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United States Patent |
6,159,264
|
Holl
|
December 12, 2000
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Composites of powdered fillers and polymer matrix
Abstract
Composite materials comprising at least 60 volume %, preferably 70 volume
%, of particles of finely powdered filler material in a matrix of
poly(arylene ether) polymer material are made by forming a mixture of the
components, forming the required bodies therefrom, and then heating and
pressing the bodies to a temperature sufficient to melt the polymer and to
a pressure sufficient to disperse the melted polymer into the interstices
between the filler particles. Surprisingly these polymer materials can
only be effective as bonding materials when the solids content is as high
as that specified, since with lower contents the resultant bodies are too
friable. This is completely contrary to accepted prior art practice which
considers that composites are progressivly weakened as the solids content
is increased, so that such content must be limited. In processes to obtain
as complete a dispersion of the components as possible they are
individually dispersed in a liquid dispersion medium containing the
polymer together with necessary additives, each mixture being ground if
required to obtain a desired particle size, the mixtures are mixed, again
ground to produce thorough dispersion, are separated from the liquid
dispersion medium and green articles formed from the resulting pasty
mixture. The green articles are then heated and pressed as described
above. Mixtures of different filler materials may be used to tailor the
electrical and physical properties of the final materials. The articles
preferably comprise substrates for use in electronic circuits.
Inventors:
|
Holl; Richard A. (Oxnard, CA)
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Assignee:
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Holl Technologies Company (Camarillo, CA)
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Appl. No.:
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345813 |
Filed:
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July 2, 1999 |
Intern'l Class: |
C22C 001/05 |
Field of Search: |
419/5,10
75/230
428/548
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References Cited
U.S. Patent Documents
5279463 | Feb., 1994 | Holl.
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5538191 | Jul., 1996 | Holl.
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5658994 | Aug., 1997 | Burgoyne, Jr. et el.
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5874516 | Feb., 1999 | Burgoyne, Jr. et al.
| |
Other References
P. 42 of "Materials and Processes for Microwave Hybrids ", by Richard
Brown, published 1989 by International Society for Hybrid Microelectronis,
Reston VA ( See p. 2, lines of this application)
|
Primary Examiner: Mai; Ngoclan
Attorney, Agent or Firm: Darrow; Christopher
Claims
I claim:
1. A method of manufacturing composite materials comprising particles of
finely powdered filler material uniformly distributed in a matrix of
polymer material, the method comprising the steps of:
mixing together from 60 to 97 volume percent of particles of the filler
material and the balance non-polar polymer bonding material consisting of
nonfunctionalized poly(arylene ether) to form a composite mixture; and
subjecting the composite mixture to a temperature sufficient to melt the
polymer material and to a pressure sufficient to uniformly disperse the
melted polymer material into the interstices between the particles of
filler material.
2. A method as claimed in claim 1, wherein the polymer material is selected
from the group consisting of polyarylene ether-2, polyarylene ether-3, and
polyarylene ether-4.
3. A method as claimed in claim 2, wherein the polymer is heated to a
temperature in the range 350-450.degree. C. to obtain cross-linking
thereof.
4. A method as claimed in claim 2, wherein the composite mixture includes a
cross-linking agent and/or an end capping agent.
5. A method as claimed in claim 1, wherein the filler material is selected
from the group consisting of particles of inorganic material, particles of
electromagnetic material, particles of a core of inorganic material
covered with a layer of a metal oxide material, particles of metal
material, and particles of low dielectric constant high melting point
polymer material, all of which particles may be hollow.
6. A method as claimed in claim 1, wherein the composite mixture is heated
to a temperature in the range 280-400.degree. C. at a pressure in the
range 3.5 to 1,380 MPa (500 to 200,000 psi).
7. A method as claimed in claim 1, comprising also the steps of:
mixing together the particles of filler material in finely powdered form,
the polymer, and a liquid dispersion medium to form a flowable composite
mixture thereof;
grinding the flowable composite mixture to uniformly disperse the particles
of the finely powdered materials in the liquid dispersion medium;
removing liquid dispersion medium from the flowable composite mixture to
produce a pasty composite mixture and forming articles from the composite
pasty mixture; and
subjecting the articles to the specified temperature and pressure.
8. A method as claimed in claim 7, wherein each of the polymer and the
filler material are mixed separately with the liquid respective dispersion
medium, and are mixed in respective drum type grinding apparatus as
disclosed in U.S. Pat. Nos. 5,279,463 and 5,538,191 to provide uniform
dispersion of the components.
9. A method as claimed in claim 7, wherein the composite mixture is mixed
in at least one drum type grinding apparatus as disclosed in U.S. Pat.
Nos. 5,279,463 and 5,538,191 to provide uniform dispersion of the
components.
10. A method as claimed in claim 1, wherein the particles of filler
material are of size in the range 0.1 to 50 micrometers and consist of a
mixture of filler materials of different chemical compositions.
11. A method as claimed in claim 1, and including the step of forming the
heated and pressurized composite mixture into a sheet, film or tape.
12. A method as claimed in claim 11, and including the step of applying a
layer of copper to a surface of the sheet, film or tape by a process
selected from sputtering and direct bonding of copper foil under heat and
pressure in a vacuum.
13. A method as claimed in claim 11, wherein the sheet, film or tape has a
thickness less than about 60 mil.
14. A method as claimed in claim 1, and including the step of applying a
layer of copper to a surface of the heated and pressurized composite
mixture by a process selected from sputtering and direct bonding of copper
foil.
15. A method as claimed in claim 1, and including the step of forming
substrates for electronic circuits from the heated and pressurized
composite mixture.
16. A method as claimed in claim 1, and including the step of enclosing
electronic circuits or devices with the heated and pressurized composite
mixture.
17. A method as claimed in claim 6, wherein the composite mixture is heated
while at a pressure in the range 70 to 1,380 MPa (10,000 to 200,000 psi).
18. A method as claimed in claim 13, wherein the sheet, film or tape has a
thickness less than about 30 mil.
19. A method as claimed in claim 13, wherein the sheet, film or tape has a
thickness less than about 10 mil.
20. A method as claimed in claim 13, wherein the sheet, film or tape has a
thickness less than about 4 mil.
21. A method as claimed in claim 13, wherein the sheet, film or tape has a
thickness less than about 1 mil.
22. A method as claimed in any one of claims 5, 6, 7, 9, 10, 15 or 16,
wherein the polymer material is selected from the group consisting of
polyarylene ether-2, polyarylene ether-3, and polyarylene ether-4.
23. A method as claimed in any one of claims 5, 6, 7, 9, 10, 15 or 16,
wherein the polymer is heated to a temperature in the range
350-450.degree. C. to obtain cross-linking thereof.
