U.S. patent application number 16/788228 was filed with the patent office on 2021-08-12 for elastomeric conductive composite interconnect.
The applicant listed for this patent is TE Connectivity Services GmbH. Invention is credited to Megan Hoarfrost Beers, Ting Gao, Lei Wang.
Application Number | 20210249150 16/788228 |
Document ID | / |
Family ID | 1000004794287 |
Filed Date | 2021-08-12 |
United States Patent
Application |
20210249150 |
Kind Code |
A1 |
Wang; Lei ; et al. |
August 12, 2021 |
Elastomeric Conductive Composite Interconnect
Abstract
An elastomeric conductive composite for use in a moldable
interconnect is provided, said composite having a polymeric matrix
containing a crosslinked polymer, a curing agent for catalyzing
crosslinking of the polymeric matrix, conductive metal particles
and nonconductive compressible rubber particles dispersed within
the polymeric matrix. The nonconductive compressible rubber
particles have a greater compressibility than an elastomeric
conductive composite that is the same as the elastomeric conductive
composite but is free of non-conductive compressible rubber
particles. A moldable interconnect containing the elastomeric
conductive composite is also provided. This interconnect can be
used to provide electrical connection between two or more opposing
contacts or arrays of contacts for establishing at least one
electrical circuit.
Inventors: |
Wang; Lei; (San Jose,
CA) ; Beers; Megan Hoarfrost; (Redwood City, CA)
; Gao; Ting; (Palo Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TE Connectivity Services GmbH |
|
|
|
|
|
Family ID: |
1000004794287 |
Appl. No.: |
16/788228 |
Filed: |
February 11, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C08L 2203/20 20130101;
H01B 1/22 20130101; C08L 2205/025 20130101; C08L 2205/18 20130101;
H05K 1/09 20130101; C08L 83/04 20130101; C08L 2312/00 20130101 |
International
Class: |
H01B 1/22 20060101
H01B001/22; C08L 83/04 20060101 C08L083/04; H05K 1/09 20060101
H05K001/09 |
Claims
1. An elastomeric conductive composite for use in a moldable
interconnect, said composite comprising: a polymeric matrix
comprising a crosslinked polymer, said polymer comprising an
elastomer a curing agent for catalyzing crosslinking of the
polymeric matrix, and conductive metal particles and non-conductive
compressible rubber particles dispersed within the polymeric
matrix, the non-conductive compressible rubber particles having a
greater compressibility than an elastomeric conductive composite
that is the same as the elastomeric conductive composite but is
free of non-conductive compressible rubber particles.
2. The elastomeric conductive composite of claim 1, wherein the
polymeric matrix comprises silicone.
3. The elastomeric conductive composite of claim 1 wherein the
polymeric matrix has a Shore A hardness of more than 20.
4. The elastomeric conductive composite of claim 1 wherein the
conductive metal particles are selected from the group consisting
of silver, silver coated particles, gold, and gold coated
particles.
5. The elastomeric conductive composite of claim 1 wherein the
conductive metal particles have an average size of less than 50
.mu.m.
6. The elastomeric conductive composite of claim 1 wherein the
concentration of the conductive metal particles is 15 vol % to 40
vol % with respect to the polymer matrix.
7. The elastomeric conductive composite of claim 1 wherein the
non-conductive compressible rubber particles comprise silicone
microspheres.
8. The elastomeric conductive composite of claim 1 wherein the
non-conductive compressible rubber particles have an average size
of 0.1 to 1000 .mu.m.
9. The elastomeric conductive composite of claim 1 wherein the
concentration of the non-conductive compressible rubber particles
in the elastomeric conductive composite is of 1 vol % to 20 vol
%.
10. The elastomeric conductive composite of claim 1 wherein the
concentration of the non-conductive compressible rubber particles
in the elastomeric conductive composite is of 20 vol % to 60 vol
%.
11. A moldable interconnect comprising an elastomeric conductive
composite for use in a moldable interconnect, said composite
comprising: a polymeric matrix comprising a crosslinked polymer,
said polymer comprising an elastomer a curing agent for catalyzing
crosslinking of the polymeric matrix, and conductive metal
particles and non-conductive compressible rubber particles
dispersed within the polymeric matrix, the non-conductive
compressible rubber particles having a greater compressibility than
an elastomeric conductive composite that is the same as the
elastomeric conductive composite but is free of non-conductive
compressible rubber particles.
