CIRCUIT MATERIALS, CIRCUIT LAMINATES, AND METHODS OF MANUFACTURE
THEREOF
BACKGROUND
[0001] This invention generally relates to circuit materials, methods for the manufacture
of the circuit materials, and articles formed therefrom, including circuits and circuit laminates.
[0002] As used herein, a circuit material is an article used in the manufacture of circuits
and multi-layer circuits, and includes circuit subassemblies, bond plies, resin coated conductive
layers, unclad dielectric layers, and cover films. A circuit laminate is a type of circuit
subassembly that has a conductive layer, e.g., copper, fixedly attached to a dielectric layer.
Double clad circuit laminates have two conductive layers, one on each side of the dielectric
layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit.
Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a
conductive wiring pattern. Typically, multilayer circuits are formed by laminating one or more
circuits together using bond plies, by building up additional layers with resin coated conductive
layers that are subsequently etched, or by building up additional layers by adding unclad
dielectric layers followed by additive metallization. After forming the multilayer circuit, known
hole-forming and plating technologies can be used to produce useful electrical pathways
between conductive layers.
[0003] A dielectric layer can comprise a polymeric dielectric composite material in
which the dielectric and physical properties are controlled by the use of mineral or ceramic
particulate fillers. Particularly where a low dielectric constant is desired, hollow glass or
ceramic microspheres can be used. For example, U.S. Patent No. 4,134,848 (Adicoff et al.)
describes a composite for a strip line board material that contains hollow, air-filled glass
microspheres in a hydrocarbon matrix. U.S. Patent 4,661,301 (Okada and Fujino) discloses a
hollow-glass microsphere-filled polymer composite made by directly extruding a molten
composition into the opening of a vertical double belt press. U.S. Patent No. 5,126,192 (Chellis et
al.) discloses a filled prepreg material having a dielectric constant below 3.2 and made using hollow
microspheres from various manufacturers. U.S. Patent No. 4,610,495 (Landi) discloses the use of a
layer of elastomer filled with hollow microspheres for controlling impedance in a solderless
connector for a microelectronic device. U.S. Patent 4,994,316 (Browne and Jarvis) discloses a
bonding layer for circuit boards containing hollow glass microspheres.
[0004] Following these earlier patents, U.S. Patent No. 8,187,696 (Paul et al.) disclosed, as
a less costly alternative to the use of synthetic microspheres in circuit products, the use of naturally
occurring hollow microspheres known as cenospheres, so long as the cenospheres meet certain
compositional requirements. Selected cenospheres were found to advantageously provide a low
D and other desired electrical properties, while maintaining the filler volume necessary for
preservation of mechanical properties. Following commercial production, however, the concern
developed that the available quantities might not be guaranteed indefinitely. This supply
constraint coupled with the variable nature of the naturally sourced product, even on a lot to lot
basis, has prompted investigation into synthetic alternatives for the use of cenospheres. A
specific desire is to obtain filler having electrical properties necessary for high frequency
applications that require a low dissipation factor in circuit subassemblies.
[0005] Hollow glass microspheres have been manufactured for a wide variety of uses in
composite materials. For example, hollow microspheres have been used as a component of
syntactic foams for the hulls of submersibles. Such microspheres have also been used for
storage and/or slow release of pharmaceuticals or hydrogen gas. The microspheres typically
have a diameter ranging from 10 to 300 micrometers and are sometimes termed microballoons
or glass bubbles.
[0006] Hollow glass microspheres can be made by a variety of processes, including
ultrasonic spray pyro lysis. The desired properties of the formed microspheres can be improved,
for certain uses, by surface treatment that can involve removing at least some of the sodium. For
example, among other reasons, producing microspheres having a clean surface can enhance the
wettability of the microspheres by various polymers. Also, sodium depletion of the
microspheres may be desirable for applications in which glass microspheres are mixed with a
chemically sensitive resin. Finally, surface treatment of microspheres can improve bonding for
coupling reactions if desired.
[0007] U.S. Patent No. 4,904,293 (Gamier) discloses treatment of glass microspheres,
after their production and recovery, to increase thermal resistance, by contacting the
microspheres with a dealkylization agent that increases the silica content, thereby reducing the
sodium content. After production, the alkaline oxide content of the microspheres is ordinarily
less than 10%. In this case, the dealkylization has the aim of bringing the alkaline oxide content
below 4% or less.
[0008] Leforge, J.W. et al, in "The Development of Silica Hollow Microspheres for Use
as a High Temperature Dielectric," Technical Report 60-899 (July 1961), prepared by Emerson
& Cuming under USAF Contract No. AF33 (616)-7263, available from Armed Services
Technical Information Agency, Arlington, Virginia, discloses the production of bulk
microsphere materials having a low dielectric constant (less than 2.0) and a low dissipation
factor (less than 0.008) that are useful at temperatures greater than 200°C. In particular, sodium
borosilicate glass microspheres ("microbubbles") were acid leached to remove sodium in order
to increase high temperature stability (1090°C). Specifically, Eccosphere® microspheres of
various densities were acid leached for various periods of time in various concentration of
H2SO4 or HC1. Following acid leaching of the microspheres, the authors found that a slightly
lower dielectric constant and equivalent loss tangent values were obtained, i.e., the authors
found that the loss tangents did not decrease in spite of removal of all extractable sodium.
[0009] In view of the above, there remains a need in the art for low dielectric constant,
low loss circuit materials having improved (lower) dissipation factor.
SUMMARY OF INVENTION
[0010] The above-described drawbacks and disadvantages are alleviated by a circuit
subassembly comprising a conductive layer disposed on a dielectric substrate layer, wherein the
dielectric substrate layer comprises, based on the volume of the dielectric layer, about 30 to
about 90 volume percent of a polymer matrix material, and about 5 to about 70 volume percent
of hollow borosilicate microspheres that are the product of a process in which the borosilicate
microspheres have been treated with an alkaline solution, thereby modifying the surface of the
microspheres, wherein the dielectric substrate layer has a dielectric constant of less than about
3.5 and a dissipation factor of less than about 0.006 at 10 GHz. The dissipation factor can be
measured by the IPC-TM-650 2.5.5.5. 1 X-band strip line method.
[001 1] Another aspect of the invention is directed to a method of making a circuit
subassembly, the method comprising combining a polymer matrix material and a filler
component to form a dielectric composite material, wherein the filler component comprises a
plurality of hollow borosilicate microspheres; wherein the borosilicate microspheres have been
treated with an alkaline solution to modify the surface thereof; forming a layer of the dielectric
composite material, thereby obtaining a dielectric substrate layer; disposing a conductive layer
on the dielectric substrate layer; and laminating the dielectric composite layer and the
conductive layer to form a dielectric substrate layer having a dielectric constant of less than
about 3.5 and a dissipation factor of less than about 0.006 at 10 GHz.
[0012] Advantageously, the circuit subassemblies can exhibit improved combination of
D , D f and PIM performance, as discussed below. The filler, because it can be synthetically
made, can be obtained in theoretically inexhaustible supply.
[0013] Also disclosed are a circuit and multilayer circuit comprising the above-described
dielectric subassembly.
[0014] The invention is further illustrated by the following drawings, detailed
description, and examples.
BRIEF DESCRIPTION OF DRAWINGS
[0015] Referring now to the exemplary drawings wherein like elements are numbered
alike in the figure:
[0016] FIGURE 1 is a schematic of a single clad laminate;
[0017] FIGURE 2 is a schematic of a double clad laminate;
[0018] FIGURE 3 is a schematic of a double clad laminate with patterned conductive
layer; and
[0019] FIGURE 4 is a schematic of an exemplary circuit assembly comprising two
double clad circuit laminates.
