Aluminum enamel board

This invention provides an aluminum enamel board having an electrically insulating glass layer formed on an Al-Si alloy plate containing at least 5 wt % of Si or a composite metal plate having an aluminum or alloy layer formed on at least one side of a metal plate made of a metal other than aluminum. Since the Al-Si alloy plate containing at least 5 wt % of Si or a composite metal plate having the aluminum or aluminum alloy layer is used as a board, the adhesion strength between the metal plate and the electrically insulating glass layer can be increased. At the same time, a predetermined material is used to form the electrically insulating glass layer, thereby minimizing a difference in thermal conductivity and thermal expansion coefficient between the electrically insulating glass layer and the metal plate. This improves the heat conduction characteristics of the board and prevents warping of the board.

BACKGROUND OF THE INVENTION 
The present invention relates to an aluminum enamel board used as a printed 
circuit board. 
Ceramic and glass-epoxy electrically insulating boards have been popular as 
conventional printed circuit boards. In recent years, however, enamel 
boards having metal plates as cores have been developed. Enamel boards 
include an iron enamel board and a stainless steel enamel board. A 
conductive or resistive paste is printed and calcined on an electrically 
insulating glass layer formed on an iron or stainless steel board to 
prepare such an enamel board which is used as a hybrid IC. Large enamel 
boards can be easily manufactured although a large board cannot be 
manufactured using a ceramic electrically insulating board. The enamel 
boards have high resistance to mechanical impact and vibration and high 
thermal resistance. 
Iron has a thermal conductivity of 50.times.10.sup.-3 W/mk, while stainless 
steel has a thermal conductivity of 15.times.10.sup.-3 W/mk. An iron or 
stainless steel enamel board has a thermal conductivity slightly higher 
than that of a ceramic board. Since an iron-based material generally does 
not have high adhesion strength with glass, an iron-based metal plate must 
be Ni-plated to form an enamel glass layer on the metal plate. In 
addition, an iron-based material has low corrosion resistance and low 
oxidation resistance, so that a glass layer must be undesirably coated on 
the entire surface of the iron-based metal plate. 
An aluminum enamel board using aluminum as a core metal has also been 
developed. In the aluminum enamel board, the thermal conductivity of 
aluminum is as high as 240.times.10.sup.-3 W/mk, the specific gravity 
thereof is as small as 2.7, and corrosion and oxidation resistance 
characteristics thereof are excellent. Aluminum can be advantageously 
oxidized to form a thin oxide film. Therefore, enamel glass can be easily 
formed thereon with high adhesion strength. 
Although aluminum has a high thermal expansion coefficient of 
24.times.10.sup.-6, the thermal expansion coefficient of enamel glass is 
low and therefore the board becomes undesirably warped after enamel 
calcination is performed. In addition, since a difference between the 
thermal expansion coefficients of aluminum and enamel glass is large, the 
surface of the enamel glass layer forms cracks or irregular wrinkles upon 
calcination. When a board having a high thermal expansion coefficient is 
used as a printed circuit board, parts may be undesirably peeled from the 
board by heat cycles upon mounting of the parts. More specifically, the 
thermal expansion coefficient of aluminum is 24.times.10.sup.-6, the 
thermal expansion coefficient of chip parts is 8.times.10.sup.-6, and the 
thermal expansion coefficient of a silicon semiconductor element is 
4.times.10.sup.-6. Therefore, the thermal expansion coefficient of the 
parts is as small as 1/3 to 1/6 that of the aluminum board. For this 
reason, when the printed circuit board is subjected to thermal hysteresis 
in the range of a low temperature to a high temperature of 100 to 
200.degree. C., bonding portions between the parts and board crack to peel 
the parts therefrom. As a result, reliability of electronic parts is 
degraded. Since the thermal conductivity of the enamel layer is 
signficantly smaller than that of aluminum, heat conduction 
characteristics of the aluminum enamel board are essentially determined by 
the enamel layer. In order to improve heat dissipation of the board, the 
thermal conductivity of the enamel layer must be increased. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an aluminum enamel 
board wherein an electrically insulating glass layer is formed on an Al-Si 
alloy plate containing at least 5% of Si or a composite metal plate having 
an aluminum or aluminum alloy layer formed on at least one side of a plate 
of a metal other than aluminum. 
