Note: Descriptions are shown in the official language in which they were submitted.
0347
INSULATING BOARDS
1 BACKGROUND OF THE INVENTION
Many insulating boards are known for use as
printed circuit boards and other applications.
Conventional printed circuit boards are often found
to have different dielectric constants when measured
in different directions within the board. This
probably results from the need to balance desired
physical properties with the dielectric constant
obtainable by conventional materials used for printed
circuit boards. It has long been recognized that a
truly isotropic printed circuit board, that is, having
substantially the same dielectric constant measurement
when measured in all or any directions, would be highly
desirable. This has been accomplished in some cases by
the use of alumina in printed circuit boards. However,
the use of alumina often results in high cost boards
which are difficult to machine and brittle,thus
detracting from their use in many applications.
It is an object of this invention to provide a
composite polymeric insulating board useful as a
printed circuit base board which insulating board
exhibits isotropic properties with respect to its
dielectric constant and loss tangent and which can be
tailored to have any one of a wide variety of pre-
selected dielectric properties.
Still another object of this invention is toprovide an insulating board in accordance with the
preceding object wherein the board can have incorporated
therein a reinforcement material and an impregnating
polymeric material resulting in a mechanically strong
yet yieldable board having good mechanical properties
for use as a printed circuit board.
It is still another object of this invention to
provide methods of forming insulating boards in
accordance with the preceding objects.
2C7~ ~C 11~0347
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l According to the invention an insulating board
has a polymeric fibrous material forming a reinforcement
layer with a predetermined dielectric constant. A
second polymeric material is impregnated into the
reinforcement layer and has a dielectric constant
closely matc~d to the dielectric constant of the
layer and preferably within a range of + l of the
dielectric constant of said polymeric layer of fibrous
material. The overall dielectric constant of the
insulating board is isotropic and preferably lies in
the range of from 2.2 to 20 when measured in any or all
directions at a temperature of 25C. and at 60 cycles.
Preferably the insulating board has a loss tangent in
the range of from 0.0009 to 0.01 when measured at 8.5
kilomegacycles (GHz) at 23C. and 50% relative humidity.
The loss tangent or dielectric constant can be
tailored to particular applications. Preferably, the poly-
meric fibrous material is selected from the group of
polymers consisting essentially of polypropylene, poly-
ethylene and polytetrafluoroethylene and copolymers con-
taining a major proportion of such olefin polymers.
Preferably, the polymer impregnated into and formed about
the reinforcement is a polybutadiene or polybutadiene co-
polymer with a minor percentage of other monomers which
are preferably vinyl unsaturated monomers.
Either the dielectric constant or the losc tangent
can be made to be different from a pure polymeric im-
pregnating material and polymeric fibrous reinforcement
containing board by the addition of fillers to the im-
pregnating material before polymerization. However, thefillers can only be used to preselect one or the other of
the dielectric constant or the loss tangent.
When variations in dielectric constant and/or loss
tangent are desired, fillers can be used to make such
,,
~ D
il;~O3~7
1 variation. Such fillers are inert and do not enter
into the exothermic reaction during curing of the
impregnated polymer about the fibrous material.
It is a feature of this invention that a wide
range of dielectric constants and/or loss tangents
can be obtained in an insulating board having good
mechanical properties permitting a multitude of uses
for the boards of this invention. The isotropic
properties with regard to both the dielectric constant
and loss tangent are extremely important when the
insulating boards are used as bases for antennas or
for conventional printed circuit base boards.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and
advantages of the present invention will be better
understood from the following description when read
in conjunction with the accompanying drawings in which:
FIG. 1 is a crGss sectional view through an
insulating board in accordance with the present
invention; and
FIG. 2 is a cross sectional view through a
modification thereof showing a copper outer layer for
use as a circuit element of a printed circuit.
DESCRIPTION OF PREFERRED EMBODIMENTS
With reference now to the drawings, an
insulating board in accordance with the present
invention is illustrated at 10 in FIG. 1 and has a
fibrous reinforcement layer 11 impregnated with a
thermoset polymeric material 12.
