Metal base board and electronic equipment using the same

A metal base board comprising a metal plate, a circuit conductor section, and an insulating layer provided between the circuit conductor and the metal plate wherein the insulating layer comprises an organic insulating material with flaky inorganic fillers added therein and the flaky inorganic fillers are stacked in the insulating layer in a stratified state.

FIELD OF THE INVENTION 
This invention relates to metal base board comprising a metallic base 
section, a circuit conductor section, and a insulating section provided 
between said circuit conductor section and the base section, and more 
particularly a metal base board used in a semiconductor device for power 
control which is incorporated in electronic equipment and generates a 
quantity of heat. 
BACKGROUND OF THE INVENTION 
In electronic control devices such as an invertor, servo amplifier, and 
spindle amplifier, a semiconductor device for power control comprises 
large capacity diodes, transistors, IGBTs, and MOSFETs. There is a 
so-called "DBC board" as a board for a circuit used in a electronic 
circuit generating a large quantity of heat such as a power control 
section. The DBC board comprises a metal plate for a conductor circuit, a 
ceramic insulating material, and a heat-conductive metal plate joined to a 
base of said ceramic insulating material. The metal plate used in the DBC 
board, for instance may be a copper plate. The ceramic insulating material 
used, for instance, may be alumina ceramic or alumina ceramic nitride. The 
heat-conductive metal plate, for instance, may be a copper plate. If such 
a material as alumina ceramic or nitriding alumina ceramic nitride, each 
with a high heat conductivity, is used as a ceramic insulating material, 
heat generated in a conductor circuit is rapidly transferred to a metal 
plate. Furthermore, as the material itself is ceramic, degradation of the 
material due to corona discharge is suppressed, and also the resistance 
against electric discharge is improved. 
In the DBC board, however, as a coefficient of thermal expansion of the 
ceramic insulating material layer is different from that of the heat 
conductive metal plate layer, the heat cycle characteristic is rather 
poor, and cracks may be generated in the joining interface. Also, due to 
restrictions in the manufacturing process of ceramics, it is difficult to 
manufacture a large size plate. Typically, a ceramic product is limited to 
a dimension of around 150 mm.times.150 mm, using the conventional 
manufacturing method. In addition, the heating of materials at a high 
temperature is required in a process of manufacturing the conventional 
board, and as a result, the manufacturing process is very complicated with 
a high production cost. 
To solve the problems with the DBC board as described above, use of a metal 
base board in which an insulating layer made of organic polymer and 
metallic foil, and adhered to a heat-conductive metal plate with an 
adhesive, has been proposed. As shown in FIG. 7, in this metal base plate 
70, a circuit conductor 3 is formed via an insulating layer 2 comprising 
an organic insulating material on the upper surface of the heat-conductive 
metal plate 1, in which a granular inorganic filler material is added to 
an insulating layer 2. This configuration is disclosed, for instance, in 
the Japanese Patent Publication No. 6235/1971 and the Japanese Patent 
Publication No. 9650/1972. A discharge path 71 is seen between conductor 3 
and plate 1. 
Generally, organic polymer constituting an insulating layer has a 
substantially lower heat conductivity as compared to that of ceramics used 
in the DBC board, but the heat conductivity is raised by adding such an 
inorganic filler material as alumina or silica. Furthermore, the heat 
conductivity is raised to a desired level by reducing the thickness of the 
insulating layer to around 7% to 30% of that of the DBC board. 
The technological documents relating to this invention includes the 
Japanese utility Model Laid Open Publication No. 106775/1991 disclosing a 
"metal base board", the Japanese Utility Model Laid Open Publication No. 
73966/1988 disclosing an "heat-radiating insulating board", the Japanese 
Utility Model Laid Open Publication No. 98253/1987 disclosing a 
"printed-circuit board", the Japanese Patent Laid Open Publication No. 
244180/1991 disclosing a "Metal base laminated plate", the Japanese Patent 
Laid Open Publication No. 27786/1990 disclosing a "low heat resistance 
circuit board", the Japanese Patent Laid Open Publication No. 232795/1989 
disclosing a "Method of Manufacturing a Metal Base Board", and the 
Japanese Patent Laid Open Publication No. 232792/1990 disclosing a 
"Circuit Board". 
