Note: Descriptions are shown in the official language in which they were submitted.
78~ (A)
3~ .
_ELECTRIC MATERIAL AND METHOD OF MAKING THE
DIELECTRIC MATERIAL
Background of the Invention:
(1) Field of the Invention
The present invention relates to dielectric
materials and a method by which such dielectric materials
are made.
~2) Description of the Prior Art
Dielectric materials may be used in many appli-
cations. Although not limited thereto in its use r dielec~
tric materials have utility in microwave circuit boards~
Also, dielectric materials may be used in capacitors.
One type of dielectric material was disclosed in a
paper presented at IEEE/NEMA 1975 Electrical Electronics
Insulation Conference at Boston, Massachusetts on November
~-~ 11, 1975, "EPSII~ 10- A New High Dielectric Constant Con-
formable Copper-Clad Laminate", M. Olyphant, Jr., D.D.
Demeny, and T.E. Nowicki. EPSIL~ 10,la product of the 3M
Company, is believed to be a composite of poly(tetrafluoro-
ethylene) (PTFE) and dielectric filler, the composite
typically having a dielectric constant between 10 and 11
and being clad on both sides by copper foil.
Prior art dielectric materials exhibit numerous dis-
advantageous properties. Prior art dielectric materials,
in general, absorb moisture in undesirable amounts. The
absorption of moisture results in at least two serious prob-
lems: the electrical properties of the dielectric materialare changed and the material may physically expand.
3~
2--
Moreover, prior art microwave dielectric materials may
tend to have a non-uniform dielectric constant throughout
the material. It is believed that this non-uniformity of
dielectric constant is due, at least in part, to incomplete
mixing of the dielectric material with the binder.
Furthermore, prior art dielectric materials may tend
to have a relatively high energy dissipation factor.
Another type dielectric material is used in a micro-
wave circuit board that is made by Rogers Corporation,
Rogers, Connecticut, and marketed under the trademark
RT/Duroid. The dielectric material, which comprlses PTFE
and glass microfibers, has desired strain relief properties
but does not have a sufficiently high dielectric constant
for some applications.
It is an object of the present invention to provide a
dielectric material having a relatively high dielectric
constant, said dielectric constant being uniform throughout
the material.
It is another object of the present invention to pro-
vide a dielectric material which is moisture-resistant,
that is, the material resists absorption of moisture.
Summary of the Invention~
.
The present invention provides a dielectric material
which is moisture-resistant, which has a relatively high
and uniform dielectric constant and which has a relatively
low dissipation factor at high frequencies. The dielectric
material comprises a composite of a polymeric binder, a
filler having a high dielectric constant and a microfibrous
material. The dielectric material is produced by blending
the particulate filler material having a high dielectric
constant and a microfibrous material in a polymer disper-
sion to form a slurry of polymer, filler and fiber. The
solids in the slurry are agglomerated to form a dough-like
material. The dough-like material may be formed into any
desired shape.
In the preferred embodiment of the invention, the
--3--
polymer dispersion comprises a dispersion of polytetrafluoro-
ethylene, the filler material comprises titania in a parti-
culate form and the microfibrous material comprises glass
microfibers. The slurry of polymer, filler and fiber is
blended to provide Eor complete mixing of the polymer filler
and fiber and for a uniform distribution of these materials
throughout the dispersion. The solid materials in the dis-
persion are agglomerated by a flocculant to produce a dough-
like material. The dough-like material is separated from
the liquid in the dispersion and dried and may be mixed with
a suitable lubricant to provide an agglomerate having better
processing properties. The agglomerate may then be formed
into any desired shape.
Detailed Description of the Invention:
__,
In the process of making the dielectric material, a dis-
persion of polymer particles is selected, the polymer being
dispersed by ionic or non-ionic surfactants. It is desir-
able that the polymer have a melting point higher than about
300C so that a circuit board made from the polymer is cap-
able of withstanding high environmental and processing tem-
peratures. The polymer may be selected from the following
classes: halogenated hydrocarbons such as poly(tetra1uoro-
ethyline), (PTFE) and fluorinated poly(ethene-co-propene),
polyolefins, polyacrylates, and substituted vinyl polymers
such as polystyrene and poly(vinylidene fluoride).