24. A method as claimed in any one of claims 6, 7, 9, 10, 15 or 16, wherein
the filler material is selected from the group consisting of particles of
inorganic material, particles of electromagnetic material, particles of a
core of inorganic material covered with a layer of a metal oxide material,
particles of metal material, and particles of low dielectric constant high
melting point polymer material, all of which particles may be hollow.
25. A method as claimed in any one of claims 7, 9, 10, 15 or 16, wherein
the composite mixture is heated to a temperature in the range
280-400.degree. C. at a pressure in the range 3.5 to 1,380 MPa (500 to
200,000 psi).
26. A method as claimed in any one of claims 9, 10, 15 or 16, and
comprising also the steps of:
mixing together the particles of filler material in finely powdered form,
the polymer, and a liquid dispersion medium to form a flowable composite
mixture thereof;
grinding the flowable composite mixture to uniformly disperse the particles
of the finely powdered materials in the liquid dispersion medium;
removing liquid dispersion medium from the flowable composite mixture to
produce a pasty composite mixture and forming articles from the composite
pasty mixture; and
subjecting the articles to the specified temperature and pressure.
27. A method as claimed in any one of claims 10, 15 or 16, wherein the
composite mixture is mixed in at least one drum type grinding apparatus as
disclosed in U.S. Pat. Nos. 5,279,463 and 5,538,191 to provide uniform
dispersion of the components.
28. A method as claimed in claim 15 or 16, wherein the particles of filler
material are of size in the range 0.1 to 50 micrometers and consist of a
mixture of filler materials of different chemical compositions.
29. A method as claimed in any one of claims 1, 2, 3, 5, 6, 7, 9, 10, 15 or
16, wherein the resulting heated and pressed composite material is ground
back down to become again finely divided particles of the filler material
intimately mixed with the polymer material, and the resultant finely
divided particles are again subjected to a heating and pressing operation
to produce a solid body of the composite material.
30. Composite materials comprising particles of finely powdered filler
material uniformly distributed in a matrix of polymer material, the
materials comprising:
from 15 to 97 volume percent of particles of the filler material and the
balance non-polar polymer material consisting of nonfunctionalized
poly(arylene ether) together forming a composite mixture;
wherein the composite mixture has been subjected to a temperature
sufficient to melt the polymer material and to a pressure sufficient to
uniformly disperse the melted polymer material into the interstices
between the particles of filler material.
31. Materials as claimed in claim 30, wherein the polymer material is
selected from the group consisting of polyarylene ether-2, polyarylene
ether-3, and polyarylene ether-4.
32. Materials as claimed in claim 30, wherein the filler material is
selected from the group consisting of particles of inorganic material,
particles of electromagnetic material, particles of a core of inorganic
material covered with a layer of a metal oxide material, particles of
metal material, and particles of low dielectric constant high melting
point polymer material, all of which particles may be hollow.
33. Materials as claimed in claim 30, wherein the particles of filler
material are of size in the range 0.1 to 50 micrometers and consist of a
mixture of filler materials of different chemical compositions.
34. Materials as claimed in claim 30, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about
60 mil.
35. Materials as claimed in claim 34, and comprising a layer of copper
applied to a surface of the sheet, film or tape by a process selected from
sputtering and direct bonding of copper foil under heat and pressure in a
vacuum.
36. Materials as claimed in claim 30, and having a layer of copper applied
to a surface by a process selected from sputtering and direct bonding of
copper foil.
37. Materials as claimed in claim 30, and comprising substrates for
electronic circuits formed from the heated and pressurized composite
mixture.
38. Materials as claimed in claim 30, and comprising electronic circuits or
devices enclosed with the composite mixture.
39. Materials as claimed in claim 34, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about
30 mil.
40. Materials as claimed in claim 34, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about
10 mil.
41. Materials as claimed in claim 34, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about 4
mil.
42. Materials as claimed in claim 34, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about 1
mil.
43. Materials as claimed in any one of claims 32, 33, 34, 37 or 38 wherein
the polymer material is selected from the group consisting of polyarylene
ether-2, polyarylene ether-3, and polyarylene ether-4.
44. Materials as claimed in any one of claims 33, 34, 37 or 38 wherein the
filler material is selected from the group consisting of particles of
inorganic material, particles of electromagnetic material, particles of a
core of inorganic material covered with a layer of a metal oxide material,
particles of metal material, and particles of low dielectric constant high
melting point polymer material, all of which particles may be hollow.
45. Materials as claimed in any one of claims 34, 37 or 38, wherein the
particles of filler material are of size in the range 0.1 to 50
micrometers and consist of a mixture of filler materials of different
chemical compositions.
46. Materials as claimed in claim 37, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about
60 mil.
47. Materials as claimed in claim 37, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about
30 mil.
48. Materials as claimed in claim 37, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about
10 mil.
49. Materials as claimed in claim 37, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about 4
mil.
50. Materials as claimed in claim 37, and having the form of a sheet, film
or tape, wherein the sheet, film or tape has a thickness less than about 1
mil.
Description
TECHNICAL FIELD
The invention is concerned with methods for the manufacture of composite
materials consisting of particles of finely powdered filler material
bonded in a matrix of polymer material, and new composite materials made
by such methods.
BACKGROUND ART
The electronics industry is an example of one which makes substantial use
of printed wiring boards and substrates as supports and dielectric
participants for electronic circuits, such substrates consisting of thin
flat pieces produced to exacting specifications as to starting material
and physical and electrical properties. The history of the industry shows
the use of progressively higher operating frequencies and currently for
frequencies up to about 800 megahertz (MHz) copper coated circuit boards
of glass fiber reinforced polymers, such as epoxies, cyanide esters,
polytetrafluoroethylene (PTFE) and polyimides, have been and are still
used. At present one popular laminate material for such applications is
FR-4, consisting of epoxy resin deposited on a woven glass fabric, because
of its ease of manufacture and low cost. Typically this material has a
dielectric constant of 4.3-4.6 and a dissipation factor of 0.016-0.022 and
is frequently used in computer related applications below about 500 MHz
frequencies. Mobile telephones now operate at frequencies of 1-40 GHz and
some computers already at 0.5 GHz, with the prospect of higher frequencies
in the future. The lowest possible value of dielectric constant is
preferred in computer applications to improve signal speed. At higher
operating frequencies above approximately 0.8 GHz, FR-4 and similar
materials are materials, despite their low cost, are no longer entirely
suitable, primarily because of unacceptable dielectric losses, heating up,
lack of sufficient uniformity, unacceptable anisotropy, unacceptable
mismatch of thermal expansion between the dielectric material and its
metallization, and anisotropic thermal expansion problems as the operating
temperatures of the substrates fluctuate. These thermal expansion problems
result from the relatively large coefficients of thermal expansion of the
polymers used as substrate material, and the unequal expansion
coefficients of reinforcing fibers in their length and thickness
dimensions. For frequencies above 800 MHz the dielectric material of the
substrates become an active capacitative participant in signal propagation
and here the current materials of choice are certain ceramics formed by
sintering or firing suitable powdered inorganic materials, such as fused
silica; alumina; aluminum nitride; boron nitride; barium titanate; barium
titanate complexes such as Ba(Mg.sub.1/3 Ti.sub.2/3)O.sub.2,
Ba(Zr,Sn)TiO.sub.4, and BaTiO.sub.3 doped with Sc.sub.2 O.sub.3 and rare
earth oxides; alkoxide-derived SrZrO.sub.3 and SrTiO.sub.3 ; and
pyrochlore structured Ba(Zr,Nb) oxides. Substrates have also been employed
consisting of metal powders, and semiconductor powders embedded in a glass
or polymer matrix, a particular preferred family of polymers being those
based on PTFE.
