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BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to microwave circuit boards and a method by
which such microwave circuit boards are produced. Generally speaking, a
microwave circuit board comprises two conductive foils having inserted
therebetween a material having a predetermined dielectric constant. The
microwave circuit boards are processed to remove portions of the
conductive foil to define a circuit pattern determined by the intended
use. The removal of parts of the foil is usually accomplished by etchants,
solvents, or the like.
(2) Description of the Prior Art
One type of prior art microwave circuit board is disclosed in a paper
presented at IEEE/NEMA 1975 Electrical Electronics Insulation Conference
at Boston, Mass. on Nov. 11, 1975, "EPSILAM 10", a product of the 3M
Company, is believed to be a composite of poly(tetrafluoroethylene) (PTFE)
and dielectric filler, the composite typically having a dielectric
constant of between 10 and 11 and being clad on both sides by copper foil.
In general, prior art microwave circuit boards exhibit numerous
disadvantageous properties. For example, prior art microwave circuit board
tend to absorb moisture to an undesirable degree. Thus, when the
conductive foil of the microwave circuit boards is removed by etchants or
solvents, the underlying dielectric material absorbs moisture in
undesirable amounts. The absorption of moisture results in at least two
serious problems: the electrical properties of the circuit board are
changed and the circuit board may physically expand, thus straining
tolerances which must be strictly observed in microwave circuit design.
The prior art circuit boards, when processed to remove portions of the
conductive foil to define circuits, tend to undergo strain relief to an
undesirable degree. Strain relief is a measurement which quantifies the
dimensional changes in the microwave circuit board after it is processed
to remove a portion of the conductive foil. As will be understood by those
skilled in the art, it is undesirable to have a circuit board which
changes in shape substantially when processed to remove a portion of the
copper foil.
Moreover, prior art microwave circuit boards tend to have a non-uniform
dielectric constant. It is believed that this non-uniformity of dielectric
constant is due, at least in part, to incomplete mixing of the dielectric
filler with the PTFE. Thus, in certain portions of the circuit board,
there may be an excess of PTFE and insufficient dielectric filler.
Furthermore, prior art microwave circuit boards tend to have a relatively
high energy dissipation factor.
Another type of prior art microwave circuit board is produced by Rogers
Corporation, Rogers, Conn., and marketed under the trademark "RT/duroid".
This product, which comprises PTFE and glass microfibers, has the 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 microwave circuit
board having a relatively high dielectric constant, said dielectric
constant being uniform throughout the circuit board.
It is another object of the present invention to provide a microwave
circuit board which is moisture-resistant, that is, the circuit board
resists absorption of moisture.
It is yet another object of the present invention to provide a microwave
circuit board having a relatively high tensile strength and an acceptable
dissapation factor.
It is a further object of the present invention to provide a microwave
circuit board having improved strain relief properties.
SUMMARY OF THE INVENTION
The microwave circuit board of the present invention comprises a sheet of
dielectric material having a layer of conductive foil adhered to either
side of the sheet. In the method of making microwave circuit boards, the
dielectric material is formed and the conductive foil is subsequently
adhered to both sides of the dielectric material. The dielectric material
is made by blending a particulate filler material and a microfibrous
material in a polymer dispersion to form a slurry of polymer, filler and
fiber. A flocculant is added to the slurry to agglomerate the polymer
particles, the filler particles and the microfibers to produce a
dough-like material. The dough-like material is formed into a sheet which
is thereafter dried. Conductive foil, such as copper foil is adhered to
both sides of the sheet to provide a microwave circuit board.
In the preferred embodiment of the invention, the polymer dispersion
comprises an anionic dispersion of poly(tetrafluoroethylene), the
particulate filler comprises titania and the microfibrous material
comprises glass microfibers. The fiber is present in a range by weight of
the unclad board of between 2.5% and 7% and preferably between 3% and 5%.
The microwave circuit board of the present invention provides for reduced
strain relief when the copper foil is removed by etchant or solvent. The
microwave circuit board is also particularly moisture resistant. In the
microwave circuit board moisture resistance is a particularly important
property because the electrical properties of the material will change if
the circuit board absorbs moisture. Moreover, the microwave circuit board
of the present invention has a relatively high dielectric constant which
is uniform throughout the entire circuit board. The microwave circuit
board also has a relatively low energy dissipation factor.