DETAILED DESCRIPTION
[0020] It has been unexpectedly discovered by the inventors hereof that use of certain
synthetic borosilicate microspheres as a particulate filler in dielectric composite materials allows
for the manufacture of circuit subassemblies having an improved dissipation factor. Such circuit
subassemblies are especially advantageous for high frequency applications. Specifically, the
borosilicate microspheres can exhibit a loss tangent of less than 0.006 at 10 GHz. Circuits and
multilayer circuits based on the presently disclosed dielectric composite material can exhibit
properties superior to other circuit materials in the prior art that comprise synthetic glass hollow
microspheres.
[0021] Borosilicate microspheres are hollow spheres having a mean diameter of less than
300 micrometers, for example 15-200 microns, more specifically 20 to 100 microns. The density
of the hollow microspheres can range from 0.1 or more, specifically 0.2 to 0.6, more specifically
0.3 g/cc to 0.5 g/cc.
[0022] Hollow microspheres are available from a number of commercial sources, for
example from Trelleborg Offshore (Boston), formerly Emerson and Cuming, Inc., W.R. Grace
and Company (Canton, MA), and 3M Company (St. Paul, MN). Such hollow microspheres are
referred to as microballoons, glass bubbles, microbubbles, or the like and are sold in various
grades, for example, which can vary according to density, size, coatings, and/or surface
treatments.
[0023] For example, microspheres can have an exterior surface chemically modified by
treatment with a coupling agent that can react with hydroxyl groups present on the surface of the
glass. In one embodiment, the coupling agent is a silane or epoxy, specifically an organosilane
having, at one end, a group that can react with hydro xyl groups present on the exterior surface of
the glass microspheres and, on the other end, an organic group that will aid in dispersibility of
the microspheres in a polymer matrix that has low polarity. A difunctional silane coupling can
have a combination of groups selected from vinyl, hydroxy, and amino groups, for example, 3-
amino-propyltriethoxy silane. Silane coatings can also minimize water absorption.
[0024] The borosilicate microspheres can be made of alkali borosilicate glass. An
exemplary oxide composition of alkali borosilicate can comprise 76.6 wt.% Si0 2, 21.3 wt.%
Na20 , 1.9 wt.% B20 3, and 0.2 wt.% other components. An exemplary soda-lime borosilicate
can comprise 80.7 wt.% Si0 2, 6.9 wt.% Na20 , 10.3 wt.% CaO, 2.1 wt.% B20 3, and 1.9% of
impurities. Thus, the composition (although mostly Si0 2 and comprising at least 1 percent
B20 3) can vary to some extent, depending on the starting materials. An exemplary XPS analysis
of borosilicate microspheres, after removal of sodium by acid washing is 98.7 wt.% Si0 2, 0.1
wt.% Na20 , and 1.2 wt.% B20 3, whereas an exemplary XPS analysis of the same or similar
microspheres, after removal of sodium by alkaline washing is 87.5 wt.% Si0 2, 3.4 wt.% Na20 ,
3.6 wt.% MgO, and 3.3 wt.% B20 3, 1.2 wt.% BaO, 0.7 wt.% CaO, and 0.4 wt.% Fe20 3. An iron
content, less than 0.50 wt.%, as measured via surface XPS analysis, therefore, can be obtained
without undue difficulty.
[0025] The production of hollow microspheres is a well-established technology. There
are several methods available to produce hollow microspheres. A common approach involves
the decomposition of a substance at very high temperature to form a gaseous composition within
liquid droplets. The rapid expansion of the gaseous composition at high temperature causes the
formation of a bubble. The hollow droplets are then rapidly cooled from the liquid state to form
hollow microspheres.
[0026] The production of hollow borosilicate microspheres can be made generally in
accordance with conventional processes in the prior art such as, for example, U.S. Patent No.
3,699,050, which discloses a dried-gel process in which a solution of glass-forming oxides is
dried to a hard residue, or gel, and then ground to a suitable particle size. The ground material is
sieved into narrow size ranges and then mechanically dropped through a high-temperature
furnace or spray tower. It appears that the chemically bound water in the gel inflates the particle
as the surface melts, forming a hollow-glass microsphere. A blowing agent can also be used.
U.S. Patent No. 4,904,293 to Gamier discloses the production of microspheres having a high
silica content, in which a starting glass is reduced to fine particles by grinding, optionally mixed
with a blowing agent, and then passed through the flame of a burner at a temperature of 1500°C
or above to form molten hollow microspheres that are then cooled to form solid hollow
microspheres.
[0027] An alternative method producing hollow microspheres involves mixing trace
amounts of a sulfur-containing compound such as sodium sulfate with a borosilicate glass which
mixture is then dropped into a hot flame that melts the powdered glass and sodium sulfate. The
melting of the sodium sulfate results in a decomposition reaction that releases minute amounts
of sulfur gas that form bubbles within the molten glass droplets. The presence of such sulfurcontaining
compounds or other such polar compounds, however, can have undesirable adverse
effects during later processing of circuit materials. Hence, the interior of the microspheres, in a
specific embodiment, comprises an inert, non-polar, sulfur-free gas composition, for example, a
composition comprising non-polar compounds such as nitrogen, carbon dioxide, oxygen and less
than 1 wt.% of compounds such as sulfur dioxide, specifically the absence of sulfur dioxide. In
one embodiment, at least 98 wt.%, specifically at least 99 wt.% of the gaseous composition in
the microspheres is inert, for example, selected from the group consisting of nitrogen, oxygen,
argon, carbon dioxide, and combinations thereof.
[0028] The microspheres, after being produced at high temperature and recovered are
contacted with a dealkylization agent to reduce the sodium content. The original sodium oxide
content of the produced and recovered microspheres can be more than about 6 wt.%, specifically
more than about 7 wt.%. The dealkylization operation has the aim to bring the sodium oxide
(Na20 ) content to an amount less than about 5.0 wt.%, specifically below about 4% wt.%, as
determined by XPS surface analysis. Thus, the wt.% of sodium oxide in the microsphere can be
0 to 5.0 wt .%, specifically 0.1 to 4.5 wt.%. In one embodiment, the wt.% of sodium oxide is 1.0
to 5.0 wt .%, specifically 2.0 to 4.5 wt.%, more specifically 2.5 to 4.0 wt.%. In one embodiment,
the sodium content is reduced to an amount of less than about 5 weight percent, based on the
weight of the microspheres, after a reduction in the total amount of sodium by at least 25 wt.%,
specifically by at least 50 wt.%.
[0029] The dealkylization treatment can be performed chemically. The dealkylization
treatment can occur at an elevated temperature or at room temperature, in a batch or continuous
process. In one embodiment, the treatment can extend for the amount of time sufficient to
obtain the desired depletion in sodium content. Accordingly, the hollow microspheres, after
formation at high temperature and after having been subjected to dealkylization to remove alkali
(sodium) ions, can have a surface layer that is depleted in sodium ions, which can extend into
the bulk of the borosilicate glass material.
[0030] As will be described in greater detail in the examples below, it was unexpectedly
found that improved circuit subassemblies, especially for high frequency applications, can be
obtained by using, as a filler for a dielectric composite material, hollow microspheres that have
been treated with an alkaline dealkylization agent, thereby obtaining a circuit subassembly
having a loss tangent of less than 0.006 at 10 GHz. More specifically, the use of alkalinewashed
borosilicate microspheres having a sodium oxide content of less than 4 weight percent,
based on XPS surface analysis of the borosilicate microspheres, was found to unexpectedly
result in a dielectric material providing desired electrical properties, particularly for high
frequency circuit applications, including a dissipation factor (Df) that is less than 0.006,
specifically less than 0.004, more specifically equal to or less than 0.0035 at 10 GHz .