Specifically, FIG. 1 or FIG. 4 in the Japanese utility Model Laid Open 
Publication No. 106775/1991 disclose a metal base board, in which lengthy 
inorganic fillers having a high heat conductivity are added in an organic 
insulating layer. The longitudinal surfaces thereof are facing the metal 
base side or are disposed at random. Because the inorganic fillers are 
added, heat radiated from parts mounted to the circuit conductor 3 of a 
metal base board 70 such as transistors, resistors, and capacitors, can 
effectively be transferred for emission to the outside. 
Furthermore, as an organic polymeric insulating material is used in these 
metal base boards, a board which is larger than a DBC board can easily be 
manufactured with a low cost. Furthermore, there is no problem with the 
formation of cracks due to a heat cycle, as in a DBC board. For the 
reasons as described above, a metal base board having the configuration as 
described above is employed in a power circuit or other similar electric 
components which generate a large quantity of heat. 
In the conventional type of metal base board as described above, however, a 
granular inorganic filler material is added to an organic polymeric 
material and the resultant mixture is used as an insulating material. 
However, the organic insulating layer is easily degraded when exposed to 
electric discharge, and the granular filler material does not work 
effectively. Namely, when degradation due to electric discharge proceeds 
up to the section with the granular inorganic filler material added 
therein, the degradation will continue to proceed along the section 
interface. As a result, the filler is not always effective in improving 
the electric discharge resistance characteristics. 
In addition, as the thickness of the insulating layer is reduced to 
approximately 7% to 30% of the thickness of an ordinary DBC board to 
insure a desirable heat radiating characteristics, the insulating 
characteristics will be affected, under high voltage. First, it should be 
noted that, if the thickness of an insulating layer is increased, the 
resistance against electric discharge is improved, but the heat radiating 
characteristics becomes lower because heat generated in the circuit 
conductor is hardly transferred to the metal plate. Also, it should be 
noted that, although a conventional type of metal base board is generally 
used in applications requiring a relatively low operating voltage, i.e., 
AC 200 V or below, recently a metal base board is often used in 
applications where a high voltage in the class of 400 V or 600 V is always 
applied. Thus, under these two conditions, the insulating layer made of 
organic polymer will be degraded due to corona electric discharge, and the 
insulation, characteristics of the insulating layer will be damaged. In 
addition, as the thickness of a plate is small, the life until the final 
breakage of insulation will be short. 
There are several reasons for this problem. First, if there is air on a 
surface along a conductor circuit section of a board, and even if the 
section has been processed with, for instance, resist or a silicon potting 
material, there exist void sections with air filled therein in the 
processed layer. Second, it is known that, if a high AC voltage is applied 
to the such air-filled void sections, corona electric discharge is 
generated. Third, in most organic polymeric materials a voltage at which 
corona electric discharge starts is in a range of approximately 400 to 500 
V, although it depends on the thickness of each board. Not only in 
electronic equipment having a rated voltage of 400 V or less, but also in 
electronic equipment for which a rated voltage is more than 400 V, if a 
voltage of more than 400 V is loaded during its service operation in 
association with electronic control of such sections as a switch, corona 
electric discharge may occur. 
Furthermore, although the "metal base board" disclosed in the Japanese 
Utility Model Laid Open Publication No. 106775/1991 is effective to 
provide thermal advantages, such as improvement of heat conductivity and 
reduction of coefficient of linear thermal expansion, degradation due to 
electric discharge will proceed along an interface, as previously 
described for the board shown in FIG. 7 with a granular inorganic filler 
material added therein when a degradation due to electric discharge 
reaches the fillers. For this reason, the filler can not improve a 
resistance against electric discharge of a metal base board. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide metal base boards which provide 
excellent heat-radiating characteristics as well as excellent resistance 
against electric discharge, and can be used even under a high voltage. It 
also is an object to provide electronic equipment which uses such metal 
base board and operates at high voltage. 
In a metal base board according to the present invention and in the 
electronic equipment using the same, an insulating layer comprises an 
organic insulating material with flaky inorganic fillers added therein. 
The flat faces of said flaky inorganic fillers are aligned basically in 
parallel with the surfaces of the circuit conductor section and the base 
section so that the inorganic flaky filler plates are stacked in a 
stratified state. With this arrangement, even if corona discharge is 
generated in a void at the surface of a circuit conductor section or 
within an insulating layer and the organic insulating material is 
degraded, the degradation due to discharge is prevented or substantially 
suppressed in the section with flaky inorganic fillers added therein. As a 
result, the discharge resistant characteristics of the entire insulating 
layer are improved. 