For example, ceramic substrates that have been used for hybrid electronic
circuit applications comprise square plates of 5 cm (2 ins) side, their
production usually involving the preparation of a "slip" (slurry) of the
finely powdered materials dispersed in a liquid vehicle, dewatering the
slip to a stiff leathery mixture, making a "green" preform from the
mixture, and then sintering the preform to become the final substrate
plate. The substrates are required to have highly uniform values of
thickness, grain size, grain structure, density, surface flatness and
surface finish, with the purpose of obtaining uniform dielectric, thermal
and chemical properties, and also to permit the uniform application to the
surfaces of fine lines of conductive or resistive metals or inks.
Such sintered products inherently contain a number of special and very
characteristic types of flaws. A first consists of fine holes created by
the entrainment of bubbles in the ceramic pre-casting slip of sizes in the
range about 1-20 micrometers; these bubbles cannot be removed from the
slip by known methods and cause residual porosity in the body. As an
example, sintered alumina substrates may have as many as 800 residual
bubble holes per sq/cm of surface (5,000 per sq/in). Another flaw is
triple-point holes at the junctions of the ceramic granules when the
substrate has been formed by roll-compacting of spray-dried powder; they
are of similar size to the bubble holes and appear in similar numbers per
sq/cm. Yet another consists of "knit-lines", which are webs or networks of
seam lines of lower density formed at the contact areas between butting
particles during cold pressing. Two other common flaws are caused by
inclusions of foreign matter into the material during processing, and the
enlarged grains caused by agglomeration of the particles despite their
initial fine grinding. The usual inclusions are fine particles due to
abrasive wear of the grinding media in the mills. Both inclusions and
agglomerates will sinter in a matrix at a different rate from the
remainder of the matrix and can result in flaws of much greater magnitude
than the original inclusion or agglomerate.
Costly mirror-finishing by diamond machining and lapping of the ceramic
surfaces is required to allow the accurate deposition of sputtered
metallization layers from which conductor lines are formed by etching.
Mirror-finishes are required because the electrical currents at
frequencies above 0.8 GHz move predominantly in the skin of a conductor
line while in the lower frequencies they occupy the entire crossection of
the conductor line. The thickness of the skin through which currents move
at GHz frquencies becomes thinner as frequencies rise and are already as
thin as 1 to 2 micrometers in copper at around 2 GHz. Any surface
roughness of the conductors on the top and bottom sides will therefor
contribute to considerable conductive losses. For example, at a frequency
of 4 GHz, the conductive loss at of the interface between conductor and
substrate is 1.65 times higher at a RMS roughness of 40 compared to a RMS
roughness of 5 (See P.42 of Materials and Processes for Microwave Hybrids,
Richard Brown, published 1989 by the International Society for Hybrid
Microelectronics of Reston, Va.)
There is therefore continuing interest in methods for manufacturing
composite materials for the production of electronic substrates and for
use as electronic packaging materials, with which sintering and the high
processing temperatures required together with their attendant
difficulties, high cost of diamond machining and lapping, and the
associated considerable costs are avoided.
The low inherent mechanical strength of the currently available matrix
forming polymers and their excessive thermal expansion coefficient has
made it necessary to embed reinforcing materials, such as woven glass
fiber cloth, into the substrate body, to strengthen it and also to
contrain its excessive thermal expansion. But such reinforcing materials
unfortunately cause unacceptable inhomogenity of the structure. For
example, the presence of such reinforcing material makes it difficult to
incorporate powdered filler materials uniformly into the body of the
substrate. Another difficulty is caused by the generally poor adhesion
exhibited by the commercially available matrix polymers toward the usual
filler materials, and extensive research and development has been
undertaken in the past, and is continuing, in connection with known
substrate-forming polymers, such as PTFE, to find coupling agents that
will provide adequate adhesion between the polymer and the powder
components, and thus satisfactory mechanical strength in the resultant
substrates.
Dielectric materials are commonly used as insulating layers between
circuits, and layers of circuits in multilayer integrated circuits, the
most commonly used of which is silica, which in its various modifications
has dielectric constants of the order of 3.0-5.0, more usually 4.0-4.5.
Low values of dielectric constant are preferred and organic polymers
inherently usually have low dielectric values in the range 1.9-3.5, so
that considerable research and work has been done to try to develop
polymers suitable for this special purpose, among which are polyimides
(frequently fluorinated), PTFE, and fluorinated poly(arylene ethers), some
of the materials having dielectric constants as low as that of air, i.e.
1.00. At the present time fluorination is the most common modification of
the polymers employed in view of the improvements obtained comprising
lowered dielectric constants, enhanced optical transparency, and reduced
hydrophilicity and solubility in organic solvents, but the fluorination
usually results in the polymers exhibiting a degree of polarization which
can cause problems in obtaining the desired dielectric properties.
U.S. Pat. No. 5,658,994, issued Aug. 19, 1997, and U.S. Pat. No. 5,874,516,
issued Feb. 23, 1999, both to Air Products and Chemicals, Inc. of
Allentown, Pa., the disclosures of which are incorporated herein by this
reference, disclose and claim a unique utility as a dielectric coating
material for micro-electronic devices of a class of poly(arylene ethers)
as a replacement for silica-based dielectric materials, wherein the
poly(arylene ether) does not have nonaromatic carbons (other than
perphenylated carbon), fluorinated substituents or significantly
polarizable functional groups. These materials, which are relatively
easily synthesized, are found surprisingly to have an excellent
combination of desirable properties, namely thermal stability, low
dielectric constant values, low moisture absorption and low moisture
outgassing.
U.S. Pat. No. 5,658,994 discloses and claims in its broadest aspect an
article of manufacture comprising a combination of a dielectric material
and a microelectronic device, wherein the dielectric material is provided
on the microelectronic device and contains a poly(arylene ether) polymer
consisting essentially of non-functional repeating units of the structure:
--{--O--Ar.sub.1 --O--Ar.sub.2 --}.sub.m --{--O--Ar.sub.3 --O--Ar.sub.4
--}.sub.n --
wherein m=0 to 1.0; and n=1.0-m; and Ar.sub.1, Ar.sub.2, Ar.sub.3 and
Ar.sub.4 are individually divalent arylene radicals selected from the
group consisting of: phenylene; biphenyl diradical; para-terphenyl
diradical; meta-terphenyl diradical; ortho-terphenyl diradical;
naphthalene diradical; anthracene diradical; phenanthrene diradical;
diradicals of 9,9-diphenylfluorene of specific type; and 4,4'-diradical of
dibenzofuran and mixtures thereof, but Ar.sub.1, Ar.sub.2, Ar.sub.3, and
Ar.sub.4, other than the diradical 9,9-diphenylfluorene, are not isomeric
equivalents.
