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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to electrical contacts and connectors made of
metal-filled or carbon-filled resilient elastomers molded on or bonded to
electrically conductive substrates and particularly to electrical
connectors having a plurality of elastomeric contacts in a predetermined
arrangement.
2. Description of the Prior Art
Electrical connectors having mechanical metal contacts which mate in some
fashion to provide a continuous electrical path often provide for some
relative motion between the mating contacts to effect the wiping action,
thus ensuring proper connection. Where a plurality of such contacts are
positioned in an electrically insulative holder or frame of sorts, some
relative motion between the contacts is also generally provided to ensure
proper alignment with the mating contacts. If a large number of contacts
are so situated in a single holder or frame for simultaneous contact or
where repeated making and breaking of the contacts is experienced,
misalignment, wear, bending, shorting and other types of circuit failure
are commonly experienced. Metal contacts have the additional difficulties
of being generally difficult to reproduceably fabricate and subject to
fatigue when under continuous use.
Any wiping action abrades the contact surfaces by removing plating and
damaging soft conductors such as indium oxide and tin oxide.
Metal-pressure contacts often become misaligned due to shock and
vibration.
Where a permanent or semipermanent electrical connection is desired, but
where soldering is impossible or undesirable, metal to metal contacts
experience surface abrasion due to the wiping action of the initial
contact which in time is subject to corrosion and increased contact
resistance. The actual contacting area of a metal to metal contact is
typically less than one-thousandth of the total surface area of the metal
contact. Moisture or other hostile atmosphere can then migrate between the
contact surfaces and rapidly deteriorate the quality of the metallic
joint.
To overcome these problems, interconnector frames carrying conductive
elastomeric contacts have been proposed to interface between two
conductors or terminals. Examples are to be found in U.S. Pat. Nos.
3,648,002 and 3,680,037. The interconnectors add additional weight, take
up usable space and increase lead length and circuit resistance. Another
patent of general interest in this area is U.S. Pat. No. 3,509,296.
The composition of conductive elastomers which can be used in contacts
according to this invention may be found in U.S. Pat. Nos. 3,140,342;
3,412,043 and 3,609,104, as well as others.
SUMMARY OF THE INVENTION
A contact of a conductive elastomer, integrally fixed to a first conductor
or conductive substrate by bonding or molding in place, is situated to
contact an opposing or mating electrical conductor under the influence of
an externally applied force. The conductive elastomer "elastically
deforms" under the influence of the external force, sealing the contacted
surface of the mating conductor against encroachment of liquids or gases
thereby inhibiting or preventing corrosion. When salt and spray tests have
been performed on the elastomeric contacts herein described, the areas of
the conductor beyond the elastomeric contact experienced corrosion while
the area under the contact shielded from the salt spray showed negligible
deterioration.
The elastomeric contacts can be precisely molded into any desired
configuration with a high degree of reproducability. The hardness, shape
and so forth of the contact can be tailored to produce a wide variety of
electrical and mechanical characteristics. Current density of 20 amperes
per square centimeter can be achieved with the correct choice of material
and design. Connectors can be as thin as 0.2 mm. and center-to-center
distances of less than 1.0 mm. are possible.
No soldering or other heat producing processes are needed to bond the
elastomeric contacts to the conductors, and therefore, heat sensitive
components can be safely attached in this manner. Since the contacts and
the conductors upon which the contacts are integrally fixed do not have to
be solderable, difficultly solderable materials such as nickel, cadmium,
stainless steel, conductive plastics, conductive coatings and tin oxide
can be used directly with no preliminary treatment. The conductive fillers
dispersed in the elastomeric contact produce little or no significant
corrosion on the conductor surfaces due to galvanic action.
The elastomeric contacts herein described are particularly adapted to the
rapid assembly of electronic packages, circuit boards and the like into
larger systems since continuity is established by merely clamping the
mating surfaces together. Automatic assembly is simplified because a
degree of misalignment in the contact pads can be tolerated. Electronic
packages having a multitude of contacts can be produced at very low cost
and at high volumes, since the costs do not rise as rapidly as the
complexity of the configuration of the contacts or their number increases.
Likewise, the disassembly of the unit and replacement of the parts is easy,
fast and virtually foolproof. Inexpensive subassemblies can be recovered
without damaging them or other components in the vicinity. The elasticity
of the connectors allows for non-parallelism in the mating boards while
lead straightening and positioning are virtually eliminated.
Whole circuit boards can be solder dipped to ensure continuity even after
the elastomeric contacts are attached, as the contacts are not wetted or
deteriorated by the solder. The molded elastomeric contacts can be used to
tie double-sided circuit boards together, eliminating the need for
plated-through holes. Extra components can be added after soldering
without loosening the previously made joints.