In a metal base board according to the present invention and the electronic 
equipment using the same therein, the insulating section comprises an 
organic insulating material with flaky inorganic fillers and a granular 
inorganic filler material, flat surfaces of the flaky inorganic fillers 
are aligned basically in parallel with surfaces of the circuit conductor 
section and the base section so that the materials can be stacked in a 
stratified state. As a result, even if discharge occurs in a void along 
the surface of the circuit conductor or in a void section within the 
insulating layer and the organic insulating material is degraded due to 
the discharge, the degradation due to discharge prevented or substantially 
is suppressed in the section with the flaky inorganic fillers added 
therein. Also, the discharge resistant characteristics of the entire 
insulating layer are improved, and because of the granular inorganic 
filler material, the heat conductivity of the insulating layer is raised. 
At the same time, the linear thermal expansion coefficient can be reduced, 
so that an insulating layer suited to conditions for practical use can be 
formed by changing a fixing ratio of the filler material. 
In a metal base board according to the present invention and electronic 
equipment using the same, the insulating section comprises an insulating 
layer made of an organic insulating material with flaky inorganic fillers 
added therein and an insulating layer made of an organic insulating 
material with a granular inorganic filler material added therein, namely 
two layers in all, the surfaces of the flaky inorganic fillers are aligned 
basically in parallel with the surfaces of the circuit conductor and the 
base section so that the filler materials can be stacked in a stratified 
state. As a result, even if the organic insulating material is degraded 
due to electric discharge, the degradation due to electric discharge is 
suppressed in the section with the flaky inorganic fillers added therein, 
or the degradation due to the electric discharge itself is prevented, and 
the electric discharge resistant characteristics are improved. Further, 
because of the insulating layer comprising a granular inorganic filler 
material, the heat conductivity of the insulating layer is raised and also 
the linear thermal expansion coefficient can be reduced. With this 
arrangement, an insulating layer suited to conditions for practical use 
can be formed by changing a ratio of thickness between the two organic 
insulating layers. 
In the metal base board according to the present invention and electronic 
equipment using the same, the insulating section comprises a plurality of 
insulating layers, each insulating layer comprises an organic insulating 
layer with a different type of flaky inorganic fillers each having a 
different average particle diameter respectively. Fillers belonging to at 
least any one type of the flaky inorganic filler materials are aligned 
basically in parallel with the surfaces of the circuit conductor and the 
base section so that the fillers can be stacked in a stratified state. As 
a result, even if electric discharge occurs in a section of the circuit 
conductor along the surface or in a void section within the insulating 
layer and the organic insulating material is degraded due to the electric 
discharge, the degradation due to electric discharge is suppressed in the 
section with the flaky inorganic fillers added therein, or the degradation 
due to electric discharge itself is prevented. In sum, the electric 
discharge resistant characteristics in the entire insulating layer are 
improved. Moreover, an insulating layer suited to conditions for practical 
use can be formed by changing a ratio of thickness between a plurality of 
organic insulating layers. 
In the metal base board and electronic equipment using the same therein, 
the insulating section comprises an organic insulating material with 
multiple different types of flaky inorganic fillers each made of a 
different raw material, surfaces of the flaky inorganic filler materials 
are aligned basically in parallel with the surfaces of the circuit 
conductor section and the base section so that the filler materials can be 
stacked in a stratified state. For this reason, even if electric discharge 
occurs in a section of the circuit conductor along the surface or in a 
void within the insulating layer and the organic insulating material is 
degraded due to the electric discharge, the degradation due to electric 
discharge is suppressed in the section with the flaky inorganic fillers 
added therein, or the degradation due to electric discharge itself is 
prevented with the electric discharge resistant characteristics in the 
entire insulating layer improved. Furthermore, properties corresponding to 
the type of filler material can be given to the insulating layer, and for 
these reasons an insulation layer more suited to conditions for practical 
use can be formed. 
In the board according to the present invention and electronic equipment 
using the same, the insulating section comprises a plurality of insulating 
layers, each layer comprises an organic insulating layer with flaky 
inorganic fillers each having a different particle diameter added therein 
respectively, surfaces of each flaky inorganic fillers are aligned 
basically in parallel with the surfaces of the circuit conductor section 
and the base section so that the fillers can be stacked in a stratified 
state. As a result, even if electric discharge occurs in a section of the 
circuit conductor along the surface or in a void within the insulating 
layer and the organic insulating material is degraded due to the electric 
discharge, the degradation due to electric discharge is suppressed in the 
section with the flaky inorganic fillers added therein, or the degradation 
due to electric discharge itself is prevented with the electric discharge 
resistant characteristics improved in the entire surface. Moreover, 
because properties corresponding to the type of filler material can be 
given to the insulation layer, an insulating layer suited to conditions 
for practical use can be formed.