The overall length and width of the board can vary
greatly depending upon the particular usage to which it
is to be put as is known in the art. Similarly, the
thickness of the board can vary depending upon its
034~
--4--
1 particular usage. For example, the board can be
made in thicknesses of from 0.004 inch to 1 inch or
more. For most applications the board has a thickness
of .05 to .1 inch. When thicknesses above .05 inch are
required, it is preferred to use composite boards such
as shown in FIG. 2 at 13 where two fibrous layers lla
and llb are used with the impregnant 12 being as
previously described. Obviously, any number of fibrous
layers can be used with additional impregnant material
to increase thickness as desired. The boards can have
copper, aluminum or other metal layers such as copper
layer 18 formed or later adhered to them or alternately
the board can be formed directly on a preformed metal
sheet. The boards can be processed into circuit
carrying bases by known techniques including etching,
masking, etc.
The fibrous material which acts as a reinforcement
i8 preferably a non-woven, polypropylene fibrous
material. In some cases, woven materials or chopped
fibrous reinforcement materials can be used as the
fibrous reinforcement. The fibers are preferably
substantially uniformly distributed throughout the
completed boards. Other olefins such as polyethylene
and polytetrafluoroethylene or copolymer~ of these
olefin~ with other materials and with each other can
be used in fiber forms. Polypropylene is preferred
since it gives the desired mechanical strength, has a
dielectric constant within a range of interest which
can be closely matched by the impregnating polymer.
It is a key feature of this invention that the
impregnant have when polymerized and cured, a dielectric
constant closely matched to that of the reinforcement.
Preferably the dielectric constants of the two are
within a range of + 1 of each other and in the case of
polybutadiene and polypropylene are within .7 at 23C.
0347
l and 60 cycles.
In a preferred embodiment, the fibrous reinforcement
is a non-woven textile such a Pelon, a trademarked
product of the Pelon Corporation of Lowell, Massachusetts,
consisting essentially of a polypropylene batting with
a thickness of from 0.002 inch to 0.025 inch. Pelon
N1251F having a weight of 30 grams per meter2 with a
thickness of about 0.009 inch and a breaking strength of
5 pounds with an elongation of 22% and a wicking rate in
30% KOH (SECS/l inch), of 40 seconds/inch is a
particularly desirable material.
The impregnating polymer to which the dielectric
constant of the Pelon is matched is a polybutadiene
which may be filled and which may comprise polybutadiene
polymerized alone or along with other monomers and
preferably vinyl unsaturated monomers.
High vinyl 1-2 liquid polybutadienes are preferred
for use as the impregnated polymer such a Ricon 150 a
trade name product of Colorado Chemical Specialties of
Golden, Colorado, consisting essentially of polybutadiene
which has a micro-structure of 70 + 5% l, 2 vinyl, a
molecular weight average of about 2,050 +200 and which is
viscous clear liquid with a viscosity as measùred in a
Brookfield Viscometer at 23C. in CPS at 40,000 + 10,000
and a specific gravity of 0.89 with a bulk density of
about 7.4 lbs/gallon and an instrinsic viscosity of
0.105 + .06. Other polybutadienes can also be used.
These polybutadienes are preferably finally cured or
polymerized after impregnation into the reinforcing
material.
The polybutadiene formulations can have comonomers
introduced therein such as vinyl unsaturated monomers
including but not limited to styrene, vinyl toluene,
t-butyl styrene, alphamethylstyrene, monochlorostyrene,
isobutylmethacrylate, methylmethacrylate, diallyl
~U34~
1 maleate, diallyphthalate. Preferably substantially
pure polybutadiene is used and in all cases the
polybutadiene is at least 50% by weight of any final
copolymers. Known cross-linkers are preferably
included to shorten curing times although in some cases
they can be eliminated. Peroxide curing or curing
induced by other conventional methods such as irradiation
can be used. Cross-linking agents include but are not
limited to divinyl benzene, trimethylolpropane trimeth-
acrylate and 1, 3 butylene dimethacrylate. Organicperoxide catalysts as known in the art can also be used
in curing. Such catalysts include benzoly peroxide,
methyl ethyl ketone peroxide, di-t-butyl-peroxide and
a,a'-bis (t-butylperoxy) diisopropyl-benzene.
