Process for producing copper-clad laminate

A process for continuously producing a copper-clad laminate by double belt pressing which comprises heating a laminating material comprising at least one resin-impregnated base of continuous length and a copper foil of continuous length superposed on at least one of the outer surfaces of the base(s) between a pair of belts under pressure, wherein a laminating material having been continuously heat-fused under reduced pressure in a vacuum chamber is used. The pressure applied to the laminating material during heating under pressure can be reduced to a range of from 10 to 30 kg/cm.sup.2.

FIELD OF THE INVENTION 
This invention relates to an improved process for continuously producing a 
copper-clad thermosetting resin laminate for use as printed circuit boards 
iin electrical or electronic equipments. 
BACKGROUND OF THE INVENTION 
Known processes for producing a copper-clad laminate include a continuous 
double belt pressing process, in which a laminating material comprising a 
desired number of resin-impregnated bases (prepregs) in continuous lengths 
and a copper foil of continuous length superposed on at least one of the 
outer surfaces of the bases is heated under pressure between a pair of 
belts as disclosed in JP-A-61-10456 (the term "JP-A" as used herein means 
an "unexamined published Japanese patent application"). 
According to this technique, resin-impregnated bases and a copper foil(s) 
both at room temperature are supplied between a pair of belts and heated 
at a temperature of about 200.degree. C. under a pressure of 50 
kg/cm.sup.2 or higher in a heating zone of the double belt press to cure 
the rein. The united laminate is then rapidly cooled under pressure in a 
cooling zone of the double belt press and delivered out of the double belt 
press. 
Application of high pressure of 50 kg/cm.sup.2 or more in the heating zone 
has been an essential condition in order to obtain a laminate free from 
bubbles, etc. However, use of such a high pressure has been attended by 
various problems. For example, in cases where a fluid, e.g., air or oil, 
is used as a pressurizing medium, a special sealing device was necessary 
to prevent the medium from leaking, making the apparatus large-sized. 
Still yet, it was difficult to completely prevent leakage. On the other 
hand, when rollers are employed to exert the presure, a large number of 
rollers having a small diameter are needed from the standpoint of assuring 
surface smoothness. However, since the load imposed on the rollers 
sufficient for applying the desired pressure would be too high for their 
strength, some devices such as a back-up mechanism should be required to 
prevent breakage of the small diameter rollers. 
In an attempt of overcoming the disadvantages associated with the 
above-stated pressurizing processes, it has ben proposed to use a molten 
metal as a pressurizing medium and the laminate is pressed by the medium 
in the form a solid formed by cooling the molten metal to a temperature 
below its melting point. This technique, however, presents another problem 
in handling of the solidified metal. 
In the above-described conventional laminating process, the 
resin-impregnated bases and copper foils supplied have room temperature. 
Immediately after curing of the resin by heating under pressure,, the 
copper-clad laminate is rapidly cooled under pressure to a temperature 
less than the glass transition point of the resin, followed by delivery. 
The resin-impregnated bases, copper foils, and steel belts for double belt 
pressing differ in coefficient of thermal expansion from each other. 
Therefore, rapid heating in the heating zone sometimes causes wrinkling of 
the copper foil due to the difference in thermal expansion. In order to 
prevent this, it has been suggested to previously apply sufficient tension 
to each of the resin-impregnated bases and the copper foils, but such a 
device was still unsatisfactory. Even if wrinkling does not occur, rapid 
cooling of the cured copper-clad laminate to a temperature less than the 
glass transition point of the resin under pressure leaves a great internal 
stress within the resulting copper-clad laminate, which would lead to 
dimensional instability. 
SUMMARY OF THE INVENTION 
One object of this invention is to provide a process for producing a 
copper-clad laminate by double belt pressing, in which the requisite 
pressure applied to the laminate is greatly reduced as compared with the 
conventional double belt pressing processes without causing bubbling. 
Another object of this invention is to provide a process for producing a 
copper-clad laminate by a double belt pressing process which neither 
causes wrinkling of the copper foil nor leaves a great residual stress in 
the resulting laminate. 
