Multilayer printed wiring board and method for producing the same

A multilayer printed wiring board produced by the buildup process. A first layer printed wiring pattern (13) is formed on a metal core (11) through a first insulation laminate (12), and a second layer printed wiring pattern (16) is formed on the first layer printed wiring pattern through through studs (15) and a second insulation laminate (14). The surface of the first layer printed wiring pattern (13) is roughened and the through studs (15) are formed by the buildup process using a conductive paste on the roughened surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a multilayer printed wiring board for use 
with electronic devices, components, etc., and more particularly to a 
multilayer printed wiring board which has a high density and excellent 
heat dissipating properties. 
2. Description of the Prior Art 
In parallel with an increasing tendency toward the miniaturization of 
devices and advancement in the electronic industry, there has been an ever 
increasing trend aiming at the production of printed wiring boards which 
permit high-density mounting. Also, there has been a tendency toward 
forming both a power supply circit and signal circuits on a single printed 
wiring board (PWB) and further there has been a demand for high-density 
mounting and improved heat dissipating properties. 
In the past, as a PWB capable of ensuring high-density mounting, a 
multilayer printed wiring board has been used widely in which a plurality 
of double-sided copper-clad glass epoxy resin boards, each formed with a 
printed wiring pattern on its laminated copper foil by the subtractive 
process, are laminated and the necessary conductive paths are provided by 
plated through-holes. However, the resin boards are made of resins which 
are inferior in heat conductivity and thick resulting in deterioration of 
the heat dissipating properties. 
Thus, while ceramic multilayer wiring boards utilizing thick film printing 
have been used widely, they have the disadvantages that they are more 
expensive, tend to be damaged by mechanical stresses and have limitations 
in size. 
Further while metal cored substrates utilizing good heat dissipating 
properties of metals have been used in the fields requiring heat 
dissipating properties, they must have multilayer structures in order to 
attain the desired high-density mounting. One method (I) to produce a 
multilayer structure is the use of plated through-holes. More 
specifically, as shown in FIGS. 2a to 2c, the method comprises laminating 
a copper foil 23 through an insulation laminate 22 to each side of an 
aluminum substrate 20 formed with through-holes 21 preliminarily (FIGS. 2a 
and 2b), forming through-holes 24 in the portions of the copper foils 23 
corresponding to the through-holes 21 (FIG. 2c), and forming a plated 
copper layer on the insides of the through-holes by an electrodeless 
plating process or an electroplating process. 
Another method (II) comprises, as shown in FIG. 3, bonding a metal foil, 
e.g., copper foil, to one side of a metal substrate 31 through an 
insulation laminate 32 and removing the unwanted portions of the copper 
foil by the subtractive process to form a first layer printed wiring 
pattern 33 of the desired design, and forming interlaminar conductors 
(hereinafter referred to as through studs), interlaminar insulation 
laminates and a second layer printed wiring pattern by the use of an 
insulating material and a conductive paste. 
More specifically, as shown in FIG. 3, a through stud 35 made of a cured 
conductive paste is formed at each of the desired positions on the first 
layer printed wiring pattern 33 on the insulation laminate 32, and a 
second layer printed wiring pattern 36 is printed with a conductive paste 
and cured through interlaminar insulation laminates 34-1 and 34-2. 
Then, in order to meet the need for a multilayer printed wiring board which 
is excellent in heat dissipating properties and low in cost, it is 
desirable to use a substrate combining the heat conductivity of a metal 
and the economy and electric insulating properties of a resin. However, of 
the previously mentioned methods of producing multilayer printed wiring 
boards of the type employing a metal cored substrate, the previously 
mentioned method (I) employing the plated through-holes is not suitable 
for the production of a high-density multilayer printed wiring board which 
attaches importance to the heat dissipating properties, since the 
essential aim of using the substrate for its excellent heat dissipating 
properties is ruined by the presence of the insulation laminates on its 
sides. 
On the other hand, the method (II) employing the conductive paste layers 
and the insulation laminates is disadvantageous in that the conductivity 
of the cured conductive paste is low and there is a limitation to the 
allowable current. 
To improve on this point, a method has been proposed in which a metal 
plating, e.g., copper plating (37 in FIG. 3), is additionally applied onto 
the cured conductive paste on the top layer printed wiring pattern. 
However, this method is also disadvantageous in that while the 
conductivity is increased greatly, a temperature rise due to the flow of a 
large current is so large that there is a limitation to the allowable 
current carrying capacity. Such temperature rise is a problem which is 
mainly concerned with the through studs. 