Other features and advantages of the elastomeric contacts herein described
will become apparent from the following description of preferred
embodiments, figures and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of conductors on a circuit board with
elastomeric contacts of this invention fixed to the conductors.
FIG. 2 is a sectional detail showing a circuit board with contacts
according to this invention.
FIG. 3 is a sectional detail of an apertured contact according to this
invention with an electrical conductor in the aperture.
FIG. 4 is a perspective detail of a circuit board with apertured and
non-apertured contacts.
FIG. 5 is a perspective view of contacts according to this invention used
on the end of a printed circuit.
FIG. 6 is a plan view of a button switch using contacts according to this
invention.
FIG. 7 is a sectional view of the button switch of FIG. 6 cut along line
7--7.
FIGS. 8a and b are diagrammatic representations of a unit cube and a unit
cylinder, respectively.
FIG. 9a and b are diagrammatic representations of the compression
experiences by the unit elements of FIGS. 8a and b respectively.
FIG. 10 is an empirical curve of compressive strain as a function of shape
factor for a fixed stress.
FIG. 11 is a diagrammatic representation of a means for making the contacts
according to this invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Electrical contacts and connectors made of resilient elastomers
incorporating a carbon or metallic filler can be molded on or bonded to
electrical conductors and adapted to serve as terminals, switches, probes
and the like. While elastomers such as polyurethane, plasticized vinyl,
neoprene, butyl rubber and EPDM, can be used in specific applications, the
preferred elastomer is silicone rubber. Silicone rubber has low
compression set, is operable over a wide temperature range (typically
-75.degree. to +200.degree.C) is resistant to most environments, and is
stable on aging.
The choice of conductive filler will depend on the resistance required,
temperature range experienced, environments, mating materials, and the
physical properties such as compression set and strength. For contacts
having a resistivity in the range of 1-5,000 ohm-cm., the conductive
particle to be incorporated in the elastomer is generally carbon.
Carbon-filled compounds have the highest strength, the greatest range of
hardness, and are easiest to mold. Carbon has the additional advantage of
being the least expensive conductive filler.
For the lowest resistivities, down to fractions of a milliohm-cm., silver
is the preferred filler. It is used in various sizes and configurations
such as powder, fibers, chopped wire and flakes having a wide range of
aspect ratios and sizes. Silver can also be coated onto glass beads,
copper powder and a variety of other metallic and non-metallic substrates
to produce less expensive fillers.
An example of such a compromise is silver plated copper. This material has
the conductivity of pure silver, but at a fraction of the cost.
Unfortunately, it cannot be used for long times above 120.degree.C, as the
copper will migrate through the silver and oxidize, thus causing a loss of
conductivity. The optimum balance of properties is obtained for each
application by choosing the correct combination of conductive and
non-conductive fillers.
For intermediate conductivities and in areas where silver might not be
compatible with other materials in the system, but where corrosion
resistance is necessary, nickel can be used as a filler. Other metals such
as tin, zinc, gold, conductive oxides and combinations of these have been
tried as conductive fillers in elastomers with varying success.
Typically an elastomer does not become conductive at metal loadings below
1.5 volume percent while the maximum necessary loading is 50 volume
percent. The proportion by weight of metal filler is dependent upon the
specific gravity of the particular metal and elastomer concerned and may
also depend on the particular size and shape of metal filler. It is
generally thought that there must be particle to particle contact to
transmit electricity through the body of the elastomer. It has been
observed that in general the thermal conductivity of the elastomer is
increased by a factor of 5 to 10 by the addition of sufficient metallic
fillers. The order and method of incorporation of the various fillers
used, mold design, storage time, fabrication technique, temperature and
time of cure, temperature and time of post-cure, can all have dramatic
effects on the conductivity of the finished elastomeric contact.
A typical electronic package shown in FIG. 1 generally as 14 consists of a
printed circuit board 16 conventionally made of a laminate such as an
epoxy-glass cloth or a phenolic-paper. The circuit board 16 has at least
one and typically a plurality of conductors 12 fixed to a surface thereof.
Conductive elastomeric contacts 10 are integrally fixed to the conductors
12 by either bonding the contact buttons 10 to the conductors with an
electrically conductive cement for adhesive, or by molding the contact
buttons in position directly on the electronic package. The choice of
materials of the circuit board 16 may in part depend upon whether the
contacts are to be bonded or molded, and if molded, the temperatures and
times of cure and post-cure necessary for the particular conductive
elastomer.