U.S. Pat. No. 5,874,516 claims poly(arylene ether) consisting essentially
of non-functional repeating units of the structure:
--{--O--Ar.sub.x --O--Ar.sub.1 --}.sub.m --{--O--Ar.sub.2 --O--Ar.sub.3
--}.sub.n --
wherein m=0.2 to 1.0; and n=1.0-m; and Ar.sub.1, Ar.sub.2, and Ar.sub.3 are
individually divalent radicals selected from the group defined in the
preceding paragraph; or essentially of non-functional repeating units of
the structure:
{--O--Ar.sub.x --O--Ar.sub.1 --}.sub.m --{--O--Ar.sub.x --O--Ar.sub.3
--}.sub.n --
wherein m=0 to 1.0; and n=1.0--m; Ar.sub.x is a special radical
9,9-bis(4-oxyphenyl)fluorene and Ar.sub.1, and Ar.sub.3 are individually
divalent radicals also selected from the group defined in the immediately
preceding paragraph.
Variations in Ar.sub.1, Ar.sub.2, Ar.sub.3 and Ar.sub.4 are stated to allow
access to a variety of properties such as reduction or elimination of
crystallinity, modulus, tensile strength, high glass transition
temperature, etc. The polymers are said to be essentially chemically
inert, have low polarity, have no additional functional or reactive
groups, and to be thermally stable at temperatures of
400.degree.-450.degree. C. in inert atmospheres. In addition to the basic
polymer structures as outlined above the polymers may also be
cross-linked, either by cross-linking itself, through exposure to
temperatures in the range of 350.degree.-450.degree. C., or by providing a
cross-linking agent, as well as end capping the polymer with known end
capping agents, such as phenylethynyl, benzocyclobutene, ethynyl and
nitrile. The ability to crosslink at elevated temperatures, with the
consequent marked increase in molecular weight and density makes the
materials particularly useful in microelectronic applications because they
can readily be applied from solution and then converted to a solvent
resistant coating by heating.
The specified polymers are non-functional in that they are chemically inert
and they do not bear any functional groups that are detrimental to their
application in the fabrication of microelectronic devices. They do not
have carbonyl moieties such as amide, imide and ketone, which promote
adsorption of water. They do not bear halogens such as fluorine, chlorine,
bromine and iodine which can react with metal sources in metal deposition
processes. They are composed essentially of aromatic carbons, except for
the bridging carbon in the 9,9-fluorenylidene group, which has much of the
character of aromatic carbons due to its proximity to aromatic structures;
for the purposes of the invention the carbon is deemed to be a
perphenylated carbon.
The polymers are proposed for use as coatings, layers, encapsulants,
barrier regions or barrier layers or substrates in microelectronic
devices. These devices may include, but are not limited to multichip
modules, integrated circuits, conductive layers in integrated circuits,
conductors in circuit patterns of an integrated circuit, circuit boards as
well as similar or analogous electronic structures requiring insulating or
dielectric regions or layers. They are also proposed for use as a
substrate (dielectric material) in circuit boards or printed wiring
boards. Such a circuit board has mounted on its surface patterns for
various electrical conductor circuits, and may include various
reinforcements, such as woven nonconducting fibers, such as glass cloth.
Such circuit boards may be single sided as well as double sided or
multilayer.
It is proposed that additives can be used to impart particular target
properties, as is conventionally known in the polymer art, including
stabilizers, flame retardants, pigments, plasticizers, surfactants, and
the like. It is also proposed that adhesion promoters can be used to
adhere the polymers to the appropriate substrates. Such promoters are
typified by hexamethyldisilazane, which can be used to interact with
available hydroxyl functionality that may be present on a surface, such as
a silica surface.
DISCLOSURE OF THE INVENTION
The principal object of the invention is to provide new methods for
manufacturing composite materials consisting of particles of finely
powdered filler material bonded together in a matrix of polymer material,
such new composite materials, and articles made from such composite
materials.
It is another object to provide such new methods with which the resultant
composite materials and articles comprises at least 60 percent by volume
of the filler material, with the remainder consisting of the polymer
material matrix together with any necessary additives.
In accordance with the invention there is provided a method of
manufacturing composite materials comprising particles of finely powdered
filler material uniformly distributed in a matrix of polymer material, the
method comprising the steps of:
mixing together from 60 to 97 volume percent of particles of the filler
material of minimum pore volume when compacted and the balance of polymer
bonding material consisting of nonfunctionalized poly(arylene ether) to
form a composite mixture; and
subjecting the composite mixture to a temperature sufficient to melt the
polymer material and to a pressure sufficient to uniformly disperse the
melted polymer material into the interstices between the particles of
filler material.
Also in accordance with the invention there are provided composite
materials comprising particles of finely powdered filler material
uniformly distributed in a matrix of polymer material, the materials
comprising:
from 60 to 97 volume percent of particles of the compacted filler material
and the balance of polymer material consisting of nonfunctionalized
poly(arylene ether) together forming a uniform composite mixture;
wherein the composite mixture has been subjected to a temperature
sufficient to melt the polymer material and to a pressure sufficient to
uniformly disperse the melted polymer material into the interstices
between the particles of filler material.
Preferably the polymer material is of maximum dimension or maximum
equivalent spherical dimension of 50 .mu.m.