DESCRIPTION OF THE EMBODIMENTS 
Now description is made hereinafter for embodiments of metal base boards 
according to the present invention and electronic equipment using the same 
with reference to the related drawings. As shown in FIG. 1, a metal base 
board 10 has a circuit conductor 3 via an insulating layer 2 on a metal 
plate 1 with flaky inorganic fillers 4, the flakes thereof having a form 
like a flat plate, added in the insulating layer 2. Such a material as Fe, 
Al, Cu, SUS, or silicon steel, for example, can be used as a material for 
the metal plate 1, but Cu is used in this embodiment. As a material for 
the circuit conductor 3, such a material as Al or Cu, for example, can be 
used, but Cu is used in this embodiment. 
In the insulating layer, an organic insulating material with flaky 
inorganic fillers added therein is used, and in this embodiment resin with 
flaky inorganic fillers each comprising an alumina filler (marketed by 
Showa Keikinzoku under the name of Low soda alumina L-13PC) added therein 
by 80 weight % against the resin is used. The flaky inorganic fillers are 
added in a quantity of 80% against the resin because, if the flaky 
inorganic fillers are mixed too significantly in the resin, viscosity of 
the resin much increases and the workability becomes low. 
Specifically, epoxy resin and a diciandiamido curing agent are mainly used 
as an organic insulating material, and alumina flakes each having a size 
approximately 0.06 mm and the thickness from 0.002 to 0.003 mm are used as 
the flaky inorganic fillers 4. Furthermore the thickness of an insulating 
layer in a board is 0.12 mm, and about 20 to 30 flake layers are formed in 
the insulating layer 2. 
The flakes in the flaky inorganic fillers may be circular, rectangular, 
polygonal, or of other form, so long as the materials are flat. 
Furthermore flat surfaces of each inorganic filler flake 4 in the organic 
insulating material are generally aligned in parallel with the surfaces of 
the circuit conductor 3 and the metal plate 1 so that the flaky inorganic 
fillers can be stacked in a stratified state. Namely, the flaky inorganic 
fillers are added so that as seen by the paths 11, any vector from a given 
point on the circuit conductor 3 to the metal plate 1 in the direction 
vertical to the circuit conductor 3 will intersect some of the flaky 
inorganic fillers. 
In the insulating layer 2, various methods may be employed for aligning the 
flaky inorganic fillers in one direction. For instance, (1) powder of 
flaky inorganic fillers, each flake having a form like a flat plate, is 
added in an organic insulating material having a low viscosity, the 
mixture is applied several times to form a thin layer, and then the thin 
layer is cured; (2) the flaky inorganic fillers are aligned in one 
direction by making use of gravity; or (3) powder of flaky inorganic 
fillers, each flake having a form like a flat plate, is disposed on a 
metal plate, and then a layer of organic insulating material is applied so 
that it becomes impregnated with the flakes. In this embodiment, powder of 
flaky inorganic fillers 4, each flake having a form like a flat plate, are 
added to an organic insulating material having a low viscosity and is 
applied several times in a form of thin layers (10 to 20.mu./l 
time.times.10 times), and the thin layers are cured by loading pressure to 
and heating the thin layers in a vacuum. 
Testing for the electric discharge resistant characteristics of the board 
obtained according to the method described above was carried out. In this 
testing for the resistance against electric discharge, samples were 
obtained by etching a 5 mm (width).times.50 mm (length) conductive pattern 
at a center of a metal base board having outer dimensions of 200 mm 
(length).times.150 mm (width). The thickness of the metal plate was 2 mm, 
the thickness of the organic polymeric insulating layer was 0.12 mm, and 
the thickness of the conductive layer was 0.1 mm. 
In the test, industrial power (AC 2 kV and AC 3 kV, 60 Hz) was applied 
between the conductive pattern and the metal plate of the metal base board 
at a peripheral temperature of 100.degree. C. The time required until the 
insulation was broken was measured. When an AC voltage is applied, air 
near the surface of the conductive pattern will result in a violent 
discharge of electricity. Results obtained in this test are shown in Table 
1. 