The present invention provides an aluminum enamel board to be used as a 
printed circuit board having a hybrid IC formed by a plate on alumina 
ceramic or a circuit formed by etching a copper-plated board. 
According to the aluminum enamel board of the present invention, in order 
to decrease thermal expansion of aluminum, electrically insulating glass 
layer 3 having a predetermined pattern is formed on Al-Si alloy plate 1 
containing at least 5% of Si, as shown in FIG. 4, or a composite metal 
plate having aluminum layer or aluminum alloy layer 2 formed on at least 
one side of a plate 1 of a metal other than aluminum and having a small 
thermal expansion coefficient, as shown in FIG. 1. 
A method of preparing an Al-Si alloy plate containing at least 5% of Si and 
used as a board according to the present invention will be described 
below. The Al-Si alloy plate is prepared such that Al and Si are mixed at 
a predetermined mixing ratio, and the resultant mixture is melted as a 
hemogeneous molten alloy and then cast in molds. During casting, a 
treatment for improving quality of a cast product is performed or a 
cooling rate is increased to prevent large Si crystal grains. Therefore, 
small Si grains are uniformly distributed. When an enamel glass paste is 
printed and calcined on an Al-Si alloy plate prepared as described above, 
an aluminum enamel board having a smooth surface can be desirable 
obtained. The resultant ingot is filleted and is hot- and cold-rolled to 
obtain a rolled plate having a thickness of 0.5 to 2 mm. In order to 
prepare a composite aluminum plate having such an Al-Si alloy plate, a 
predetermined Al alloy plate is bonded to the surface of the filleted 
ingot, the ends of the ingots are welded, and the resultant structure is 
hot- and cold-rolled to obtain a rolled composite material plate having a 
thickness of about 0.5 to 2 mm. 
An Al-Si alloy plate must contain at least 5 wt % of Si. If the content of 
Si is less than 5 wt %, the thermal expansion coefficient cannot be 
sufficiently lowered. Even if electrically insulating glass layer 3 is 
formed on an Al-Si alloy plate, board warping and peeling of glass layer 3 
cannot be satisfactory eliminated. 
However, if the content of Si exceeds 12 wt % although the upper limit is 
not specified, it is difficult to roll the alloy because an Si crystal 
grain size is excessively increased. When such a material is rolled, many 
pores may be formed on the surface because large Si grains are removed 
from the surface. In this state, when an enamel glass paste is printed and 
calcined on the porous surface, the paste is expanded at pore portions, 
resulting in many problems. For example, it is difficult to obtain a board 
having a smooth surface. If an Al-Si alloy plate containing 12 wt % or 
more of Si is used, an Al plate or an Al alloy plate having a composition 
different from that of the Al-Si alloy plate is applied as cladding layer 
4 on metal plate 1 of an Al-Si alloy, thereby obtaining a composite 
aluminum plate, as shown in FIG. 2. In this case, it is most preferable to 
clad pure aluminum. A glass paste is printed and calcined on cladding 
layer 4 to form electrically insulating glass layer 3, thereby preparing 
an aluminum enamel board. 
The ratio of the thickness (if cladding layers are respective formed on the 
upper and lower surfaces, the thickness must be a total thickness of the 
cladding layers) of cladding layer 4 to that of the composite aluminum 
plate (to be referred to as a cladding ratio hereinafter) preferably falls 
within the range of 0.2% to 10%. If the ratio is less than the value 
corresponding to 0.2%, crystal defects such as pinholes cannot be 
completely eliminated. However, if the ratio exceeds the value 
corresponding to 10%, the thermal expansion coefficient cannot be 
satisfactory decreased. 
