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Description  |
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BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to high capacitance laminates made from thin films
of polytetrafluoroethylene filled with large amounts of high dielectric
filler, in which the films are plated or clad with copper or other
conductive foils and sheets.
2. Description of The Prior Art
High dielectric laminates have been known heretofore, as has their use in
printed circuit boards, especially boards for use in microwave
applications. Laminates of filled polytetrafluoroethylene and clad with
copper are of value in these applications because they reduce, at a given
frequency, the wavelength travelling along a conductive path on the
laminate. Filled polytetrafluoroethylene has advantages in this use in
part because it absorbs little moisture (which affects electrical
properties), exhibits low loss, and provides a chemically inert, high
temperature resistant material.
Filled polytetrafluoroethylene (PTFE) laminates of high dielectric that are
clad with copper are disclosed in U.S. Pat. No. 4,518,737, specifically at
column 1, line 63 to column 2, line 11. However, such laminates can
generally not be made thin enough for many digital printed circuit board
applications where thin high dielectric laminates used to increase board
capacitance would be of value.
In digital systems, speed of computation is highly valued. To achieve high
speed computation, highly integrated, very fast simultaneously switching
integrated circuits (IC's) have been developed. To operate properly these
IC's reference known ground and power voltages that are required to remain
within given ranges. When these IC's switch they require current for a
very short duration. This causes a localized drop in voltage on the power
plane. Historically, this drop in voltage has been moderated by the use of
surface capacitors. These discrete capacitors provide a store of charge
that can be fed to the power plane if the voltage begins to drop. As the
speed of IC's increases, this becomes an ineffective way to solve this
problem. With today's high performance IC's, the duration that current is
needed is very short. Discrete capacitors cannot be positioned close
enough to the IC and the path made low enough in inductance to satisfy the
needs of the IC, therefore, the voltage drops. If this drop, or spike, in
voltage is large enough, errors can be generated when the IC references
this reduced voltage. Very thin high dielectric laminates used for the
power/ground layers (when located in sequential layers) increase the
capacitance of the printed wiring board. This increases the density of
charge stored for a given voltage differential, reducing the voltage
swings relative to a lower capacitance laminate with discrete capacitors
caused by the fast switching IC's improving the fidelity of the signal.
However, thin polytetrafluoroethylene films filled with high dielectric
material are not ordinarily obtainable. Filled polytetrafluoroethylene
films are generally made by calendering. As the films become thinner and
thinner in the calendering process, rheological problems causes pinholes
or tears in the filled film. Pinholes and tears, of course, cause
electrical problems.
It is desirable to provide laminates of a very thin highly filled films of
polytetrafluoroethylene of high dielectric constants and conductive
metals, such as copper. It is also desirable to provide a printed circuit
board that utilizes such a laminate. It is further desirable to use
materials of high tensile strength.
SUMMARY OF THE INVENTION
One aspect of the invention is a laminate comprising:
a. A thin film of filled polytetrafluoroethylene that:
1. contains 25-85 volume percent particulate filler having a high
dielectric constant,
2. has a film thickness of between 0.0001 and 0.005 inches,
3. is substantially free of visual pinholes, and
4. has a matrix tensile strength of at least 2600 psi;
b. A film of conductive metal attached to at least one side of the film
defined in part a.
Another aspect of the invention is a printed circuit board in which at
least one layer of the board is comprised of the laminate defined above.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a laminate of the invention that is copper clad on one side.
FIG. 2 depicts a laminate of the invention that is copper clad on both
sides.
FIG. 3 depicts a printed circuit board of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The fillers useful herein include any commonly known filler particulate
that has a high dielectric constant. By "particulate" is meant individual
particles of any aspect ratio and thus includes fibers and powders.
Preferably the filler will be smaller than 40 microns and most preferably
less than 20 microns average size, and preferably will be titanium dioxide
or barium titanate or a ferroelectric complex. Filler concentration in the
film will be between about 25-85 volume percent, and the dielectric
constant will be at least 7.
In order to obtain the desired degree of thinness, namely between 0.0001
and 0.005 inches, it is preferred to make the filled films by:
(a) mixing 25-85 volume percent particulate filler of an average size of 40
micron or less with polytetrafluoroethylene in aqueous dispersion,
(b) cocoagulating the filler and the polytetrafluoroethylene.
(c) lubricating the filled polytetrafluoroethylene with lubricant and paste
extruding the lubricated material to form a film,
(d) calendering the lubricated film,
(e) expanding said film by stretching it so as to form a porous
polytetrafluoroethylene having said filler distributed therein,
(f) in either order, laminating the conductive metal, and densifying the
stretched material by compressing it until the desired thickness is
obtained.
By expanding the polytetrafluoroethylene, as described in U.S Pat. No.