DESCRIPTION OF THE DRAWINGS
Methods and apparatus for the production of the new composite materials,
and new composite materials and articles made of such new composite
materials, produced using such methods and apparatus, that are particular
preferred embodiments of the invention will now be described, by way of
example, with reference to the accompanying diagrammatic drawings wherein:
FIG. 1 is the first part of a block flow diagram of the specific method and
apparatus for the manufacture of the composite materials and articles of
the invention, particularly for the manufacture of flat rectangular copper
clad substrates intended for use for electronic circuits;
FIG. 2 is side elevation of a mixer/solvent evaporation mill shown in
outline in FIG. 1;
FIG. 3 is a cross-section through the mill of FIG. 2, taken on the line
A--A therein;
FIG. 4 is another part of the block flow diagram, continuing from FIG. 1;
FIG. 5 is a further part of the block flow diagram, continuing from FIG. 4;
and
FIGS. 6 and 7 are respective part cross sections to a greatly enlarged
scale through a small piece of a typical material of the invention in
order to show the grain structure thereof, and showing respectively a
layer of metal in position to be applied to a surface, and applied to the
surface.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
I have discovered that unexpectedly a particular sub-family of a known
family of polymers, namely poly(arylene ethers), exhibit unusually high
inherent adhesiveness toward finely ground filler materials of the kind
that can be employed in combination with matrix materials to produce
electronic substrates and that, also unexpectedly, the production of
useful composite materials requires a complete reversal of approach from
that which has previously been employed in the production of composite
materials. A major problem in the prior art processes of forming composite
materials, and in the substrates obtained thereby, is the progressive loss
of mechanical strength that results as the filler solids content is
increased, and hitherto attempts to incorporate more than about 40 volume
percent generally has resulted in composites which are so friable that
they literally collapse to a heap of sand-like material if in testing they
are stressed to the degree required in commercial practice. Moreover, it
has been found difficult with prior art processes to incorporate as much
as 40 volume percent solids material, since the mixtures become so viscous
that uniform mixing is virtually impossible. Consequently, the approach
has of necessity been to incorporate only as much filler material as will
result in a substrate of adequate mechanical strength, and to accept the
lower desired electrical characteristics that result. I have discovered
however that with the methods of the invention, for the successful
production of composite materials, the solids content must instead be
increased to values well beyond those of the conventional prior art. An
acceptable minimum for my new composite materials is 60 volume percent, in
that such materials are of the required minimum mechanical strength, it
being found that the mechanical strength increases with increased solids
content, instead of decreasing, up to the value of about 95-97 volume
percent, or beyond which the proportion of polymer is reduced below the
minimum value required to maintain adequate adhesion between the uniformly
distributed filler particles. It is my belief that a possible explanation
for this highly unexpected result, although other explanations may be
possible, and therefore I do not intend that the invention be limited
thereby, is that although the chosen polymers exhibit unusually high
adhesion, especially toward oxide materials such as silica, aluminum
oxide, metal powders and boron nitride, they are not particularly
mechanically strong, and therefore are most effective in this new and
special application if employed in the form of very thin adherent layers
interposed between the filler particles, such as can only be obtained with
the methods of the invention and when the solids content is sufficiently
high. It is difficult to specify with any degree of accuracy the optimum
thicknesses for the interposed layers; it is known that layers of 1-3
micrometers are very successful in giving superior adhesion with adequate
strength, and a possible upper limit is 40 micrometers (0.001 in).
Composite materials of the invention can be made by mixing together the
required portion by weight, or by volume, of particles of the chosen
non-polar, nonfunctionalized polymer material of sufficiently small
dimension, or equivalent spherical dimension, e.g. in the range 0.1 to 50
micrometers, with the corresponding portion by weight or by volume of the
chosen filler material, again of sufficiently small dimension, or
equivalent spherical dimension, e.g. in the range 0.1 to 50 micrometers,
and subjecting the mixture to a temperature sufficient to melt the polymer
material, e.g. in the range 280-400.degree. C. and to a pressure, e.g. in
the range 3.5 to 1,380 MPa (500 to 200,000 psi), preferably 70 to 1,380
MPa (10,000 to 200,00 psi), sufficient to disperse the melted polymer
material into the interstices between the particles of filler material. By
equivalent spherical diameter is meant the diameter of a completely
spherical particle having the same volume as the specified particle. In
alternative processes which are described in more detail below the polymer
may be added in the form of a solution thereof, provided steps are taken
to remove all of the solvent once the filler and polymer materials have
been uniformly mixed together. The polymer material preferably is selected
from the group comprising polyarylene ether-2, polyarylene ether-3, and
polyarylene ether-4, which materials are described in more detail below,
while the filler material is selected from the group comprising particles
of inorganic material, particles of electromagnetic material, particles of
a core of inorganic material, covered with a layer of a metal oxide
material, particles of metal material particles of magnetic material, and
particles of low dielectric constant high melting point polymer material,
all of which particles may be hollow.
The resultant heated and pressurized composite mixture may be formed into a
sheet, film or tape, onto a surface of which a layer of copper may be
applied, either by sputtering or by direct bonding of copper foil under
heat and pressure in a vacuum, the sheet, film or tape being formed by a
thermoplastic extrusion process. Alternatively, green bodies can be cut
from sheet or tape before the heat and pressure step and these green
bodies then converted to heated and pressed bodies by a thermoplastic
compression process, again to a surface of which a layer of copper can be
applied by sputtering or by direct bonding of copper foil under heat and
pressure in a vacuum. The resultant bodies may comprise substrates for
electronic circuits or enclosures for electronic circuits or devices. The
processes of the invention will be described in detail below in connection
with the manufacture of such thin flat plates, but it will be apparent
that they are applicable also to any shape of molded article with which
direct production of superior surface finish, highly uniform
micro-structures, and high dimensional uniformity from finished article to
article is desired.
With microelectronic devices, and with the higher frequencies now employed,
the problems of adequate uniformity of physical and chemical constitution
and physical and electrical properties of the substrates have been
exacerbated, and the simple mixing processes described above usually will
not provide sufficient uniformity, and in such case it becomes necessary
to employ a method and apparatus as described in detail below.
Referring now to FIG. 1, in this particular process it is assumed that a
mixture of different filler materials are to be used, especially in view
of the opportunity this provides of specifically tailoring the mechanical
and electrical characteristics of the resultant substrates for the end
product. The polymer is used in the form of a solution thereof (usually of
about 10% concentration) in a suitable solvent such as cyclohexanone, and
the opportunity is taken of employing this solvent also as a liquid
dispersion and suspension vehicle for the filler materials. A preliminary
mixture is first formed of each of the selected finely powdered filler
materials, usually inorganic materials, with the selected polymer
solution, although in other processes other vehicles may of course also be
used. The filler material or mixture of materials may be obtained
respectively by precipitation or coprecipitation from solutions of
suitable precursors, and however obtained should have the required purity,
dielectric constant, loss tangent, and particle size distribution. In this
embodiment up to four different powdered materials can be fed from a
delivery and metering system comprising a plurality of hoppers 10, 12, 14
and 16 respectively, while the solution of the polymer in the
cyclohexanone is fed from its hopper 18, and suitable surface active
functional additives, if required, such as surfactants, plasticizers and
lubricants, are fed from a respective hopper 20. Each powdered material
can be fed directly to the respective hopper 10, 12, 14, and 16, or
alternatively obtained from respective precipitation or coprecipitation
systems 22, 24, or 26 (a system for the contents of the hopper 16 is not
shown). If the polymer is employed in the form of a powder then it will be
fed from the hopper 18, while the dispersion vehicle will be fed from a
respective hopper, or perhaps from the hopper 20 along with the additives.
The flow of each filler powder from its hopper is continuously precision
metered by a respective meter 28, that of the polymer solution is metered
by meter 32, that of the surface active additives is metered by meter 34,
and those of the combined polymer solution/filler or additive flows are
metered by respective meters 36. Each preliminary mixture of polymer
solution, powders and additives is delivered into a respective drum type
mixer/grinding mill 38, described in more detail below.