The polymer dispersion is preferably an aqueous dis-
persion which is diluted to between about 10 and about 35
weight percent solids, most preferably about twenty weight
percent solids.
A dielectric filler is added to the polymer dispersion
and mixed so as to uniformly distribute the filler in the
dispersion. The filler can comprise from about ten weight
percent to about 65 weight percent of the dielectric mate-
rial. For a dielectric constant of 10.5 using nonpigmentary
titania filler, the filler should comprise between about
60 and 65 weight percent of the dielectric material. The
filler and the amount of the filler in the dielectric
material is selected depending upon the desired electrical
properties of the dielectric material. Although it should
be understood that the dielectric filler can be selected
from any of a number of known dielectric materials, exem-
plary materials include titania, alumina, zirconia, ground
quartz, amorphous or crystalline silica and ferrite cera-
mics in powder form~ The upper limitation on the weight
percent of the filler in the dielectric material is the
amount at which the dielectric material would exhibit un-
desirable porosity or impaired strength. The filler mate~
rial should be used in particle form and the average par-
ticle size should be below about 50 micrometers, and, most
preferably, between about 1 and 20 micrometers. The pre-
ferred filler material is titania. It should also be un-
derstood that a mixture of different filler materials may
be used. For example, it may be desirable to use a titania
filler and modify the magnetic permeability of the resul-
ting material by including ferrite fillers. The filler
added may be a mixture of filler and liquid, thereby simpli
fying handling of the filler and providing intimate mix-
ture of the filler with the polymer dispersion.
Micro~ibers are then added to the polymer and filler
slurry and are mixed in the slurry ~o provide a slurry of
polymer, filler, and microfibers. Although it is preferred
that the fiber comprise microfibrous glass, the fiber could
be made from other compositions, such as fibrous aluminum
silicate, fibrous microcrystalline materials, such as a po-
tassium titanate whisker material. Examples of other non-
conductive microfibers include quartz fiber and zirconia fibers.
It is preferred that the fibers have an average diameterbelow about 2 micrometers and preferably have an average
diameter of between about 0.1 and 1.0 micrometers. It is
preferred that the fibers have, on the average, a relatively
short length, preferably below about 3 millimeters. Since
many of the fibers provided from conventional sources exceed
these desired lengths, the fibers may be broken by any con-
ventional mechanical means, such as grinding the fibers, or
pressing the fibers to crush the fibers. The amount of
microfibrous material included in the dielectric material
should be sufficient to provide desired strain relief and
rigidity. In general, it is preferred that the microfibers
be added in an amount less than about 10 weight percent of
the dielectric material. A particularly preferred source
of fi~er is a fiber sold by Johns Manville 7 under the de-
signation 10~E, a fiber believed to comprise a low sodium
and potassium content borosilicate glass. The fibers may
be added in dry form or may be added in a liquid-fiber
slurry to facilitate handling.
It should be understood that the polymer, the filler
material, and the fibers may be mixed in any order. However,
it is desirable to mix the afore-mentioned materials in such
a manner to provide uniform distribution of the materials.
This is believed to be desirable in order to provide a di-
electric material having a relatively uniform dielectric
constant, uniform strain relief and uniform moisture resis-
tance. ~lthough it is envisioned that other li~uids may be
used in the slurry, it is particularly preferred that the
afore-mentioned slurry have an aqueous base. Once the slur-
- ry is mixed in any conventional manner to a point wherein
the fibers, ~he particles of filler material, and the poly-
mer are intimately and uniformly mixed, the materials in the
slurry are agglomera~ed to provide a dough-like mass.
In order to agglomerate the mixture of polymer, filler
and fibers, a flocculant is added to the mixture. It should
be understood that the chemical composition of the floccu-
lant used is dependent upon the polymer chosen and the man-
ner by which the polymer is dispersed.