TABLE 1 
__________________________________________________________________________ 
Thickness 
of Insulting 
Time until insul. Broken 
Layer (Hr) 
(.mu.m) 
2 k V loaded 
3 k V loaded 
__________________________________________________________________________ 
Flaky Inorganic Filler Materials 
120 9500 4300 
Granular Inorganic Filler Materials 
120 250 80 
__________________________________________________________________________ 
As clearly shown in Table 1, the metal base board in which flaky inorganic 
fillers are used has far superior electric discharge resistant 
characteristics as compared to that of a conventional product (a metal 
base board in which a granular inorganic filler material is used). Namely, 
in a metal base board 10 having the configuration as described above, an 
electric discharge generated in the side of the circuit conductor 3 
degrades the insulating layer 2, and the degradation proceeds up to the 
section with the flaky inorganic fillers 4 added in the insulating layer 
2. As described above, surfaces of flaky inorganic fillers 4 each 
constituting a portion of the insulating layer 2 are aligned basically in 
parallel with the surfaces of the circuit conductor 3 and the metal plate 
1, and in addition the fillers are stacked in stratified layers. With this 
structure, although electric discharge occurs in the flaky inorganic 
filler material section, progress of degradation is very slow because the 
flaky inorganic fillers 4 provide excellent electric discharge resistant 
characteristics. As a result, the performance as shown in Table 1 is 
provided. Namely, the metal base board having a circuit conductor 3 formed 
over the insulating layer 2 on the surface of the metal plate 1, in which 
the flaky inorganic fillers 4 are added in the insulating layer 2 with the 
surfaces of each flake of the flaky inorganic filler material 4 aligned 
generally in parallel with the surfaces of the circuit conductor 3 and the 
metallic body 1 and the aligned flakes 7 inorganic filler stacked in a 
stratified state, has superior electric discharge resistant 
characteristics as compared to that of a conventional type of board. 
Semiconductor devices, such as transistors, resistors, and capacitors for 
power control which generate a large quantity of heat, are mounted on a 
metal base board having the configuration as described above to build a 
variety of electronic equipment. 
Now description is made hereinafter of a second embodiment of the present 
invention. As shown in FIG. 2, a metal base board 20 has the circuit 
conductor 3 via the insulating layer 2 on the metal plate 1 with flaky 
inorganic filler materials 4 and a granular inorganic filler material 
added in the insulating layer 2. In this embodiment, a resin with flaky 
alumina inorganic fillers (marketed by Showa Keikinzoku under the name of 
Low soda alumina AL-13PC) by about 40 weight % against the weight of resin 
and granular inorganic filler materials (Alumina filler having a spheric 
form produced by Showa Denko) by about 50 weight % against the weight of 
resin added therein, respectively, is used. 
In the organic insulating material with the flaky inorganic fillers 4 and 
the granular inorganic filler materials 5 added therein, the surfaces of 
the flaky inorganic fillers are aligned basically in parallel with the 
surfaces of the circuit conductor 3 and the metal plate 1, and are stacked 
in a stratified state. Namely, any vector in the insulating section 2 from 
a given point on the circuit conductor 3 to the metal plate 1 in a 
direction vertical to the circuit conductor 3, as seen by the paths 21, 
will intersect flakes in the flaky inorganic fillers 4. 
Various methods can be employed for aligning the flaky inorganic fillers 4 
in one direction. However, in this embodiment, the flaky inorganic filler 
4 and a granular inorganic filler material 5 are added in powder form to 
an organic insulating material having a low viscosity. The mixture is 
applied several times in a form of thin layers. Then, the thin layer is 
cured in a vacuum by applying pressure and heat. As a result, the 
viscosity of the resin could be reduced to about 70% as compared to a case 
where only the flaky inorganic fillers 4 described in relation to the 
first embodiment was added, and also the workability could be improved. 
Accordingly, the viscosity of an insulating material can be reduced and 
also the linear thermal expansion coefficient as well as the heat 
conductivity can easily be adjusted in a manufacturing process. 
Testing for the electric discharge resistant characteristics of the board 
obtained according to the method as described above was carried out in the 
same procedure as that in the first embodiment. The result of this testing 
are as shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Thickness 
of Insulting 
Time until insul. Broken 
Layer (Hr) 
(.mu.m) 
2 k V loaded 
3 k V loaded 
__________________________________________________________________________ 
Flaky + Granular Inorganic Materials 
120 7800 3080 
Granular Filler Materials 
120 250 80 
__________________________________________________________________________ 
As clearly shown in Table 2 above, for a metal base board with flaky 
alumina filler materials and granular alumina filler materials added in an 
organic insulating material thereof, the electric discharge resistant 
characteristics are substantially better as compared to that in the 
conventional products. The mechanism for improving the electric discharge 
resistant characteristics in this embodiment is basically the same as that 
in the first embodiment. 