One of the aspects of the invention that also distinguishes it from prior
art processes is that it is preferred to use low cost powders of a
relatively wide range of particle sizes in order to obtain optimum packing
together of the particles, and resultant minimization of the interposed
polymer layers, as contrasted with the highly uniform size, and
consequently expensive, powders which were required, particularly for the
production of fired ceramic substrates to achieve adequate uniformity of
processing. Prior to the formation of each mixture the respective powder
particles usually consist of particles of a range of sizes and
agglomerates of many finer particles that can vary even more widely in
size, and this must be corrected, particularly the reduction of the
agglomerates back to their individual particles. Each mixer/mill 38
operates to mix the components and produce complete dispersion of the
powdered material in the liquid vehicle, and also as a grinding mill to
mill the respective powder particles and agglomerates to a required size
distribution to a obtain a required degree of uniformity, but with a
distribution that will also result in a minimum pore volume when
compacted.
The proportions of the powder, polymer solution and additives from the
hoppers are such as to obtain a solids content in the respective
preliminary mixture in the range of 40-95% by volume, the quantities of
the dispersing vehicle and the functional additives being kept as low as
possible, but sufficient for the consistency of the mixture to be kept to
that of a relatively wet paste or slurry, to permit its free flow through
the relatively narrow processing flow passages of the respective mill 38,
and the subsequent machines. A viscosity in the range of about 100-10,000
centipoises will usually be satisfactory. In the methods of the invention
preferably such grinding, deagglomeration and dispersion of each
preliminary mixture is carried out simultaneously in its respective mill
38, using for this purpose a special mill which is the subject of my U.S.
Pat. No. 5,279,463, issued Jan. 18, 1994, and U.S. Pat. No. 5,538,191,
issued Jul. 23, 1996, the disclosures of which are incorporated herein by
this reference.
These special mills may be of two major types, in a first of which the mill
has two circular coaxial plate members with a processing gap formed
between them; the axis of rotation can be vertical or horizontal. It is
preferred however to use the second type of mill, which consists of an
inner cylindrical member rotatable about a horizontal axis inside a
stationary hollow outer cylindrical member, the axes of the two cylinders
being slightly displaced so that the facing walls are more closely spaced
together at one longitudinal location around their periphery to form,
parallel to the axes, what is referred to as a processing or micro gap,
and are more widely spaced at the diametrically opposed longitudinal
location to form, again parallel to the axes, what is referred to as a
complementary or macro gap. The mixture flows through the processing gap
producing so-called "supra-Kolmorgoroff" mixing eddies in the portion of
the slurry at and close to the macro gap and so-called "sub-Kolmorgoroff"
mixing eddies in the micro or processing gap. Ultrasonic transducers may
be mounted on the stationary member which apply longitudinal pressure
oscillations into the processing gap and reinforce the "sub-Kolmorgoroff"
mixing eddies. Such apparatus is capable of processing relatively thick
slurries of sub-micrometer particles in minutes that otherwise can take
several days in conventional high shear mixers and ball or sand mills.
The separate preliminary mixtures are now mixed together to form a combined
mixture having the consistency of a uniform slurry or wet paste by passing
them into a mixer/mill 40, in which the combined mixture is subjected to
another grinding, deagglomerating and uniform dispersing operation. The
mixer/mill 40 is again one of the above-mentioned special mills which are
the subject of my U.S. Pat. Nos. 5,279,463 and 5,538,191, being also of
the type comprising an inner cylindrical member rotatable inside a
stationary hollow outer cylindrical member. Although only a single
mixer/mill 40 is employed in this embodiment, in some processes it may be
preferred to employ a chain of two or more such mills depending upon the
amount and rate of grinding, deagglomeration and mixing that is required.
The milled slurry from the mill 40 passes to a mixer/solvent evaporation
mill 42 which again is of the type comprising an inner cylindrical member
44 rotatable inside a stationary hollow outer cylindrical member 46, the
paste being carried on the outer cylindrical surface of the member 44 in
the form of a thin film 47. In the mill most of the cyclohexanone solvent
is removed while the paste is vigorously mixed, the paste becoming
continuously thicker as it travels in a helical path from the feed entry
point 48 of the evaporation mill to its discharge outlet 50 as more and
more solvent is withdrawn through solvent discharge outlet 52, from which
it passes to a condenser (not shown) for recovery and reuse. The
evaporation of the solvent from this mill is facilitated by heat from a
row of cartridge heaters 54 in the base of the machine, their output being
such as to obtain a temperature in the tape body of about 150.degree. C.
Near to the discharge outlet of the mill the paste is of sufficient
stiffness that it can be extruded into a coherent thin tape 56 of the
desired dimension in thickness and width using a conventional paste
extruding machine 58. Since this tape still contains small amounts of
solvent and the additives, it must be subjected to a further heating
process in a tunnel dryer oven, and to this end the tape is deposited on
an endless conveyor 60, which passes it through a drying oven 62, during
which passage the solvent and as much as possible of the additives are
removed to leave the strip or tape consisting only of a thoroughly and
uniformly dispersed composite mixture of the particles of the filler
material or materials and the polymer or polymers. A suitable temperature
for such an oven is, for example, in the range 150-250.degree. C., the
heating being carried out slowly to avoid as far as possible the formation
of bubble holes by the exiting dispersion medium and additives or additive
breakdown products.
The tape 56 of dried paste is passed through a cutting station 64, in which
it is severed into individual "green" substrate preforms 66, usually of
rectangular shape and of the size required for the electronic circuit
board substrate, if that is the use for which the materials are intended.
The preforms are deposited manually or automatically into the cavity of a
heated compression mold comprising heated upper and lower platens 68 and
70, the cavity being located in the lower heated platen 70 to facilitate
the loading process. Once the preform is loaded the mold cavity is closed
by the downward moving heated top platen 68 which protrudes into the
cavity to compress the preform to its required dimensions and density. The
temperature to which the preforms are heated in the mold is sufficient to
melt the polymer so that it will flow freely under the pressure applied to
completely fill the interstices and coat the filler material particles,
while the maximum is that at which the polymer will begin to degrade
unacceptably. The minimum pressure to be employed is coupled with the
choice of temperature, in that it must be sufficient for the melted
polymer to flow as described, the pressure and time for which the mold is
closed being sufficient for the material of the preforms to attain maximum
compaction and density. During the heat and pressure cycle the melted
polymer will flow relatively freely and the temperature and pressure are
maintained for a period sufficient to ensure that the polymer can
completely fill the relatively small interstices between the solid
particles in the form of correspondingly very thin layers. Typically the
temperature is in the range 280-400.degree. C., while the pressure is in
the range 70 MPa to 1,380 MPa (10,150 to 200,000 psi), although a more
commercially likely pressure is about 345 MPa (50,000 psi), while
pressures as low as 3.5 MPa (500 psi) may be usable. The surfaces of the
platens that contact the preforms are mirror-finished or better to assist
in obtaining the smooth surfaces that are desired for electronic
substrates intended for microwave frequency applications.