12. The moldable interconnect of claim 11, wherein the elastomeric
conductive composite is in the form of an array of elements held in
an insulating substrate.
13. The moldable interconnect of claim 12, wherein the elements are
pillars having opposite first and second ends, the pillars being
conductive between the first and second ends.
14. The moldable interconnect of claim 13, wherein the resistance
through each pillar is less than 1 Ohm
15. The moldable interconnect of claim 14, wherein the insulating
substrate has opposite first and second outer surfaces, the first
and second outer surfaces being planar and parallel to each
other.
16. The moldable interconnect of claim 11, wherein when the
elements are made of elastomeric conductive composite comprising at
least 20 vol % of non-conductive compressible rubber particles, the
compressibility of the elements is increased by at least 10% at a
given applied force compared to the compressibility of the elements
made of an elastomeric conductive composite similar to that of
claim 1 that is free of non-conductive compressible rubber
particles.
17. Use of a moldable interconnect comprising an elastomeric
conductive composite, said composite comprising: a polymeric matrix
comprising a crosslinked polymer, said polymer comprising an
elastomer a curing agent for catalyzing crosslinking of the
polymeric matrix, and conductive metal particles and non-conductive
compressible rubber particles dispersed within the polymeric
matrix, the non-conductive compressible rubber particles having a
greater compressibility than an elastomeric conductive composite
that is the same as the elastomeric conductive composite but is
free of non-conductive compressible rubber particles for an
electrical interconnect device used to provide electrical
connection between two or more opposing arrays of contacts for
establishing at least one electrical circuit.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention generally relates to an improved
elastomeric conductive composite for use in a moldable
interconnect.
Introduction to the Invention
[0002] Interconnect devices are used to provide electrical
connection between two or more opposing contacts or arrays of
contacts for establishing at least one electrical circuit, where
the respective arrays may be provided on a silicon package, an
electronic device, a printed circuit board or similar solutions. In
one interconnect technique, the electrical connection is provided
by an interconnect device that is physically interposed between
corresponding electrical contacts of the opposing arrays of
contacts.
[0003] Some known interconnect devices use an array of conductive
elastomeric columns supported on a substrate, columns which may be
compressed to establish reliable contact and provide the electrical
connection between the opposing contacts. In other known
interconnect devices, the elastomeric columns are non-conductive
and the electrical connection is provided via a separate contact or
trace.
[0004] Conductive elastomers offer several advantages as materials
for interconnect devices, for socketing high speed silicon
packages. They are moldable and easily formed into custom patterns
with inexpensive tooling. Furthermore, it is possible to achieve
small pitches and contact heights, as small as 0.5 mm for each, and
perhaps smaller. The contact resistance of molded contacts can be
very low, <15 m.OMEGA./pin, and the signal integrity is
excellent compared to metal contacts. For all of these reasons,
they are sought out for various applications, especially for those
that generally require low volumes of custom patterns along with
excellent electrical performance.
[0005] U.S. Pat. No. 6,271,482 (David R. Crotzer et al.), discloses
several types of electrical interconnects based on conductive
elastomers formed of a non-conductive elastic material, having a
quantity of conductive flakes and conductive powder granules
dispersed therein.
[0006] U.S. Pat. No. 7,726,976 and U.S. Publication Nos.
2012/0257366A1 and 2012/0258616A1 offer further examples of
metallized-particle interconnects (MPI) devices also based on
conductive elastomers.
[0007] However, the material requirements for moldable
interconnects are very challenging. Very low contact resistance
must be coupled with excellent mechanical properties and
processability. Specifically, the materials must have good
compressibility, especially for large arrays, to compensate for the
imperfect flatness of the substrates between which they are
compressed. In addition, they must maintain their performance for
long times under compression and heat exposure, meaning the polymer
chains in the elastomer must have minimal relaxation. As well, they
must be molded into contact array geometries with good mold release
properties.
[0008] In the past, alternative silicone materials have been
compounded with conductive metal particles to meet these
challenging mechanical and electrical requirements. The resulting
conductive composites can be molded into contact arrays. However,
the forces needed to compress the contact arrays are larger than
desired, and the durability of the contact arrays is less than
desired due to the creep and relaxation performance of the
material. Furthermore, the material formulations and geometries
that can be molded are limited by the mold release properties of
the materials.