In order to form aluminum layer 2 on metal plate 1 of a metal other than 
aluminum (to be referred to as "metal plate 1" hereinafter) in a composite 
metal plate used as a board according to the present invention, the 
following methods can be utilized: a molten aluminum plating in which 
metal plate 1 is dipped in molten aluminum and is removed therefrom and 
rolled; a method in which direct chemical plating is performed for a metal 
plate by usig a non-aqueous aluminum solution; a cladding method in which 
a thin aluminum plate is pressed on metal plate 1 and these plates are 
mechanically bonded by rolling; a method in which a thin aluminum plate is 
pressed on metal plate 1, and the resultant structure is heated to bond 
them by diffusion or by forming a reactive phase; a physical deposition 
method such as vacuum deposition, sputtering, and ion plating; a chemical 
deposition method; and fire spraying. An optimal method can be employed 
according to the type of metal plate 1. 
Aluminum layer 2 according to the present invention may be made of pure 
aluminum or an aluminum alloy. The thickness of aluminum layer 2 is not 
limited to a specific value but preferably falls within the range of 0.3 
mm or less. However, aluminum layer 2 may be a thin film having a 
thickness of 1 .mu.m or less. 
Examples of metal plate 1, other than an aluminum plate, in the composite 
metal plate according to the present invention are metal plate 1 of Fe, 
Ni, Cu, W, Mo, or Si, an alloy plate of at least two of these metal 
elements, and a composite material plate of at least two of the metal 
elements and alloys which are prepared as described above. The composite 
materials are defiend as a Cu-W composite material prepared by a W or Mo 
plate impregnated with molten Cu, a Cu-Mo composite material, a composite 
material prepared by coating Cu on an Fe-Ni alloy plate, and a composite 
material prepared by coating an Fe-Ni alloy on a Cu plate. 
Metal plate 1 of an Fe-Ni alloy plate, a W plate, an Mo plate, an Si plate, 
or a plate of a composite material of Cu and one of the above-mentioned 
metals as a major constituent has a thermal expansion coefficient similar 
to that of a semiconductor element mounted on a printed circuit board and 
is suitable for great improvement in electrical reliability of an 
electronic unit in heat cycles. 
An enamel glass paste is applied by printing, spraying or the like to the 
resultant Al-Si alloy plate or composite metal plate having an aluminum or 
aluminum alloy layer formed on at least one side of a plate of a metal 
other than aluminum. The enamel glass paste is then calcined to form 
electrically insulating glass layer 3, thereby preparing an aluminum 
enamel board. 
Electrically insulating glass layer 3 according to the present invention 
need not be formed by a ceramic material, but can be formed by a 
combination of a ceramic material and a metal powder or a combination of a 
ceramic material and a surface-oxidized metal powder. As a result, thermal 
expansion of the enamel layer can be increased to decrease a difference 
between the thermal expansion coefficients of aluminum and enamel glass. 
The thermal conductivity of an aluminum plate or an aluminum alloy plate is 
high, while the thermal conductivity of the enamel layer is much lower 
than that of aluminum. For this reason, heat conduction characteristics of 
the aluminum enamel board are determined by the enamel layer. Therefore, 
it is important to increase heat conductivity of the enamel layer in order 
to improve heat dissipation of the board. From this point of view, a 
composite ceramic layer with a metal having a high thermal conductivity is 
formed on a metal plate. 
