Dielectric composite

A composite material is disclosed. The material comprises a polymeric matrix and from about 20 volume percent to about 70 volume percent inorganic particles distributed throughout the matrix. Suitable inorganic particles include hollow inorganic microspheres and porous inorganic particles. The inorganic particles are coated with a surface coating. The composite material of the present invention exhibits a dielectric constant of less than about 2.5 and a thermal coefficient of expansion of less than about 70 ppm/.degree.C.

CROSS REFERENCE TO RELATED APPLICATION 
This application is related to copending application Ser. No. 483,501, now 
U.S. Pat. No. 5,024,871, "Ceramic Filled Fluoropolymeric Composite 
Material" by D. J. Arthur and G. Swei, filed on the same date herewith. 
SUMMARY OF THE INVENTION 
Commonly assigned U.S. Pat. No. 4,849,284 ('284 patent), the disclosure of 
which is incorporated herein by reference, describes a ceramic filled 
fluoropolymer matrix electrical substrate material sold by Rogers 
Corporation under the trademark RO-2800. This material preferably 
comprises a polytetrafluoroethylene matrix filled with amorphous fused 
silica and microfiberglass. The silica filler is coated with a silane 
coating material which renders the surface of the silica hydrophobic and 
provides improved tensile strength, peel strength and dimensional 
stability to the composite material. The electrical substrate material is 
well suited for forming a rigid printed wiring board substrate and 
exhibits improved electrical performance over other printed wiring board 
substrate materials. 
Two properties are of particular interest with regard to the performance of 
an electrical printed wiring board substrate material. A low dielectric 
constant, i.e. (DK) DK.ltoreq.about 3 is particularly desirable in such 
materials. The low significantly improves the electrical performance of 
PWB material by a reduction in propagation delay, cross-talk, and rise 
time degradation in digital and microwave PWB applications. It is also 
important that PWB substrate materials exhibit a low Z axis coefficient of 
thermal expansion, i.e. (CTE) CTE.ltoreq.about 70 ppm/.degree.C. The 
substrate of the 284 patent comprises a fluoropolymer matrix and a 
silane-coated ceramic filler. The substrate exhibits a DK of about 2.8 and 
a Z-axis CTE of about 24 ppm/.degree.C. 
There is a constant and intensive effort in the art to develop PWB 
substrate materials which exhibit an advantageous balance of mechanical, 
thermal and electrical properties. 
DISCLOSURE OF THE INVENTION 
A composite material is disclosed. The composite material, comprises a 
fluoropolymer matrix, from about 20 volume % to about 70 volume % first 
coated inorganic particles distributed throughout the matrix. The first 
coated inorganic particles comprise hollow inorganic microspheres and a 
hydrophobic coating on the microspheres. The material exhibits a DK of 
less than about 2.5 and a Z-axis CTE of less than about 50 ppm/.degree.C.

DETAILED DESCRIPTION OF THE INVENTION 
The fluoropolymer matrix of the composite material of the present invention 
may comprise any fluoropolymer that exhibits a low dielectric constant. 
Polytetrafluoroethylene (PTFE), hexafluoropropene (HFP), 
tetrafluoropolyethylene (TFE), and perfluoroalkylvinyl ether (PAVE), are 
examples of suitable fluoropolymer matrix materials. It will be 
appreciated that PTFE, HFP, TFE and PAVE are all thermoplastic 
fluoropolymers. PTFE is the preferred matrix material. 
A PTFE powder known as Teflon.RTM. 6C, available from DuPont, and a PTFE 
dispersion known as Fluon.RTM. AD704, available from ICI, have been found 
to be particularly well suited for use in the present invention. 
The first inorganic particles of the present invention may comprise hollow 
inorganic microspheres and a hydrophobic coating on the microspheres. It 
is preferred that the first inorganic particles exhibit low ionic 
contamination. 
Hollow inorganic microspheres having a density of less than about 1 
gram/cubic centimeter (g/cm.sup.3) are preferred. For a given microsphere 
material, the theoretical dielectric constant of the microsphere decreases 
with decreasing density. It should be noted that since the microspheres 
become increasingly fragile and susceptible to mechanical damage during 
processing with decreasing microsphere density, a balance between 
performance and processability must be determined, based on the particular 
application of the material of the present invention. 
Hollow inorganic microspheres are available from a number of commercial 
sources. Hollow glass microspheres and hollow ceramic microspheres are 
suitable for use in the present invention. Silica microspheres and 
borosilicate microspheres are preferred. Hollow silica microspheres, known 
as SI Eccospheres and manufactured by Emerson & Cuming, Dewey & Almy 
Chemical Division of W. R. Grace & Co. of Canton, Mass., hollow 
borosilicate microspheres, known as H-50/10,000 Glass Bubbles, 
manufactured by 3M Company of St. Paul, Minn. and, hollow high silica 
microspheres known as SDT-60 Eccospheres, also manufactured by Emerson and 
Cuming, have been found to be particularly suitable for use in the 
practice of the present invention. The SI Eccospheres are hollow silica 
microspheres having an average particle size of about 80 .mu.m and an 
average particle density ranging from about 0.14g/cm.sup.3 to about 
0.8g/cm.sup.3. The H-50/10,000 are hollow borosilicate microspheres having 
an average particle density of about 0.5g/cm.sup.3 to about 0.6g/cm.sup.3 
and are available in different particle size distributions. The SDT-60 
Eccospheres are hollow high silica microspheres having an average particle 
density of about 0.28g/cm.sup.3 and particles ranging in size from about 5 
.mu.m to about 40 .mu.m. 