DETAILED DESCRIPTION OF THE INVENTION
The microwave circuit board of the present invention comprises a sheet of
dielectric material having conductive foil clad to one and usually both
sides of the sheet. In the method of making the circuit boards, the
dielectric material is produced and, subsequently, the conductive foil, a
copper foil by way of example, is adhered to opposite sides of the sheet.
In the process of making the dielectric material, a dispersion of polymer
particles is selected, the polymer being dispersed by ionic or non-ionic
surfactants. It is desirable that the polymer have a melting point higher
than about 300.degree. C. so that a circuit board made from the polymer is
capable of withstanding high environmental and processing temperatures.
The polymer may be selected from the following classes: halogenated
hydrocarbons such as polytetrafluoroethylene (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 dispersion which is diluted
to between about 10 and 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 10 weight percent to about 75 weight percent of the dielectric
material. 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, exemplary materials include titania, alumina,
zirconia, ground quartz, amorphous or crystalline silica and ferrite
ceramics in powder form. The upper limitation of the weight percent of the
filler in the dielectric material is the amount at which the dielectric
material would exhibit undesirable porosity or impaired strength. The
filler material should be used in particle form and the average particle
size should be below about 50 micrometers, and, most preferably, between
about 1 and 20 micrometers in diameter. The preferred filler material is
titania and particularly 3030 grade titanium dioxide. It should also be
understood 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 resulting material by including ferrite
fillers. The filler added may be a mixture of filler and liquid, thereby
simplifying handling of the filler and providing intimate mixture of the
filler with the polymer dispersion.
Microfibers are then added to the polymer and filler slurry and are mixed
in the slurry to 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 or fibrous micro-crystalline materials such as a potassium
titanate whisker material. Examples of other non-conductive microfibers
include quartz fiber and zirconia fibers.
It is preferred that the fibers have an average diameter below 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 this desired
length, the fibers may be broken by any conventional 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
range between an amount greater than 7 and less than 20 weight percent of
the total non-filler content of the dielectric with the preferred range
being between 8.5 and 14 weight percent. Particularly good success has
been achieved employing a fiber sold by Johns Manville Co. under the
designation 104E and 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
aforementioned materials in such a manner as to provide uniform
distribution of the materials. This is necessary in order to provide a
dielectric material having a relatively uniform dielectric constant,
uniform strain relief and uniform moisture resistance. Although it is
envisioned that other liquids may be used in the slurry, it is
particularly preferred that the aforementioned slurry have an aqueous
base. Once the slurry is mixed in any conventional manner to a point
wherein the fibers, the particles of filler material, and the polymer are
intimately and uniformly mixed, the materials in the slurry are
agglomerated 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 flocculant used is dependent upon the polymer
chosen and the manner by which the polymer is dispersed.
The preferred flocculating agent for formulations based on PTFE is
poly(ethyleneimine), (PEI), a commercially available water soluble polymer
having the repeating unit:
##STR1##
and available as an aqueous solution. It is understood that aqueous
solutions combine with H.sub.2 O to form a polycationic material with the
repeating unit:
##STR2##
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
effective with ionically stabilized polymer dispersions is the
hydrolyzable inorganic compounds that form aqueous solutions of polyvalent
ions. These function by reducing the ionic double layer repulsion between
polymer particles.
The liquid is removed from the agglomerated material by any conventional
technique. A preferred method of removing the liquid from 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 batch of 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.degree. to 200.degree. C. for 16
to 24 hours or at any temperature and time sufficient to remove the
remaining liquid from the agglomerated material.
The agglomerate or dough comprising the polymer, filler, and microfibers
are formed into any desired shape. This shape is then further processed by
the application of heat and pressure which causes densification of the
shape and causes the polymer to wet the filler and fibre. The applied
temperature and pressure preferably should respectively range between
600.degree. F. and 800.degree. F. and 100 p.s.i. and 900 p.s.i., with the
preferred temperature being 760.degree. F. and the preferred pressure
being approximately 700 p.s.i.
In the case of an aqueous slurry, it should be understood that the 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 calendering apparatus. In cases where it is difficult to
form the desired shapes from the aqueous agglomerate, it is preferred 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 calendering or paste extrusion.
The lubricant can be selected from various conventional lubricants. It is
particularly preferred that the lubricant be non-toxic as a liquid or a
vapor and have a relatively low volatility so that, at forming
temperatures, the liquid lubricant will not vaporize. However, it should
be understood that for particular forming method, 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
include Stoddard solvent, a mixture of aliphatic hydrocarbons commercially
available as a dry cleaning fluid, a liquid polyisobutylene sold by Exxon
under the "Vistanex" trademark and esters such as dioctylphthalate.