[0009] U.S. Pat. No. 5,904,978 discloses an electrically conductive
composite article which is continuously conductive throughout the
structure of the material and is therefore suitable for
applications such as EMI shielding requiring a flexible and
conformable material. The composite comprises electrically
conductive particles and electrically nonconductive expanded hollow
polymeric particles. However, the resulting composite is deformable
under a low compression load. Additionally, the composite described
is solvent-processed and cannot be molded.
[0010] Several approaches have been tried such as adjusting the
crosslinking density of the conductive composites. The
compressibility of the conductive composites can be improved by
reducing the crosslinking density. Still, the creep and relaxation
performance of conductive composites becomes worse as the
crosslinking density is reduced.
[0011] Thus, new approaches are needed to improve the
compressibility of the conductive composites without deteriorating
other properties such as creep resistance, relaxation resistance,
compressibility, process ability and contact resistance.
BRIEF SUMMARY OF THE INVENTION
[0012] The present invention relates to an elastomeric conductive
composite for use in a moldable interconnect, said composite
comprising a polymeric matrix comprising a crosslinked polymer, a
curing agent for catalyzing crosslinking of the polymeric matrix,
conductive metal particles and nonconductive compressible rubber
particles dispersed within the polymeric matrix, the nonconductive
compressible rubber particles having a greater compressibility than
an elastomeric conductive composite that is the same as the
elastomeric conductive composite but is free of non-conductive
compressible rubber particles. The polymeric matrix can be filled
with a high loading of conductive and non-conductive particles by
conventional mixing technologies. The resulting composite offers
the advantage that the amount of compression at a given normal
force is greater for this formulation comprising nonconductive
compressible rubber particles when compared to a similar
formulation without such nonconductive compressible rubber
particles. The addition of soft compressible rubber particles into
the conductive composite allows excellent compressibility while
maintaining other key properties of the conductive composite. While
soft rubber particles have been used in the past to improve impact
resistance, tactile softness, light diffusion, and many other
material characteristics, in this invention the good
compressibility of soft rubber particles is used to improve the
compressibility of conductive composites. The resulting composite
is also easy to manufacture into an interconnect device.
[0013] According to a further embodiment, a moldable interconnect
is provided comprising the elastomeric conductive composite as
described above.
[0014] In a further embodiment, the use of the moldable
interconnect is provided for an electrical interconnect device used
to provide electrical connection between two or more opposing
arrays of contacts for establishing at least one electrical
circuit.
[0015] Accordingly, the primary object of the present invention is
to provide an elastomeric conductive composite with very low
contact resistance and good mechanical properties. It is also an
object of the present invention to provide an improved moldable
interconnect comprising the elastomeric conductive composite and
the use of the moldable interconnect for an electrical interconnect
device.
[0016] The above primary object as well as other objects, features
and advantages of the present invention will become readily
apparent from the following detailed description which is to be
ready in conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 illustrate an electrical interconnect system.
[0018] FIG. 2 is a graph of compression (%) v. force (g/pin)
depicting the results of testing performed on a texture analyzer
for two embodiments of the moldable interconnect article of the
present invention
DETAILED DESCRIPTION OF THE INVENTION
[0019] In the present invention, an elastomeric conductive
composite for use in a moldable interconnect is provided which
comprises a polymeric matrix comprising a crosslinked polymer, a
curing agent for catalyzing crosslinking of the polymeric matrix,
conductive metal particles and nonconductive compressible rubber
particles dispersed within the polymeric matrix. The nonconductive
compressible rubber particles have a greater compressibility than
an elastomeric conductive composite that is the same as the
elastomeric conductive composite but is free of non-conductive
compressible rubber particles.
[0020] Such a material may be effectively employed in a variety of
useful applications including moldable interconnect devices for the
data connection industry or socketing high speed silicon packages
where traditional structures struggle to support the signal
integrity performance requirements.