The first inorganic particles comprise between about 10 volume % and about 
70 volume % of the composite material of the present invention. Preferably 
the first inorganic particles comprise between about 15 volume % and about 
65 volume % of the composite material of the present invention. In 
applications which low DK is of primary importance, it is preferred that 
the composition of the present invention comprises from about 35 volume 
percent to about 65 volume percent hollow inorganic microspheres. 
The second inorganic particles of the present invention may comprise any 
particulate inorganic filler material that exhibits a low coefficient 
thermal expansion and low DK. It is preferred that the second inorganic 
particles exhibit low ionic contamination. The second inorganic particles 
may comprise porous second inorganic particles or nonporous second 
inorganic particles. 
The porous inorganic particles of the present invention may be any porous 
inorganic particles having a dielectric constant of less than 3.5. It is 
preferred that the porous inorganic particles have void space greater than 
about 20 volume % and internal surface of the voids being area greater 
than about 150 m.sup.2 /g. It is preferred that the porous inorganic 
particles comprise porous silica particles. The preferred range of 
particle size of the porous inorganic particles is dependent upon the 
particular application of the composite material. Glass particles, known 
as Corning VYCOR.RTM. 7930, manufactured by Corning Glass Works, Corning, 
N.Y. have been found to be particularly well suited for use as the porous 
second inorganic particles of the present invention. VYCOR.RTM. 7930 glass 
particles have a dielectric constant of about 3.0, a void space of about 
28 volume %, a surface area of about 250 m.sup.2 /g, an average pore 
diameter of about 40 Angstroms and a specific gravity of about 1.5. 
Suitable nonporous second inorganic particles include glass particles, 
ceramic particles, glass fibers and ceramic fibers. While continuous 
fibers may be suitable for use in some embodiments of the present 
invention, discontinuous fibers are preferred. 
Silica particles are particularly preferred nonporous second inorganic 
particles and amorphous fused silica particles are most preferred 
nonporous second inorganic particles. It is preferred that the nonporous 
second inorganic particles of the present invention exhibit an average 
particle size less than about 30 microns. Amorphous fused silica particles 
known as GP-7I, manufactured by Harbison-Walker were found to be 
particularly well suited for use as nonporous second inorganic particles 
of the present: invention. 
The second inorganic particles comprise up to about 55 volume % of the 
composite material of the present invention. The second inorganic 
particles preferably comprise less than about 40 vol. % of the composite 
material of the present invention for application in which low DK is of 
primary importance. 
The hydrophobic coating of the present invention may comprise any coating 
material that is thermally stable, exhibits a low surface energy, and 
improves the moisture resistance of the composite of the present 
invention. Suitable coating materials include conventional silane 
coatings, titanate coatings and zirconate coatings. Preferred silane 
coatings include: phenyltrimethoxysilane, phenyltriethoxysilane, 
3,3,3-trifluropropyl) trimethoxysilane, 
(tridecafluoro-1,1,2,2-tetrahydrodecyl)-1,1 triethoxysilane 
(heptadecafluoro-1,1,2,2-tetrahydrodecyl) 1-triethoxysilane and mixtures 
thereof. Suitable titanate coatings include: 
neopentyl(diallyl)oxytrineodecanoyl titanate, 
neopentyl(diallyl)oxytri(dodecyl)benzene-sulfonyl titanate and 
neopentyl(diallyl)oxytri(dioctyl)phosphate titanate. Suitable zirconate 
coatings include: neopentyl(diallyl)oxytri (dioctyl)pyrophosphate 
zirconate and neopentyl(diallyl)oxytri(N-ethylenediamino)ethyl zirconate. 
The hydrophobic coating is used in an amount effective to render the 
surfaces of the filler particles hydrophobic and compatible with the 
matrix material. The amount of coating relative to the amount of inorganic 
particles coated will vary with the surface area coated and density of the 
inorganic particles. Preferably, the coated inorganic particles of the 
present invention include from about 3 parts by weight (pbw) hydrophobic 
coating: 100 pbw inorganic particles to about 15 pbw hydrophobic coating: 
100 pbw inorganic particles. 
The filler materials are coated by agitating the filler materials in a 
solution of the coating material, removing the filler material from the 
solution and finally heating the filler material to evaporate solvents 
from the coating and to react the coating with the surface of the filler 
material. 