Small amounts of other compounding agents can be
used to induce curing as known in the art. In all
cases, the final product is a thermoset polybutadiene
polymeric material.
It is important to maintain physical properties
in the finally cured product as well as maintaining
isotropic properties as to loss tangent and dielectric
constant over a wide range of operating frequencies and
other conditions. The desirable physical properties
include high strength, some small degree of flexibility,
good support for conventional printed circuit and
antenna materials and other known desirable physical
properties of printed circuit boards.
Since in some cases it is desirable to tailor the
dielectric constant and/or loss tangent, various fillers
can be used to provide tailoring. Preferred fillers
include polyethylene, polytetrafluoroethylene, alumina,
barium titanate, titanium dioxide, and strontium
titanate. Other fillers can be used. The fillers are
used to raise the dielectric constant in an amount
necessary to balance the properties of the materials
11;~0347
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1 in the overall board. The fillers can also be used
to adjust the loss tangent of the materials. Normally
one can design a board of this invention to meet a
predetermined dielectric constant value or a pre-
determined loss tangent value by selection of theamount and particular filler used. However, it is
usually not possible to preselect both values by
addition of a filler. Thus either loss tangent or
dielectric constant to be set is selected and filler
added to obtain that value in the amount required as
known in the art and whatever value results for the
other value not designed for is accepted. In all cases
the fillers are inert during the chemical reaction to
cure the impregnant about the reinforcing material.
The fillers are preferably in particle form of less
than micron size and never more than 95~ by weight of
the total composition.
Generally the insulating boards of this invention
are prepared by mixing together the monomeric butaliene
of low polymers of butaliene with the catalyst and one
or more comonomers if used. Similarly if filler is used,
that is premixed with the monomer. Homogeneous
distribution of the ingredients is preferred. A
reinforcing polymeric fabric material is then selected
and cut to size. The monomeric mixture of resin and
catalyst is then poured onto the reinforcing material
to thoroughly soak it. If more than one layer of
reinforcement is to be used, plural layers are stacked
until the desired number of layers and desired
thickness is obtained. Generally one reinforcing
layer of .007 inch thickness for each .010 finish
thickness is used. The soaked reinforced layer or
layers are then placed in an evacuation chamber to
remove all air and to insure complete wetting of the
reinforcing fibrous layer. The impregnated fibrous
034~
--8--
1 layer is then placed in a press at 275F. to 300F.
and pressed to remove air and allow the resin to cure
for time periods of from preferably 30 minutes to 2
hours. The completed molded material is in a thermoset
form, is substantially homogeneous and exhibits
isotropic properties as to dielectric constant values
and loss tangent values.
The following non-limiting specific examples are
provided as illustrative of the invention:
Example 1
One hundred grams of Ricon 150 are homogeneously
admixed with a catalyst obtained from Lucidol
Corporation under the trade name Lupersol 101 which is
a peroxide catalyst having a boiling point at 119C.
and known to be useful for high temperature curing of
polybutadiene. The mixture is made and then 30 grams
of it is poured over each of three 10" long by 10" wide
sheets of Pelon having a thickness of 0.008" each and
comprising 8.34 grams. After thorough soaking, the
sheets are stacked, placed in an air evacuation chamber
to remove all air and to assure complete wet out of the
fabric. The impregnated fabric is then placed in a
press at room temperature and the press is brought
down to stops so as to produce a 0.025" thick finished
product. The product is maintained in the press for
one hour at 285F. to produce a final insulating board
having a dielectric constant of 2.3 at 8.5 GHz and 23C.
The loss tangent of the material is 0.002 at 8.5 GHz and
23C. The material exhibits good mechanical properties
for use as a printed circuit board. That is, it has
some flexibility yet is resistant to temperatures of up
to at least 275F. while maintaining its physical
properties. The board is a thermoset composite material
that exhibits isotropic properties as to loss tangent and
dielectric constant.