[0031] Dissipation factor (Df) is a measure of loss-rate of energy of an electrical
oscillation in a dissipative system. Electrical potential energy is dissipated to some extent in all
dielectric materials, usually in the form of heat. Df will vary depending on the dielectric
material and the frequency of the electrical signals. Dissipation factor (Df) and loss tangent are
the same for present purposes. Another property of relevance to circuit subassemblies is Passive
Intermodulation (PIM), which is the generation of unwanted frequencies due to non-linearities in
the current-voltage relationship of passive elements. PIM is a growing issue for cellular
networks. PIM can create interference that reduces the sensitivity of a cellular system. The
presence of ferromagnetic materials such as iron can contribute to generation of significant
amounts of PIM.
[0032] Dissipation factor and PIM are especially relevant to PCB (printed circuit board)
antennas. Antennas are a critical component in any transmission system or wireless
communication infrastructure, for example, cellular base station antennas. As such, careful
consideration is given to the properties of high frequency laminates used in PCB antennas. For
such applications, a D of less than 3.5, a Df of less than 0.004, and a PIM of less than -153 dBc
are desirable. In one embodiment, the circuit subassembly exhibits a dissipation factor of
0.0030 to 0.0035 at 10 GHz, a PIM of less than -153 dBc, and a dielectric constant between 2.5
and 3.5.
[0033] The size of the microspheres and their size distribution can vary, depending on
the desired characteristics of the dielectric composite material. In an exemplary embodiment,
the borosilicate microspheres of the particulate filler exhibit a mean particle diameter of about
20 to about 100 micrometers, specifically about 25 to about 75 micrometers, more specifically
about 20 to about 70 micrometers, for example 30 to 65 or 40 to 55, most specifically about 35
to about 60 micrometers. The size distribution can be bimodal, trimodal, or the like.
[0034] The borosilicate microspheres are present in the dielectric composite material of
the circuit subassembly in an amount effective to lower the dielectric constant of the
composition to the desired level without a significant negative effect on dissipation factor.
Furthermore, the dielectric composite material can be used in high frequency circuit
applications.
[0035] In some cases, it is desirable to fine-tune the dielectric constant of a circuit
substrate to a predetermined value, while maintaining a volume loading of filler that achieves a
low coefficient of thermal expansion. In such cases, the desired effect can be obtained by
loading levels of the borosilicate microspheres as low as about 10 volume percent. In one
embodiment, the borosilicate microspheres are present in the dielectric composite material in an
amount of about 1 to about 70 volume percent (vol. %), based on the total volume of the
composition, specifically about 5 to about 50 vol. %, depending on the desired dielectric
constant. For example, in a 2.55-D dielectric composite material, the microspheres can be
present in an amount of 27.0 to 29.5 volume percent, whereas in a 3.0 Dk dielectric composite
material, the microspheres can be present in an amount of 14.5 to 16.5 volume percent.
[0036] The dielectric composite material can optionally include one or more additional
particulate fillers other than the borosilicate microspheres. Use of additional types of fillers
allows the dielectric constant, dissipation factor, coefficient of thermal expansion, and other
properties of the dielectric composite material to be fine-tuned. Examples of secondary
particulate fillers include, without limitation, titanium dioxide (rutile and anatase), barium
titanate, strontium titanate, silica (including fused amorphous silica), corundum, wollastonite,
Ba2Ti 0 2o, solid glass spheres, synthetic glass or ceramic hollow spheres, quartz, boron nitride,
aluminum nitride, silicon carbide, beryllia, alumina, alumina trihydrate, magnesia, mica, talcs,
nanoclays, and magnesium hydroxide. A single secondary filler, or a combination of secondary
fillers, can be used to provide a desired balance of properties. Optionally, the fillers can be
surface treated with a silicon-containing coating, for example, an organo functional alkoxy silane
coupling agent. Alternatively, a zirconate or titanate coupling agent can be used. Such coupling
agents can improve the dispersion of the filler in the polymeric matrix and reduce water
absorption of the finished composite circuit substrate.
[0037] The dielectric composite can also optionally contain constituents useful for
making the material resistant to flame. Such constituents are typically present in overall
composite volumes ranging from 0 to 30 volume percent. These flame retarding agents can be
halogenated or not. The choice of flame retardant can influence the loading required to achieve
the desired level of flame resistance.
[0038] The total filler component used to manufacture the dielectric composite material
can accordingly comprise from 5 to 70 vol. % of the borosilicate microspheres and from 1 to 90
vol. % of one or more secondary fillers, specifically from 25 to 75 vol. % of secondary filler,
based on the total composition of 100 percent. In one embodiment, the filler component
comprises 5 to 50 vol. % of the borosilicate microspheres and 70 to 30 vol. % of fused
amorphous silica as secondary filler.
[0039] The borosilicate microspheres are dispersed in a dielectric polymer matrix
material to form the dielectric composite material for a circuit subassembly. Exemplary
dielectric polymer matrix materials include low polarity, low dielectric constant and low loss
polymer resins, including those based on thermosetting and thermoplastic resins such as 1,2-
polybutadiene (PBD), polyisoprene, polybutadiene-polyisoprene copolymers, polyetherimide
(PEI), fluoropolymers such as polytetrafluoro ethylene (PTFE), polyimide, polyetheretherketone
(PEEK), polyamidimide, polyethylene terephthalate (PET), polyethylene naphthalate,
polycyclohexylene terephthalate, polybutadiene-polyisoprene copolymers, polyphenylene ether
resins, and those based on allylated polyphenylene ether resins. These materials exhibit the
desirable features of low dielectric constant that can be further improved (i.e., reduced) by
addition of the borosilicate microspheres. Combinations of low polarity resins with higher
polarity resins can also be used, non-limiting examples including epoxy and poly(phenylene
ether), epoxy and poly(ether imide), cyanate ester and poly(phenylene ether), and 1,2-
polybutadiene and polyethylene.
[0040] Suitable fluoropolymer matrix materials for the dielectric layer include
fiuorinated homopolymers, e.g., polytetrafiuoroethylene (PTFE) and polychlorotrifluoro ethylene
(PCTFE), and fiuorinated copolymers, e.g. copolymers of tetrafluoroethylene with a monomer
selected from the group consisting of hexafluoropropylene and perfiuoroalkylvinylethers,
copolymers of tetrafluoroethylene with a monomer selected from the group consisting of
vinylidene fluoride, vinyl fluoride and ethylene, and copolymers of chlorotrifluoro ethylene with
a monomer selected from the group of hexafluoropropylene, perfiuoroalkylvinylethers,
vinylidene fluoride, vinyl fluoride and ethylene. Blends of these fluoropolymers and
terpolymers formed from the above listed monomers can also be used as the polymer matrix
material.