As a result of extensive investigations, the inventors have found a process 
of using a laminating material having been previously haat-fused under 
reduced pressure. It has further been found that a copper-clad laminate 
free from wrinkles of the copper foil and having a reduced residual 
internal stress can be obtained by previously heating the 
resin-impregnated bases and copper foils and controlling the temperature 
for cooling the cured copper-clad laminate in the cooling zone above the 
glass transition point of the resin, thereby establishing a process for 
providing a copper-clad laminate whose dimensional changes are minimized 
while using a reduced pressure for pressing. 
That is, a first invention relates to an improvement in a process for 
continuously producing a copper-clad laminate by double belt pressing 
which comprises heating a laminating material comprising at least one 
resin-impregnated base of continuous length and a copper foil of 
continuous length superposed on at least one of the outer surfaces of the 
base(s) between a pair of belts under pressure, said improvement comprises 
using a laminating material having been continuously heat-fused under 
reduced pressure in a vacuum chamber. In a preferred embodiment of the 
first invention, the reduced pressure for continuous heat-fusion is not 
more than 500mmmHg, and the air in the vicinity of the inlet and outlet of 
the vacuum chamber for continuous heat-fusion, through which the 
resin-impregnated bases and copper foils are introduced and delivered, has 
a cleanness class of 100,000 or less. 
A second invention relates to an improvement in a process for continuously 
producing a copper-clad laminate by the above-mentioned double belt 
pressing process, said improvement comprises using a laminating material 
having been continuously heat-fused under reduced pressure in a vacuum 
chamber and controlling temperature changes among the heat-fused 
laminating material, the pair of belts, and the copper-clad laminate in 
the heating zone and the cooling zone of the doube belt press within a 
range of 100.degree. C. In a preferred embodiment of the second invention, 
the resin-impregnated bases and copper foils are heat-fused to a 
temperature not less than 100.degree. C. and supplied between the pair of 
belts while being kept at that temperature; the pair of belts at the inlet 
for supplying the resin-impregnated bases and copper foils are heated to a 
temperature not less than 100.degree. C.; and the temperature of the 
copper-clad laminate while being delivered out of the cooling zone of the 
double belt press is at a temperature higher than the glass transition 
point of -the resin by at least 20.degree. C. 
According to the improved process of this invention, the pressure condition 
of th double belt press can be greatly relieved to 10 to 30 kg/cm2, and 
the resulting copper-clad laminate is excellent in dimensional stability, 
i.e., freedom from dimensional change or warpage.

DETAILED DESCRIPTION OF THE INVENTION 
The resin-impregnated base to be used in the present invention can 
generally be prepared by continuously impregnating a varnish or 
solvent-free thermosetting resin composition comprising a phenolic resin, 
an epoxy resin, an unsaturated polyester resin, etc. into a reinforcing 
base in continuous lengths, such as kraft paper, linter paper, glass 
fabric, glass mat, etc., followed by solvent removal by drying or the like 
means. 
The copper foil which can be laminated on the resin-impregnated base 
includes electrolytic copper foil, rolled copper foil, etc., whose back 
side (the side to contact with the resin-impregnated base) may be treated 
to provide a good adhesive surface, or whose back side may have provided 
thereon an adhesive layer. A suitable copper foil is selected from among 
them in accordance with the kind of the impregnating resin used. 
The double belt press may be any of types using a fluid (e.g., air or oil) 
as a pressurizing medium, small diameter rollers, a combination of the 
pressurizing medium and the small diameter rollers, or the like means. A 
pressure of from 10 to 30 kg/cm.sup.2 would be enough for pressing. The 
pressing temperature and time (retention time) are determined 
appropriately depending on the kind of the resin-impregnated base used. 
From the standpoint of productivity, it is preferable to set the pressing 
temperature at around 200.degree. C., somewhat higher as compared with 
usual laminate molding, to thereby reduce the retention time. 
