Glass fibers having low dielectric constant

Glass fibers having a low dielectric constant composed of a glass having a dielectric constant of not more than 5 and comprising: PA1 SiO.sub.2 45-65% by weight, PA1 Al.sub.2 O.sub.3 9-20% by weight, PA1 B.sub.2 O.sub.3 13-30% by weight, PA1 CaO+MgO+ZnO 4-10% by weight, PA1 Li.sub.2 O+Na.sub.2 O+K.sub.2 O 0-5% by weight, Based on the total weight of the glass, the total proportion of SiO.sub.2, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, CaO, MgO, ZnO, Li.sub.2 O, Na.sub.2 O, and K.sub.2 O being at least 95% by weight.

This invention relates to glass fibers having a low dielectric constant, 
and particularly to glass fibers having a low dielectric constant for use 
in reinforcing printed circuit boards. 
As electronics devices and appliances have decreased in size and increased 
in density and performance in recent years, printed circuits have been 
required to possess higher density, faster signal speed and higher 
reliability. To increase signal speed, printed circuit boards should have 
a low dielectric constant, and this, in turn, leads to the need for grass 
fibers having a low dielectric constant for reinforcing printed circuit 
boards. 
Presently, "E glass" is widely used as glass fibers for reinforcing printed 
circuit boards. E glass comprises 52-56 SiO.sub.2, 12-16 Al.sub.2 O.sub.3, 
6-13 B.sub.2 O.sub.3, 16-25 CaO and 0-6 MgO in percent by weight, and has 
a dielectric constant of about 6 which is lower than the dielectric 
constants of common glasses. But glass having a much lower dielectric 
constant has been required. 
D glass and S glass are known to give glass fibers having a low dielectric 
constant. Since, however, these glasses have very high viscosity, melting 
at very high temperatures for long periods of time is required to obtain 
homogeneous glass with no bubbles. Furthermore, since the fiberization 
temperature for these glasses is considerably high, a platinum bushing 
used in fiberization has an extremely short life. 
Japanese Laid-Open Patent Publication No. 130365/1984 describes a process 
for producing glass fibers having a low dielectric constant which 
comprises fiberizing and weaving E glass, for example, and dissolving 
components other than silica in the resulting glass woven cloth by 
treating it with an acidic solution followed by washing with water and 
moisture removal, to give high-silica fibers. This process requires 
additional steps, such as acid treatment, after the glass is melted, spun 
and woven. Furthermore, since the components other than silica are 
dissolved, fine voids occur at those parts which have previously been 
occupied by the removed components, and this results in a reduction in the 
strength of the glass fibers. 
Japanese Laid-Open Patent Publication No. 151345/1983 describes a glass 
composition having a low dielectric constant comprising a SiO.sub.2 
-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -BaO type glass and a heat-resistant 
filler. 
It is an object of this invention to provide glass fibers having a low 
dielectric constant which can be produced at relatively low melting and 
fiberization temperatures with excellent productivity and are suitable for 
reinforcing high-performance printed circuit boards. 
According to this invention, the above problem of the prior art is solved 
by producing glass fibers having a low dielectric constant of 5 or less 
from a glass comprising 45 to 65% of SiO.sub.2, 9 to 20% of Al.sub.2 
O.sub.3, 13 to 30% of B.sub.2 O.sub.3, 4 to 10% of CaO+MgO+ZnO, and 0 to 
5% of Li.sub.2 O+Na.sub.2 O+K.sub.2 O, the total proportion of SiO.sub.2, 
Al.sub.2 O.sub.3, B.sub.2 O.sub.3, CaO, MgO, ZnO, Li.sub.2 O, Na.sub.2 O 
and K.sub.2 O being at least 95%, all percentages being by weight. 
The limitation of the composition of the glass to the above range is for 
the following reasons. 
SiO.sub.2 : 
If the proportion of SiO.sub.2 is less than 45% by weight, the dielectric 
constant of the glass fibers becomes high. If it is larger than 65% by 
weight, the viscosity of he glass becomes so high that it is difficult to 
dissolve or draw the glass into fibers. The preferred proportion of 
SiO.sub.2 is 48 to 60% by weight. 
Al.sub.2 O.sub.3 : 
If the proportion of Al.sub.2 O.sub.3 is less than 9% by weight, the 
chemical durability of the glass is reduced. If it is larger than 20% by 
weight, the dielectric constant and viscosity of the glass become too 
high. 