[0021] In general, a polymeric matrix material includes a
polymerization product of one or more monomers and a crosslinking
agent. The polymeric matrix of the present invention comprises an
elastomeric crosslinked polymer. Examples of polymers useful as a
matrix for the elastomeric composite of the present invention
include silicone, fluorosilicone, fluoroelastomer, epoxy, ethylene
propylene diene monomer rubber (EPDM), polyolefin (PO), or
polyurethane (PU). A silicone matrix offers the advantage of
providing an excellent combination of strength, elasticity,
performance over wide temperature range.
[0022] In a further embodiment, the polymeric matrix that comprises
an elastomeric crosslinked polymer, before adding conductive metal
particles or nonconductive compressible rubber particles, has a
Shore A hardness of more than 20, so as to ensure that after the
addition of the metal particles, required mechanical properties,
e.g., the balance between mechanical strength, compressibility and
creep and relaxation resistance, are obtained. The Shore A hardness
of the polymeric matrix before mixing with the conductive metal
particles should be of more than 20, preferably more than 30 and
most preferable comprised between 40 to 90.
[0023] The catalyzing agent, which can also equivalently be called
curing agent or hardener, functions as a catalyst to facilitate the
bonding of the molecular components of the material, and can for
example be a catalyst such as a peroxide, platinum, or tin
catalyst.
[0024] The polymeric matrix can take a high loading of conductive
metal particles by standard mixing technologies, such as batch,
roller, or centrifugal mixing. The high loading is such as to
exceed the percolation threshold of the conductive metal
particles.
[0025] According to another aspect of the present invention the
conductive metal particles should have very low electrical
resistivity and good stability to heat and humidity. Preferred
conductive particles include silver, nickel, aluminum, platinum,
copper, stainless steel, carbon, and gold. The particles may
comprise a metal coating to enhance conductivity. Preferred metal
coatings include silver, nickel, copper, and gold. Most preferred
are particles selected from the group consisting of silver, silver
coated particles such as silver-coated glass, silver-coated copper,
silver-coated nickel, silver-coated aluminum, gold, and gold coated
particles.
[0026] The conductive metal particles should be small so that they
can form conductive networks in small form factors. The average
size should desirably be less than 50 .mu.m, particularly less than
40 .mu.m, more preferably less than 30 .mu.m, especially less than
20 .mu.m, most preferably less than 12 .mu.m with the advantage
that a good conductivity in small geometries (the resulting contact
pins are generally <1 mm in diameter) is obtained and also
maintained after environmental conditioning such as heat,
heat/humidity, etc.
[0027] The conductive metal particles may be a blend of multiple
types of morphologies and shapes to facilitate higher conductivity.
For example, they can be spherical, flakes, granules, fibers,
layers, or other shapes, and combinations of these.
[0028] In one embodiment of the present invention, the
concentration of the conductive metal particles is 15 vol % to 40
vol %, preferably more than 20%, more preferably 20 vol % to 35 vol
%, more preferably 30 vol % to 35 vol %, more preferably 20 vol %
to 30 vol %, more preferably 25 vol % to 30 vol %, more preferably
20 vol % to 25 vol % of the polymer matrix. Those ranges of
concentrations allow achieving a percolated network of conductive
particles without sacrificing processability.
[0029] According to another improvement of the invention, preferred
nonconductive compressible rubber particles (also called soft
rubber particles) to be dispersed within the polymeric matrix are
silicone particles or silicone microspheres. The soft rubber
particles can be in the form of spheres, microspheres, powders,
flakes, tubes, cubes, ellipsoids and any other shape or morphology
which makes them suitable to be mixed in the polymeric matrix.
Available microspheres are essentially ball-shaped particles
adapted to be deformed under the action of a certain pressure. The
microspheres can be easily deformed by pressure and recover their
shape after the release of pressure, so that when distributed
within the matrix this will have an impact on the mechanical
behavior of the resulting composite.
The nonconductive compressible rubber particles have an average
size of 0.1 to 1000 .mu.m, preferably from 1 .mu.m to 100 .mu.m,
more preferably from 5 .mu.m to 30 .mu.m. The size needs to be
small enough not to disrupt conductive network of metal particles
in the contact array geometry (contacts are generally <100 .mu.m
in diameter) or negatively impact the mechanical integrity but also
large enough to provide compressability benefit.