The composite material of the present invention is compounded by the 
procedure outlined in U.S. Pat. No. 4,335,180 the disclosure of which is 
incorporated by reference. Briefly, the process includes mixing silane 
coated inorganic particle with a fluoropolymer dispersion, coagulating the 
mixture using a flocculating agent, filtering the coagulated the mixture 
and then consolidating mixture to form a composite substrate at elevated 
temperature (600.degree. F. to 800.degree. F.) and pressure (100 psi to 
900 psi) Alternatively, a fluoropolymer powder may be mixed with the 
coated filler particles and the mixture so formed may be consolidated 
under elevated temperature and pressure to form the composite substrate. 
It should be noted that consolidation pressures must be limited to 
pressures below the compressive strength of the microspheres to avoid 
excessive breakage of the microspheres. 
EXAMPLE 
Samples of the composite material of the present invention were prepared. 
A hydrolyzed silane coating solution was made by dissolving 3 wt % of 
(tridecafluoro-1,1,2,2 tetrahydrooctyl)1 triethoxysilane in isopropanol. 
The solution was diluted with water to produce a solution having a water 
to silane stoichiometric ratio of 3 to 1. The pH of the solution was 
adjusted to 5 with acetic acid. The solution was then stirred for 24 
hours. 
The inorganic particles were coated with the silane coating solution. The 
particles were immersed in the solution and agitated in a Patterson-Kelly 
mixer for 40 minutes. The mixture was stored for 8 hours in a closed 
container The particles were removed from the solution and dried at 
120.degree. C. to evaporate the solvents from the mixture and to cure the 
silane coating onto the surfaces of the particles. 
The particles were combined with a fluoropolymer matrix to make sample 
composite materials. The coated filler particles were mixed with 
fluoropolymer particles (Teflon.RTM. 6C) and a lubricant (dipropylene 
glycol) to form a paste. The paste was extruded and calendered at room 
temperature to form sheets. The sheets were lot pressed at 700.degree. F. 
and 500 pounds per square inch (psi) to consolidate the composite 
material. 
The above procedure was repeated to produce samples of different 
compositions. The composition of each of the samples by volume fraction is 
given in Table 1. 
TABLE 1 
______________________________________ 
A B C D E F G H 
______________________________________ 
FLUOROPOLYMER 
MATRIX 
PTFE 40 40 40 40 40 40 40 40 
FIRST INORGANIC 
TICLES 
silica microspheres.sub.2 
60 -- -- -- -- 50 -- -- 
borosilicate -- 60 -- -- -- -- 30 -- 
microspheres.sub.3 
borosilicate -- -- 60 -- -- -- -- 40 
microspheres.sub.4 
silica microspheres.sub.5 
-- -- -- 30 30 -- -- -- 
SECOND INORGANIC 
TICLES 
porous silica -- -- -- -- -- -- -- 20 
particles.sub.6 
amorphous fused 
-- -- -- 30 30 10 30 -- 
silica.sub.7 
HYDROPHOBIC 
COATING 
silane coupling 
+ + + + -/+.sub.9 
+ + + 
agent.sub.8 
______________________________________ 
.sub.1 Teflon 6C, DuPont 
.sub.2 SDT60, Emerson & Cuming 
.sub.3 H50/10,000 (325 Mesh), 3M 
.sub.4 H50/10,000 (500 mesh), 3M 
.sub.5 SI Eccospheres, Emerson Cuming 
.sub.6 Vycor 7930, Corning Glass Works 
.sub.7 GP7I, HarbisonWalker 
.sub.8 Tridecafluoro1,1,2,2-tetrahydrooctyl-,1-triethoxysilane 
.sub.9 Coupling agent on second particles only 
The DK and CTE and water absorption of the composite materials so produced 
were measured and are set out in Table 2 below. 
TABLE 2 
______________________________________ 
A B C D E 
______________________________________ 
Dielectric 
1.938 1.943 2.029 2.42 2.38 
Constant 
CTE 47 50 41 29 29 
(ppm/.degree.C.) 
Water 2.84.sub.1 
1.72.sub.2 
1.91.sub.1 
0.56.sub.1 
0.68.sub.1 
Absorption 
______________________________________ 
F G H 
______________________________________ 
Dielectric 
2.109 2.31 2.20 
Constant 
(@ 10 GHz) 
33 20 32 
CTE 
(ppm/.degree.C.) 
Water -- 2.85.sub.1 1.80.sub.1 
absorption 
______________________________________ 
.sub.1 = 48 hours, 50.degree. C. 
.sub.2 = 24 hours, 50.degree. C. 
Comparison of samples A, B, and C shows the effect of difference choices of 
first organic particles. Comparison of samples A, F, G, and H show the 
effect of introducing second inorganic particles. Comparison of the water 
absorption results of samples D and E demonstrates the benefit of using 
the coupling agent of the present invention. 
The DK and Z axis CTE for a number of conventional dielectric substrate 
materials are presented in FIG. 1 for comparison with the values tabulated 
in Table 2. None of the conventional substrate materials exhibit a DK of 
less than about 2.5 and a z-axis CTE of less than about 50. 
While preferred embodiments have been shown and described, various 
modifications and substitutions may be made thereto without departing from 
the spirit and scope of the invention. Accordingly, it is to be understood 
that the present invention has been described by way of illustrations and 
not limitation.