The preferred flocculating agent for formulations basedon PTFE is poly(ethyleneimine), (PEI), a commercially avail-
able water soluble polymer having the repeating unit:
~ CH2CH2NH ~
--6--
and available as an aqueous solution. It is understood
that aqueous solutions combine with H2O to form a poly-
cationic material with the repeating unit:
_ _~
_ ~ CH2C~I2NH2 - __
_ OH
A large number of other polycationic flocculating agents
could also be used. It is believed that these materials
flocculate the mixture by attaching to anionic groups on
the surfaces of the polymer particles, the fibers, and the
fillers. Another type of flocculant that would be effec-
tive with ionically stabilized polymer dispersions is theuse of hydrolyzable inorganic compounds that form aqueous
solutions of polyvalent ions. These fun~tion by reducing
the ionic double layer repulsion between polymer particles.
The liquid is then removed from the agglomerated mass
by any given conventional means. A preferred method of
removing the liquid ~rom the agglomerated material is to
transfer the agglomerated material to a nylon fabric filter
bag and allow gravity drainage of the material. By this
method there is produced a wet crumbly dough having about
60 weight percent solids. The batch may be then spread
thinly in shallow trays and allowed to dry in an oven at a
temperature of 100 to 200C for 16 to 24 hours or at any
temperature and time sufficient to remove the liquid from
the agglomerated material.
The agglomerate or dough comprising the polymer, filler,
and microfibers can be directly formed into a desired shape.
In the case of an aqueous slurry, it should be understood
that ~he forming or shaping of the agglomerate into sheets
or other desired shapes is difficult because the agglomerate
tends to be sticky and clog extrusion dyes or stick to calen-
dering apparatus. In cases where it is difficult to form
the desired shapes from the aqueous agglomerate, it is pre-
ferred that the agglomerate be dried and then mixed with a
suitable lubricant, the lubricant allowing for shaping of
the agglomerate by any conventional means such as calen-
dering or paste extrusion.
The lubricant can be selected from various convention-
al lubricants. It is particularly preferred that the lub-
ricant be non-toxic as a liquid or a vapor and have a re-
latively low volatility so that at forming temperatures,the liquid lubricant will not vaporize. ~owever, it should
be understood that for particular forming methods, it may
be necessary to use a toxic lubricant which may also have
a relatively high volatility. The particularly preferred
lubricant is dipropylene glycol (DPG) manufactured by
Union Carbide Corporation. Other types of lubricants in-
clude Stoddard solvent, a mixture of aliphatic hydrocarbons
commercially available as a dry cleaning fluid/ a liquid
polyisohutylene sold by Exxon under the Vistanex trademark
and esters such as dioctylphthalate.
In the preferred embodiment of the invention, the poly-
mer dispersion is an aqueous dispersion of PTFE particles
stabilized by an added nonionic surface activ~;age~t. The
filler material is a ceramic grade titanium ~ ~ and
the fibers are borosilicate glass fibers, all of which are
believed to have a negative charge.
A flocculating agent is added to the mixture to agglom--
erate the filler, the PTFE particles and the fibers. The
water is removed from the agglomerate to provide a dried
crumb dough.
The lubricant is mixed with the dried crumb dough so
as to uniformly disperse the lubricant and to break up the
large aggregates of the dough. The mixture of the lubricant
and the dou~h provides a material which is still dry in
appearance and in a crumb or fibrous particle form. The
material may then be formed by conventional methods, such
as, for example, paste extrusion and/or calenderin~. After
the dough is formed into the desired shape, the formed shape,
whether a sheet or some other shape, is dried in a vented
forced air circulation oven for 16 to 24 hours at 200 to
300C. The dried dielectric material may be cut or trimmed
to desired dimensions.
3~
8--
The dielectric materials provided by the process of
the present invention have various properties that make
the dielectric materials particularly useful.
One important aspect of the present invention is
the provision of a dielectric material which provides
for reduced strain relief in microwave circuit boards in-
corporating the dielectric material. Strain relief is a
measurement of the dimensional changes of the microwave
circuit board after a portion o-f the conductive foil has
been removed by an etchant or a solvent. As would be
understood by one skilled in the art, it is highly desir-
able to reduce the dlmensional changes of the circuit
board because of the high tolerances within which these
circuit boards are to be used.