Electronic equipment are constructed by packaging semiconductor devices for 
power control, such as transistors, resistors, and capacitors, which 
generate a quantity of heat, on a metal base board having the 
configuration as described above. 
Now description is made for a third embodiment of the present invention. As 
shown in FIG. 3, a metal base board has the circuit conductor 3 formed via 
the insulating layer 2 on the upper surface of the metal plate 1, the 
insulating layer 2 comprises a first insulating layer 2a and a second 
insulating layer 2b, the first insulating layer 2a is filled with the 
flaky inorganic filler materials 4, and the second insulating layer 2b is 
filled with the granular filler materials 5. 
In the first insulating layer 2a, an organic insulating material such as an 
epoxy resin with the flaky inorganic filler materials 4 added therein is 
used. In this embodiment, alumina filler flaky inorganic fillers are added 
by about 80% against the weight of resin (available for Showa Keikinzoku 
under the name of Low soda alumina AL-13 PC). 
In the second insulating layer 2b, an organic insulating material such as 
an epoxy resin with a granular inorganic filler material added is used. In 
this embodiment, alumina filler granular inorganic fillers (Ultra-granular 
alumina, spheric product sold by Showa Denko) are added by about 90 weight 
% against the weight of resin. 
Also, in the metal base board according to this embodiment, the first 
insulating layer 2a made of an organic insulating material with the flaky 
inorganic filler materials 4 added therein is formed in the insulating 
layer at the side adjacent the circuit board 3. This arrangement is 
effective for improving the electric discharge resistant characteristics 
thereof. In a manufacturing process, the second insulating layer 2b with 
the granular inorganic filler materials 5 added therein is first formed on 
the metal plate 1, then the first insulating layer 2a with the flaky 
inorganic fillers 4 added therein is formed, and finally the circuit 
conductor 3 is adhered. 
Surfaces of the flaky inorganic fillers 4 added in the first insulating 
layer 2a are aligned basically in parallel with the surfaces of the 
circuit conductor 3 and the metal plate 1 in the organic insulating 
material, and also the flaky inorganic fillers are stacked in a stratified 
state. Namely, the flaky inorganic fillers are added so that any vector 
from a given point on the circuit conductor 3 to the metal plate 1 in the 
direction vertical to the circuit conductor 3, as seen in paths 31, will 
intersect a flake in the flaky inorganic filler in the insulating section 
2. 
Various methods can be employed for aligning inorganic filler materials in 
one direction. In this embodiment, at first a powder of granular filler 
material 5 is added to an organic insulating material having a low 
viscosity, and the mixture is applied several times in a form of thin 
layer, which is cured to the thickness of 0.06 mm by applying pressure and 
heat. Furthermore, powder of the flaky inorganic filler material 4 is 
added in an organic insulating material having a low viscosity, and the 
mixture is applied several times in a form of thin layers, which are cured 
to the thickness of 0.06 mm by applying pressure and heat in a vacuum. 
Thus, an insulating layer having the total thickness of 0.12 mm is formed 
and the circuit conductor 3 is adhered thereto. 
Testing for the electric discharge resistant characteristics of the board 
obtained as described above was carried out according to the same 
procedure as in the first and second embodiments above. The results of 
this testing are as shown in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Thickness 
of Insulting 
Time until insul. Broken 
Layer (Hr) 
(.mu.m) 
2 k V loaded 
3 k V loaded 
__________________________________________________________________________ 
2-layer Structure Based on the Flaky 
120 8800 3800 
and Granular Inorganic Filler Materials 
Granular Inorganic Filler Materials 
120 250 80 
__________________________________________________________________________ 
As clearly shown in Table 3, the metal base board according to this 
embodiment is far superior in he electric discharge resistant 
characteristics as compared to that of the conventional products. The 
mechanism for improving the electric discharge resistant characteristics 
is basically the same as that in the first embodiment. However, as the 
thickness of the first insulating layer 2a with the flaky inorganic filler 
materials added therein is smaller as compared to that of the insulating 
layer in the first embodiment described above, the time until breakage of 
insulation becomes shorter. 
Electronic equipment are built by packaging semiconductor devices for power 
control as transistors, resistors, and capacitors which generate a 
quantity of heat on the metal base board having the configuration as 
described above. 