Another unexpected advantage of the nonfunctionalised poly(arylene ethers)
employed is that, since they may be cross-linked by exposure to
temperatures in the range of 350.degree.-450.degree. C. in the presence of
oxygen, it is possible to take the finished substrate through a cycle in
which initially the polymer is melted again and thoroughly diffused
throughout the body, the polymer at this stage being relatively fluid, and
thereafter the temperature is increased until cross-linking and
corresponding densification of the polymer takes place. Alternatively, the
composite mixture may include as an additive a cross-linking agent and/or
an end capping agent, so that the desired densification will take place at
lower temperatures. As described above this ability to crosslink and/or
end cap at elevated temperatures makes the materials particularly useful
in microelectronic applications because they can readily be applied as low
viscosity materials, e.g. even from solution as described, and then
converted to a solvent resistant material of maximum density by the
heating.
The substrates 66 issuing from the press are fed to a multi-stand, heated,
flattening roller mill 72 in which they are rolled to an accurately
controlled thickness and flattened. The surfaces of these rolls are also
mirror-finished, or better, again in order to obtain the desired final
smooth surfaces. The sheet, film or tape from which the preforms have been
cut usually has a thickness less than about 60 mil, can be less than about
30 mil, may be less than about 10 mil, may be less than about 4 mil, and
can even be less than about 1 mil. Substrates intended for use in
electronic circuits will usually be of thickness in the range 0.125 mm to
1.5 mm (5-60 mil), and if intended for thick film usage are usually
required to be smooth to about 0.75-0.90 micrometer (22-40 microins),
while if intended for thin film usage must be flat to better than 0.05
micrometer (2 microins). The preforms are now fed to a heated laminating
press 74 in which they are each laminated on one or both sides with a thin
flat smooth piece 76 of copper sheet of the same size, which subsequently
is etched to produce the electric circuit. These sheet copper pieces are
obtained by cutting from a strip 78 supplied from a roll thereof (not
shown) which is cut into pieces at a cutting and mirror-finish surfacing
station 80. The surfacing means comprises a hot press in which the cut
pieces are pressed between a pair of heated platens, the platen surfaces
being mirror-finished or better so that a corresponding finish is imparted
to the surfaces of the pieces. The mirror-finishing of the substrate
surfaces and those of the copper pieces is especially important in
ultrahigh-frequency applications since, as described above, the currents
tend to flow only in the surface layers of the conductors, and uniformity
in characteristics of the etched conductors is facilitated by such smooth
surfaces.
With the processes of the invention the volume percentage of the filler
material can be 60% or more, the minimum value being that at which the
interposed layers of polymer are somewhat too thick to have the required
mechanical strength for the substrate to have the corresponding amount of
mechanical strength. The maximum value is set by the amount of the
particular polymer required to adequately bind the particular filler
material to form a strong coherent body. Thus, they enable the production
of composite materials in which the solids content is easily and
economically in the range 60%-97% by volume, preferably 70%-97% by volume.
The volume fraction of the polymer in the mixtures is only that needed to
adhere the filler material particles together while filling the pores left
in the inorganic powder after its compression to minimum pore, preferably
pore-free, high density. The relatively small amounts of polymer present
in the composite materials must be extremely well and evenly dispersed
among the fine particles, and this is readily achievable with the
processes employed virtually independently of the particle size of the
filler material.
The process and apparatus described above are particularly suited for high
volume production of composite materials, but simpler processes requiring
less apparatus are also within the scope of the invention. For example, as
described above it is also possible to mix together the finely divided
filler material and polymer, the dispersion medium, and its necessary
additives and thereafter rely upon its processing in one or a series of
mixer/mills 38 and/or 40 to produce the required thorough dispersion,
while at the same time obtaining the preferred range of particle sizes,
the dispersed mixture that is produced thereafter being passed to the
drying oven 46 etc., as with the prior process.
In many applications the degree of uniformity required in the material of
the finished substrate is such that even the extensive specific process
described above may not be sufficient, and it may be necessary to apply an
additional series of steps in which the substrates are broken and ground
back down to about the original particle size distribution, with the
difference that the filler material particles are now intimately
associated with particles and thin coatings of the polymer. This finely
divided material is again ground and dispersed in a suitable medium by use
of one or a chain of the special mills, such as the mills 38 and 40
described above, until the maximum possible uniformity is obtained, when
the dispersion medium is removed and the resultant material again
subjected to a heating and pressing operation to produce the desired
substrates, the polymer being sufficiently thermoplastic at the
temperatures required for this to be possible.
FIGS. 6 and 7 are respective photo micrograph cross sections through a
material of the invention, respectively before and after the mirror
finished piece 76 of copper sheet is attached to the mirror-finished
surface of the substrate, the material consisting of closely packed
particles 82 of the filler material, of irregular size and shape, coated
and bound together by polymer material 84 that no longer exists as
discrete particles but as thin intervening films and interstice-filling
masses. As an indication of the size of the particles, etc. involved the
square section of FIG. 6 measures 5 micrometers each side. The
adhesiveness of the polymers of the invention is sufficient to ensure
adequate bonding without the need for reinforcing fibers or fiber-cloth.
A particular currently preferred group of the selected poly(arylene ether)
polymers, in which the repeating unit is biphenyl diradical linked with
the 4,4'-diradical of 9,9-diphenylfluorene, are designated PAE-2, while
another currently preferred group, in which the repeating unit is
para-terphenyl diradical linked with the 4,4'-diradical of
9,9-diphenylfluorene, are designated PAE-3, and third currently preferred
group, in which the repeating unit is a combination of the units of PAE-2
and PAE-3, are designated PAE-4. Methods for the production of these
polymers are disclosed in the above-mentioned U.S. Pat. Nos. 5,658,994 and
5,874,516, to which reference may be made. Samples of these polymers are
found to have the following principal characteristics:
______________________________________
PAE-2 PAE-3 PAE-4
______________________________________
Weight average molecular
65,300 45,400 75,800
weight Mw
Number average molecular 20,700 11,400 25,700
weight Mn
Mw/Mn 2.58 3.98 2.95
Glass transition temperature 257.degree. C. 271.degree. C. 273.degree.
C.
Tg via DSC
Tensile modulus (dynes/cm.sup.2) 1.45 .times. 10.sup.10 1.45 .times.