SUMMARY OF THE INVENTION 
With a view to overcoming the foregoing deficiencies in the prior art, it 
is the primary object of the present invention to provide a multilayer 
printed wiring board in which conductor paths or through studs between the 
respective layer printed wiring patterns are high in reliability and 
excellent in heat dissipating properties and ensure high-density mounting. 
In accordance with the invention, there is thus provided a multilayer 
printed wiring board including a metal core made of aluminum, iron, copper 
or the like, an insulation laminate disposed on the metal core and at 
least two layers of printed wiring patterns laminated to the insulation 
laminate through another insulation laminate. Particularly, the first 
layer printed wiring pattern closer to the metal core is a metal layer 
whose surface on the second layer printed wiring pattern side is formed 
into a rough surface such that the average value of the differences in 
height between its peaks and valleys is 1 .mu.m or more and the second 
layer printed wiring pattern and through studs between the printed wiring 
patterns are made of a cured conductive paste. 
A method for producing such multilayer printed wiring board comprises the 
steps of forming a first layer printed wiring pattern by the subtractive 
process on a metal foil layer of a metal-clad insulating metal cored 
substrate made by bonding together, through an insulation laminate, a 
metal foil formed at least on one side thereof with a rough surface whose 
average value of the differences in height between its peaks and valleys 
is 1 .mu.m or more and a metal core, with the rough surface serving as a 
nonbonding surface, forming a plurality of through studs made of cured 
resin-type conductive paste to provide conduction between the first layer 
printed wiring pattern and a second layer printed wiring pattern to be 
formed next and a second insulation laminate for providing insulation 
spaces of the first layer printed wiring pattern and insulation between 
the first and second printed wiring patterns, and forming the second layer 
printed wiring pattern by screen printing a desired pattern on the surface 
of the through studs and the second insulation laminate by the use of a 
resin-type conductive paste, curing the paste and applying an 
electrodeless plating onto the cured paste. 
In accordance with the invention, by virtue of the fact that the through 
studs made of the cured resin-type conductive paste are provided on the 
roughened surface of the first layer printed wiring pattern to ensure a 
satisfactory contact between the conductive metal powder in the paste and 
the first layer printed wiring pattern, the generation of heat is 
decreased and the allowable current value is increased, thereby fully 
ensuring the effects of the use of the metal core, which has excellent 
heat dissipating properties. 
The above and other objects, features and advantages of the invention will 
become more apparent from the following detailed description taken in 
conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the invention which is applied to the production of a 
two-layer printed wiring board will now be described with reference to 
FIGS. 4a to 4h. 
In FIGS. 4a to 4h a metal core 11 and a metal foil 13 are bonded together 
under the application of heat and pressure through an insulation laminate 
12 having good adhesive properties to obtain a metal foil clad-insulated 
metal cored substrate (FIG. 4a). While such materials as aluminum, copper, 
iron and their alloys may be used as materials for the metal core, 
aluminum and its alloys, which are relatively low in cost, excellent in 
heat conductivity and require no corrosion resisting treatment and the 
like, are preferable materials. Also, these materials may be used in the 
form coated with an oxide layer on the surface thereof. 
The insulation laminate must possess adhesive properties for bonding the 
metal core and the metal foil together and electrical insulating 
properties and a composite material mainly composed of a resin and an 
inorganic material is preferred. Suitable resins may include, for example, 
epoxy, phenol, polyamide-imide, polyimide, nitrile-rubber modified phenol, 
butyral modified phenol resin, etc., and the inorganic material used for 
the purpose of providing the desired molding properties and educing the 
thermal coefficient in addition to the heat conductivity may be composed 
of glass cloth or powdered talc, quartz, alumina, titania or the like. 
Such an insulation laminate is preliminarily applied and dried on either 
or both of the metal core and the metal foil and then the bonding is 
effected. If necessary from the characteristic point view, it is effective 
to apply a different one or ones onto it. The thickness of the insulation 
laminate 12 should preferably be between 0.03 and 0.1 mm in dependence on 
the desired electrical insulation and heat conductivity. 