[0041] Other specific polymer matrix materials include thermosetting polybutadiene
and/or polyisoprene resin. As used herein, the term "thermosetting polybutadiene and/or
polyisoprene resin" includes homopolymers and copolymers comprising units derived from
butadiene, isoprene, or mixtures thereof. Units derived from other copolymerizable monomers
can also be present in the resin, for example, optionally in the form of grafts. Exemplary
copolymerizable monomers include, but are not limited to, vinylaromatic monomers, for
example substituted and unsubstituted monovinylaromatic monomers such as styrene, 3-
methylstyrene, 3,5-diethylstyrene, 4-n-propylstyrene, alpha-methylstyrene, alpha-methyl
vinyltoluene, para-hydroxystyrene, para-methoxystyrene, alpha-chlorostyrene, alphabromostyrene,
dichlorostyrene, dibromostyrene, tetra-chlorostyrene, and the like; and substituted
and unsubstituted divinylaromatic monomers such as divinylbenzene, divinyltoluene, and the
like. Combinations comprising at least one of the foregoing copolymerizable monomers can
also be used. Exemplary thermosetting polybutadiene and/or polyisoprene resins include, but
are not limited to, butadiene homopolymers, isoprene homopolymers, butadiene-vinylaromatic
copolymers such as butadiene-styrene, isoprene-vinylaromatic copolymers such as isoprenestyrene
copolymers, and the like.
[0042] The thermosetting polybutadiene and/or polyisoprene resins can also be
modified. For example, the resins can be hydroxyl-terminated, methacrylate-terminated,
carboxylate-terminated resins or the like. Post-reacted resins can be used, such as epoxy-,
maleic anhydride-, or urethane -modified butadiene or isoprene resins. The resins can also be
crosslinked, for example by divinylaromatic compounds such as divinyl benzene, e.g., a
polybutadiene-styrene crosslinked with divinyl benzene. Exemplary resins are broadly
classified as "polybutadienes" by their manufacturers, for example, Nippon Soda Co., Tokyo,
Japan, and Cray Valley Hydrocarbon Specialty Chemicals, Exton, PA. Mixtures of resins can
also be used, for example, a mixture of a polybutadiene homopolymer and a poly(butadieneisoprene)
copolymer. Combinations comprising a syndiotactic polybutadiene can also be useful.
[0043] The thermosetting polybutadiene and/or polyisoprene resin can be liquid or solid
at room temperature. Suitable liquid resins can have a number average molecular weight greater
than about 5,000 but generally have a number average molecular weight of less than about 5,000
(most preferably about 1,000 to about 3,000). Thermosetting polybutadiene and/or polyisoprene
resins include resins having at least 90 wt. % 1,2 addition, which can exhibit greater crosslink
density upon cure due to the large number of pendent vinyl groups available for crosslinking.
[0044] The polybutadiene and/or polyisoprene resin can be present in the polymer matrix
composition in an amount of up to 100 wt. %, specifically up to about 75 wt. % with respect to
the total resin system, more specifically about 10 to about 70 wt. %, even more specifically
about 20 to about 60 or 70 wt. %, based on the total polymer matrix composition.
[0045] Other polymers that can co-cure with the thermosetting polybutadiene and/or
polyisoprene resins can be added for specific property or processing modifications. For
example, in order to improve the stability of the dielectric strength and mechanical properties of
the electrical substrate material over time, a lower molecular weight ethylene propylene
elastomer can be used in the resin systems. An ethylene propylene elastomer as used herein is a
copolymer, terpolymer, or other polymer comprising primarily ethylene and propylene.
Ethylene propylene elastomers can be further classified as EPM copolymers (i.e., copolymers of
ethylene and propylene monomers) or EPDM terpolymers (i.e., terpolymers of ethylene,
propylene, and diene monomers). Ethylene propylene diene terpolymer rubbers, in particular,
have saturated main chains, with unsaturation available off the main chain for facile crosslinking.
Liquid ethylene propylene diene terpolymer rubbers, in which the diene is
dicyclopentadiene, can be used.
[0046] The molecular weights of the ethylene propylene rubbers can be less than 10,000
viscosity average molecular weight. Suitable ethylene propylene rubbers include an ethylene
propylene rubber having a viscosity average molecular weight (MV) of about 7,200, which is
available from Lion Copolymer, Baton Rouge, LA, under the trade name Trilene® CP80; a
liquid ethylene propylene dicyclopentadiene terpolymer rubbers having a molecular weight of
about 7,000, which is available from Lion Copolymer under the trade name of Trilene® 65; and
a liquid ethylene propylene ethylidene norbornene terpolymer, having a molecular weight of
about 7,500, which is available from Lion Copolymer under the name Trilene® 67.
[0047] The ethylene propylene rubber can be present in an amount effective to maintain
the stability of the properties of the substrate material over time, in particular the dielectric
strength and mechanical properties. Typically, such amounts are up to about 20 wt. % with
respect to the total weight of the polymer matrix composition, more specifically about 4 to about
20 wt. %, even more specifically about 6 to about 12 wt. %.
[0048] Another type of co-curable polymer is an unsaturated polybutadiene- or
polyisoprene-containing elastomer. This component can be a random or block copolymer of
primarily 1,3 -addition butadiene or isoprene with an ethylenically unsaturated monomer, for
example a vinylaromatic compound such as styrene or alpha-methyl styrene, an acrylate or
methacrylate such a methyl methacrylate, or acrylonitrile. The elastomer can be a solid,
thermoplastic elastomer comprising a linear or graft-type block copolymer having a
polybutadiene or polyisoprene block and a thermoplastic block that can be derived from a
monovinylaromatic monomer such as styrene or alpha-methyl styrene. Block copolymers of this
type include styrene-butadiene-styrene triblock copolymers, for example, those available from
Dexco Polymers, Houston, TX under the trade name Vector 8508M®, from Enichem
Elastomers America, Houston, TX under the trade name Sol-T-6302®, and those from Dynasol
Elastomers under the trade name Calprene® 401; and styrene-butadiene diblock copolymers and
mixed triblock and diblock copolymers containing styrene and butadiene, for example, those
available from Kraton Polymers (Houston, TX) under the trade name KRATON Dl 118.
KRATON Dl 118 is a mixed diblock / triblock styrene and butadiene containing copolymer that
contains 33% by weight styrene.
[0049] The optional polybutadiene- or polyisoprene-containing elastomer can further
comprise a second block copolymer similar to that described above, except that the
polybutadiene or polyisoprene block is hydrogenated, thereby forming a polyethylene block (in
the case of polybutadiene) or an ethylene-propylene copolymer block (in the case of
polyisoprene). When used in conjunction with the above-described copolymer, materials with
greater toughness can be produced. An exemplary second block copolymer of this type is
KRATON GX1855 (commercially available from Kraton Polymers, which is believed to be a
mixture of a styrene-high 1,2-butadiene-styrene block copolymer and a styrene-(ethylenepropylene)-
styrene block copolymer.
[0050] Typically, the unsaturated polybutadiene- or polyisoprene-containing elastomer
component is present in the resin system in an amount of about 2 to about 60 wt. % with respect
to the total polymer matrix composition, more specifically about 5 to about 50 wt. %, or even
more specifically about 10 to about 40 or 50 wt. %.
[005 1] Still other co-curable polymers that can be added for specific property or
processing modifications include, but are not limited to, homopolymers or copolymers of
ethylene such as polyethylene and ethylene oxide copolymers; natural rubber; norbornene
polymers such as polydicyclopentadiene; hydrogenated styrene-isoprene-styrene copolymers
and butadiene-acrylonitrile copolymers; unsaturated polyesters; and the like. Levels of these
copolymers are generally less than 50 wt. % of the total polymer in the matrix composition.
[0052] Free radical-curable monomers can also be added for specific property or
processing modifications, for example to increase the crosslink density of the resin system after
cure. Exemplary monomers that can be suitable crosslinking agents include, for example, di, tri-
, or higher ethylenically unsaturated monomers such as divinyl benzene, triallyl cyanurate,
diallyl phthalate, and multifunctional acrylate monomers (e.g., Sartomer® resins available from
Sartomer USA, Newtown Square, PA), or combinations thereof, all of which are commercially
available. The crosslinking agent, when used, can be present in the resin system in an amount of
up to about 20 wt. %, specifically 1 to 15 wt. %, based on the total polymer matrix composition.