[0030] The concentration of the nonconductive compressible rubber
particles should be below the upper limit that would destroy the
conductive network and/or the mechanical integrity and geometry of
the conductive composite so to maintain mechanical integrity and
conductivity, while should also be such as to optimize the
moldability and mold release properties and reduce flash. The
concentration should be 1 vol % to 60 vol % of the elastomeric
conductive composite, should preferably be from 1 vol % to 20 vol %
of the total conductive composite, most preferably from 5 vol % to
10 vol %, or from 20 vol % to 60 vol %, more preferably from 20 vol
% to 50 vol %, more preferably from 25 vol % to 35 vol %, most
preferably from 30 vol % to 40 vol %. Specifically, when using a
concentration of 5% to 10%, small pins of conductive composite
which are .about.0.7 mm tall and -0.3 mm wide can be obtained.
[0031] The nonconductive compressible rubber particles have a
greater compressibility than an elastomeric conductive composite
that is the same as the elastomeric conductive composite as
described above but is free of non-conductive compressible rubber
particles. The good compressibility of soft rubber particles allows
improving the compressibility of the conductive composite. It is
understood that the term compressibility can be defined as the %
that the height of an element is reduced under 40 g compressive
force.
[0032] When the elements, for example pillars that are 1.1 mm tall
with center diameters of 0.66 mm and top/bottom diameters of 0.46
mm, are made of elastomeric conductive composite comprising at
least 20 vol % of nonconductive compressible rubber particles, the
compressibility of the elements is increased by at least 10% at a
given applied force compared to the compressibility of the elements
made of an elastomeric conductive composite that is free of
non-conductive compressible rubber particles.
[0033] In order to form a conductive array, into the polymeric
matrix a desired amount of nonconductive compressible rubber
particles and the catalytic agent are added and mixed uniformly.
The desired amount of conductive particles is then added and mixed
uniformly. The elastomeric conductive composite can be then molded
into a conductive array which is made up of pillars for example
each roughly 1.14 mm tall with center diameters of 0.66 mm and
top/bottom diameters of 0.46 mm and then the compressibility can be
tested on a texture analyzer or dynamic mechanical analyzer. The
conductive array can also have different geometries, while the
pillars can also have different heights and diameters.
[0034] Mixing may occur by any suitable means including dry
blending of powders, wet blending, centrifugal mixing, compounding,
roll milling, etc.
[0035] FIG. 1 illustrates a moldable interconnect 100 comprising an
interconnect device 106 made of the elastomeric conductive
composite comprising the polymeric matrix 124, conductive metal
particles 126 and non-conductive compressible rubber particles 128
dispersed within the polymeric matrix in accordance with an
exemplary embodiment. Interconnect devices such as the one depicted
in FIG. 1 may be a board-to-board, board-to-device, or
device-to-device type of interconnect device, and are used to
provide electrical connection between two or more opposing arrays
of contacts for establishing at least one electrical circuit, where
the arrays may be provided on a silicon package, an electronic
device, a printed circuit board or the like. In particular, FIG. 1
is a cross sectional view of a portion of an interconnect device
106 interconnecting the electrical components 102 and 104 of the
moldable interconnect. For example, electrical component 104 may be
a circuit board, and electrical component 102 is an electronic
package such as a chip or processor, while interconnect device 106
can be a socket mounted to the circuit board and configured to
receive the chip. The first electrical component 102 and second
electrical component 104 are shown in a mated state. In an
exemplary embodiment, the interconnect device 106 includes a
plurality of compressible pillars 108 of elastomeric conductive
composite arranged in and held in a contact array of elements. The
wording `pillars` should be interpreted to have the same meaning of
`elements` columns' or `pins` and it's used to describe the molded
contact shape, which is frustoconical with a center diameter in
correspondence of a wider mid-section and with top/bottom diameters
in correspondence of narrower opposite first end 110 and second end
112. The pillars 108 within the array are arranged in a
predetermined pattern. The pillars 108 have opposite first ends 110
and second ends 112 with the pillars being conductive between the
first and second ends. The interconnect device 106 is mounted to
the second electrical component 104 such that the first end 110 of
each elastomeric column 108 engages the mating contact 114 provided
on the first electrical component 102, while the second end 112 of
each elastomeric column 108 engages the mating contact 116 provided
on the second electrical component 104. In other alternative
embodiments, the interconnect device 106 may be secured to the
second electrical component 104, such as by using latches,
fasteners or other means to mechanically hold the interconnect
device 106 on the second electrical component 104.