In the first invention, heat-fusion of the resin-impregnated bases and 
copper foils under reduced pressure is performed in a vacuum chamber 
containing therein a heating means and a pressing means. A desired number 
of resin-impregnated bases and a copper foil(s) are introduced into the 
vacuum chamber and heated under reduced pressure to melt the impregnating 
resin and, at the same time, remove air and any residual volatile content 
present in the individual resin-impregnated bases, between the 
resin-impregnated bases, and between the resin-impregnated base and the 
copper foil. Then, the heated resin-impregnate bases and copper foils are 
fused into one body by pressing. In order to maintain the vacuum chamber 
under reduced pressure, the inlet and outlet of the chamber through which 
the resin-impregnated bases and copper foils are introduced or delivered 
are sealed by using one or more pairs of rolls. Nevertheless, it is 
difficult to completely prevent air from entering into the vacuum chamber 
through the sealing. If the air in the vicinities of the inlet and outlet 
should contain dust, the dust is carried with the air and sticks to the 
surfaces of the resin-impregnated bases and copper foils in contact with 
the sealing. In particular, dust adhered to the copper foil surface is 
liable to cause unevenness of the foil on pressing under pressure. In 
order to avoid this, it is preferable that the cleanness of air in the 
vicinities of the inlet and outlet of the vacuum chamber is controlled to 
class 100,000 or less, particularly 10,000 or less, by adding an 
appropriate device, for example, by installing the whole apparatus in a 
cleaned room or by blowing cleaned air from the vicinities of the inlet 
and outlet of the vacuum chamber. 
As mentioned above, the heat treatment of the resin-impregnated bases and 
copper foils in the vacuum chamber aims not only at fusion while removing 
air among the rsin-impregnated bases or between the base and the copper 
foil but, in a preferred mode, at removal of bubbles which have been 
necessarily generated in the individual resin-impregnated bases during the 
preparation thereof. Accordingly, it is particularly desirable that the 
resin-impregnated bases and copper foils introduced into the vacuum 
chamber are heated while being once separated apart from each other to 
sufficiently remove the volatile content and the like, and then fused 
together by hot-pressing. 
In the present invention, the requisite pressure to be applied can be 
reduced to a range of from 10 to 30 kg/cm.sup.2, which is much lower than 
the conventionally employed pressure of 50 k/cm.sup.2, by using the thus 
heat-fused laminating material. 
In the second invention, the above-described heat-fusion under reduced 
pressure is combined with control of temperature change among the 
heat-fused laminating material, the pair of belts, and the resulting 
copper-clad laminate within a range of 100.degree. C. In the present 
invention, the resin is usually cured by heating at about 200.degree. C. 
under pressure. Accordingly, the temperature change during the stages of 
from introduction of the heat-fused laminating material through curing of 
the resin on heating under pressure can be controlled within 100.degree. 
C. by maintaining both the heat-fused laminating material and the pair of 
belts at a temperature of 100.degree. C. or more and also heating a pair 
of feed drums of the double belt press (the drums for introducing the 
laminating material into between the belts) at a temperature of 
100.degree. C. or higher. Further, it is preferable that the high 
temperature copper-clad laminate in which the resin has been cured by 
heating under pressure be cooled under pressure in a cooling zone to a 
temperature not less than 100.degree. C., particularly to a temperature 
higher than the glass transition point of the cured resin by at least 
20.degree. C. Such cooling temperature control can easily be achieved by 
decreasing the amount of a cooling medium to be fed to the cooling zone or 
using no cooling medium at all. 
The copper-clad laminate having passed through the cooling zone is then 
cooled to a temperature below the glass transition point of the cured 
resin by either allowing to cool at substantially zero pressure within the 
doube belt press or by blasting cool air (forced cooling) in the outside 
of the double belt press or the like cooling means. 
Thus, the pressure condition during double belt pressing can be greatly 
reduced to 10 to 30 kg/cm.sup.2, and dimensional stability of the 
resulting copper-clad laminate can greatly be improved. 
The heating means to be employed for heat fusion under reduced pressure or 
preliminary heating may be conventional and is not particularly limited. 
For example, radiation heating by far infrared rays, etc. or conductive 
heating by means of heating rolls, etc. can be used. Radiation heating by 
means of a non-contact far infrared heater or a lamp is preferred. 