The elastomeric contacts are useful for connecting conductors on opposite
sides of a circuit board, as shown in FIG. 2. A circuit board 16 carrying
a conductor 12 on at least a portion of a surface of the board can have
bonded, molded, or otherwise integrally fixed to a surface 18 of the
conductor 12 an elastomeric contact button 10. The elastomeric contact may
be given greater strength by integrally fixing a contact, such as 20,
through a penetration of the conductor 12 and the circuit board 16. This
is particularly advantageous where the conductive elastomeric contact is
to be subjected to tangential or other motion which may cause shearing
forces to develop. Where it is not feasible that the button be designed to
penetrate the circuit board, the same advantage can be obtained by
designing the button to cover both the conductor and some of the
surrounding nonconductive area to increase the adhesion of the contact
button to the electronic package assembly.
A conductive elastomeric contact such as 22 is useful for connecting
conductors on opposite sides of a single circuit board, thus replacing the
conventional plated-through holes in a circuit board which has often
proved to be unreliable as a result of differential thermal expansion
between the plating and the substrate or imperfect plating. The contact 22
penetrates the circuit board 16 and the two opposite conductors 12 and 24
and is integrally fixed to at least one of the conductors.
The contact 20 or 22 of FIG. 2 can be modified or made to receive a lead
26, as shown in FIG. 3. The contact 28 can either be sufficiently
penetrable or apertured to receive the conductor 26, essentially coaxially
with that portion of the contact penetrating the circuit board. Contacts,
or sockets, such as 28 can be advantageously used in connection with
"bread boarding" and temporary testing circuits, as well as in finished
"assembly line" construction. No pretreatment except cleaning of the
conductor 26 is necessary to achieve satisfactory low contact resistance.
Conductive paths 30 and resistive paths 32 can be screened onto a printed
circuit board 16, as shown in FIG. 4. The conductive elastomeric contact
buttons 34 and apertured contact buttons 36 can then be affixed to the
conductive paths or molded through appropriate holes in the conductive
path and supporting circuit board. The conductive button can be designed
to also introduce an appropriate contact resistance, if desired, to match
the requirements of the circuit.
Conductive elastomeric contacts can also be used as edge connectors for
printed circuit boards, liquid crystal displays, flat etched cable, etc.,
as in FIG. 5. It is particularly advantageous to use the screw or snap-in
type retaining mechanism or other means 38 for maintaining the physical
proximity between the two members to be joined in this fashion. The
conductive contacts 40 can be fixed to either the edge of the board 42 or
the surface of the board 44. In either instance, the contacts are
integrally fixed to the conductors of the first board and are adapted to
abut the conductors of a second mating board in such a manner as to
electrically connect the conductors of the first and second board.
One of the two mating boards can be replaced by a housing containing an
integrated circuit, capacitors, coils, or other appropriate subassemblies.
In the case of integrated circuits, conductive elastomeric contacts can
also be molded on or otherwise integrally fixed to the wire ends
projecting from a conventional integrated circuit protective case.
The conductive elastomeric contacts can be used to make a switch as shown
in FIGS. 6 and 7, wherein the elastomeric buttons 46 will serve both as
the compliant contacts for the actuator 48 and as the elastic member of
the switch holding the actuator 48 in position. The actuator need only
have a conductive surface 50 to complete the circuit between elastomeric
contacts 46 and a third elastomeric contact 52. The compliant contacts 46
can be designed to produce a wide range of closing forces. Switches have
been made using this concept, wherein the total button height before
compression was in the neighborhood of 0.7mm and the buttons were
depressed a distance of from 0.1 to 0.4 mm. This is not to be viewed as
the complete range of operability, but rather a typical situation
employing the buttons as the elastic member of the switch as well as the
contact for the switch. Different compliance situations will require that
the buttons have different stress relaxation as a function of time,
strain, composition and shape.
The button shape and height is generally dictated by the function which the
button will serve, the modulus of the elastomer, the number of buttons
making simultaneous contacts, the required substrate flatness, the
conductivity required, and the force available to make contact. It will be
appreciated that a great variety of contact button shapes are available,
including round, semi-cylindrical, square and concentric circular, as well
as parallelepiped and truncated pyramids.
In order to be able to design the elastomeric contacts for optimum utility,
several variables, such as shape factor, must be taken into account. Shape
factor is the ratio of the area of the loaded surface to the total area of
the unloaded surface free to expand. FIG. 8a shows an elastomer in the
shape of a cube with the length of each edge being a unit dimension. The
area of the surface to be loaded 54 is then equal to the unit dimension
squared. The total area of the surfaces which are free to expand 56 is
equal to four times the unit dimension squared. Thus, the shape factor for
a cube is 0.25, that is, the area of surface 54 divided by the total area
of surfaces 56.