[0036] The array of elements or elastomeric pillars 108, is held in
an insulating substrate 118. The pillars 108 may be molded or
otherwise disposed within the insulating substrate 118. The
insulating substrate has opposite first and second outer surfaces,
the first and second outer surfaces 120 and 122 being planar and
parallel to each other.
[0037] In an enlarged view, a detail of each column is shown, with
the elastomeric conductive composite being illustrated as a
polymeric matrix 124 comprising, dispersed within, conductive metal
particles 126 and non-conductive compressible rubber particles
128.
[0038] Other examples of application for interconnect devices next
to connectors are sockets or interposers. Examples of other
applications of the elastomeric conductive composite are
electromagnetic interference (EMI) shielding gaskets, tubing and
cable jacket products.
[0039] The following procedure was used to determine the properties
of the material created in the following example:
Example
[0040] Two formulations were prepared based on a silicone polymeric
matrix. In the first formulation 30 vol % (total, based on the
final formulation composition) silicone microspheres with average
diameter 30 .mu.m were added and mixed uniformly using a
centrifugal mixer. In the second formulation no microspheres were
added. Then, in both, 25 vol % (with respect to the amount of
silicone) silver-coated copper powder was added and mixed in
uniformly. The compounds were then molded into conductive array
samples made up of pillars roughly 1.14 mm tall, with center
diameters of roughly 0.66 mm and top/bottom diameters of roughly
0.46 mm. The conductive array samples were then tested on a texture
analyzer. As shown in FIG. 2, the amount of compression at a given
normal force was greater for the formulation with microspheres
compared to the formulation without. When the samples were made of
an elastomeric conductive composite comprising at least 25 vol % of
non-conductive compressible rubber particles, the compressibility
of the elements was increased by at least 10% at a given applied
force compared to the compressibility of the elements made of a
similar elastomeric conductive composite not containing
non-conductive compressible rubber particles. The compressibility
is defined as the % that the height of an element is reduced under
a 40 g compressive force. The molded pillars made with the
formulation with microspheres have only a slightly higher
resistance of 241 mOhm/pin, compared to 75 mOhm/pin for the pillars
made with the formulation without microspheres. The resistance
through each pillar is less than 1 Ohm. The resistance was measured
from the top to the bottom of a pillar, and includes the contact
resistance with flat, gold-plated electrodes.
[0041] A further formulation was prepared based on a silicone
polymeric matrix, in which 5 vol % (total, based on the final
formulation composition) silicone microspheres with average
diameter 5 .mu.m were added and mixed uniformly. In this case, 0.69
mm tall elements with center diameters of 0.24 mm and top/bottom
diameters of 0.17 mm were obtained which could easily be released
from the mold, indicating an improvement in the moldability
compared to a similar formulation without silicone microspheres.
The flash between pins that resulted from the molding process was
reduced compared to a similar formulation without silicone
microspheres.
[0042] It is to be understood that the above description is
intended to be illustrative, and not restrictive. For example, the
above-described embodiments (and/or aspects thereof) may be used in
combination with each other. In addition, many modifications may be
made to adapt a particular situation or material to the teachings
of the invention without departing from its scope. Dimensions,
types of materials, orientations of the various components, and the
number and positions of the various components described herein are
intended to define parameters of certain embodiments, and are by no
means limiting and are merely exemplary embodiments. Many other
embodiments and modifications within the spirit and scope of the
claims will be apparent to those of skill in the art upon reviewing
the above description. The scope of the invention should,
therefore, be determined with reference to the appended claims,
along with the full scope of equivalents to which such claims are
entitled. In the appended claims, the terms "including" and "in
which" are used as the plain-English equivalents of the respective
terms "comprising" and "wherein." Moreover, in the following
claims, the terms "first," "second," and "third," etc. are used
merely as labels, and are not intended to impose numerical
requirements on their objects. Further, the limitations of the
following claims are not written in means--plus-function format and
are not intended to be interpreted based on 35 U.S.C. .sctn. 112,
sixth paragraph, unless and until such claim limitations expressly
use the phrase "means for" followed by a statement of function void
of further structure.
* * * * *