Preferred embodiments of the process according to the present invention are 
illustrated below by referring to the accompanying drawings. 
FIG. 1 is a schematic view of an apparatus for carrying out the process of 
the first invention. FIG. 2 is a schematic view of an apparatus for 
carrying out the process of the second invention, in which the same 
mechanism of FIG. 1 further includes a means for preliminary heating. 
In FIG. 1, 8 sheets of a resin-impregnated base (10) rolled in continuous 
lengths and 2 sheets of a copper foil (11) are introduced into a vacuum 
chamber (2) kept at reduced pressure by means of a vacuum pump (7) via a 
pair of vacuum sealing rolls (20). In the vacuum chamber (2), the bases 
and copper foils are heated from both sides thereof under reduced pressure 
by means of a far infrared heater (23) and then united by means of two 
pairs of hot-press rolls (22). The thus fused laminating material (12) is 
discharged from the vacuum chamber (2) via a pair of vacuum sealing rolls 
(21) and fowarded to a double belt press (3), wherein it is heated under 
pressure to cure the resin and to unite the resin impregnated bases and 
the copper foils into a laminate. The resulting copper-clad laminate is 
delivered from the double belt press (3) and cut to lengths with a cutter 
(4). 
In front of the the sealing rolls (20) and at the rear of the sealing rolls 
(21) is provided a simple enclosure, into which clean air is blown through 
inlets (24) to keep the airing these areas clean, whereby dust in the 
atmosphere is prevented from entering into the vacuum chamber to cause 
defects of the copper-clad laminate. 
FIG. 2 shows an embodiment comprising the same mechanism as in FIG. 1, 
except that an auxiliary heater (25) is additionally provided between a 
vacuum chamber (2) and a double belt press (3), the feed rolls for 
introducing the laminating material into the double belt press are feed 
drums (34) equipped with a heating means, and a cooling fan (42) is 
provided at the rear of the outlet of the double belt press. 
The laminating material (12) fused under reduced pressure is kept at the 
fusing temperature or further heated by means of the auxiliary heater (25) 
and supplied between a pair of steel belts (33) heated by means of the 
feed drums (34). The laminating material is continuously pressed at about 
200.degree. C. under a pressure of 10 to 30 kg/cm.sup.2 in the heating 
zone (31) of the double belt press (3) whereby the resin is cured and the 
laminating material is united into a copper-clad laminate. The copper-clad 
laminate is then cooled in the cooling zone (32) of the double belt press 
(3) to a temperature slightly higher than the glass transition point of 
the resin. After the pressure is released, the laminate is discharged from 
the double belt press, quickly cooled by means of the cooling fan (42), 
and cut to lengths with a cutter (4). 
The present invention is now illustrated in greater detail with reference 
to the following Examples, but it should be understood that the present 
invention is not deemed to be limited thereto. In these examples, all the 
percents are by weight unless otherwise indicated. 
EXAMPLE 1 
Glass fabric of 0.2 mm in thickness was impregnated with an epoxy resin 
varnish and dried to prepare prepregs of continuous length having a resin 
content of 40%. 
Eight sheets of the prepregs and two 35 .mu.m thick copper foils were laid 
up in the manner as shown in FIG. 1 to constitute a laminating material 
with the copper foil on both sides thereof. The laminating material was 
introduced into a vacuum chamber, where it was hot-pressed to fuse and 
unite into one body. The fused laminating material was introduced into a 
double belt press, where it was heated under pressure to prepare a 
copper-clad laminate of continuous length, which was delivered from the 
double belt press and cut with a cutter to prepare a cut-to-size 
copper-clad laminate. 
In this example, the whole apparatus was installed in a room having a 
cleanness class of 100,000 or less, and the cleanness of air in the 
vicinities of the inlet and outlet of the vacuum chamber was ensured by 
blowing dry air having a cleanness class o 10,000 or less. The retention 
time and pressure in the vacuum chamber were 0.3 minute and 100 mmHg, 
respectively. The surface temperature of the prepreg during fusion was 
100.degree. C. The retention time, temperature, and pressure in the double 
belt press were 1.0 minute, 200.degree. C., and 25 kg/cm.sup.2, 
respectively. 