[0053] A curing agent can be added to the resin system to accelerate the curing reaction
of polyenes having olefinic reactive sites. Specifically useful curing agents are organic
peroxides such as, for example, dicumyl peroxide, t-butyl perbenzoate, 2,5-dimethyl-2,5-di(tbutyl
peroxy)hexane, a,a-di-bis(t-butyl peroxy)diisopropylbenzene, and 2,5-dimethyl-2,5-di(tbutyl
peroxy) hexyne-3, all of which are commercially available. Carbon-Carbon initiators can
be used in the resin system, for example, 2,3-dimethyl-2,3 diphenylbutane. Curing agents or
initiators can be used alone or in combination. Typical amounts of curing agent are from about
1.5 to about 10 wt. % of the total polymer matrix composition.
[0054] In one embodiment, the polybutadiene or polyisoprene polymer is carboxyfunctionalized.
Functionalization can be accomplished using a polyfunctional compound having
in the molecule both (i) a carbon-carbon double bond or a carbon-carbon triple bond, and (ii)
one or more of a carboxy group, including a carboxylic acid, anhydride, amide, ester, or acid
halide. A specific carboxy group is a carboxylic acid or ester. Examples of polyfunctional
compounds that can provide a carboxylic acid functional group include maleic acid, maleic
anhydride, fumaric acid, and citric acid. In particular, polybutadienes adducted with maleic
anhydride can be used in the thermosetting composition. Suitable maleinized polybutadiene
polymers are commercially available, for example from Cray Valley under the trade names
RICON 130MA8, RICON 130MA13, RICON 130MA20, RICON 131MA5, RICON 131MA10,
RICON 131MA17, RICON 131MA20, and RICON 156MA17. Suitable maleinized
polybutadiene-styrene copolymers are commercially available, for example, from Sartomer
under the trade names RICON 184MA6. RICON 184MA6 is a butadiene-styrene copolymer
adducted with maleic anhydride having styrene content from 1 to 27 wt.% and number average
molecular weight (M ) of about 9,900 g/mole.
[0055] The relative amounts of the various polymers, for example, the polybutadiene or
polyisoprene polymer and other polymers, can depend on the particular conductive metal layer
used, the desired properties of the circuit materials and circuit laminates, and like considerations.
For example, it has been found that use of a poly(arylene ether) can provide increased bond
strength to the conductive metal layer, particularly copper. Use of a polybutadiene or
polyisoprene polymer can increase high temperature resistance of the laminates, particularly
when these polymers are carboxy-functionalized. Use of an elastomeric block copolymer can
function to compatibilize the components of the polymer matrix material. Determination of the
appropriate quantities of each component can be done without undue experimentation,
depending on the desired properties for a particular application.
[0056] In addition to the polymeric matrix material, the dielectric composite material can
optionally further include an unwoven or woven, thermally stable web of a suitable fiber,
specifically glass (E, S, and D glass) or high temperature polyester fibers. Such thermally stable
fiber reinforcement provides a circuit laminate with a means of controlling shrinkage upon cure
within the plane of the laminate. In addition, the use of the woven web reinforcement renders a
circuit substrate with a relatively high mechanical strength.
[0057] The dielectric composite material can be produced by means known in the art.
The particular choice of processing conditions can depend on the polymer matrix selected. For
example, where the polymer matrix is a fluoropolymer such as PTFE, the polymer matrix
material can be mixed with a first carrier liquid. The mixture can comprise a dispersion of
polymeric particles in the first carrier liquid, i.e. an emulsion, of liquid droplets of the polymer
or of a monomeric or oligomeric precursor of the polymer in the first carrier liquid, or a solution
of the polymer in the first carrier liquid. If the polymer component is liquid, then no first carrier
liquid may be necessary.
[0058] The choice of the first carrier liquid, if present, is based on the particular
polymeric matrix material and the form in which the polymeric matrix material is to be
introduced to the dielectric composite material. If it is desired to introduce the polymeric
material as a solution, a solvent for the particular polymeric matrix material is chosen as the
carrier liquid, e.g., N-methyl pyrrolidone (NMP) would be a suitable carrier liquid for a solution
of a polyimide. If it is desired to introduce the polymeric matrix material as a dispersion, then a
suitable carrier liquid is a liquid in which the matrix material is not soluble, e.g., water would be
a suitable carrier liquid for a dispersion of PTFE particles and would be a suitable carrier liquid
for an emulsion of polyamic acid or an emulsion of butadiene monomer.
[0059] The filler component can optionally be dispersed in a suitable second carrier
liquid, or mixed with the first carrier liquid (or liquid polymer where no first carrier is used).
The second carrier liquid can be the same liquid or can be a liquid other than the first carrier
liquid that is miscible with the first carrier liquid. For example, if the first carrier liquid is water,
the second carrier liquid can comprise water or an alcohol. In an exemplary embodiment, the
second carrier liquid is water.
[0060] The filler dispersion can include a surfactant in an amount effective to modify the
surface tension of the second carrier liquid to enable the second carrier liquid to wet the
borosilicate microspheres. Exemplary surfactant compounds include ionic surfactants and
nonionic surfactants. Triton X-100®, commercially available from Rohm & Haas, has been
found to be an exemplary surfactant for use in aqueous filler dispersions. Generally, the filler
dispersion comprises from about 10 vol. % to about 70 vol. % of filler and from about 0.1 vol. %
to about 10 vol. % of surfactant, with the remainder comprising the second carrier liquid.
[0061] The mixture of the polymeric matrix material and first carrier liquid and the filler
dispersion in the second carrier liquid can be combined to form a casting mixture. In an
exemplary embodiment, the casting mixture comprises from about 10 vol. % to about 60 vol. %
of the combined polymeric matrix material and borosilicate microspheres and optional
secondary filler and from about 40 vol. % to about 90 vol. % combined first and second carrier
liquids. The relative amounts of the polymeric matrix material and the filler component in the
casting mixture are selected to provide the desired amounts in the final composition as described
below.
[0062] The viscosity of the casting mixture can be adjusted by the addition of a viscosity
modifier, selected on the basis of its compatibility in a particular carrier liquid or mixture of
carrier liquids, to retard separation, i.e. sedimentation or flotation, of the hollow sphere filler
from the dielectric composite material and to provide a dielectric composite material having a
viscosity compatible with conventional laminating equipment. Exemplary viscosity modifiers
suitable for use in aqueous casting mixtures include, e.g., polyacrylic acid compounds, vegetable
gums, and cellulose based compounds. Specific examples of suitable viscosity modifiers
include polyacrylic acid, methyl cellulose, polyethyleneoxide, guar gum, locust bean gum,
sodium carboxymethylcellulose, sodium alginate, and gum tragacanth. The viscosity of the
viscosity-adjusted casting mixture can be further increased, i.e., beyond the minimum viscosity,
on an application by application basis to adapt the dielectric composite material to the selected
laminating technique. In an exemplary embodiment, the viscosity-adjusted casting mixture
exhibits a viscosity between about 10 cp and about 100,000 cp; specifically about 100 cp and
10,000 cp. It will be appreciated by those skilled in the art that the foregoing viscosity values
are room temperature values.
[0063] Alternatively, the viscosity modifier can be omitted if the viscosity of the carrier
liquid is sufficient to provide a casting mixture that does not separate during the time period of
interest. Specifically, in the case of extremely small particles, e.g., particles having an
equivalent spherical diameter less than 0.1 micrometers, the use of a viscosity modifier may not
be necessary.