10.sup.10 .39 .times. 10.sup.10
Weight loss % at 400.degree. C. 0.36
0.57 0.65
after 6 hrs
Weight loss % at 450.degree. C. 0.91 1.65 1.26
after 6 hrs
Wt % gain moisture at 0.279 0.301 0.274
85.degree. F./85 RH
______________________________________
In the above-mentioned U.S. patents these materials are described as having
improved properties, as compared with prior art fluorinated poly(arylene
ether) materials designated PAE-1, a particular sample of which has the
following comparable characteristics:
______________________________________
PAE-1
______________________________________
Weight average molecular weight Mw
20,000
Number average molecular weight Mn 7,700
Mw/Mn 2.58
Glass transition temperature Tg via DSC 166.degree. C.
Tensile modulus (dynes/cm.sup.2) 1.23 .times. 10.sup.10
Weight loss % at 400.degree. C. after 6 hrs 0.72
Weight loss % at 450.degree. C. after 6 hrs 3.16
______________________________________
Substrates made using PAE-2 have been very successful; the material does
not oxidise in air, is highly adhesive without the use of coupling agents,
and has a loss tangent in the frequency range of particular interest (1-10
GHz) less than 0.0008, as compared to most other thermoset polymer
materials presently used for electronic circuit applications, namely
0.02-0.005. The polymer is thermoplastic and can be processed at
280-300.degree. C., and by post treating the substrates at 300-400.degree.
C. to establish cross-linking they can be rendered thermoset, when the
loss tangent drops below 0.0008. Polymers of weight average molecular
weight below about 30,000 are regarded as less desirable for use with the
methods of the invention, since even more than the PAE-2/3/4 materials
they are not able to form adequately structurally strong films, sheets or
any other substantially three-dimensional body, because of a tendency of
these relatively thick structures to shatter into a multitude of smaller
fragments. I have discovered however that surprisingly even the lower
molecular weight materials remain intact and cohesive as thin film
depositions in the low micrometer range thicknesses of about 1-3
micrometers and can therefore be used, although the higher molecular
weight materials are to be preferred.
The relative proportions of the filler materials and of the polymer depend
at least to some extent upon the use to which the substrate is to be put;
if a very high frequency circuit is to be installed then it will be
preferred to have the maximum amount of filler dielectric material and the
minimum amount of polymer. As has been described above, the minimum amount
of polymer is set by that required to fill the intergrain interstices when
the interstitial volume is at its minimum value, and to ensure sufficient
coating of the grains for the resulting composite to have the required
mechanical strength. For this reason the composites usually require a
minimum of 3% by volume of polymer to be present as long as the optimum
particle packing of the filler material has been obtained, the remaining
97% solids content comprising the filler dielectric material, residual
surface active and coupling agents, and organic or inorganic reinforcing,
strength-providing fibers and whiskers, when these are provided.
Materials of relatively small particle sizes are preferred, particularly
for the filler starting materials, and also for the polymer if a solid
polymer or polymers is employed. The preferred particle size range for the
filler starting materials is from 0.01 to 50 micrometers, while that for
the polymer is from 0.1 to 50 micrometers. As described above, the
presence of particles of filler material of a range comprising different
sizes is preferred, since this improves the capability of dense packing in
a manner that reduces the interstitial volume, and consequently
facilitates the production of the very thin highly adhesive layers that
are characteristic of the invention, besides reducing the amount of
polymer required to fill the interstices and adhere the particles
together. It can be shown theoretically that the minimum interstitial
volume that can be obtained when packing spheres of uniform size is about
45%. Owing to the wider particle size distribution that can be employed,
this volume can be reduced considerably further, down to the specified
value of about 3%.
As described above, there are a number of important parameters for the
resultant substrates which must be considered in making a selection of the
fillers and polymers to be used. Among those which require the highest
possible values are tensile strength; peel strength; solder joint
reliability; compliance i.e. low modulus; plated through hole reliability;
dielectric constant; chemical inertness; dimensional stability and Q
factor. Among those which require the lowest possible values are water
absorption; crosstalk v line spacing; and loss tangent or dissipation
factor (reciprocal of Q factor).
The methods of this invention are particularly applicable to the production
of composite materials in which the finely powdered filler material
consists of any one or a mixture of the "advanced" materials that are now
used in industry for the production of fired ceramic substrates for
electronic circuits, the most common of which are aluminium nitride;
barium titanate; barium-neodymium titanate; barium copper tungstate; lead
titanate; lead magnesium niobate; lead zinc niobate; lead iron niobate;
lead iron tungstate; strontium titanate; zirconium tungstate; the chemical
and/or physical equivalents of any of the foregoing; alumina; fused
quartz; boron nitride; metal powders; and semiconductors. Another
important group is compositions comprising powdered ferrites and like
inductive materials in a polymer matrix that have already been produced,
used for example in magnetic passive products such as transformers,
inductors and ferrite core devices, but the methods used add the powdered
filler material into the polymer matrix and their solids contents have
generally been limited to not more than about 50% by volume. The invention
permits the production of such composite materials of higher solids
content, e.g. 80% by volume and above.
At this time the only ceramic materials with temperature stable dielectric
constants that are available have values in the ranges 2.6 to 12, 37 to 39
and 80 to 90, whereas in the quickly expanding market of wireless
telecommunication, which is based on microwave frequencies ranging from
800 MHz to over 30 GHz, and in which small size and low weight are of
increasing importance, the preferred dielectric constant values need to be
tailored to be anywhere between 8 and 2000, according to choices dictated
by the optimum circuit architecture, instead of, as at present, the
circuit architecture being dictated by the very limited ranges of
dielectric constants that are available. In microwave or GHz frequencies
signal propagation depends mainly on the waveguide character of the
circuitry and consequently only such high dielectric constant materials
allow significant miniaturization, permitting the use of narrower
conductor line widths and shorter lengths. For example, coaxial dielectric
resonators, at this time used in more than 25 million cellular telephones
worldwide, could be reduced in size and weight by more than half and in
cost by more than two thirds if the dielectric constant of the substrate
material could be raised from the present value of alumina of about 9 to
over 400 and its dielectric loss (loss tangent) kept below 0.0005.
It is possible with these processes to fabricate composite materials in
which the powdered filler material is a tailored blend of two or more
individual materials. The requirements for substrate materials, especially
for very high frequency applications, are very exacting, requiring
consideration of a large number of physical properties including filler
material content, bulk density (range), surface finish, grain size
(average), water absorption (%), flexural strength, modulus of elasticity,
coefficient of linear thermal expansion, thermal conductivity, dielectric
strength, dielectric constant, dissipation factor, and volume resistivity.
The possibility of such blending makes it possible to tailor the
properties of the substrates to their specific tasks in a manner which is
not possible with a sintered ceramic as in most cases the sintering phase
rules would be violated and the resulting fired material would fall apart.
One of the main reasons for combining filler materials in any given ratio
is to obtain a mixture with a tailored dielectric constant, which constant
will remain uniform over a temperature range from say -50.degree. C. to
+200.degree. C., and with a very high Q factor (equivalent to a very low
loss tangent) desirably above 500 and if possible as high as 5,000.
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