 
denotes a resin support which may be made of the same material as the 
resin support 101 of the circuit board shown in FIG. 1. Reference numeral 
202 denotes a sintered conductor, which may be comprised of the same 
materials as the sintered conductor 102 of the circuit board shown in FIG. 
1. Reference numeral 203 denotes an adhesive layer which may also be made 
of the same material as the adhesive layer 101 of the circuit board shown 
in FIG. 1. In this embodiment, the sintered conductor 202 is so formed 
that part thereof protrudes from the resin support 203. Reference numeral 
204 denotes a window through which an IC is attached. Reference numeral 
205 denotes what is called a lead, which is utilized for electrical 
connection to an external circuit such as the IC. 
FIGS. 3A to 3C illustrate a flow sheet to show a process of fabricating the 
sintered conductor circuit board shown in FIG. 1 or 2. In FIG. 3A, 
reference numeral 301 denotes the releasing substrate previously 
described. The releasing substrate may preferably be made of a boron 
nitride of hexagonal system (h-BN). Alternatively, the releasing substrate 
may be those comprising an aluminum substrate patternwise coated thereon 
with a water-based graphite coating composition as exemplified by AQUADUG 
(trade name; available from Acheson Japan, Ltd.) followed by drying, which 
can similarly give the sintered conductor wiring and also has a good 
releasability when it is further patternwise coated with, e.g., a Cu 
paste, followed by drying and then sintering. Still alternatively, as the 
releasing substrate, it is also possible to use an aluminum substrate 
itself which is usually of wide use in hybrid ICs, which can be readily 
released under appropriate control of sintering conditions or the 
composition of the sinterable conductor paste. 
Reference numeral 102 denotes the wiring formed of the sintered conductor, 
which has been sintered by applying the sinterable conductor paste on the 
above releasing substrate by screen printing or the like, followed by 
firing at the high temperature as previously described. The wiring may be 
formed of the sintered conductor with a high conductivity as previously 
exemplified by gold, silver and copper. The method of forming this wiring 
is well known to those skilled in the field of ceramics. Next, as shown in 
FIG. 3B, the resin support or substrate 101 having the adhesive layer 103 
is superposed on the sintered conductor wiring, which are then hot-pressed 
at the temperature previously described. Thereafter, the resin support 101 
is peeled from the releasing substrate, so that the sintered conductor 
wiring is transferred to the resin support by virtue of the adhesive layer 
103 and releasing substrate 301. FIG. 3C illustrates the state the 
sintered conductor wiring has been transferred to the resin support. The 
releasing substrate 301 can be again used in further fabrication. 
The sintered conductor circuit board shown in FIG. 2, provided with the 
lead 205, can be obtained by transferring the sintered conductor in such a 
way that it may protrude from the resin support in an appropriate length. 
It is also possible to fabricate a double-sided sintered conductor circuit 
board, which is prepared by transferring the sintered conductor to both 
sides of the resin support of the sintered conductor circuit board shown 
in FIG. 1. 
FIGS. 4A to 4E illustrate a flow sheet to show a process of fabricating 
such a double-sided sintered conductor circuit board prepared by 
transferring the sintered conductor to both sides of a resin support. 
FIGS. 4A to 4C are the same as FIGS. 3A to 3C except for making holes in 
the circuit board and also making use of a resin support 401 having 
adhesive layers 103 on its both sides. FIG. 4D shows the step of burying a 
conductive connecting material 402 in the resin support at its holes. The 
conductive connecting material 402 is rubbed in them by means of a 
squeegee 403. Thereafter, the steps shown in FIGS. 4A to 4C are once 
repeated to give a double-sided sintered conductor circuit board as shown 
in FIG. 4E. Reference numeral 404 denotes a sintered conductor provided on 
the back side. The conductive connecting material 402 may be comprised of 
a low-melting metal as exemplified by solder, or a paste incorporated with 
metal powder as exemplified by silver paste. 
FIG. 5 cross-sectionally illustrates a multi-layered sintered conductor 
circuit board according to the present invention. To the surfaces of a 
plurality of resin supports 501 in which holes have been made, sintered 
conductors 502 are respectively transferred by virtue of adhesive layers 
503, and then the holes in the resin support are filled with a conductive 
connecting material 504. Thereafter, the respective resin supports 501 
with the sintered conductors 502 are laminated. This multi-layered circuit 
board can be fabricated by superposing one another sheets of sintered 
conductor circuit boards having been processed up to the step of FIG. 4D 
and a sheet of double-sided sintered conductor circuit board having been 
processed up to the step of FIG. 4E, which are superposed with 
registration of holes and then hot-presses to give a laminate. 
In the illustration in FIG. 5, the sintered conductors are buried in the 
adhesive layers. Such a configuration is obtained upon hot-pressing. This 
can give a mechanically strong multi-layered sintered conductor circuit 
board. 
In the foregoing descriptions, the releasing substrate has a flat surface. 