[0064] A layer of the viscosity-adjusted casting mixture can be cast on a substrate by
conventional methods, e.g., dip coating, reverse roll coating, knife-over-roll, knife-over-plate,
and metering rod coating. Examples of carrier materials can include metallic films, polymeric
films, ceramic films, and the like. Specific examples of carriers include stainless steel foil,
polyimide films, polyester films, and fluoropolymer films. Alternatively, the casting mixture
can be cast onto a glass web, or a glass web can be dip-coated.
[0065] The carrier liquid and processing aids, i.e., the surfactant and viscosity modifier,
are removed from the cast layer, for example, by evaporation and/or by thermal decomposition
in order to consolidate a dielectric layer of the polymeric matrix material and the filler
comprising the hollow microspheres.
[0066] The layer of the polymeric matrix material and filler component can be further
heated to modify the physical properties of the layer, e.g., to sinter a thermoplastic matrix
material or to cure and/or post cure a thermosetting matrix material.
[0067] In another method, a PTFE composite dielectric material can be made by the
paste extrusion and calendaring process taught in U.S. Patent 5,358,775.
[0068] Useful conductive layers for the formation of the circuit laminates, multi-layer
circuit laminates can include, without limitation, stainless steel, copper, gold, silver, aluminum,
zinc, tin, lead, transition metals, and alloys comprising at least one of the foregoing, with copper
being exemplary. There are no particular limitations regarding the thickness of the conductive
layer, nor are there any limitations as to the shape, size or texture of the surface of the
conductive layer. Preferably however, the conductive layer comprises a thickness of about 3
micrometers to about 200 micrometers, specifically about 9 micrometers to about 180
micrometers. When two or more conductive layers are present, the thickness of the two layers
can be the same or different.
[0069] In an exemplary embodiment, the conductive layer is a copper layer. Suitable
conductive layers include a thin layer of a conductive metal such as a copper foil presently used
in the formation of circuits, for example, electrodeposited copper foils.
[0070] The circuit subassemblies, e.g., laminates, can be formed by means known in the
art. In one embodiment, the lamination process entails placing one or more layers of the
dielectric composite material between one or two sheets of coated or uncoated conductive layers
(an adhesive layer can be disposed between at least one conductive layer and at least one
dielectric substrate layer) to form a circuit substrate. The conductive layer can be in direct
contact with the dielectric substrate layer or optional adhesive layer, specifically without an
intervening layer, wherein an optional adhesive layer is less than 10 percent of the thickness of
the dielectric substrate layer. The layered material can then be placed in a press, e.g., a vacuum
press, under a pressure and temperature and for duration of time suitable to bond the layers and
form a laminate. Lamination and curing can be by a one-step process, for example using a
vacuum press, or can be by a multi-step process. In an exemplary one-step process, for a PTFE
polymer matrix, the layered material is placed in a press, brought up to laminating pressure (e.g.,
about 150 to about 400 psi) and heated to laminating temperature (e.g., about 260 to about
390°C). The laminating temperature and pressure are maintained for the desired soak time, i.e.,
about 20 minutes, and thereafter cooled (while still under pressure) to below about 150°C.
[0071] In an exemplary multiple-step process suitable for thermosetting materials such
as polybutadiene and/or isoprene, a conventional peroxide cure step at temperatures of about
150°C to about 200°C is conducted, and the partially cured stack can then be subjected to a highenergy
electron beam irradiation cure (E-beam cure) or a high temperature cure step under an
inert atmosphere. Use of a two-stage cure can impart an unusually high degree of cross-linking
to the resulting laminate. The temperature used in the second stage is typically about 250°C to
about 300°C, or the decomposition temperature of the resin. This high temperature cure can be
carried out in an oven but can also be performed in a press, namely as a continuation of the
initial lamination and cure step. Particular lamination temperatures and pressures will depend
upon the particular adhesive composition and the substrate composition, and are readily
ascertainable by one of ordinary skill in the art without undue experimentation.
[0072] In accordance with an exemplary embodiment, Figure 1 shows an exemplary
circuit subassembly, in particular a single clad laminate 110 comprising a conductive metal layer
112 disposed on and in contact with a dielectric layer 114. The dielectric substrate layer 114
comprises a polymer matrix material having a particulate filler content of about 10 to about 70
volume percent, wherein the particulate filler comprises borosilicate microspheres. An optional
glass web (not shown) can be present in dielectric substrate layer 114. It is to be understood that
in all of the embodiments described herein, the various layers can fully or partially cover each
other, and additional conductive layers, patterned circuit layers, and dielectric layers can also be
present. Optional adhesive (bond ply) layers (not shown) can also be present, and can be
uncured or partially cured. Many different multi-layer circuit configurations can be formed
using the above substrates.
[0073] Another embodiment of a multilayer circuit assembly is shown at 210 in Figure
2. Double clad circuit layer 210 comprises conductive layers 212, 216 disposed on opposite
sides of a dielectric substrate layer 214 comprising borosilicate microspheres. Dielectric
substrate layer 214 can comprise a woven web (not shown).
[0074] A circuit subassembly 310 is shown in Figure 3, comprising a circuit layer 318
and a conductive layer 316 disposed on opposite sides of a dielectric substrate layer 314.
Dielectric substrate layer 314 can comprise a woven web (not shown).
[0075] Figure 4 shows an exemplary multilayer circuit assembly 410 having a first
double clad circuit 430, a second double clad circuit 440, and a bond ply 442 disposed there
between. Double clad circuit 430 comprises a dielectric substrate 434 disposed between two
conductive circuit layers 436, 438. Double clad circuit 440 comprises a dielectric substrate 444
disposed between two conductive circuit layers 446, 448. At least one, and preferably both, of
dielectric substrates 434, 444 comprises borosilicate microspheres as a filler. Each dielectric
substrate layer 434, 444 can comprise a nonwoven glass reinforcement (not shown). Two cap
layers 450, 460 are also shown. Each cap layer 450, 460, includes a conductive layer 452, 462
disposed on a bond ply layer 454, 464.
[0076] Another aspect of the present invention relates to a method of making a circuit
subassembly comprising combining a polymer matrix material and a filler component to form a
dielectric composite material treating borosilicate microspheres with an alkaline dealkylization
agent, specifically an alkaline solution to reduce the sodium content, forming a layer of the
dielectric composite material, disposing a conductive layer on the dielectric composite layer, and
laminating the dielectric composite layer and the conductive layer to form a circuit subassembly
having a dielectric constant of less than about 3.5 and a dissipation factor of less than about
0.006.
[0077] The borosilicate microspheres can be treated with an alkaline solution.
Specifically, the borosilicate microspheres can be treated with an alkaline solution to reduce the
sodium oxide content, more specifically by subjecting the borosilicate microspheres to an
aqueous alkaline solution (for example, having a pH greater than 8.0, more specifically greater
than 10.0), the method comprising effectively leaching or washing the borosilicate microspheres
for a time sufficient to reduce the level of sodium below a preselected amount. In one
embodiment, the borosilicate microspheres are treated with an alkaline solution to reduce the
total sodium oxide content (as determined by XPS surface analysis) by at least 25 wt.%, more
specifically by at least 50 wt.%, of the original amount (in the formed and recovered
microspheres), specifically to reduce the sodium content by at least 2 wt.% to a final amount less
than 5.0 wt.%, for example, 1 wt.% to 4 wt.%. According to one embodiment, the borosilicate
microspheres are washed with an alkaline dealkylization agent (as compared to an acidic
dealkylization agent) in order to reduce the sodium oxide content by such amounts. For
example, aqueous ammonium hydroxide, at a concentration effective to reduce the sodium oxide
level to a desired amount, as determined by XPS surface analysis, can be used.