A ceramic material used in the present invention is not limited to any 
specific one. A preferable ceramic material is prepared by adding an oxide 
to a glass powder containing PbO-B.sub.2 O.sub.3 -SiO.sub.2 or ZnO-B.sub.2 
O.sub.3 -SiO.sub.2 as a major constituent and has a softening point of 
400.degree. to 600.degree. C. A homogeneous molten ceramic material is 
cooled and solidified, or components constituting the ceramic material are 
mixed. The prepared material is pulverized by a ball mill or the like to 
prepare a fine powder having a grain size of several microns. The fine 
ceramic powder and a metal powder are kneaded with an organic vehicle 
prepared by mixing high boiling alcohol and ethyl cellulose to prepare a 
coating material. The coating material is applied to the surface of the 
aluminum plate by screen printing and is calcined at a temperature of 
400.degree. C. or more, thereby preparing the aluminum enamel board of the 
present invention. The metal powder is an aluminum powder, an aluminum 
alloy powder, or a surface-oxidized aluminum powder. The aluminum powder 
may be a spherical or flake-like powder and has a grain size of 1 to 10 
microns, and preferably 5 to 15 microns. The content of the aluminum 
powder with respect to the ceramic material is 5 to 50 vol %, and 
preferably 10 to 20 vol %. If the content is less than 5 vol %, thermal 
conductivity is low and warping still occurs. However, if the content 
exceeds 50 vol %, the surface of the enamel board becomes nonuniform. The 
present inventors found that an enamel layer of the enamel board was 
preferably a composite material ceramic layer prepared by mixing a ceramic 
material and an aluminum powder. However, the material mixed with the 
ceramic material is not limited to aluminum, but can be replaced with a 
Ti, Mg, or Pb powder. 
A method of forming electrically insulating glass layer 3 on aluminum layer 
2 according to the present invention is not limited to a specific one. For 
example, a glass frit of PbO-B.sub.2 O.sub.3 -SiO.sub.2 or ZnO-B.sub.2 
O.sub.3 -SiO.sub.2 is formed into a paste and the paste is applied to 
aluminum layer 2 by printing or spraying and is calcined in air at a 
temperature of 500.degree. to 600.degree. C. A ceramic powder such as an 
Al.sub.2 O.sub.3 or AlN powder may be contained in glass layer 3 in order 
to improve thermal conductivity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described by way of its examples and 
comparative examples. 
EXAMPLE 1 
0.05-mm thick aluminum films were formed on both sides of a 1.0-mm thick Cu 
alloy plate containing 1 wt % of Be according to the cladding method to 
prepare an Al-coated Cu alloy plate. A glass paste of PbO-B.sub.2 O.sub.3 
-SiO.sub.2 was applied on the Al-coated Cu alloy plate and was calcined at 
600.degree. C., and 100-.mu.m thick electrically insulating glass layers 
having a thermal expansion coefficient of 14.times.10.sup.-6 /.degree.C. 
were formed on both sides of the calcined body, thereby preparing a 
circuit board. 
In order to evaluate the adhesion strength of the glass layers on the 
board, a 200-g copper ball was dropped from a height of 45 cm on the glass 
layer to check peeling of the glass layers according to an adhesion 
strength test complying with JIS R4301 (5.5). The surface of the aluminum 
layer was not exposed at all. 
In order to measure the adhesion between the glass layers and the aluminum 
layer, the glass paste was screen-printed on the Al-coated Cu alloy plate 
prepared as in the above method and was calcined at 600.degree. C. to 
prepare a 100-.mu.m glass layer having a diameter of 5 mm. The resultant 
glass layer served as a specimen subjected to adhesion strength 
measurement. As shown in FIG. 3, aluminum layers 2 were respectively 
fomred on both surfaces of metal plate 1 and electrically insulating glass 
layer 3 as the specimen was formed on one of aluminum layers 2. 3-cm long 
alumina rod 4 having a diameter of 5 mm was bonded to glass layer 3 
through low melting glass 6 having a softening point of 350.degree. C. 
Alumina rod 5 was bonded to aluminum layer 2 through epoxy resin adhesive 
7, thus preparing a test sample. The adhesion strength of glass layer 3 
was measured to be 5.8 kg/mm.sup.2 (n=5). 
EXAMPLE 2 
A 100-.mu.m thick glass layer of PbO-B.sub.2 O.sub.3 -SiO.sub.2 having a 
thermal expansion coefficient of 11.times.10.sup.-6 /.degree.C. was formed 
on one of the surfaces of a 1.0-mm thick Al-coated iron plate having 
100-.mu.m thick Si-containing (8 wt %) Al alloy layers formed on an iron 
plate by plating with a molten aluminum alloy to prepare a circuit board, 
following the same procedures as in Example 1. An adhesion strength test 
by dropping a copper ball was performed following the same procedures as 
in Example 1. The surface of the aluminum layer was not exposed at all. 