Alternatively, the surface may be partly depressed or recessed so that any 
upper and lower sintered conductors in the multi-layered sintered 
conductor circuit board can be more surely connected. 
FIGS. 6A and 6B are cross sections to show the shape of a sintered 
conductor and how it is connected to other sintered conductor, in an 
instance in which part of the surface of a releasing substrate has been 
recessed. In FIG. 6A, reference numeral 601 denotes a releasing substrate 
provided with recesses on its surface. To this releasing substrate, the 
sinterable conductor paste as previously described is applied by screen 
printing or the like, followed by firing under the conditions as 
previously described. Subsequently, a resin support with holes 
positionally corresponding with the recesses is superposed on the 
releasing substrate having the conductor, which are then hot-pressed in 
the same manner as previously described. Then the resin support is peeled 
from the releasing substrate, so that the sintered conductor is 
transferred to the resin support. As a result, projections 602 are formed 
on the sintered conductor at its parts corresponding to the recesses of 
the releasing substrate. The holes of the resulting sintered conductor 
circuit board are filled with the conductive connecting material in the 
same manner as previously described. This circuit board is prepared in 
plurality. The plural circuit boards are superposed one another and then 
laminated by hot pressing with registration of the projections 602 to the 
holes to produce a multi-layered sintered conductor circuit board. These 
holes positionally correspond with the holes so made in the resin support 
as to enable electrical connection of the sintered conductors on the 
respective sintered conductor circuit boards. FIG. 6B is an enlarged view 
of the part at which a sintered conductor is connected to other sintered 
conductor. In FIG. 6B, reference numeral 603 denotes the resin support in 
which the holes have been made; 604, the sintered conductors; and 605, the 
conductive connecting material. Upon lamination of the circuit boards, the 
projections 602 made of the sintered conductor come through the holes and 
press against the conductive connecting material 605 to make the 
connection surer. 
The projections 602 may also be provided by printing or the like on the 
sintered conductor 102 shown in FIG. 1. 
The present invention will be described below in greater detail by giving 
Examples. 
EXAMPLE 1 
To a releasing substrate comprising a hexagonal boron nitride plate (trade 
name: TG-GRADE; available from Denka K.K.) with a flattened surface, a 
sinterable conductor paste having the composition as shown in Table 1 was 
applied by screen printing in stripe lines with a stripe width of 300 
.mu.m. 
TABLE 1 
______________________________________ 
Composition of Conductor Paste 
Material components 
Amount (% by weight) 
______________________________________ 
Cu powder (1.7 .mu.m)* 
79.9 
CuO (2.5 .mu.m)* 2.55 
Glass flit (1.9 .mu.m)* 
2.55 
Vehicle (binder + solvent) 
15.5 
Total 100% by weight 
______________________________________ 
*average particle diameter 
This conductor paste was prepared by adding to the above inorganic 
components 1.0 parts by weight of ethyl cellulose as a binder, a component 
of the vehicle, and dissolving them in 15 parts by weight of a solvent 
terpinenol to give a vehicle composition, followed by kneading using a 
three-roll mill. 
The resulting printed stripe lines (circuit pattern) was dried at 
120.degree. C. for 10 minutes, and then fired in a mesh belt furnace in a 
nitrogen atmosphere. The firing was carried out under conditions of 
900.degree. C., which was maintained for 10 minutes, and for a period of 
60 minutes from the time the substrate with the printed circuit pattern 
was put in the furnace to the time the fired product was withdrawn 
therefrom. As a result, a pattern of the sintered conductor (sintered 
copper) was formed in a good precision without separation of the pattern 
from the substrate. The sintered conductor pattern had a stripe width of 
about 300 .mu.m, which underwent almost no widthwise shrinkage. The 
sintered conductor had a thickness of about 15 .mu.m. A thicknesswise 
shrinkage was seen. 
Next, a polyimide sheet with an epoxy adhesive (available from Toray 
Industries, Inc.; a cover-lay sheet for FPG (flexible printed circuit); 
hereinafter simply "polyimide sheet") was superposed on the above 
substrate with the sintered conductor in such a manner that part of the 
sintered conductor protrudes from an end of the polyimide sheet by about 1 
mm, which were then hot-pressed at 160.degree. C. After drying, the 
polyimide sheet was peeled to find that the sintered conductor had adhered 
to, i.e., transferred to, the polyimide sheet. The protruded portion of 
the sintered conductor was in the state of a finger-like projection, thus 
forming the commonly known TAB (tape automated bonding). 
EXAMPLE 2 
The procedure in Example 1 was repeated to form a sintered conductor 
stripes on the releasing substrate hexagonal boron nitride plate, except 
that the sintered conductor was made to have a three-layer structure 
comprised of a gold conductor, a copper conductor and a gold conductor in 
this order. The gold conductor layer was formed using a gold conductor 
paste prepared by adding the above vehicle to inorganic components 
comprised of 80% by weight of gold powder (average particle diameter: 
about 1 .mu.m) and 1.0% by weight of glass flit (average particle 
diameter: about 1.9 .mu.m). The copper conductor layer was formed using 
the same copper conductor paste as in Example 1. Both the pastes were 
superposingly applied by screen printing to the releasing substrate in 
order of gold, copper and gold. 