Strain relief is measured by determining the dimen-
sional change of a strip specimen due to removal of the
conductive foil Sometimes the change in dimension is re-
tarded by viscoelastic behavior of the composite. It has
been found that a brief heat exposure after foil removal
accelerates the change in dimension.
In the case of PTFE, titania and microfiber glass
composites described as the preferred embodiment of this
invention as well as other microwave circuit board mate-
rials based on PTFE, the following procedure for measuring
strain relief is believed to be satisfactory-
1. A 25 millimeter strip of the material is cut, taking
care to avoid flexing or other mishandling that would
impose strains on the specimen.
2. Use a sharp stylus to mark two points on the metal
foil, about 300 mm apart, on the same side of the
specimen.
3. Condition the specimens for 24 hours in standard labor-
atory conditions, 23C, 50~ R.H.
4. Measure in the same atmosphere the distance between
centers of the marks using an optical method capable
of 5 micrometer resolution.
~23~
g
5. Mask an area encompassing each mark about 12 milli-
meters square using a press~re sensitive tape capable
of withstanding etching conditions.
6. Chemically etch away the metal foil, except for the
masked areas. This can be done with various etchants
known to those versed in the art, such as solutions
of ferric chloride or ammonium persulphate. Rinse the
etched specimen in clean water.
7. Remove the masks.
8. Bake the specimen for one hour at 150C followed by
conditioning as in step 3.
9. Measure the distance between centers of the marks.
10. Calculate dimension change as mm change per metex of
original length between marks.
Another particularly important property of the di-
electric material provided by the present invention is
that the dielectric material is moisture-resistant.
Moisture resistance is measured by weighing specimens o
the dielectric material from which the metal foil has
been removed by etching followed by washing and drying
for 1 hour at 150C. The weighing is done before and
after a water immersion test and the amount of water ab-
sorbed is determined. Water immersion is for 48 hours at
50C. It has been ound that with the preferred embodiment
of this invention, it is possible to fabricate circuit
boards that absorb less than about 0.3 weight percent water
when subjected to the afore-mentioned test.
A further important property of the dielectric mate-
rial provided by the method of the present invention is
that a dielectric material has a relatively high dielec-
tric constant and the dielectric constant is quite uniform
throughout the entire material. It has been found that
with the described method, it is possible to provide a
dielectric material having a dielectric constant in the
range of about 10 to about 11 and having a uniformity o~
+.25.
"3l~
--10--
The dielectric constant of the material is determined
by measuring the dielectric constant of a circuit board
incorporating the material. The dielectric constant is
measured at microwave frequencies by employing adaptations
of one of the test methods described in American Society
of Testing and Materials Standard Methods D 3380 or D-2520.
Wi-th these as background information we have found that an
effective method involves etching two resonator elements
of differing lengths with their appropriate probe lines
onto a given specimen. The etched specimen together with
a matching specimen etched free of foil are clamped be-
tween conductive plates to form an assembly containing two
stripline resonators. The resonant frequencies in the 8
to 12.5 GHz range and the lengths of these resonators are
determined at a clamping stress of 6.9 MPa. ~rom these
data one may then calculate the fringing correction for
the resonator length and the dielectric constant. The cal-
culation is based on the known fact that the propagation
velocity of a transverse electrical mode electromagnetic
wave through a dielectric medium having a magnetic perma-
bility of unity is related directly to the square root of
the inverse of the dielectric constant.
Another important property of the dielectric material
provided by the method of the present invention is that a
circuit board incorporating the material has a relatively
low energy dissipation factor at high frequencies. At 10
G Hertz, the circuit board usually has a dissipation fac-
tor of less than about .005. The dissipation factor is
measured as follows:
The half power width of the resonantfrequency peak in
the stripline resonator method described before is divided
by the resonant frequency to give a dissipation ratio for
both metal and dielectric. An estimated value for the
metal is calculated from this value and subtracted from
the ratio to give dissipation factor of the dielectric mate-
rial.
EXAMPLE 1.