Copper and aluminum foils of good conductivity may be used for the metal 
foil and particularly the copper foil having good working properties is 
suited for the purpose. While it is conceivable to use a method of 
effecting the bonding by employing a metal foil whose surface on the 
second layer pattern side is preliminarily roughened in such a manner that 
the average value of the differences in height between its peaks and 
valleys is 1 .mu.m or more and another method of roughening the surface of 
the metal foil after the bonding operation, it is preferable to use a 
preliminarily roughened foil since the roughening after the formation of a 
pattern deteriorates the dimensional accuracy of the wiring and since it 
is difficult to uniformly roughen the surface of the pattern. 
While any of such chemical methods as chemical etching, electrochemical 
solution and electrodeposition and such physical methods as honing and 
sandblasting may be used as the method of roughening the surface of the 
metal foil, the method of electrodeposition is preferred from the 
productivity point of view. The roughness of the roughened surface is 
given in terms of the average value of the differences in height between 
its peaks and valleys corresponding to the 10-point average roughness 
according to the JIS Specification B601 and its value must be 1 .mu.m or 
more, and preferably in the range between 2 .mu.m and 6 .mu.m. The 
roughened surface roughness of less than 1 .mu.m results in an 
unsatisfactory contact between the roughened surface of the first layer 
printed wiring pattern of the metal foil and the metal particles in the 
conductive paste, thus failing to increase the current carrying capacity. 
On the other hand, any unnecessarily large surface roughness is not 
preferred since there is the danger of deteriorating the dielectric 
strength of the insulation laminate, although the current carrying 
capacity can be increased. 
Then, the undesired portions of the metal foil are removed by etching in 
accordance with the conventional method so as to form a first layer 
printed wiring pattern 13 as shown in FIG. 4b. 
Then, as shown in FIG. 4d, a conductive paste is cured to provide through 
studs 15 forming conductive paths between the first-layer and second-layer 
printed wiring patterns and also an insulation laminate 14 made of an 
insulating material is provided for insulation between the portions of the 
first layer printed wiring pattern and between the two printed wiring 
patterns. The conductive paste may be such that it has a specific 
resistance between 1.times.10.sup.-5 .OMEGA.cm and 1.times.10.sup.-3 
.OMEGA.cm after curing. The conductive metal powder contained in the paste 
may be silver, copper, silver-plated copper, silver-plated nickel or the 
like and copper powder is well suited from the cost point of view. In 
particular, it is preferable to use a resin-type copper paste employing 
spherical copper powder having holes therein. This type of paste is well 
suited, because forming the copper powder into particularly a spherical or 
granular shape has the effect of remarkably improving the printing 
performance and moreover the presence of holes inside the copper powder 
has the effect of decreasing the specific gravity of the copper powder to 
prevent the copper powder from sinking in the paste and achieving the 
desired porosity that satisfactorily ensures the probability of contact 
between the copper particles in the cured paste with a relatively small 
loading weight (as compared with spherical powder having no holes), 
thereby ensuring stable high conductivity. 
In addition to the composite material compositions described by way of 
examples in connection with the insulation laminate 12, such resins as 
ultraviolet-curing resins, e.g., acrylic resins and such inorganic 
materials as clays, barium sulfate and calcium carbonate may be used as 
insulating materials for forming such insulation laminate. 
Where necessary from the characteristic point of view, the use of a 
laminate of different composite materials is effective. 
It is to be noted that while there is no predetermined order in which 
through studs and an insulation laminate are formed, where an insulation 
laminate is formed by screen printing and curing a thermosetting-type 
insulating paste, the formation of through studs should preferably be 
effected as shown in FIG. 4c. The reason is that if the screen printing of 
the insulating paste is effected first, there is the danger of the 
insulating paste bleeding over the first layer printed wiring pattern of 
the metal foil and forming a thin insulating film, thereby deteriorating 
the reliability in conduction between the first layer printed wiring 
pattern and the through studs. However, this does not apply to a case 
where a UV curing development-type insulating paste, dry film or the like 
having a good resolution is used. 
Then, as shown in FIG. 4f, a second layer printed wiring pattern is formed 
by a buildup process and curing a conductive paste. In this case, it is 
desirable to preliminarily make the lower layer surface as flat as 
possible by mechanical grinding as shown in FIG. 4e. The reason is that 
the printing accuracy of the conductive paste is enhanced with a decrease 
in the irregularities on the undercoat. Also, where the surface of the 
through studs is completely covered with an insulating material, the 
through stud surface on the second printed wiring pattern side can be 
fully exposed by grinding. In addition, the compositions mentioned in 
connection with the description of the through studs can be used for the 
conductive paste and similarly a resin-type conductive paste employing 
spherical copper powder having holes is well suited. Further, a printed 
wiring pattern formed by the use of this conductive paste can be used 
advantageously as a chemically plated undercoat pattern. In other words, 
generally the amount of conductive metal powder in a resin-type conductive 
paste shows a loading weight representing a minimum specific resistance. 