[0078] The above-described dielectric compositions and methods provide a circuit
subassembly or circuit laminate with excellent properties. In one embodiment, the circuit
laminate has a dielectric constant of less than about 3.5 measured at 10 GigaHertz. In another
embodiment, the resultant circuit laminate has a dissipation factor of less than about 0.006
measured at 10 GigaHertz. In yet another embodiment, the circuit laminate has a dielectric
constant of less than about 3.5 and a dissipation factor of less than about 0.006 measured at 10
GigaHertz.
[0079] The invention is further illustrated by the following non-limiting Examples.
EXAMPLES
[0080] The materials listed in Table 1 were used in the following examples.
Specifically, the following filler materials were assembled for evaluation.
Table 1
[0081] The interior of all hollow spheres in Table 1 contain only an inert gaseous
mixture and are free of sulfur containing compounds. In addition to the assembled hollow
spheres listed in Table 1, the following materials were used in dielectric formulations.
Table 2
[0082] The synthetic microspheres are formed from sodium borosilicate glass. Once the
hollow spheres are formed, at least some of the sodium is chemically removed from the ceramic
matrix via an alkaline or acidic washing process. With regard to the commercially available
products, the acidic removal process is relatively more efficient, with resulting in virtually no
sodium, or less than 0.5 wt.%, remaining in the synthetic microspheres, as determined by surface
X-Ray photoelectron spectroscopy (XPS) analysis. The alkaline treatment, tending to be
comparatively less efficient for removing sodium, results in about 3.5% w/w sodium oxide
remaining near the surface of the microspheres, as measured using XPS analysis. Bulk
compositions are higher in sodium content. Appropriate methods for quantification of the
overall sodium content include atomic absorption spectroscopy (AAS) and inductively coupled
plasma spectroscopy (ICP).
[0083] The interior of the alkaline-washed synthetic microspheres is reported to contain
gaseous composition comprising nitrogen (53.3% v/v), oxygen (4.80% v/v), argon (0.16% v/v),
carbon dioxide (42,6% v/v), and hydrogen (0. 12% v/v), within a detectable limit of 0.01 based
on Mass Spectral Analysis.
[0084] Initial experimental trials were conducted primarily with the acid-washed
microspheres. These filler materials were substituted on an equal basis for standard cenospheres
(natural microspheres), which are used as a low-D filler in a dielectric substrate formulated by
Rogers Corporation for antenna applications. This dielectric substrate represented a challenging
comparison case due to its high loading of the low-Dk microspheres. The substrate exhibits a
nominal dielectric constant of 2.55 when tested at 10GHz in accordance with the IPC-TM-650
(X-band strip line method). Specifically, the incumbent cenosphere filler in a control
formulation E was replaced with each of the synthetic microsphere candidate materials in the
formulations A to D as detailed below:
Table 3
[0085] For testing purposes, the solids in the above formulations were dispersed in
xylene at a consistency of approximately 35 wt.% and coated onto 1080 style woven glass fabric
to a dry solids basis weight ranging from 1.7 to 2.6 g/100 cm2. Observations regarding the ease
of dispersibility and difficulties in making prepregs are reported in Table 4 below. During
manufacture of the prepregs, 8 to 12 plies were stacked to form approximately 0.060- inch thick
laminates and were laminated to copper foil in a flat-bed press for the duration of 1.17 hours at a
temperature of 475°F and a pressure of 1100 pounds per square inch to form the cured laminates.
[0086] The laminated sheets were removed from the press and the copper foil was
removed by dissolution to form test coupons. The dielectric constant and dissipation factor were
tested at 10 GHz using the clamped strip line test method described in IPC-TM-650 2.5.5.5.1.
D and Df results are summarized in Table 4 below.
Table 4
[0087] In view of the results in Table 4, none of the formulations (A-D), containing
synthetic microsphere fillers that used the acidic-wash treatment to reduce sodium were
successful from the standpoint of either dissipation factor (Df) or processing. A dissipation
factor of less than about 0.0035 at 10 GHz is desirable for high frequency applications
[0088] In comparison, synthetic microsphere fillers that received an alkaline treatment,
specifically which had been subjected to alkaline washing, yielding a different microsphere
surface (which is believed to produce a different interaction between filler and resin matrix)
were then tested. The formulations containing the alkaline-treated synthetic microspheres are
detailed in Table 5 below. The same procedures as described previously were followed.
Table 5
[0089] Observations regarding the ease of dispersibility and difficulties in making
prepregs using the formulations in Table 5 are provided in Table 6.
Table 6
[0090] As evident by the results in Table 5, the shift towards the alkaline treatment had a
significantly positive impact with regard to dissipation factor (or loss tangent) and, to a lesser
extent, prepreg processing.
[0091] Without wishing to be bound by theory, treatment with an alkaline wash can
yield improved results, specifically relative to acid wash, at least partly because of possible
surface incompatibilities between the borosilicate spheres and the resin system and/or the
presence of bound polar groups to the surface of the spheres. According to one theory, the
hydrogen protons from the acid wash (which may replace, to some extent, leached sodium atoms
in the microspheres), which hydrogen protons then reside near the surface of the leached
borosilicate microspheres, may bind or attract water molecules, the presence of which can
adversely affect loss tangent or dissipation factor, specifically higher dissipation factor over
time.
[0092] Beginning with the alkaline-treated Synthetic Microsphere 5 (composition F), the
results implied significant progress with regard to loss tangent or dissipation factor, but still
presented an issue regarding prepreg processing. The alkaline-treated Synthetic Microsphere 6
(Composition G) was then evaluated in light of the favorable processing performance noted with
the acid-treated Synthetic Microsphere 3 version. This improved the prepreg processing
compared to composition F, but still further improvement was desired. Finally, the use of
Synthetic Microspheres 7 and 8 resulted in both improved loss tangent and improved prepreg
processing.
[0093] Thus, in view of the results in Table 6, the use of alkaline-treated synthetic
microspheres provided a significant improvement with respect to dissipation factor. In addition,
a move to a relatively smaller particle size via the alkaline-treated Synthetic Microsphere 7
(Composition H) resolved processing issues. Finally, removing the epoxy functional silane
coating, as on alkaline-treated Synthetic Microsphere 8 (Composition I), which used an in-situ
silanation, was found to reduce the dissipation factor even further.
[0094] Given the successful results with the alkaline-washed Synthetic Microsphere 8
filler, laminate samples were sent out for PIM testing using a Summitek Instruments 1900B®
PIM Analyzer. The circuits were tested with new and reworked Type 1 connectors from
FIR IC (Changzhou Wujin Fengshi Communication Equipment Co.), and the results shown in
Table 7.
Table 7
[0095] In view of the results in Table 7, PIM for the candidate material Synthetic
Microsphere 8 filler were quite good, with an average value of -166 dBc, and were similar to a
PTFE control. This is a significant differentiating characteristic in favor of the synthetic
microspheres.
[0096] Ranges disclosed herein are inclusive of the recited endpoint and are
independently combinable. "Combination" is inclusive of blends, mixtures, alloys, reaction
products, and the like. Also, "combinations comprising at least one of the foregoing" means that
the list is inclusive of each element individually, as well as combinations of two or more
elements of the list, and combinations of one or more elements of the list with non-list elements.
The terms "first," "second," and so forth, herein do not denote any order, quantity, or
importance, but rather are used to distinguish one element from another. The terms "a" and "an"
herein do not denote a limitation of quantity, but rather denote the presence of at least one of the
referenced item. The modifier "about" used in connection with a quantity is inclusive of the
state value and has the meaning dictated by context, (e.g., includes the degree of error associated
with measurement of the particular quantity). In addition, it is to be understood that the
described elements can be combined in any suitable manner in the various embodiments.