Now description is made hereinafter for a fourth embodiment of the present 
invention. As shown in FIG. 4, a metal base board 40 has the circuit 
conductor 3 disposed the insulating layer 2 on the upper surface of the 
metal plate 1. The insulating layer 2 comprises the first insulating layer 
2a and the second insulating layer 2b. The flaky inorganic fillers 4 are 
added in the first insulating layer 2a, and flaky inorganic fillers 6 
having the average size different from that of the flaky inorganic fillers 
4 added in the first insulating layer 2a are added in the second 
insulating layer 2b. 
In the metal base board according to this embodiment, disposing the first 
insulating layer 2a made of an organic insulating material with the flaky 
inorganic filler materials 4 having a relatively large average size 
adjacent the circuit board 3 is effective in improving the electric 
discharge resistant characteristics of the assembly. Accordingly, at first 
the second insulating layer 2b with the flaky inorganic fillers 7 having a 
short average dimension is formed on the metal plate 1. Then the first 
insulating layer 2a with the flaky inorganic fillers 4 having a lengthy 
average size added therein is formed there on. Finally the circuit 
conductor 3 is adhered. 
Surfaces of the flaky inorganic fillers 4 in the insulating layer 2a and 
the flaky inorganic fillers 6 in the second insulating layer 2b are 
aligned basically in parallel with the surfaces of the circuit conductors 
3 and the metal plate 1, and the fillers are stacked in a stratified 
state. Accordingly, any vector in the insulating section from a given 
point on the circuit conductor 3 to the metal plate 1 in the direction 
vertical to the circuit conductor 3, as seen along paths 41 will intersect 
flakes in the flaky inorganic filler materials 4 and 6. 
Also in this configuration, the electric discharge resistant 
characteristics are far better than that of conventional products. The 
mechanism for improving the electric discharge resistant characteristics 
in this embodiment and the method of aligning the direction of the 
inorganic filler materials are basically the same as those in the first 
and the third embodiments. 
Semiconductor devices for power control such as transistors, resistors, and 
capacitors, which generates a large quantity of heat, are mounted on the 
metal base board having the construction as described above to build 
electronic equipment. 
Now description is made for a fifth embodiment of the present invention. As 
shown in FIG. 5, a metal base board 50 has the circuit conductor 3 formed 
via the insulating layer 2 on the upper surface of the metal plate 1. 
Flaky inorganic filler material 4 of a first type and a second type of 
flaky inorganic material are disposed in mixed form in the insulating 
layer 2. In an organic insulating material with the random mix of flaky 
inorganic fillers 4 and the flaky inorganic fillers 7 added therein, 
surfaces of the flaky inorganic filler materials 4 and 7 are aligned 
basically in parallel with the surfaces of the circuit conductor 3 and the 
metal plate 1, and the inorganic filler flakes are stacked in a stratified 
state. Namely any vector from a given point on the circuit conductor 3 to 
the metal plate 1 in the direction vertical to the circuit conductor 3 as 
seen from paths 51 will intersect with flakes of the flaky inorganic 
fillers 4 and 7. Any of, for instance, mica, boron nitride, flaky glass, 
and flaky alumina is used as a raw material for the flaky inorganic 
fillers 4 and 7 described above. 
Also in this configuration, the electric discharge resistant 
characteristics are far better than that of conventional products. The 
mechanism for improving the electric discharge resistant characteristics 
in this embodiment and the method of aligning the direction of the 
inorganic fillers in one direction are basically the same as those in the 
first embodiments. 
Semiconductor devices for power control such as transistors, resistors, and 
capacitors, which generates a large quantity of heat, are mounted on the 
metal base board having the construction as described above to build 
electronic equipment. 
Next description is made of a sixth embodiment of the present invention. As 
shown in FIG. 6, a metal base board 60 has the circuit conductor 3 formed 
via the insulating layer on the upper surface of the metal plate 1. The 
insulating layer 2 comprises the first insulating layer 2a and the second 
insulating layer 2b. The flaky inorganic fillers 4 are added in the first 
insulating layer 2a, and flaky inorganic fillers 8 made of a raw material 
different from that of the flaky inorganic fillers 4 added in the first 
insulating layer 2a are added in the second insulating layer 2b. With this 
arrangement, any vector from a given point on the circuit conductor 3 to 
the metal plate 1 in the direction vertical to the circuit conductor 3, as 
seen from paths 61 will intersect a flake of the flaky inorganic fillers 4 
and 8. Any of, for instance, mica, boron nitride, flaky glass, and flaky 
alumina is used as a raw material for the flaky inorganic fillers 4 and 8 
described above. 