B.sub.2 O.sub.3 : 
This component produces a great effect of decreasing the dielectric 
constant of the glass and is also effective as a flux component. If its 
proportion is less than 13% by weight, the dielectric constant of the 
glass is high, and the viscosity of the glass also becomes high. If it is 
above 30% by weight, the chemical durability of the glass becomes very 
poor. Furthermore, its volatilization becomes vigorous, and it is 
difficult to produce homogeneous glass. The excessive use of B.sub.2 
O.sub.3 also quickens breakdown of the bushing used in fiberization. The 
preferred proportion of B.sub.2 O.sub.3 is more than 20% and up to 30% by 
weight. 
CaO, MgO, ZnO: 
If the total proportion of these three components is below 4% by weight, 
the chemical durability of the glass if reduced. If it exceeds 10% by 
weight, the glass is liable to be devitrified, and also has a high 
dielectric constant. Among these components, CaO is most effective for 
preventing devitrification. Preferably, therefore, the glass contains at 
least 1% by weight of CaO. MgO and ZnO reduce the dielectric constant of 
the glass more than CaO, and from the standpoint of dielectric constant, 
MgO and ZnO are preferred. 
In consideration of both devitrification and dielectric onstant, it is 
preferred that the mole ratio of Mgo+Zno to CaO+MgO+ZnO be from 0.4 to 
0.7. When MgO and ZnO are contained singly in the same mole %, ZnO reduces 
dielectric constant more than MgO. When these two components are contained 
together, they further reduce the dielectric constant. It is preferred 
that the mole ratio of MgO to MgO+ZnO be from 0.25 to 1, more preferably 
from 0.35 to 0.75. 
Li.sub.2, Na.sub.2 O, K.sub.2 O: 
These components are effective as a flux. If the total proportion of these 
components s larger than 5% by weight, the dielectric constant of the 
glass becomes high. 
Optionally, the glass in accordance with this invention may contain not 
more than 5% by weight of BeO, not more than 2% by weight of SrO, not more 
than 1.5% by weight of BaO and not more than 1.5% by weight of PbO. Since 
these components act to increase dielectric constant, their proportions 
should not exceed the aforesaid limits. 
Various impurities may possibly get into the glass, for example oxides of 
Ti and Fe from the raw materials, oxides of Zr, Ti, Fe and Cr from the 
refractory of the glass melting furnace, and an oxide of Mo from the 
electrode of the electric melting furnace. Oxides of Mo, Fe, Cr, Zr and Ti 
may permissibly be present in the glass if the amount of each of them is 
not more than 1% by weight. 
F.sub.2 used as a flux component may be contained in the glass in an amount 
of up to 2% by weight. As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 used as a 
clarifier may be contained in the glass in an amount of up to 2% by 
weight. 
In any case, the total proportion of SiO.sub.2, Al.sub.2 O.sub.3,B.sub.2 
O.sub.3, CaO, MgO, ZnO, Li.sub.2 O, Na.sub.2 O and K.sub.2 O should be at 
least 95% by weight based on the total weight of the glass. 
The glass in accordance with this invention can be obtained by melting a 
mixture of raw materials in predetermined proportions to a temperature of 
about 1500.degree. C. or higher to melt the mixture. Melting is preferably 
effected by electrical heating by utilizing the Joule's heat of the glass 
itself. 
The glass can be fiberized by using a conventional method of producing 
glass fibers. One example will be described below in a general manner. 
The molten glass is molded into the form of a marble. The marble is 
re-melted at a temperature of about 300.degree. to 1400.degree. C. The 
molten glass is drawn continuously from platinum nozzles to form filaments 
having a diameter of about 5 to 13 micrometers. The filaments are bundled 
and wound up. 
For use in reinforcing printed circuit boards, the glass fibers are formed 
into a mat and impregnated with a resin in advance. 
The glass of this invention has a sufficiently low dielectric constant of 
not more than 5.0, and therefore the glass fibers of this invention are 
suitable for reinforcing printed circuit boards of high performance that 
permits high signal speeds. 
The glass of this invention has a little tendency to devitrification. In 
other words, the devitri 
fication temperature T.sub.L of the glass of this invention is 25 
sufficiently lower than its working temperature T.sub.W. The glass working 
temperature T.sub.W is defined as a temperature at which log .eta. where 
.eta. is the viscosity of the glass in poises is 3. If the T.sub.L of the 
glass is equal to, or greater than, T.sub.W, devitrification of glass 
occurs during the production of fibers from the glass. It is difficult 
therefore to produce glass fibers, and the resulting glass fibers have an 
inferior quality. The glass of this invention can circumvent such 
troubles. 