Another Al-coated iron plate was prepared following the same procedures as 
described above and a test sample was formed following the same procedures 
as in Example 1. The adhesion strength of the glass layer was measured to 
be 5.2 kg/mm.sup.2 (n=5). 
EXAMPLE 3 
Pure aluminum was cladded on both surfaces of an Fe-Ni alloy plate having a 
thickness of 1.0 mm and containing 42 wt % of Ni to prepare an Al-coated 
Fe-Ni alloy plate coated with 0.05-mm thick aluminum layers. A glass paste 
of PbO-B.sub.2 O.sub.3 --SiO.sub.2 was applied to the Al-coated Fe-Ni 
alloy plate and was calcined at 600.degree. C. to form 100-.mu.m thick 
electrically insulating glass layers having a thermal expansion 
coefficient of 4.1.times.10.sup.-6 /.degree.C., thereby preparing a 
circuit board. An adhesion strength test was performed by dropping a 
copper ball, following the same procedures as in Example 1. The surface of 
the aluminum layer was not exposed at all. 
Another Al-coated Fe-Ni alloy plate was prepared following the same 
procedures as described above and a test sample was formed following the 
same procedures as in Example 1. The adhesion strength of the glass layer 
was measured to be 6.1 kg/mm.sup.2 (n=5). 
EXAMPLE 4 
An Al layer was spattered on one side of a composite material plate 
prepared by forming 0.1-mm thick Cu layers on both sides of a 0.8-mm thick 
Fe-Ni alloy plate (invar alloy) containing 36 wt % of Ni. A 100-.mu.m 
thick glass layer having the same composition as in Example 3 was formed 
on the resultant aluminum layer and was calcined (in an N.sub.2 gas 
containing 10 ppm of O.sub.2) to prepare a circuit board. An adhesion test 
by dropping a copper ball was performed for the resultant circuit board, 
following the same procedures as in Example 1. The surface of the aluminum 
layer was not exposed at all. The adhesion strength of the glass layer was 
5.4 kg/mm.sup.2 (n=5). 
COMATIVE EXAMPLES 1-4 
A glass paste as in Examples 1 to 4 was applied to metal plates (without 
aluminum layers) used as boards in Examples 1 to 4 to form glass layers, 
and the adhesion strength of these glass layers was evaluated. The glass 
paste was calcined in an N.sub.2 gas atmosphere containing 20 ppm of 
O.sub.2 in order to prevent oxidation of the metal plates. Evaluation 
results are summarized in Table 1. An adhesion test by dropping a copper 
ball was performed, and the glass layers peeled from all samples in 
Comparative Examples 1 to 4. 
TABLE 1 
______________________________________ 
Adhesion Strength 
Metal Plate (kg/mm.sup.2) 
______________________________________ 
Comparative 
Same as in Example 1 
0.5 
Example 1 
Comparative 
Same as in Example 2 
0.8 
Example 2 
Comparative 
Same as in Example 3 
2.1 
Example 3 
Comparative 
Same as in Example 4 
0.5 
Example 4 
______________________________________ 
The following effects of the present invention are derived from Examples 1 
to 4 and Comparative Examples 1 to 4. Since an aluminum layer is formed on 
the surface of a metal plate serving as a core or base in the aluminum 
enamel board of the present invention, the aluminum layer can be tightly 
bonded to a glass layer serving as an electrically insulating layer. 
Therefore, the aluminum enamel board can be effectively used as a hybrid 
IC circuit board. In addition, when a metal plate having a low thermal 
expansion coefficient is used as a core or base, semiconductor elements 
can be directly mounted on the board. Therefore, the packing density of 
the parts can be increased. 
Examples 5 and 5 and Comparative Examples 5, which use Al-Si alloy plates 
as boards, will be described below. 