The ingredients were slurried as follows: 45 liters
of tap water were placed in a 20-gallon tank and mixed
with 148 grams of microfiber (Johns ~anville's 104E glass
fiber pre-crushed by rolling). The water and microfibers
were mixed for about ten minutes. The filler containing
2520 grams of solids, a titania filler sold by National
Lead Industries under the trademark Titano~ 3030 was added
and mixed with the microfiber and water mixture for five
minutes. A polymer dispersion of PTFE, believed prepared
by emulsion polymerization of TFE in the presence of a per-
fluoroalkane carboxy salt emulsifying agent and stabilized
after polymerization by the addition of about 0.7% nonionic
surface action agent poly(ethyleneoxy) nonyl phenol, sold
by ICI under the trademark Fluon AD-704 was added and was
mixed for about 10 minutes. The level of water was brought
up to 50 liters with additional water. The slurry was mix-
ed for five minutes and then a flocculant was added. The
flocculant used in this example was poly(ethyleneimine),
(PEI) in a one-weight percent solution. Eighty grams of
1% solution was added to the slurry and the slurry was
mixed for about 1/2 minute. Additional -flocculant solu-
tion was added in small increments until the flocculation
resulted in clear water between the flocs. The total a-
mount of flocculant solution added was about 120 grams.
The flocculated solids suspended in water were trans-
ferred to a nylon fabric filter bag to allow gravity drain-
age of the water, thereby providing a wet, crumbly dough
having approximately 68% solids. The crumb was then spread
in a one-inch thick layer in shallow trays and dried for 24
hours at 160C in a forced air circulation oven. The dried
crumb was in the form of small chunks. Thereafter, the
dried crumb was mixed with a lubricant. The lubricant used
was dipropylene glycol (DPG) sold by Union Carbide Corpor-
ation. DPG is non-toxic as a liquid or as a vapor and has
a relatively low volatility at room temperature. A blen-
der was used to mix the aggregates of dried crumb and uni-
12-
formly disperse the lubrican~. For 3,900 grams of crumb,
688 grams of lubricant was added.
The lubricated dough was then formed into sheets.
First, the lubricated dough was formed into a billet
having dimensions 38 mm diameterby about 40 mm height.
The billets were then ex~rusion-pressed at about 12,000
psi at a speed of about 3.0 inches per minute through a
4.8 mm diameter die to produce a rope-like extrudate of
about 5 mm diameter. This was then passed through a 2
roll calender with a .25 mm gap setting to produce a rib-
bon. The X direction is considered parallel to the extru-
sion and the Y direction perpendicular to the X direction
in the plane of the ribbon. These co-ordinates are dis-
cussed in connection with the finished clad panels.
Several layers of ribbon were combined in two calen-
dering operations in the X direction and the spacing be-
tween the rollers of the calender was .045 inches and .035
inches respectively. The calender roll force against gap
determining stops was set at 90 psig on the 2 eight-inch
diameter cylinders of the calender. Sheets 12.8 inches
long were cut from the e~truded ribbon and were extended
by repeated calendering passes in the Y direction, to form
sheets about 12 inches wide and at least 20 inches lon~.
The sheets were laid in a stack on clean, aluminum
trays and dried in a vented forced air circulation oven for
24 hours at 246C whereafter the sheets were trimmed accur-
ately to an 11 by 18 inch sheet.
The sheets were clad with copper foil rolled to a
thickness of about 34 micrometers and surface treated for
adhesion on one side. The foil was cut to sheet size of
18.5 by 11.5 inches.
The composite sheets were stacked to attain the de-
sired thickness and assembled between copper foil and stain-
less steel caul plates to form a layup or laminating package
that was then wrapped in an aluminum foil envelope folded
and rolled at the edges to exclude air. The package was
clamped between cold platens in a laminating press at about
3~1L
-13-
3.4 MPa. This pressure was maintained through a heating
and cooling procedure -that caused the composite to under-
go crystalline melt and limited flow to accomplish densi-
fica-tion and adhesion of the sheets to adjacent sheets or
adjacent foil in the layup. The heating was done by elec-
trical heating elements in the platens controlled therma-
statically to maintain a package temperature of 396C for
45 minutes. At the end of this period the heating circuits
were turned off and the platens and package allowed to cool
over an additional 3 to 4 hour period to a temperature be-
low 150C at which point the press was opened and the lam-
inated panels were removed from between the plates.
