Noble metal and solid-phase lubricant composition and electrically conductive interconnector

A noble metal and solid-phase lubricant composition and an an electrically conductive interconductor including the electrically conductive composition are disclosed. The electrically conductive composition includes a noble metal component and a solid-phase lubricant component. The solid-phase lubricant component is present in an amount sufficient to cause the electrically conductive composition to have a coefficient of friction which is significantly lower than the coefficient of friction of the noble metal component without causing the electrically conductive composition to be significantly less malleable than the noble metal component, nor to be significantly less corrosion resistant than the noble metal component. The electrically conductive composition can form a contact layer of the electrically conductive interconnector. The contact layer is bonded to a diffusion barrier which, in turn, is bonded to a bulk electrical conductor of the electrically conductive interconnector.

BACKGROUND OF THE INVENTION 
Many electrical and electronic devices (such as electronic connectors and 
switches) must exhibit very high reliability. For example, switches that 
are used to trigger the release of automobile air bags often are required 
to remain operational, despite non-use, over extended periods of time. In 
another example, electronic connectors used in high-speed data 
transmission at conditions which include relatively low-voltage and 
low-current generally must operate without failure in order to prevent 
interruptions in data transmission. However, electrically conductive 
interconnectors within such devices typically are formed of metals which 
can corrode after wear at surfaces exposed to the atmosphere. Corrosion at 
surfaces where contact is made often significantly reduces the lifetime 
reliability of electronic devices which include such interconnectors. 
One attempt to improve the reliability of electronic devices is to bond a 
relatively non-corrosive electrically conductive contact layer to 
electrically conductive interconnectors at surfaces where contact, such as 
during switch closure. Contact layers are typically formed of a noble 
metal or an alloy thereof. However, noble metals are relatively expensive. 
As a result, contact layers generally are fabricated to be as thin as 
possible without causing failure under expected use-conditions. Also, 
noble metals are relatively soft and, therefore, can wear away during 
repeated operation of electronic devices. The relatively corrosive metal 
beneath the contact layers can thereby be exposed to the atmosphere, 
ultimately causing failure of these electronic devices. 
Liquid lubricants have been applied to surfaces of contact layers in an 
attempt to reduce wear. However, many liquid lubricants are considered 
hazardous, especially during their application, which often involves use 
of volatile chlorinated hydrocarbon dispersants. In addition, liquid 
lubricants can become unevenly distributed on contact layer surfaces and 
can evaporate or creep away, thereby causing portions of the contact 
layers to be exposed to conditions which can result in excessive wear and 
consequent premature failure. Additionally, liquid phase lubricants 
typically attract dust and abrasive particles from the atmosphere which 
accelerate wear and corrosion in the contact area, thereby resulting in 
significantly reduced contact reliability. Also, many liquid lubricants 
are relatively poor electrical conductors, thereby causing relatively high 
electrical resistance across closed contact surfaces and possible failure 
of electronic devices which include such contact surfaces. 
Solid-phase lubricants have also been applied to the surfaces of contact 
layers in an attempt to reduce wear. Commonly used solid-phase lubricants 
include graphite, molybdenum disulfide and various plastics. Typically, 
these have been applied by air-spraying, sputtering and ion plating. 
However, the wear durability of these surface coatings is limited because 
the motion of sliding contacts tends to plow away the solid-phase 
lubricant from the wear track, thereby leaving a pile-up of lubricant and 
wear-debris at the ends of the wear track. Also, solid-phase lubricants 
typically are poor electrical conductors, thereby causing high electrical 
resistance across contact surfaces which come to rest upon a particle of 
the solid-phase lubricant. 
Thus, a need exists for an electrically conductive composition and an 
electrically conductive interconnector which overcome or minimize the 
above-mentioned problems. 
SUMMARY OF THE INVENTION 
The present invention relates to a new electrically conductive composition 
and a new electrically conductive interconnector for an electrical 
circuit. 
An electrically conductive composition includes a noble metal component and 
a solid-phase lubricant component. The solid-phase lubricant component is 
present in an amount sufficient to cause the electrically conductive 
composition to have a coefficient of friction which is significantly lower 
than the coefficient of friction of the noble metal component without 
causing the electrically conductive composition to be significantly less 
malleable than the noble metal component. 
An electrically conductive interconnector for an electrical circuit 
includes a bulk electrical conductor and a diffusion barrier which is 
bonded to a surface of the bulk electrical conductor, whereby significant 
diffusion of the bulk electrical conductor across the diffusion barrier is 
prevented. A contact layer is bonded to the diffusion barrier, the contact 
layer being formed of an electrically conductive composition including a 
noble metal component and a solid-phase lubricant component. The 
solid-phase lubricant component is present in an amount sufficient to 
cause the electrically conductive composition to have a coefficient of 
friction which is significantly lower than the coefficient of friction of 
the noble metal component without causing the electrically conductive 
composition to be significantly less malleable than the noble metal 
component. 