The glass of this invention has sufficiently high water resistance. During 
fiberization, the glass fibers may be coated with an aqueous binder, or 
after fiberization, the fibers may be treated with an aqueous liquid to 
remove the binder. Furthermore, the glass fibers may be exposed to 
atmospheric air during perforation of printed circuit boards reinforced 
with these fibers. Accordingly, the glass fibers are required to have 
water resistance. The water resistance can generally be expressed by the 
amount of glass dissolved after glass powder is immersed in water at a 
predetermined temperature for a predetermined period of time. The amount 
of the glass of this invention dissolved has been found to be sufficiently 
smaller than that of conventional D glass. 
The following examples illustrate the present invention more specifically. 
The following raw materials were used in the following examples. 
Source of SiO.sub.2 : silica sand (High Silica VSH-2, a product of Nichitsu 
Kogyo Co., Ltd.) 
Source of Al.sub.2 O.sub.3 : Al(OH).sub.3 (special reagent grade, produced 
by Katayama Chemical Co., Ltd.) 
Source of B.sub.2 O.sub.3 : H.sub.3 BO.sub.3 (special reagent grade, 
produced by Katayama Chemical Co., Ltd.) 
Source of CaO: CaCO.sub.3 special reagent grade, produced by Katayama 
Chemical Co., Ltd.) 
Source of MgO: basic magnesium carbonate (special reagent grade, produced 
by Katayama Chemical Co., Ltd.) 
Source of ZnO: ZnO (special reagent grade, produced by Katayama Chemical 
Co., Ltd.) 
Source of Li.sub.2 O: Li.sub.2 CO.sub.3 (special reagent grade, produced by 
Kishida Chemical Co., Ltd.) 
Source of Na.sub.2 O: Na.sub.2 CO.sub.3 (special reagent grade, produced by 
Futaba Chemical Co., Ltd.) 
Source of K.sub.2 O: K.sub.2 CO.sub.3 (special reagent grade, produced by 
Katayama Chemical Co., Ltd.) 
In the following examples, the devitrification temperature T.sub.L, the 
working temperature T.sub.W (the temperature at which log .eta. where 
.eta. is the viscosity of the glass in poises is 3) and the water 
resistance of the glass (the amount dissolved) were measured by the 
following methods. 
Devitrification temperature T.sub.L 
The glass was pulverized and sieved to obtain test samples having a 
particle diameter of 1400 to 1700 micrometers. An electrical furnace was 
set so that it had a continuous temperature gradient within a 
predetermined temperature range, for example a temperature range of 
250.degree. C., in accordance with the presumed devitrification 
temperature. A number of glass samples were placed along the temperature 
gradient of this electric furnace, and maintained for a predetermined 
period of time, for example for 4 hours. 
The samples were withdrawn, and cooled. The samples were then examined 
under a polarizing microscope for the presence or absence of crystals. The 
highest temperature among those at which the devitrified samples were 
maintained is defined as the devitrification temperature. 
Working temperature T.sub.W 
A platinum spherical body was suspended in the molten glass, and the load 
exerted on the spherical body at the time of pulling it up was measured 
(the "spherical body pull-up method"). The viscosity of the glass was 
calculated in accordance with the Stokes law. The viscosity .eta. can be 
calculated from the following equation. 
EQU .eta.=K(W.sub.1 -W.sub.0)/v 
K: the container coefficient 
W.sub.1 : the weight of the spherical body 
W.sub.0 : the load required for the spherical body to be balanced in the 
liquid 
v: the average velocity under the load W.sub.1 
The viscosities (poises) were measured over a predetermined temperature 
range, and a viscosity-temperature curve was drawn. The temperature 
corresponding to .eta.=10.sup.3 poises is defined as the working 
temperature T.sub.W. 
Amount dissolved (water resistance) 
Measured by a method substantially in accordance with the alkali dissolving 
test in JIS R3502 (Method of Testing Glass Instruments and Devices Used 
for Chemical Analysis). Specifically, the glass sample having a 
predetermined particle diameter was held in boiling water for a 
predetermined period of time, and then, the aqueous solution containing 
the dissolved matter was quantitatively analyzed by plasma emission 
analysis, flame photometry, etc., and the amount of the sample dissolved 
was calculated. This method permits quantitative determination of all the 
matter dissolved from the glass including alkali.

EXAMPLE 1 
CACO.sub.3, H.sub.3 BO.sub.3, Al(OH).sub.3 and silica sand (SiO.sub.2) were 
weighed so that the weight ratio of CaO-B.sub.2 O.sub.3 -Al.sub.2 O.sub.3 
-SiO.sub.2 was 7.5:25.0:15.0:52.5. They were then mixed to produce a 
glass-forming batch. 
The batch was melted at 1500.degree. C. for 4 hours and molded into the 
form of a marble and a plate, and then annealed. 