Surfaces of the flaky inorganic filler materials 4 in the insulating layer 
2a and the flaky inorganic fillers 8 in the second insulating layer 2b are 
aligned basically in parallel with the surfaces of the circuit conductors 
3 and the metal plate 1, and the fillers are stacked in a stratified 
state. Also in this configuration, the electric discharge resistant 
characteristics is far better than that of conventional products like in 
each of the embodiments described above. The mechanism for improving the 
electric discharge resistant characteristics in this embodiment and the 
method of aligning direction of the inorganic fillers in one direction is 
basically the same as those in the first and the third embodiments. 
Semiconductor devices for power control such as transistors, resistors, and 
capacitors, which generates a large quantity of heat, are mounted on the 
metal base board having the construction as described above to build 
electronic equipment. 
It should be noted that the metal base boards according to the present 
invention are not limited to those described in relation to the above 
embodiments and can be modified within the spirit of the present 
invention. In the embodiments described above, a circuit conductor is 
formed on only one side of the metal plate, but circuit conductors may be 
formed via an insulating layer on two sides of the metal plate. Also the 
metal base board shown in above third embodiment, fourth embodiment and 
sixth embodiment is comprised by two layers including an insulating layer, 
but the metal base board according to the present invention are not 
limited to two layer structure, and can be modified to the metal base 
board having plural layers. 
As described above, in a metal base board and electronic equipment using 
the same therein, an insulating section comprises an organic insulating 
material with flaky inorganic fillers added therein, and the flaky 
inorganic fillers are stacked in a stratified state, so that, even if the 
insulating layer is degraded due to electric discharge, the degradation 
due to electric discharge is prevented by the flaky inorganic fillers 
added therein, and for this reason the electric discharge resistant 
characteristics is improved. 
Also, in the metal base board according to the present invention and 
electronic equipment using the same therein, the insulating section 
comprises an organic insulating material with flaky inorganic fillers and 
a granular inorganic filler material added therein, and the flaky 
inorganic fillers are stacked in a stratified state. As a result, even if 
the insulating layer is degraded due to an electric discharge, the 
degradation due to electric discharge is prevented by the section with the 
flaky inorganic fillers added. For this reason, the electric discharge 
resistance characteristics are improved with the workability also raised. 
Further, the required performance can be satisfied by mixing inorganic 
fillers having various characteristics without being restricted by the 
flaky inorganic fillers. 
Also in the metal base board according to the present invention and 
electronic equipment using the same therein, the insulating section 
comprises a first insulating layer made of an organic insulating material 
with flaky inorganic fillers added therein and a second insulating layer 
made of an organic insulating material with granular inorganic filler 
material added therein, and the flaky inorganic filler materials are 
stacked in a stratified state. As a result, even if the insulating layer 
is degraded due to electric discharge, the degradation due to electric 
discharge is prevented by the flaky inorganic filler materials. For this 
reason, the electric discharge resistance characteristics is improved with 
the workability also raised, and a required performance can be satisfied 
by mixing inorganic filler materials having various characteristics 
without being restricted by the flaky inorganic filler materials. 
Also in the metal base board according to the present invention and 
electronic equipment using the same therein, the insulating section 
comprises an organic insulating material with flaky inorganic fillers made 
of a plurality of different types of materials added therein, and the 
flaky inorganic fillers are stacked in a stratified state. Accordingly, 
even if the insulating layer is degraded due to electric discharge, the 
degradation due to electric discharge is prevented by the section with the 
flaky inorganic fillers added. For this reason the electric discharge 
resistance characteristics are improved with the workability also raised, 
and a required performance can be satisfied by mixing inorganic filler 
materials having various characteristics without being restricted by the 
flaky inorganic fillers. 
Also in the metal base board according to the present invention and 
electronic equipment using the same therein, the insulating section 
comprises a plurality of insulating layers, the insulating layers comprise 
an organic insulating material with flaky inorganic filler materials made 
of a different material respectively added therein, and the flaky 
inorganic fillers are stacked in a stratified state. Accordingly, even if 
the insulating layer is degraded due to electric discharge, the 
degradation due to electric discharge is prevented by the section with the 
flaky inorganic fillers added therein. For this reason the electric 
discharge resistance characteristics are improved with the workability 
also raised, and a required performance can be satisfied by mixing 
inorganic filler materials having various characteristics without being 
restricted by the flaky inorganic filler materials. 
Although the invention has been described with respect to a specific 
embodiment for a complete and clear disclosure, the appended claims are 
not to be thus limited but are to be construed as embodying all 
modifications and alternative constructions that may occur to one skilled 
in the art which fairly fall within the basic teaching herein set forth.