The present invention has many advantages. The noble metal component is 
relatively non-corrosive, thereby preventing significant corrosion at the 
contact layer. The solid-phase lubricant component will not evaporate or 
creep away. In addition, the solid-phase lubricant component causes the 
contact layer to have a coefficient of friction which is significantly 
lower than that of the noble metal component of the composition. Wear of 
the contact layer during opening and closing of an electronic device 
including an electrically conductive interconnector of the invention is 
thereby significantly diminished. As a result, the probability of failure 
of the contact layer and subsequent failure of the electronic device is 
significantly reduced. Also, the amount of solid-phase lubricant component 
present does not cause the malleability of the composition to be 
significantly less than that of the noble metal component of the 
composition. Contact layers formed of the electrically conductive 
composition can thereby be fabricated using known methods of forming 
contact layers which include noble metals.

DETAILED DESCRIPTION OF THE INVENTION 
The features and other details of the composition and of the electrically 
conductive interconnector of the invention will now be more particularly 
described with reference to the accompanying drawings and pointed out in 
the claims. The same number present in different figures represents the 
same item. It will be understood that the particular embodiments of the 
invention are shown by way of illustration and not as limitations of the 
invention. The principle features of this invention can be employed in 
various embodiments without departing from the scope of the invention. 
In one embodiment of the invention, shown in FIG. 1, an electrically 
conductive composition 10 includes a noble metal component 12 and a 
solid-phase lubricant component 14. Electrically conductive composition 10 
is suitable for use as a contact layer of an electrically conductive 
interconnector in an electronic device, not shown. 
A suitable noble metal component 12 can include, for example, noble metals 
and alloys thereof which are suitable for forming an electrically 
conductive contact layer of an electrical interconnector. Examples of 
suitable noble metals for use in noble metal component 12 include gold, 
silver, platinum, palladium, etc#An example of a suitable noble metal 
alloy is a noble metal including about sixty-nine percent gold, about 
twenty-five percent silver and about six percent platinum, by weight. In a 
particularly preferred embodiment, noble metal component 12 is gold. 
A suitable solid-phase lubricant component 14 is a solid at the expected 
use-conditions of an electrically conductive interconnector and can cause 
electrically conductive composition 10 to have a coefficient of friction 
which is significantly lower than the coefficient of friction of noble 
metal component 12 without causing significantly less malleability of 
electrically conductive composition 10 than noble metal component 12. 
Also, the solid-phase lubricant does not cause the resulting electrically 
conductive composition to be significantly less corrosion resistant than 
the noble metal component. In a particularly preferred embodiment, the 
amount of solid-phase lubricant present is sufficiently low to cause the 
electrical resistance of the electrically conductive composition to be 
less than ten percent greater than the electrical resistance of the noble 
metal component of the electrically conductive composition. 
A "significantly lower coefficient of friction," as that phrase is used 
herein, means a coefficient of friction which is sufficiently lower than 
the coefficient of friction of noble metal component 12 to allow 
significantly reduced wear of electrically conductive composition 10 
during formation of an electrical interconnection. Preferably, the 
coefficient of friction of electrically conductive composition 10 is less 
than about 50% that of noble metal component 12. 
An example of wear is loss of a portion of electrically conductive 
composition 10 of a contact layer by contacting the contact layer with a 
mating contact surface, not shown, of an electrical conductor to thereby 
form an electrical interconnection. In one embodiment, wear is 
significantly reduced when the contact layer can contact a bulk electrical 
conductor at least twice as many times as can a contact layer formed of 
noble metal component 12 alone, without exposing the material to which the 
contact layer is bonded to conditions sufficient to corrode the material 
in an amount sufficient to prevent electrical conduction across the bulk 
electrical conductor. 
"Without causing significantly less malleability than the noble metal 
component," as that phrase is used herein, means that malleability is 
sufficient to allow forming of a material, such as electrically conductive 
composition 10, into a contact layer having the same thickness as a 
contact layer formed only of noble metal component 12. Preferably, 
electrically conductive composition 10 has a malleability which is 
sufficient to allow bonding and rolling without cracking. A typical 
measure of malleability is a bend test, such as the standard Longitudinal 
Bend Test (ASTM E290, Arrangement C, FIG. 6, described by the American 
Society for Testing and Materials (hereinafter "ASTM")). Material which 
meets this test is capable of forming a 180.degree. bend angle with a bend 
radius equal to the material thickness without cracking of the materials. 