The microwave circuit boards were tested for the var-
ious properties indicated in Table 1.
EXAMPLE 2.
A lubricated dough compound was prepared by exactly
the same method as in example 1 except that a slight change
was made in the proportion o~ polymer, fiber, and filler
so that the~ were present in 62.8, 3.7, and 33.5 parts by
weight respectively.
The lubricated dough was then molded into bricks
having dimensions of 50 mm by 150 mm by 50 mm height.
These were then press extruded through a slit die having
a slit opening of about 2.5 mm by 15Q mm to produce a rib-
bon shaped extension which was then calendered one pass in
the X direction and cut into 318 mm lengths which were
then calendered in the Y direction to produce sheets of
about 500 mm length ~ direction by about 300 mm in the
X direction.
The sheets were then dried and laminated into panels
the same as example 1 except that the clamping stress used
in the press was about 5.2 MPa; the temperature was about
388C and the time at temperature was about 225 minutes.
Data on typical panels prepared b~ this example are
shown in Table II.
3~
EXAMPLE 3.
A series of formulations were processed to the con-
dition of a wet dough by a procedure similar to that of
example 1 except For slight changes in the relati~e pro-
portions of polymer~ filler and fiber as shown in Table
III.
Instead of drying the wet dough before processing it
into sheets these formulations were formed into sheets
by evenly spreading the wet crumb onto a carrier sheet of
polyester plastic film and passing this through the nip of
a two roll calender se~eral times to form a self-support-
ing cohesive sheet which was then peeled from the plastic
film and subjected to further calender passes until a
suitable sheet of desired thickness and size was obtained.
This more difficult procedure was necessary because it
was found -too difficult to extrude the wet dough into a
rope or ribbon.
The formed sheets were then dried for sixteen hours
at 105 to 204C in a forced air circulator oven. The
dried sheets were assembled with 34~ m thick copper foil
and stainless steel caul plates into laminating packages.
Laminating was accomplished by the following steps:
1. The package was precompressed by subjecting it to a
stress of 6.9 MPa for 1 minute in a press with platens
at about 23C.
2. The package was heated and sheets and foil were bonded
together by clamping the package at 1.7 MPa in a press
with platens already heated to 388C. The package was
held in this condition for a period of 50 minutes.
3. The package was densified and cooled by rapidl~ trans-
ferring it to a press with platens at about 23C where
it was clamped at 3.4 MPa until the package tempera-
ture was below 150C. Table III summarizes the for-
mulations and test results of eight panels prepared
from four formulations.
23~
-15-
TABLE I
PANELS CLAD WITH 34~ ~ COPPER FOIL
Panel identification lA lB
Thickness, average of 20 values mm .618 .612
Uniformity (std. dev. as % of avg.) 2.45 2.57
Specific gravity by immersion of 2.830 2.806
dielectric only
Peel strength of foil bond after 20
seconds floak in solder at 260
Average minimum value per 3 mm strip, 1.17 1.12
kN/m
Strain relief afker etching away foil
mm/m
X direction - 1.89 - 2.13
Y direction - .53 - .69
Dielectric constant at X band
Resonator in X direction 10.26 10.22
Resonakor in Y direction 10.15 10.03
Q of resonator at X band
As is condition of dielectric 204 192
Dielectric soaked 48 hours in 172 154
50C water
Water absorption, % weight gain of
dielectric specimen after 48
hours in 50C water 0.25 0.23
' ;
-16-
TABLE II
Panel identification 2A 2B 2C
Strain relief mm/m
X direction - 1.10 - .97 - 1.05
Y direction - 1.27 - 1.05 - 1.25
5 Peel strength, kN/m
after 20 sec. float in 260C 9-4 8.2 6.8
solder
std. deviation of 4 readings .6 .3 .0
Dielectric constant at lOGHz 10.43 10.60 10.58
Q of resonator 313 283 304
--17--
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-18-
While preferred embodiments have been shown and
described, various modifications and substitutions may
be made thereto without departing from the spirit and
S scope of the invention. Accordingly, it is to be under-
stood that the present invention has been described by
way of illustration and not limitation.
.