In the preferred embodiment of the invention, the polymer dispersion is an
aqueous dispersion of PTFE particles stabilized by an added nonionic
surface active agent. The filler material is a ceramic grade titanium
dioxide 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 agglomerate 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 dough provides a material which is
still dry in appearance and in a crumb or fibrous particle form. The
material is then formed into any desired shape by conventional methods,
such as, for example, paste extrusion and/or calendering. 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
example for 16 to 24 hours at 200.degree. to 300.degree. C. This dried
shape is then subjected to heat and pressure. The finished shape may then
be cut or trimmed to desired dimensions. The preferred shape is a sheet
having one or both opposing surfaces coated with a conductive layer
forming a microwave circuit board. This conductive layer is customarily
applied during the pressurized heating step by placing a thin sheet of
adhesive coated conductive foil in contact with the sheet of dielectric
material before the application of heat and pressure. The end product is a
laminate of conductive foil and dielectric material.
The dielectric materials provided by the process of the present invention
have various properties that make such dielectric materials particularly
useful.
One important aspect of the present invention is the provision of a
dielectric material which provides both for reduced strain relief in
microwave circuit boards incorporating the dielectric material and for
nearly equal dimensional change in both the X and Y directions as a result
of strain relief. Strain relief is a measurement of the dimensional
changes of the microwave circuit board in both the X and Y directions
after a portion of the conductive foil has been removed by an etchant or a
solvent. As will be understood by those skilled in the art, it is
essential to reduce the dimensional changes of the circuit board because
of the requisite high tolerances which must be maintained if these circuit
boards are to be used at microwave frequencies.
Strain relief is measured by determining the dimensional change of a strip
specimen due to removal of the conductive foil. Sometimes the change in
dimension is retarded 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 materials 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 laboratory conditions,
23.degree. C., 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.
5. Mask an area encompassing each mark about 12 millimeters square using a
pressure 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 150.degree. C. followed by
conditioning as in step 3.
9. Measure the distance between centers of the marks.
10. Calculate dimension change as mm change per meter of original length
between marks.
Another particularly important property of the dielectric materials of the
present invention is their high degree of moisture-resistance. Moisture
resistance is measured by weighing specimens of the dielectric material
from which the metal foil has been removed by etching followed by washing
and drying for 1 hour at 150.degree. C. The weighing is done before and
after a water immersion test and the amount of water absorbed is
determined. Water immersion is for 48 hours at 50.degree. C. It has been
found that with the preferred embodiment of this invention, it is possible
to fabricate circuit boards that absorb less than about 0.5 weight percent
water when subjected to the afore-mentioned test.
A further important property of the dielectric material provided by the
method of the present invention is that it has a relatively high
dielectric 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 of .+-.0.25.
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 D2520. Employing the
aforementioned methods as background information it has been 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 and a matching specimen etched free of foil are clamped
between 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. From these data one may then calculate the fringing correction for
the resonator length and the dielectric constant. The calculation is based
on the known fact that the propagation velocity of a transverse electrical
mode electromagnetic wave through a dielectric medium having a magnetic
permability of unity is related directly to the square root of the inverse
of the dielectric constant.
Another important property of the dielectric materials of the method of the
present invention is that a circuit board incorporating these materials
has a relatively low energy dissipation factor at high frequencies. At 10
GHz, the circuit board usually has a dissipation factor of less than about
0.005. The dissipation factor is measured as follows:
The half power width of the resonant frequency peak in the stripline
resonator utilized in the dielectric constant measuring method described
above is divided by the resonant frequency to give a dissipation ratio for
both metal and dielectric. An estimated value for the metal is calculated
and subtracted from the ratio to give dissipation factor of the dielectric
material.
The following examples and tables demonstrate various physical properties
for the preferred laminate of conductive foil and dielectric layer:
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
Manville'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 (titanium dioxide) filler sold by National Lead
Industries under the trademark "Titanox 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 perfluoroalkane carboxy salt emulsifying agent and stabilized after
polymerization by the addition of about 0.7% nonionic surface active 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 mixed
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 this 1% solution was added to the slurry and the
slurry was mixed for about 1/2 minute. Additional flocculant solution was
added in small increments until the flocculation resulted in clear water
between the flocs. The total amount of flocculant solution added was about
120 grams.