In one embodiment, solid-phase lubricant 14 is a suitable carbon-containing 
compound. Preferably, the carbon-containing compound is graphite having a 
particle size of less than about one micron following formation of 
electrically conductive composition 10. The amount of solid-phase 
lubricant component 14 present in electrically conductive composition 10 
is sufficient to cause electrically conductive composition 10 to have a 
coefficient of friction which is significantly lower than that of noble 
metal component 12 without causing electrically conductive composition 10 
to be significantly less malleable than noble metal component 12. For 
example, when noble metal component 12 includes gold and solid-phase 
lubricant component 14 includes graphite, the graphite is preferably 
present in electrically conductive composition 10 in an amount in the 
range of between about 0.01 and about ten percent by weight. In a 
particularly preferred embodiment, the graphite is present in an amount in 
the range of between about 0.1 and about one percent by weight. 
Noble metal component 12 and solid-phase lubricant component 14 are 
combined to form electrically conductive composition 10 by a suitable 
method, such as by powder compaction, a method known in the art. For 
example, in one illustration of forming electrically conductive 
composition 10, a gold powder having a particle size in the range of 
between about 2 and about 20 microns is mixed by a suitable method with 
graphite powder having a particle size of about 10 microns. The combined 
gold and graphite powder can be mixed in a suitable powder mixture 
apparatus, such as is known in the art. 
The mixture of gold and graphite powder is poured into a metal die or a 
rubber mold and exposed to a pressure of about 1.times.10.sup.5 psi by a 
suitable means to form a powder compact which is suitable for sintering. 
Preferably, the powder compact has dimensions of about one by two by 
twelve inches. Alternatively, the powder may be compacted in the form of a 
cylinder having a diameter of about four inches and a length of about 
twelve inches. An example of a suitable means for compressing the gold and 
graphite powder mixture is an isostatic hydraulic press, such as is known 
in the art. 
The powder compact is then sintered in an inert atmosphere, such as argon 
or nitrogen, in a suitable sealed furnace to form a sintered bar. An 
example of a suitable furnace is an electrically heated furnace, such as 
is known in the art. The powder compact is sintered in the furnace at a 
temperature in the range of between about 800.degree. C. and about 
1000.degree. C. and at about atmospheric pressure for a period of time 
sufficient to cause the powder compact to be formed into a sintered bar 
having a density which is at least ninety-eight percent of the theoretical 
density of the gold and graphite mixture. Preferably the powder compact is 
sintered for a period of time in the range of between about one and about 
twelve hours. 
The sintered bar is subsequently cooled to about room temperature and 
rolled by a suitable rolling mill under a pressure of at least about 
1.times.10.sup.5 psi to form a rolled bar. An example of a suitable 
rolling mill is a Stanat Model TA-315 rolling mill, commercially available 
from Stanat Manufacturing Co., Inc. The thickness of the sintered bar is 
reduced by rolling from about one inch to about one-half inch. 
Following rolling, the rolled bar can be machined by a suitable means if 
needed to remove rough edges to form a rolled and machined bar. An example 
of a suitable means for machining the rolled bar is a Model 146 rotary 
shear slitting machine, commercially available from Ruesch Machine Co. 
The rolled and machined bar is then annealed by exposing the bar to a 
temperature in the range of between about 800.degree. C. and about 
1000.degree. C. in an inert atmosphere for a period of time in the range 
of between about one and about four hours. Rolling, slitting and annealing 
are repeated until a contact layer strip is formed of electrically 
conductive composition 10, wherein the contact layer strip has a thickness 
in the range of between about 3.times.10.sup.-3 and about 
3.times.10.sup.-2 inches. Preferably, the sequence of rolling, slitting 
and annealing is repeated between about five and about seven times. 
The contact layer strip can be rolled after the last annealing iteration. 
In addition, the contact layer strip can be flattened by a suitable 
method, such as is known in the art, to remove waves and ripples from the 
strip. The contact layer strip is then slit to a suitable width for 
forming a contact layer. 
In one embodiment of the invention, shown in FIG. 2, electrically 
conductive interconnector 16 includes contact layer 18 which is formed of 
the electrically conductive composition of the invention, as described 
above. Contact layer 18 is bonded to diffusion barrier 20 which is, in 
turn, bonded to bulk conductor 22. Electrically conductive interconnector 
16 is suitable for forming an electrical interconnection, such as in an 
electronic device, to close a circuit, not shown. Contact is established 
during formation of the electrical interconnection at contact layer 18 so 
that an electrical current can be conducted across electrically conductive 
interconnector 16. 
Electrically conductive interconnector 16 is formed from bonded metal strip 
24, which is also shown in FIG. 2. Bonded metal strip 24 includes bonded 
inlay strip 26, which is formed of contact layer strip 28 and diffusion 
barrier strip 30. Contact layer strip 28 is formed by the method described 
above and includes the electrically conductive composition of the 
invention, which is also described above. 