Characteristic properties of the resulting copper-clad laminate are shown 
in Table 1. 
COMATIVE EXAMPLE 1 
A copper-clad laminate was produced in the same manner as in Example 1, 
except that the prepregs and copper foils were not heat-fused under 
reduced pressure and that the pressure exerted for double belt pressing 
was increased to 50 kg/cm.sup.2. 
Characteristic properties of the resulting copper-clad laminate are shown 
in Table 1. 
EXAMPLE 2 
A laminating material having the same structure as used in Example 1, 
except that the copper foil thickness was 18 .mu.m, was heat-fused under 
reduced pressure in the same manner as in Example 1. The fused laminating 
material was further heated up to 120.degree. C. by means of a far 
infrared heating means as shown in FIG. 2 and then introduced into a 
double belt press. The feed drums of the double belt press were heated to 
120.degree. C. by a heat transfer medium, and a pair of belts were thereby 
heated to 120.degree. C. The heated laminating material was forwarded to a 
heating zone of the double belt press set at 200.degree. C., where it was 
press-molded at a maximum temperature of 200.degree. C. at a pressure of 
25 kg/cm.sup.2. The laminate was forwarded from the heating zone to a 
cooling zone, where it was cooled under pressure to 160.degree. C., and 
then cooled to 130.degree. C. at zero pressure within the double belt 
press. The laminate discharged out of the double belt press was rapidly 
cooled to 30.degree. C. by means of air fans to obtain a copper-clad 
laminate. 
Characteristic properties of the resulting copper-clad laminate are shown 
in Table 1. 
COMATIVE EXAMPLE 2 
A copper-clad laminate was produced in the same manner as in Comparative 
Example 1, except for changing the thickness of the copper foil to 18 
.mu.m. 
Characteristic properties of the resulting copper-clad laminate are shown 
in Table 1. 
TABLE 1 
______________________________________ 
Compar- 
ative Compar- 
Laminating Example ative 
Condition: 
Example 1 1 Example 2 
Example 2 
______________________________________ 
Copper Foil 
35 35 18 18 
Thickness (.mu.m) 
Heat-Fusion 
done not done done not done 
under Vacuum 
Preliminary 
not done not done done not done 
Heating 
Forced Cooling 
done done not done 
done 
under Pressure 
Pressure of 
25 50 25 50 
Double Belt 
Press (kg/cm.sup.2) 
Properties of Copper-Clad Laminate: 
Residual not observed not observed 
Bubbles* observed observed 
Surface State 
good good good wrinkles 
of Copper Foil observed 
Dimensional 
Change** (%) 
Longitudinal 
0.000 -0.010 0.000 -0.010 
Crosswise -0.005 -0.020 -0.002 -0.020 
Warpage*** 
0.3 2.4 0.2 2.4 
(mm) 
______________________________________ 
Note: 
*The copper foil was removed by etching, and the etched surface was 
observed under a microscope. 
**Dimensional changes between before and after heat treatment at 
170.degree. C. for 30 minutes. 
***A maximum warpage of the copperclad laminate (250 .times. 500 mm) 
heated at 170.degree. C. for 30 minutes on a horizontal plate. 
As can be seen from the foregoing description, according to the present 
invention, a copper-clad laminate free from bubbles can be obtained 
without requiring a high pressure for double belt pressing by using a 
laminating material preliminarily fused under reduced pressure. As a 
result, the pressurizing mechanism and other means to be used for double 
belt pressing can be produced with great ease and excellent economy. 
In addition, by controlling the changes of temperature of a laminating 
material while passing in the double belt press within a range of 
100.degree. C., a copper-clad laminate having improved dimensional 
stabilit and excellent surface conditions of the copper foil can be 
obtained. 
It can thus be understood that the present invention provides an excellent 
process of practical use and industrial value from the viewpoint of both 
apparatus for continuous laminate molding and performance of copper-clad 
laminates produced. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.