The flocculated solids suspended in water were transferred to a nylon
fabric filter bag to allow gravity drainage 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 160.degree. C. 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 Corporation. DPG is non-toxic as a liquid or as a
vapor and has a relatively low volatility at room temperature. A blender
was used to mix the aggregates of dried crumb and uniformly disperse the
lubricant. 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 diameter by about
40 mm height. The billets were then extrusion-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 extrudate was
then passed through a 2 roll calender with a 0.25 mm gap setting to
produce a ribbon. The X direction is considered parallel to the extrusion
and the Y direction perpendicular to the X direction in the plane of the
ribbon. These co-ordinates are discussed in connection with the finished
clad panels.
Several layers of ribbon were combined in two calendering operations in the
X direction and the spacing between the rollers of the calender was 0.045
inches and 0.035 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
extruded ribbon and were extended by repeated calendering passes in the Y
direction, to form sheets about 12 inches wide and at least 20 inches
long.
The sheets were laid in a stack on clean, aluminum trays and dried in a
vented forced air circulation oven for 24 hours at 246.degree. C.
whereafter the sheets were trimmed accurately 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 desired thickness and
assembled between copper foil and stainless steel caul plates to form a
layup or laminating package that was then wrapped in an aluminum foil
envelope folder and rolled at the edges to exclude air. The package was
clamped between cold platens in a laminating press at about 3.4 MPa. This
pressure was maintained through a heating and cooling procedure that
caused the composite to undergo crystalline melt and limited flow to
accomplish densification and adhesion of the sheets to adjacent sheets or
adjacent foil in the layup. The heating was done by electrical heating
elements in the platens controlled thermostatically to maintain a package
temperature of 396.degree. C. 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 below
150.degree. C. at which point the press was opened and the laminated
panels were removed from between the platens.
The microwave circuit boards were tested for the various properties
indicated in Table 1. Table 1 shows a vast improvement over the prior art
with strain relief properties which are nevertheless less than optimum.
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 of
polymer, fiber, and filler so that they 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 150 mm to produce a ribbon
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 Y 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 388.degree. C. and the time at temperature was about
225 minutes.
Data on typical panels prepared by this example are shown in Table II and
here again the strain relief properties, which significantly improved when
compared to the prior art, are still less than optimum.
EXAMPLE 3
A series of formulations were processed to the condition of a wet dough by
a procedure similar to that of Example 1 except for slight changes in the
relative proportions 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 several times to form a self supporting
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.degree. to
204.degree. C. in a forced air circulator oven. The dried sheets were
assembled with 34 .mu.m thick copper 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 23.degree. C.
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
388.degree. C. The package was held in this condition for a period of 50
minutes.
3. The package was densified and cooled by rapidly transferring it to a
press with platens at about 23.degree. C. where it was clamped at 3.4 MPa
until the package temperature was below 150.degree. C.
Table III summarizes the formulations and test results of eight panels
prepared from four formulations in accordance with Example 3.
TABLE I
______________________________________
PANELS CLAD WITH 34 .mu.m COPPER FOIL
______________________________________
Panel identification 1A 1B
Thickness, average of 20 values mm
.618 .612
Uniformity (std. dev. as % of avg.)
2.45 2.57
Specific gravity (by immersion of
dielectric only) 2.830 2.806
Peel strength of foil bond after 20
seconds float in solder at 260.degree. C.
(average minimum value per 3 mm
strip, kN/m) 1.17 1.12
Strain relief after 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
Resonator 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
50.degree. C. water
Water absorption, % weight gain of
dielectric specimen after 48
hours in 50.degree. C. water
0.25 0.23
______________________________________
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
Peel strength, kN/m
after 20 sec. float in 260.degree. C.
9.4 8.2 6.8
solder
std. deviation of 4 readings
.6 .3 .0
Dielectric constant at 10GHz
10.43 10.60 10.58
Q of resonator 313 283 304
______________________________________
TABLE III
__________________________________________________________________________
Panel Identification
3A 3B 3C 3D 3E 3F 3G 3H
Composition in parts by weight
Polymer 33.6
33.6
33.6
33.6
31.6
31.6
31.6
31.6
Filler 63 63 63 63 65 65 65 65
Fiber 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4
Fiber crush method
press
press
roll
roll
press
press
roll
roll
Test results of laminated panels
1. Avg. Thickness, mm
.74 .72 .68 .69 .71 .71 .73 .72
2. Thickness uniformity
(std. dev. as % of avg.)