~lZ0347
g
Example 2
1 Example 1 is repeated; however, 56 grams of the
Ricon Lupersol mixture is further admixed with 144
grams of strontium titanate in powder form to form a
uniform mixture. 140 grams of this mixture is then
impregnated into four sheets of Pelon as previously
described which is then molded and cured as described
in Example 1 to form a final product having a thickness
of 0.040 inches. The dielectric constant of this
material at 8.5 GHz and 23C. is 10 and a loss tangent
of about 0.0032 at 8.5 GHz and 23C.
Example 3
Example 1 is repeated; however, 40 grams of the
Ricon and Lupersol mixture is further admixed with 160
grams of strontium titanate. 150 grams of this mixture
is then impregnated into three sheets of Pelon 1251F.
The three sheets of Pelon are disposed above each other
and cured in the mold as previously described. The
final product exhibits a dielectric constant of 16 at
8.5 GHz and 23C and a loss tangent of about 0.0029 at
8.5 GHz and 23C.
Example 4
Example 2 is repeated except that 155 grams of
barium titanate (BaTiO3) is substituted for the 144
grams of strontium titanate. An insulating board results
having good mechanical properties with a dielectric
constant of 10 at 8.5 GHz and 23C and a loss tangent of
about 0.008 at 8.5 GHz and 23C.
In all of the above examples, the dielectric
constant and loss tangent of the resulting material are
isotropic.
When any one of the formulations of Examples 1-4
are formed by placing the impregnated sheets on a copper
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--10--
1 layer, the impregnating polymeric material polymerizes
and adheres to the copper forming an integrally printed
circuit board suitable for etching of circuitry as
shown in FIG. 2 of the drawing.
While specific examples of the present invention
have been shown and described, many variations are
possible. In all cases it is necessary to match the
resin, fiber and particulate filler to give the desired
dielectric constant and/or loss tangent of the resulting
board. Both the impregnating thermoset material and the
reinforcement layer must have a dielectric constant
under 3 and a loss tangent of less than 0.002 at
operating temperatures such as, for example, 60 cycles
per second and 23C. In all cases the final board also
has a dielectric constant of less than 3 unless the
impregnating polymeric material i8 filled and is capable
of withstanding substained operating temperatures of at
least 135C. without melting. The reinforcement layer
of polyolefin is preferably a non-woven fibrous
material which ~wells or otherwise is distributed
uniformly throughout the board from top to bottom
surfaces thereof to give homogeneity to the insulating
board. Mats of Pelon or like materials are preferred
because the fibers swell and give substantially
homogeneous distribution. Such fibers often have
lengths of at least 1/8 inch and diameters of less than
0.001 inch although these dimensions may vary.
The dielectric constant and loss tangent values
of any particular material can of course vary somewhat
with the frequency at which it is measured. The
values given are for the frequencies and temperatures
of operation of use. In many cases the variation is
minor as for example the polypropylene reinforcing mat
has a dielectric constant of 2.2 at 60 cycles, 103 cycles
and at 106 cycles while its loss tangent - tan ~ is
llZ(~347
about 0.0005 at 60 cycles, 0.0008 at 103 cycles and
0.0018 at 106 cycles. Polybutadiene which has a
dielectric constant when measured at 23C. and 60 cycles
of about 2.9, 2.78 at 106 and 2.9 at 101 has a loss
tangent - tan ~ at 60 cycles of 0.0001, at 10V 0.0029,
and at iol 0.004. These materials when combined in
the present invention give an overall insulating
board dielectric constant of 2.315 at 101 and a loss
tangent of 0.0015.
Preferably the polymeric impregnating material
eomprises from 70 to 98~ by weight of the binder to
reinforcing material total and the reinforcing material
eomprises from 2 to 30% by weight. The insulating boards
ean have thieknesses of from 0.005 inch and up but in
most cases will be less than one inch thick.