FIG. 8b shows a cylindrical shape having a height of the same unit length
and an area for the surface to be loaded, 54, equal to one unit squared,
which is the same as in FIG. 8a. If surface 54 of FIG. 8b has an area of
one unit squared, then the radius of surface 54 must be approximately
0.564. The area 56, which is free to expand under a load, is then equal to
3.54 units square and the cylinder has a shape factor of 0.282.
FIGS. 9a and 9b diagrammatically represent the elastomers of FIG. 8a and
8b, respectively, when placed under a load of 1,000 psi. It is noted that
the elastomer having the larger surface area in which to expand, that is a
smaller shape factor, will experience a greater compression under the same
load conditions.
FIG. 10 is a graph showing the compressive strain, that is, the deflection
experienced expressed as a percent, as a function of the shape factor.
This curve was achieved by examining the compression characteristics for a
DuPont polyurethane elastomer having a shore A hardness of 60 under a
pressure of 200 psi. It will be noted that as the shape factor decreases,
the compressive strain increases; that is, as the ratio of free area to
expand 56 to load area 54 increases, the compressive strain also
increases, but not in a linear fashion. Curves for conductive elastomers
are similar in shape, but not identical. If the physical dimension of a
particular elastomeric contact are set by other considerations, such as
travel space, or available compressive force, then changes in button
characteristics can be achieved by the selection of differing elastomers.
If, on the other hand, the choice of elastomers is set by reason of
availability of materials or curing temperature considerations, proper
operating characteristics may be achieved by modifying the button shape to
achieve the proper shape factor.
FIG. 11 diagrammatically depicts an apparatus for transfer molding the
elastomeric contacts onto an electronic assembly circuit board or other
substrate 58, which may generally be carrying a plurality of electric
conductors 60 as well as other electronic components. The transfer molding
apparatus consists of a base template 62 and a transfer template 68, each
having appropriate depressions 64 and 70, respectively, for defining an
elastomeric contact surface. The base template 62 and transfer template 68
together form a mold cavity in which the substrate 58 is positioned. The
transfer template 68 also consists of a charging chamber 72 for receiving
the charge of elastomer previous to molding and communicating channels 74
between the charging chambers 72 and depressions 70 for permitting the
elastomers to be transferred. Ram 76 snuggly seals the charging chamber 72
for exerting the proper pressure on the elastomeric charge to plasticize
and force the elastomer through the channels 74. A means for heating the
upper template and elastomeric charge is located in the upper template 68.
Means 66 for heating the lower template and for assisting in curing the
elastomer when transferred is positioned below the lower template.
In operation, a circuit or other substrate 58 is positioned within the mold
cavity formed by the upper template 68 and lower template 62 and the two
templates joined together securing the substrate 58 in position. A charge
of conductive elastomer is placed in the charging cavity 72, heating means
66 and heating means 78 are then brought to the appropriate temperature.
Ram 76 is then forced into the charging chamber or transfer pot 72,
transferring the elastomer from the transfer chamber through the channels
74 and into the contact area formed between the depression surfaces 64 and
70 the substrate 58. The heating means 66 and 78 continue heating while
the pressure on the ram remains at a prefixed amount for curing the
elastomer in place in the circuit board. The pressure on the ram 76 is
then released, the mold cavity taken apart, and the circuit board with
elastomeric contacts removed from the mold cavity.
The following is an example of the production of an electronic package
having silver-filled silicone buttons molded onto the leads of the
package. The compound used was:
TABLE 1
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Material Weight
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Silicone rubber compound
Devolatilized methyl vinyl polysiloxane gum
containing reforcing fillers, processing
aids and stabilizing agents
(Rhodia Corp. Compound RE-650)
19.8%
Dicumyl peroxide carried on carrier of
precipitated calcium carbonate, 40% active
(Hercules, Inc., Di-Cup 40C)
0.2%
Silver coated copper powder
6% by weight Silver on copper powder
Particle size, 10-50 microns
(Boronite Corp. Silvered Copper 5315)
65.0%
Silver powder
Average particle diameter 3.0-4.0 microns
Apparent density 16-19 gms/in.sup.3
(Metz Metallurgical Corp., EG-200)
5.0%
Silver flake
Average particle diameter 10.0 microns
Average particle thickness 1.5 microns
Apparent density 20-27 gms/in.sup.3
(Metz Metallurgical Corp., Ag Flake No. 6)
10.0%
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An electronic package was positioned in the appropriate mold cavity and the
mold assembled. The molding compound listed above was placed in the
transfer pot. The material was transferred and cured at a pressure of
20,000 psi, and at a temperature of 340.degree.C. for 10 minutes. The cull
was removed, the mold opened and the assembly removed. Twenty-four
conductive rubber buttons had been formed on the 24 copper lead ends with
a resistance of less than 1 ohm from each button to the internal wiring.
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