Thus, in the past, where an electrodeless plating is applied to the 
surface of a printed wiring pattern formed with a conductive paste so as 
to impart a high degree of conductivity, a paste filled with more 
conductive metal powder than the loading weight representing the minimum 
specific resistance is used and the metal is exposed considerably to the 
surface of the pattern formed with the conductive paste. 
In the case of the resin-type conductive paste employing spherical copper 
powder having holes, the copper powder is not easily caused to sink due to 
the presence of the holes therein and moreover their spherical shape tends 
to facilitate the exposure of the heads of the individual copper 
particles. Thus, without increasing the copper powder loading weight, it 
is possible to provide a uniform plated wiring pattern with a high 
deposition rate and high plate adhesion to the conductor surface. 
Then, as shown in FIG. 4g, a metal layer 17 is formed on the second layer 
printed wiring pattern 16 by electrodeless plating. This operation is 
performed for the reason that the conductivity of the conductive pase is 
low as compared with the metal, thus limiting the allowable current of the 
resulting wiring pattern, and the operation may be eliminated if the 
required current is low. In addition, there is no particular limitation to 
the method and type of the electrodeless plating. While the electrodeless 
plating of gold, silver, copper, nickel or the like is applicable, a 
method of applying a nickel-boron alloy electrodeless plating 17-1 and 
then applying an electrodeless copper plating 17-2 to form a printed 
wiring pattern is excellent in solderability and free from the occurrence 
of any soldering erosion. Also, it is to desirable perform a treatment, 
such as mechanical grinding, solvent treating or chemical treating as a 
preliminarily treatment for the electrodeless plating. 
The following are specific examples of the actual production of multilayer 
printed wiring boards according to the present invention. 
EXAMPLE 1 
After the surface of an aluminum plate which was 1.5 mm thick and 500 mm 
square had been activated by alkali etching, a copper foil (35 .mu.m 
thick) having one side (referred to as the M side) roughened to a 10 point 
average roughness of 8 .mu.m according to the JIS Specification B0601 and 
the other side (referred to as the S side) roughened to 3.2 .mu.m was 
bonded with the M side serving as a bonding surface to the aluminum plate 
under the application of heat 170.degree. C. and pressure of 50 
kg/cm.sup.2 through an insulation laminate formed by applying and curing a 
resin composition including alumina 50% (weight %, the same applies 
hereinafter) and polyamide-imide resin 50% to a thickness of 50 .mu.m and 
an epoxy resin composition containing silica 50%, thereby preparing a 
copper-clad aluminum plate. Then, a piece was cut from the resulting 
copper-clad aluminum plate to a size of 100 mm long and 150 mm wide and 
then in accordance with the conventional method a resist was formed with a 
dry film and etched by a ferric chloride solution, thereby forming a first 
layer printed wiring pattern. Note that the M and S sides were roughened 
by an electrochemical deposition process. 
Then, using a resin-type copper paste prepared by mixing 85% of spherical 
copper powder having holes and an average particle size of 10.8 .mu.m, 15% 
of resol-type phenol resin (nonvolatile content) and a small amount of 
oleic acid with a solvent consisting of ethyl Carbitol, a through stud of 
1.0 mm in diameter and 0.03 mm in height was formed at each of given 
portions by the buildup process and cured at 180.degree. C. for 30 
minutes. 
Note that the spherical copper powder with holes was produced by the 
following method. Molten copper of 1700.degree. C. was sprayed under a 
pressure of 1 kg/cm.sup.2 against a water bed turning along with a drum 
within a rotary drum and a high-pressure nitrogen gas of 20 kg/cm.sup.2 
was sprayed against the molten copper from the oblique side to effect a 
primary atomization of it. Then, the molten copper was impinged on the 
water bed to effect a secondary atomization, collected from the water, 
dried, placed in a hydrogen gas of 250.degree. C. for 30 minutes so as to 
reduce its surface oxidation and classified by a classifier, thereby 
obtaining spherical copper powder having the maximum particle size of 50 
.mu.m and the average particle size of 10.8 .mu.m. The porosity was 10.5% 
and the apparent density was 3.52 g/cm.sup.3. 