Example 5 
Al and an Al-Si alloy were mixed, melted and cast to prepare an Al-10 wt 
%Si alloy, and the resultant alloy ingot was hot- and cold-rolled to form 
1.5-mm Al-Si alloy plate 1. This plate had a thermal expansion coefficient 
of 20.5.times.10.sup.-6 /K and was cut into square pieces each having a 
side of 50 mm. These pieces were cleaned using ultrasonic waves. An 
aluminum enamel glass frit available from Nippon Ferro Co. was pulverized 
to obtain a powder having a mean grain size of 1.5 .mu.m. The powder was 
then dispersed in water, and the resultant dispersion was sprayed and 
dried on the surface of each piece. The spraying/drying cycle was repeated 
to set the thickness of the dried glass layer to be 300 .mu.m. The piece 
with the glass layer was calcined in a furnace at 550.degree. C. to 
prepare an aluminum enamel board having 150.mu.m thick electrically 
insulating glass layer 3 (FIG. 4). No cracks or wrinkles were found on the 
surface of the aluminum enamel board. The glasslayer had a thermal 
expansion coefficient of 15.times.10.sup.-6 /K. 
EXAMPLE 6 
Al and an Al-Si alloy were mixed, melted and cast to prepare an Al-18%Si 
alloy plate, and the plate was cut into pieces. A pure Al plate was made 
to adhere to the surface of each piece, and the resultant structure was 
hot- and cold-rolled to prepare a 1.5-mm thick cladding material (FIG. 2) 
having Al-18%Si layers having a cladding ratio of 5% and formed on both 
surfaces. The cladding material had a thermal expansion coefficient of 
19.0.times.10.sup.-6 /K. This plate was further cut into square pieces 
each having a side of 50 mm, and the pieces were cleaned using ultrasonic 
waves. An aluminum enamel board was prepared following the same procedures 
as in Example 5. 
No wrinkles nor cracks were found on the surface of the aluminum enamel 
board. 
COMATIVE EXAMPLE 5 
A 1.5-mm thick Al plate having a side of 50 mm and an Al-2wt %Si alloy 
plate having the same dimensions as those of the Al plate were prepared 
following the same procedures as in Example 5. The Al plate had a thermal 
expansion coefficient of 25.0.times.10.sup.-6 /K, while the Al-2wt %Si 
alloy plate had a thermal expansion coefficient of 23.2.times.10.sup.-6 
/K. Following the same procedures as in Example 5, 150-.mu.m thick 
electrically insulating glass layers were respectively formed on these 
plates to prepare aluminum enamel boards. Wrinkles were formed on these 
boards, and the boards were also greatly warped. 
The following effects of the present invention can be derived from Examples 
5 and 6 and Comparative Example 5. Since at least 5 wt % of Si are added 
to aluminum as the metal plate of the conventional aluminum enamel board, 
the aluminum enamel board of the present invention can have a low thermal 
expansion coefficient which is similar to that of the electrically 
insulating glass layer, thereby preventing warping of the board and 
peeling of the electrically insulating glass layer. When the content of Si 
is increased in the Al-Si layer, the surface of the Al-Si alloy plate 
cannot be smooth. As a result, even if an enamel glass paste is printed 
and calcined on the Al-Si alloy paste, the resultant aluminum enamel board 
does not have a smooth surface. 
This problem can be solved by cladding the Al-Si alloy plate with a pure 
aluminum. Since a thermal expansion coefficient of the overall board can 
be decreased, many problems encountered upon soldering of elements on the 
board can be solved. 
Examples 7, 8, and 9 and Comparative Example 6 will be described in which a 
metal plate material is selected from W, Mo, Si, and a composite material 
of Cu and one of these metals as a major constituent. 
EXAMPLE 7 
Glass having a composition of 20wt %PBO-30wt %SiO.sub.2 10wt %B.sub.2 
O.sub.3 --20wt %Na.sub.2 O--20wt %TiO.sub.2 was mixed with ethyl alcohol, 
and the resultant mixture was pulverized by a ball mill to prepare a glass 
powder having a mean grain size of 2.0 .mu.m. Spherical pure aluminum 
powder having a mean grain size of 7.0 .mu.m was kept at 500.degree. C. 
for 36 hours to oxidize the surface layer of the powder to a depth of 0.5 
m. 20 vol % of the surface-oxidized aluminum powder were added to the 
above glass powder, ethyl cellulose and butyl Carbitol were mixed, and the 
resultant mixture was sufficiently kneaded by a triple roll mill, thereby 
preparing an aluminum powder-containing glass paste. An Al-10wt %Si alloy 
plate having a thickness of 1.5 mm, a width of 100 mm, and a length of 100 
mm was sufficiently cleaned, and the aluminum powder-containing glass 
paste was applied to the aluminum plate by screen printing. The resultant 
structure was dried at 100.degree. C. for 30 minutes and was calcined in 
air at 530.degree. C. 