[0097] All cited patents, patent applications, and other references are incorporated herein
by reference in their entirety. However, if a term in the present application contradicts or
conflicts with a term in the incorporated reference, the term from the present application takes
precedence over the conflicting term from the incorporated reference.
[0098] While the invention has been described with reference to several embodiments
thereof, it will be understood by those skilled in the art that various changes can be made and
equivalents can be substituted for elements thereof without departing from the scope of the
invention. In addition, many modifications can be made to adapt a particular situation or
material to the teachings of the invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the particular embodiments
disclosed as the best mode contemplated for carrying out this invention, but that the invention
will include all embodiments falling within the scope of the appended claims.

What is claimed is:
1. A circuit subassembly, comprising
a conductive layer disposed on a dielectric substrate layer, wherein the composition of
the dielectric substrate layer comprises, based on the volume of the dielectric substrate layer:
about 30 to about 90 volume percent of a polymer matrix material; and
about 5 to about 70 volume percent of hollow borosilicate microspheres
wherein the borosilicate microspheres are a product of a process of subjecting the
borosilicate microspheres to an alkaline solution; and wherein the dielectric substrate layer has a
dielectric constant of less than about 3.5 and a dissipation factor of less than about 0.006 at 10
GHz.
2. The circuit subassembly of claim 1, wherein the circuit subassembly has a
dissipation factor of less than about 0.0035 at 10 GHz.
3. The circuit subassembly of any one of claims 1 or 2, wherein the circuit
subassembly has a passive intermodulation (PIM) that is less than -154 dBc.
4. The circuit subassembly of any one of claims 1 to 3, wherein the mean particle
size of the microspheres is less than 70 micrometers.
5. The circuit subassembly of any one of claims 1 to 4, wherein the microspheres
further comprise an inert gas within the hollow microspheres.
6. The circuit subassembly of any one of claims 1 to 5, wherein the microspheres
have a ferric oxide content of about 0.5 weight percent, as measured by XPS surface analysis,
based on the total weight of the composition.
7. The circuit subassembly of any one of claims lto 6, wherein the microspheres
have a density of 0.20 to 0.60 g/cc.
8. The circuit subassembly of any one of claims 1 to 7, wherein the microspheres
have a sodium oxide content of not more than about 5 weight percent, as measured by XPS
surface analysis.
9. The circuit subassembly of any one of claims 1 to 8, wherein the microspheres
have a sodium oxide content of 2.5 to 4.5 weight percent, as measured by XPS surface analysis.
10. The circuit assembly of any one of claims 1 to 9, wherein the borosilicate
microspheres are a product made by a process in which treatment with the alkaline solution
reduced the sodium oxide content below a preselected amount.
11. The circuit assembly of claim 10, wherein the borosilicate microspheres are a
product of a process comprising washing the microspheres in a strong aqueous alkaline solution
until the measured sodium oxide content is reduced by at least 25 wt.% of the original amount.
12. The circuit assembly of claim 8, wherein the borosilicate microspheres are a
product of a process in which the content of sodium oxide is reduced by at least 2 wt.% as
measured by XPS surface analysis.
13. The circuit assembly of any one of claims 1 to 12, wherein the borosilicate
microspheres are a product of a process that does not comprise washing or leaching the
microspheres with an acidic solution to obtain the final sodium oxide content of the
microspheres.
14. The circuit subassembly of any one of claims 1 to 13, wherein the borosilicate
microspheres have a median particle diameter of 20 to 100 micrometers.
15. The circuit subassembly of any one of claims 1 to 14, wherein the dielectric
substrate layer comprises one or more additional fillers, in an amount from 20 to 80 vol. %,
based on the total volume of the filler component.
16. The circuit subassembly of claim 15, wherein additional filler is silica, fused
amorphous silica, or a combination thereof.
1 The circuit subassembly of any one of claims 1 to 16, wherein the polymer matrix
material comprises 1,2-polybutadiene, polyisoprene, polyetherimide, a fluoropolymer,
polytetrafluoroethylene, polyphenylene ether, polyimide, polyetheretherketone, polyamidimide,
polyethylene terephthalate, polyethylene naphthalate, polycyclohexylene terephthalate, or a
combination comprising at least one of the foregoing.
18. The circuit subassembly of any one of claims 1 to 17, wherein the polymer matrix
material is polytetrafluoroethylene.
19. The circuit subassembly of any one of claims 1 to 17, wherein the polymer matrix
material comprises 1,2-polybutadiene, polyisoprene, or a combination of 1,2-polybutadiene and
polyisoprene.
20. The circuit subassembly of any one of claims 1 to 17, wherein the polymer matrix
material comprises poly(arylene ether).
2 1. The circuit subassembly of claim 19, wherein the polybutadiene or polyisoprene
polymer is carboxy-functionalized, and comprises butadiene, isoprene, or butadiene and
isoprene, and less than 50 weight percent of a co-curable monomer.
22. The circuit subassembly of any one of claims 1 to 21, further comprising a
second conductive layer disposed on a side of the dielectric substrate layer opposite a first said
conductive layer.
23. The circuit subassembly of any one of claims 1 to 22, wherein the conductive
layer is a copper foil.
24. The circuit subassembly of any one of claims 1 to 23, wherein the conductive
layer is etched to provide a circuit.
25. The circuit subassembly of any one of claims 1 to 24, wherein the conductive
layer is in direct contact with the dielectric substrate layer or optional adhesive layer, without an
intervening layer, wherein an optional adhesive layer is less than 10 percent of the thickness of
the dielectric substrate layer.
26. The circuit subassembly of any one of claims 1 to 25, wherein a bond ply is
disposed between and in adjacent contact with two patterned conductive layers, wherein each
conductive layer is attached to a dielectric layer.
27. A circuit comprising the circuit subassembly of any one of claims 1 to 26.
28. A multilayer circuit comprising the circuit subassembly of any one of claims 1 to
26.
29. The multilayer circuit of claim 28, wherein the circuit subassembly is used in an
antenna.
30. A method of making a circuit subassembly, the method comprising:
treating hollow borosilicate microspheres with an alkaline solution;
combining the hollow borosilicate microspheres with a polymer matrix material to form
a dielectric composite material;
forming a layer of the dielectric composite material, thereby obtaining a dielectric
substrate layer;
disposing a conductive layer on the dielectric substrate layer; and
laminating the dielectric substrate layer and the conductive layer, wherein the dielectric
substrate layer exhibits a dielectric constant of less than about 3.5 and a dissipation factor of less
than about 0.006 at 10 GHz.
31. The method of claim 30, wherein treating the hollow borosilicate microsphere
with an alkaline solution comprises washing the borosilicate microspheres with an aqueous
alkaline solution to effectively reduce the sodium oxide content of the borosilicate microspheres
to a preselected amount, as determined by XPS surface analysis.
32. The method of claim 31, wherein treating the borosilicate microspheres with an
alkaline solution comprises reducing the sodium oxide content by at least 25 wt.% of the
original amount, as determined by XPS surface analysis.
33. The method of any one of claims 30 to 32, wherein treating the borosilicate
microspheres with an alkaline solution comprises reducing the sodium content by at least 1
wt.%, as determined by XPS surface analysis.
34. The method of any one of claims 30 to 33, wherein the borosilicate microspheres
are not washed with a strong acidic solution to effectively lower and obtain a final sodium oxide
content in the borosilicate microspheres, as determined by XPS surface analysis.