Then, an insulating paste of an epoxy resin containing 30% of silica was 
printed by the screen printing process over the surface, excluding the 
through stud portions, and cured at a temperature of 150.degree. C. for 30 
minutes. 
Then, the surface of the assembly was ground by the use of a three-reel 
scrub grinder having three buffing rolls #600, #600 and #800. 
Then, a resin-type copper paste of the previously mentioned type was used 
to print a second layer printed wiring pattern by the buildup process and 
similarly cured at 180.degree. C. for 30 minutes, thereby forming a 
printed wiring pattern layer of about 20 .mu.m. 
Then, after the printed wiring board had been immersed in 10% HCl for 10 
minutes to clean its surface, the printed wiring board was immersed in an 
electrodeless nickel plating bath (NICRAD-741: Okuno Chemical Ind. Co., 
Ltd.) (65.degree. C., pH7.1) for 15 minutes, thereby applying an NiB (B 
content was about 0.9%) plating about 3.5 .mu.m thick. Then, the printed 
wiring board was immersed in an electrodeless copper plating bath (ELC-HS; 
Uemura Kogyosha K.K.) (65.degree. C.) for 2 hours to apply a copper 
plating about 8 .mu.m thick. 
EXAMPLE 2 
A multilayer printed wiring board according to the invention was produced 
in the same manner as in Example 1 except that a copper-clad aluminum 
plate was formed by using a copper foil (35 .mu.m) having the same M side 
as in Example 1 and a smooth S side and then the 10-point average 
roughness of the S side was made to be 1.1 .mu.m by the sandblasting 
treatment. 
EXAMPLE 3 
After the surface of an aluminum plate had been coated with an oxide film 6 
.mu.m thick by anodic oxidation, a copper foil 35 .mu.m thick having the M 
side roughened to 8 .mu.m (average roughness of JIS-B0601) and the S side 
roughened to 4.5 .mu.m was bonded with the M side serving as a bonding 
surface to the aluminum plate under the application of heat and pressure 
through an insulation laminate formed by coating and curing an epoxy resin 
composition including alumina 65 weight % to a thickness of about 70 
.mu.m, thereby preparing a copper-clad aluminum plate of 250 mm.times.250 
mm square. Then, a first layer printed wiring pattern was formed in the 
same manner as in Example 1. 
Then, using a resin-type copper paste prepared in the same manner as in 
Example 1 using spherical copper powder of an average particle size of 2.7 
.mu.m, a through stud of 1.0 mm in diameter and 0.03 mm in height was 
formed at each of given portion on the pattern by the buildup process and 
cured at 150.degree. C. for 30 minutes. 
Then, an insulating resin paste of an epoxy resin containing 30% of silica 
was printed by the buildup process over the surface, excluding the through 
stud portions, and cured at a temperature of 150.degree. C. for 30 
minutes. 
Then, after the surface of the assembly was ground by the use of an 
abrasive paper, a second layer printed wiring pattern was formed by the 
buildup process using the resin-type copper paste of the same type as of 
Example 1 and cured at 180.degree. C. for 30 minutes, thereby forming a 
printed wiring pattern layer about 20 .mu.m thick. After that, the NiB 
plating was applied to the layer in the same manner as in Example 1. 
Comparative Example 
A comparative multilayer printed wiring board was produced in the same 
manner as in Example 1 except that the S side roughness of a copper foil 
was 0.4 .mu.m. 
With the multilayer printed wiring boards produced in these Examples and 
Comparative Examples, a dc current was supplied to the wiring of the 
following construction and the temperature of each printed wiring board 
was measured by a thermocouple. The results are shown in terms of the 
differences from room temperature in the following Table 1. 
Note that the circuit construction is as shown in FIG. 5 and the wiring 
dimensions are as follows. 
First layer printed wiring pattern (41): width 0.5 mm, total length 60 mm. 
Second layer printed wiring pattern (42): width 1 mm, total length 30 mm. 
Through studs (43): 1 mm, four studs. 
TABLE 1 
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PWB Temperature Rises 
Temperature rise due 
Electric 
to current flow (.degree.C.) 
resistance 
5 (A) 10 (A) (.OMEGA.) 
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example 1 14 55 0.17 
example 2 15 58 0.18 
example 3 14 57 0.19 
comparative 
51 Blowout 0.18 
example 
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With the foregoing examples, it is ascertained that the allowable current 
flowing through the printed wiring of the PWB can be increased in 
accordance with the present invention.