The marble-like glass was then put in a glass fiberization furnace, melted 
at a temperature of 1300.degree. to 1350.degree. C., and drawn into glass 
filaments having a diameter of 5 to 13 micrometers. During the drawing, 
filament breakage attributed to the presence of undesired nonmelted 
material and to devitrification did not occur, and the glass composition 
in this example was found to be suitable for fiberization. 
The dielectric constant, viscosity at high temperatures, devitrification 
temperature and dissolved amount of the plate-like glass produced 
simultaneously with the production of the marble-like glass were measured. 
The glass was found to have a dielectric constant of 4.90, a working 
temperature of 1339.degree. C., and a dissolved amount of 0.65 mg. In the 
measurement of the devitrification temperature, the glass was maintained 
at various temperatures in the range of 1140.degree. to 1440.degree. C. 
for 4 hours, but no devitrification was observed in any of the samples 
tested. Accordingly, the devitrification temperature of the glass was 
found to be lower than 1140.degree. C. 
EXAMPLES 2-11 AND COMATIVE EXAMPLE 
Glass marbles were individually produced as in Example 1 from glass 
composition shown in Examples 2 to 11 in Table 1 and, for comparison, E 
glass and D glass. The glass marbles were individually spun at a 
temperature near the point at which log .eta. was 3 to form glass 
filaments. The glass compositions of Examples 2 to 11, like that in 
Example 1, showed good meltability and drawability. 
The dielectric constants, high-temperature viscosities, devitrification 
temperatures and dissolved amounts of these glasses were measured, and the 
results are shown in Table 1. 
As shown in Table 1, the glasses obtained in the examples of the invention 
have a dielectric constant of 5 or less which is much lower than the 
dielectric constant of E glass which is 6.8. The temperature (working 
temperature) at which log .eta. is 3.0 was, for example, about 
1260.degree. C. in Example 10 which is higher than E glass but at least 
200.degree. C. lower than that of D glass (1508.degree. C.). 
The devitrification temperatures of the glasses of the invention are lower 
than their working temperatures, showing good workability. Furthermore, 
the water resistances (dissolved amount) of the glasses of the invention 
are generally comparable to E glass and superior to D glass. 
As stated above, the glass in accordance with this invention is relatively 
easy to melt and draw and gives glass fibers having excellent water 
resistance and effective electrical properties represented by its 
dielectric constant of not more than 5. 
TABLE 1 
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Comparison 
E D 
Examples 
glass 
glass 
1 2 3 4 5 6 7 8 9 10 11 12 
__________________________________________________________________________ 
Li.sub.2 O 
0 0.18 
0.18 
0.18 
0.18 
0.18 
0.18 
0.18 
0.18 
1.8 0.18 
Na.sub.2 O 
0.4 0.12 
0.12 
0.12 
0.12 
0.12 
0.12 
0.12 
0.12 
1.2 0.12 
K.sub.2 O 
0.24 1.5 0 0 0.14 
F.sub.2 
0.45 
CaO 22.8 0.5 7.5 4.1 8.0 
4.4 4.1 7.0 
5.0 4.9 3.6 
3.6 1.3 6.3 
MgO 0.41 0.2 1.5 1.9 3.7 1.3 
1.3 0.9 
ZnO 0 3.0 2.6 
2.5 1.8 
SiO 0.21 
Al.sub.2 O.sub.3 
14.2 15.0 17.0 
16.0 
10.6 
9.9 14.0 
15.1 
14.8 
17.0 
16.7 
9.6 13.6 
B.sub.2 O.sub.3 
6.67 22.7 25.0 15.0 
19.9 
25.0 
29.9 
29.9 
25.1 
24.7 
24.9 
25.6 
29.1 
20.5 
SiO.sub.2 
55.2 74.7 52.5 59.5 
55.8 
59.8 
55.8 
48.9 
52.7 
51.7 
50.3 
49.9 
54.3 
59.3 
Di- 6.79 3.77 4.90 4.94 
4.97 
4.42 
4.33 
4.85 
4.76 
4.80 
4.79 
4.80 
4.78 
4.78 
electric 
constant 
Working 
1190 1508 1339 1424 
1372 
1446 
1391 
1264 
1313 
1307 
1294 
1264 
1222 
1430 
temper- 
ature 
(.degree.C.) 
Devitri- 
1150 above below 1187 
be- 
below 
below 
be- 
below 
below 
1256 
1260 
below 
below 
fication 1400 1140 low 
1140 
1140 
low 
1050 
1050 1050 
1100 
temper- 1140 1100 
ature 
(.degree.C.) 
Water 
0.30 1.75 0.65 not 0.23 
not not 1.49 
0.43 
0.31 
0.25 
not not 0.22 
resistance mea- mea- 
mea- mea- 
mea- 
(dissolved sured sured 
sured sured 
sured 
amount) 
(mg) 
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