The subsequent procedure in Example 1 was further repeated to give a 
sintered conductor of three-layer structure, except that the firing was 
carried out under conditions of 850.degree. C. in a nitrogen atmosphere, 
which was maintained for 10 minutes. 
The sintered conductor of three-layer structure thus prepared was 
transferred to the polyimide sheet in the same manner as in Example, to 
give a TAB structure having a finger covered with sintered gold. 
EXAMPLE 3 
To the surface of the same sintered conductor as prepared in Example 1 on 
the hexagonal boron nitride substrate, a solder flux was applied, followed 
by dipping in a molten solder. As a result, the sintered conductor, 
without separation from the substrate, had a surface on which the solder 
had been applied. This sintered conductor was transferred to the polyimide 
sheet in the same manner as in Example 1, to give a TAB structure having a 
finger provided with solder. 
EXAMPLE 4 
Circuit pattern of the sintered conductor was formed in the same manner as 
in Example 1. A polyimide sheet having holes at the desired positions was 
superposed thereon with registration, followed by hot pressing. Thereafter 
the sintered conductor was transferred to the polyimide sheet to give a 
sintered conductor circuit board. From the back of this sintered conductor 
circuit board, i.e. the side on which the sintered conductor is not 
provided, a thermoplastic binder resin silver paste was rubbed into the 
holes, and then the paste was dried. Another circuit pattern of the 
sintered conductor was also formed on that surface in the same manner as 
in Example 1, and the sintered conductor circuit board previously prepared 
was superposed thereon with registration, followed by hot pressing so that 
the sintered conductor was transferred. Thus the double-sided sintered 
conductor circuit board as shown in FIG. 4E was obtained. The sintered 
conductors on the both sides were electrically connected through the 
silver paste filled in the holes. 
EXAMPLE 5 
Using a releasing substrate comprising a hexagonal boron nitride substrate 
provided with recesses on its surface, the same sinterable conductor paste 
as used in Example 1 was applied by screen printing and firing in the same 
manner as in Example 1 to form a pattern of the sintered conductor on the 
releasing substrate. Subsequently, a polyimide sheet with holes 
positionally corresponding with the recesses was superposed thereon with 
registration, which were then hot-pressed in the same manner as in Example 
1. Then the polyimide sheet was peeled from the releasing substrate, so 
that the sintered conductor was transferred to the resin support. As a 
result, projections as shown in FIG. 6 by reference numeral 602 were 
formed on the sintered conductor at its parts corresponding to the 
recesses of the releasing substrate. The holes of the resulting sintered 
conductor circuit board were filled with a conventionally known 
thermoplastic resin silver paste. 
The foregoing procedure was repeated to prepare sintered conductor circuit 
boards of the same structure, which were superposed one another and then 
laminated by hot pressing in the same manner as in Example 1, with 
registration of the projections to the holes. A multi-layered sintered 
conductor circuit board having basically the same structure as shown in 
FIG. 5 was thus obtained. The respective layers of this multi-layered 
sintered conductor circuit board was confirmed to be perfect. 
EXAMPLE 6 
On an aluminum substrate, a graphite dispersion AQUADUG (trade name; 
available from Acheson Japan, Ltd.) was coated, followed by drying to 
produce a releasing substrate. On this releasing substrate, a Cu paste was 
patternwise printed, followed by drying, and then firing was carried out 
in the same manner as in Example 1. As a result, the graphite had 
disappeared and a pattern of the sintered conductor was obtained on the 
aluminum substrate. The same polyimide sheet as used in Example 1 was 
superposed thereon followed by hot pressing in the same manner as in 
Example 1, so that the sintered conductor was transferred to the polyimide 
sheet to give a sintered conductor circuit board. 
EXAMPLE 7 
Using as an inorganic component only copper powder having an average 
particle diameter of about 1 .mu.m, a copper conductor paste was prepared 
by kneading the powder together with the same vehicle as used in Example 
1. This cooper conductor paste was applied to the surface of an aluminum 
substrate by screen printing. The inorganic component of this conductor 
paste contained no additives for aiding adhesion to the substrate. The 
firing was carried out in the same manner as in Example 1. The sintered 
conductor thus formed was in a weak adhesion to the aluminum substrate, 
and hence, after the superposing of a polyimide sheet and hot pressing in 
the same manner as in Example 1, transferred to the polyimide sheet. Thus 
a sintered conductor circuit board was obtained. 
As having been described above, the present invention makes it possible to 
obtain a high-density circuit board in a high precision. In addition, the 
circuit board can be obtained by a simple process, there is no step of 
causing industrial waste, and the fabrication equipment can be made 
compact, resulting in a low cost and high productivity.