3,543,566, to form an expanded porous film comprised of nodes
interconnected with fibrils, the filler particles appear to collect around
the nodes and thus do not rub or roll to any appreciable extent when
subjected to compaction. Thus the expanded, filled PTFE can be densified
to form very thin films that are substantially free of pinholes or tears.
Once the thin filled film is obtained, or before densification, as the case
may be. a conductive metal, such as copper, is laminated to one or both
sides using conventional lamination procedures. The procedure can be a
batch procedure such as pressing at 1000 psi, 350.degree. C., under vacuum
for two hours or can be a continuous procedure such as is described in
U.S. Pat. No. 3,082,292. The copper can be formed into desired circuitry
configurations, and the laminate can thus be used as a power/ground layer.
When copper is laminated to both sides, the laminate can be used as a
capacitor, thus providing a reservoir of stored charge that supplies
charge to areas of the circuitry that become depleted as current is
demanded by the IC. By maintaining a high density of charge throughout the
circuit board by usage of the laminate of this invention, the voltage
spikes that ordinarily occur as the current pulses are reduced, thus
improving the fidelity of high speed signals.
By using the laminate of this invention as a distributive capacitor, the
need for individual capacitors in the circuit board is reduced or
eliminated, thus saving space on the surface of the board.
If desired, an organic polymer, such as a thermoset resin, can be present
in the thin film. Presence of such a polymer can lower lamination
temperatures and improve adhesion of conductive metal to the film.
In addition, reinforcing fillers can also be present to provide dimensional
control. Such fillers can be low dielectric ceramic fillers, such as
silicon dioxide.
Referring now to the drawings, FIGS. 1 and 2 depict laminates of the
invention, where 1 is the layer of filled polytetrafluoroethylene and 2 is
a layer of copper on one side of the PTFE film.
FIG. 3 represents a multilayer circuit board where the laminate of the
invention is denoted by 10 and comprises high dielectric filled PTFE 11
laminated to copper foils 12 and 13. which are of different voltages, 14
and 15 are copper grounds, 16, 17, 18 and 19 are copper signals, and 20,
21, 22 and 23 are low dielectric insulating material; 24 and 25 represent
low dielectric material and can be the same or different from those of
layers 20, 21, 22 and 23.
Test Procedures:
Matrix tensile strength testing was carried out on an Instron Model 1122.
Samples were one inch wide. Gauge length (distance between clamps) was two
inches. Samples were pulled at a rate of 500% per minute. Matrix tensile
strength is determined by the following equation:
##EQU1##
Wherein: .rho..sup.I =Intrinsic Density
.rho..sup.B =Bulk Density
TS Bulk=Bulk Tensile at Break
V.sub.I % PTFE=Intrinsic Volume % PTFE
EXAMPLES
EXAMPLE 1
A slurry of 11,866.8 g of Tioxide HPB titanium dioxide and 30 liters of
de-ionized H.sub.2 O was run through a colloid mill at 0.0025 cm. setting.
13.85 liters of de-ionized water were then added to the slurry under
agitation. While the slurry was agitated at 120 rpm, 7,273 g. of
polytetrafluoroethylene in a 16.0% solids PTFE dispersion was rapidly
poured into the slurry. The PTFE dispersion was obtained from ICI
Americas. Co. Within 35 seconds, the co-coagulation was complete. After 10
minutes, the coagulum had settled to the bottom of the mixing vessel and
the water was clear.
The coagulum was dried at 160.degree. C. in a convection oven. The material
dried in small, cracked cakes approximately 2 cm thick and was chilled to
under 10.degree. C. The chilled cake was hand ground using a tight,
circular motion and minimal downward force through a 0.635 cm mesh
stainless steel screen, then 0.46 cc of polypropylene glycol per gram of
powder was added. The mixture was chilled again, passed through a 0.635
cm. mesh screen, tumbled for 10 minutes, then allowed to sit at 18.degree.
C. for 48 hours and was retumbled for 10 minutes. It contained 42 volume %
filler.
The pellet was then ram extruded in tape form. The extrudate was calendered
through heated rolls at a reduction of 25% per pass to 0.057 cm. The
material was then stretched transversely 3.5 to 1 with the lubricant still
present. The lubricant was evaporated by running the tape across heated
rolls. The film was then stretched 5:1 transversely at 295.degree. C. and
133%/sec.
The expanded filled film was then layed up four (4) plies between copper
foil and pressed at 1000 psi in a vacuum assisted platem press at a
temperature of 350.degree. C. for two (2) hours then cooled under
pressure. This resulted in a copper laminate having a dielectric constant
of 10 and a 0.0025 inches dielectric film thickness, and a matrix tensile
strength of 3500 psi. The capacitance of the laminate is 900 picofarads
per square inch.
In general, the laminates of the invention will have a capacitance of
greater than 650 picofarads per square inch.
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