The above cycle was repeated to prepare an aluminum enamel board having 
100-.mu.m thick enamel layers. The board had a thermal conductivity of 170 
W/mk, and warping was measured to be 0.2 mm/100 mm. The board also had a 
breakdown voltage of 1.2 kV/mm, and a resistivity of 3.5.times.10.sup.12 
.OMEGA.cm. 
EXAMPLE 8 
An aluminum powder which was not surface-oxidized was mixed with glass to 
prepare a paste and hence an enamel board, following the same procedures 
as in Example 7. Since the surface of the aluminum powder was not 
oxidized, the paste reacted slightly with ethyl cellulose and butyl 
Carbitol and did not have an appropriate viscosity. For this reason, the 
resultant enamel board, prepared such that the paste was applied by screen 
printing to an Al-Si alloy plate as in Example 7, dried at 100.degree. C. 
for 30 minutes, and calcined in air at 530.degree. C., did not have a 
smooth surface. The enamel board had a thermal conductivity of 175 W/mk, 
and its warping was measured to be 0.2 mm/100 mm. The board had a 
breakdown voltage of 6 kV/mm and a resistivity of 1.3.times.10.sup.11 
.OMEGA.cm. 
EXAMPLE 9 
15 vol % of a stainless steel powder subjected to 2-hour surface oxidation 
at 1,250.degree. C. and having a mean grain size of 10 .mu.m was mixed 
with glass used as in Example 7, and ethyl cellulose and butyl Carbitol 
were added thereto. The resultant mixture was sufficiently kneaded by a 
triple roll mill to prepare a stainless steel powder-containing glass 
paste. An Al-10wt %Si alloy plate having a thickness of 1.5 mm, a width of 
100 mm, and a length of 100 mm was sufficiently cleaned, and the stainless 
steel powder-containing glass paste was applied to the aluminum plate by 
screen printing. The resultant structure was dried at 100.degree. C. for 
30 minutes and was calcined in air at 530.degree. C. 
The above cycle was repeated to prepare an aluminum enamel board having 
100-.mu.m thick enamel layers. The board had a thermal conductivity of 120 
W/mk, and warping was measured to be 0.25 mm/100 mm. The board also had a 
breakdown voltage of 1.5 kV/mm, and a resistivity of 4.1.times.10.sup.12 
.mu.m. 
COMATIVE EXAMPLE 6 
A glass paste which did not contain an aluminum powder was prepared 
following the same procedures as in Example 7 and was calcined on an 
aluminum plate having a thickness of 1.5 mm, a width of 100 mm, and a 
length of 100 mm to prepare an aluminum enamel board having a 100-.mu.m 
enamel layer of only glass. The enamel board had a thermal conductivity of 
95 W/mk, and its warping was measured to be 1.2 mm/100 mm. The enamel 
board had a breakdown voltage of 1.2 kV/mm and a resistivity of 
4.5.times.10.sup.12 .mu.m. 
The following effects of the present invention can be derived from Examples 
7, 8, and 9 and Comparative Example 6. Since the enamel layer of the 
aluminum enamel board of the present invention comprises a composite 
ceramic material of ceramic and a metal powder, the thermal conductivity 
and the thermal expansion coefficient of the enamel layer can be similar 
to those of the aluminum core. Therefore, there is provided an aluminum 
enamel board free from warping and having good heat conduction 
characteristics. Therefore, unlike in the conventional circuit board, the 
packing density of electronic parts which generate heat can be increased.