2.11
1.31
1.42
3.04
1.27
1.14
1.10
1.59
3. Specific gravity by immersion
2.904
2.936
2.936
2.905
2.966
2.941
2.900
2.945
4. Dielectric constant at X band
10.07
10.16
9.62
9.58
10.88
10.84
9.00
9.88
5. Q of resonator at X band
189 183 176 173 178 165 190 176
__________________________________________________________________________
As the above demonstrates the absorption of water is minimized despite the
fact that fibers added to the composition. This is an unexpected result
since it was previously believed that increasing the fiber content would
increase the porosity of the final product drastically. One suggested
theory for this result is that the fiber disrupts the flow of the formed
shape during the heat and pressure application step. This would allow the
application of greater pressures at higher temperatures and thus form a
denser end product. It should be noted that the above theory is only
suggestive and should not be taken to limit the present invention.
Another unexpected result is that the dielectric constant in the end
product is isotropic. This isotropic state was unanticipated since it was
expected that the fibers would orientate, under the applied pressure, and
inhibit an even distribution of the dielectric filler material.
The enhanced strain relief properties combined with the unexpected
improvement in the dielectric distribution and the minimized water
absorption result in the dielectric material of the present invention
comprising a significant improvement when compared to prior art dielectric
materials intended for similar applications.
It has been determined that a dielectric material in accordance with the
present invention should have a strain relief which closely aproaches 1
mm/m in both the X and Y directions, and which preferably is less than 1
mm/m in both the X and Y directions, and a water absorption of less than
0.5%. By extrapolation of the test results set forth in Table IV, this
fiber content lies between a percentage by weight of fibers to total
non-filler in the range of 7% to 20%. It may also be seen from Table IV
that the initial addition of fiber, 0.15% by weight, increases the water
absorption as expected. However, within the range of fiber addition lying
between 0.15% and 5.00% by weight there is an unexpected diminishing in
water absorption before the resumed expected increase. In the critical
range of fiber content, accordingly, there is the unexpected result that
the amount of water absorption does not increase in direct proportion to
the amount of fiber present. It was expected that the moisture absorption
would continue to increase at the rate defined by the values determined
for fiber content of 0% and 0.15% with the maximum acceptable value of
fiber content by weight thus lying between 1% where the strain relief
properties are unacceptable. However, because of the unexpected dip in
moisture absorption, it is possible to take advantage of the reduction in
strain relief afforded by up approximately 20% fiber in the total
non-filler included in the dielectric. For the samples tested, the optimum
range of fiber content was between 3% and 5% by weight or in a weight
ratio of 0.07 to 0.20 of the total non-filler.
All of the samples in Table IV were prepared, to form a dielectric sheet
material, by exactly the same method as in Example 2 except that the
intermediate step of forming the dough into bricks prior to extrusion was
not performed. Also, the initial removal of liquid from the agglomerated
material was accomplished by draining over a screen. The binder was PTFE,
the filler grade 3030 titanium dioxide and the microfibers were glass.
Sheets of each sample composition were laminated between copper foils at
200 p.s.i., 400 p.s.i. and 707 p.s.i. at 760.degree. F. These sheets were
then subjected to the previously mentioned tests for water absorption,
strain relief, determining the dissipation factor at 10 GHz and
determining changes in both the dissipation factor and dielectric
constant.
Although not shown in Table IV, the samples were also subjected to tensile
strength tests to determine the modules in kpsi, and the ultimate stress,
in psi. For use in microwave applications, a high resistance to
deformations; i.e., stiffness; without being brittle and the modules of
the material is a measure of this resistance to deformation. Similarly,
the dielectric sheet material should show a high degree of resistance to
damage during fabrication and use and the ultimate stress is a measure of
this resistance to damage. It was known that the addition of ordinary
fiber fillers to prior art polymer compounds would increase both the
modules and ultimate stress up to a point with an excess of fibers beyond
this point degrading the properties of interest. The maximum fiber content
before a reduction in modules and/or ultimate stress has been encountered
has historically been quite high. Most unexpectedly, it has been found
that the maximum modules and ultimate stress for the dielectric material
of the present invention occurs at a very low microfiber content. In fact,
the maximum modules and ultimate stress for the samples of Table IV lie
within the 7% to 20% fiber/non-filler range discussed ab | | |