Diffusion barrier strip 30 is formed of an electrically conductive material 
which is suitable for forming diffusion barrier 20. Diffusion barrier 20 
prevents significant diffusion of an electrically conductive bulk 
conductor material of bulk conductor 22 across diffusion barrier 20 to 
contact layer 18. Examples of suitable materials for forming diffusion 
barrier strip 30 include nickel, palladium, silver, or an alloy thereof. 
Preferably, the material includes nickel having a purity of at least 99.8% 
by weight. In one embodiment, diffusion barrier strip 30 has about the 
same width as contact layer 18 and has a thickness in the range of between 
about 1.times.10.sup.-4 and about 1.times.10.sup.3 inches. Preferably, the 
diffusion barrier strip 30 has a thickness of about 5.times.10.sup.-4 
inches. 
Contact layer strip 28 is bonded to diffusion barrier strip 30 by a 
suitable method, such as is known in the art. An example of a suitable 
method of bonding contact layer strip 28 to diffusion barrier strip 30 is 
by metallurgical adhesion, wherein contact layer strip 28 and diffusion 
barrier strip 30 are overlaid and co-rolled by a suitable rolling mill 
under a pressure of at least about 1.times.10.sup.5 psi. Preferably, the 
pressure applied during rolling reduces the combined thickness of contact 
layer strip 28 and diffusion barrier strip 30 by an amount in the range of 
between about 50% and about 70%. Rolling causes contact layer strip 28 to 
adhere to diffusion barrier strip 30, thereby forming bonded inlay strip 
26. 
Bonded inlay strip 26 is then successively annealed and rolled by the same 
method described above with regard to contact layer strip 28 until contact 
layer strip 28 has a suitable thickness to form contact layer 18. For 
example, bonded inlay strip 26 has a thickness after rolling and annealing 
which is in the range of between about 1.times.10.sup.-3 and about 
1.times.10.sup.-2 inches. After rolling and annealing, bonded inlay strip 
26 is slit to remove burrs and rough edges of bonded inlay strip 26. 
Bonded inlay strip 26 is then inlaid into metal strip 32 within recessed 
portion 34 of metal strip 32. Metal strip 32 is formed of a material which 
is suitable for forming bulk electrical conductor 22. Examples of suitable 
materials of metal strip 32 include copper and alloys thereof, nickel and 
alloys thereof, etc. Preferably, the material includes copper. 
Particularly preferred materials include UNS C19400, C51000, C72500. In 
one embodiment, metal strip 32 includes copper and has a width of about 
six inches and a thickness of about 0.1 inches. 
Recessed portion 34 is formed by a suitable method, such as is known in the 
art. An example of a suitable method of forming recessed portion 34 is 
skiving. The depth of recessed portion 34 is about equal to the thickness 
of bonded inlay strip 26. 
Bonded inlay strip 26 is then inlaid into recessed portion 34 of metal 
strip 32. Bonded inlay strip 26 and metal strip 32 are subsequently rolled 
and annealed to bond diffusion barrier strip 30 to metal strip 32 and to 
form bonded metal strip 24 into the finished thickness. Preferably, the 
finished thickness of bonded metal strip 24 is in the range of between 
about 5.times.10.sup.-3 and about 5 .times.10.sup.-2 inches, and contact 
layer 18 has a thickness in the range of between about 5.times.10.sup.-6 
and about 1.5.times.10.sup.-3 inches. In a particularly preferred 
embodiment, contact layer has a thickness of about 5.times.10.sup.-5 
inches. Bonded metal strip 24 can then be formed by suitable methods, such 
as punching, blanking, stamping, drawing, bending, as is known in the art, 
to form electrically conductive interconnector 16. 
In another illustration of the invention, shown in FIG. 3, electrical 
circuit 36 includes electrical interconnection device 38. Electrical 
device 38 has electrically conductive interconnectors 40,42, which are 
oriented so that contact layers 44,46 are facing each other. Contact 
layers 44,46 are formed of the electrically conductive composition of the 
invention, described above. 
Diffusion barriers 48,50 are interposed between contact layers 44,46 and 
bulk electrical conductors 52,54 of electrically conductive 
interconnectors 40,42. Bulk electrical conductors 52,54 are configured to 
allow positive normal force by electrical conductors 52,54 on electrical 
conductors 56 to cause contact between contact layers 44,46 and electrical 
conductor 56 during advancement of electrical conductor 56 in a direction 
illustrated by arrow 58. Electrical circuit 38 is thereby directed from a 
position wherein electrical circuit 36 is opened, as shown in FIG. 3, to a 
position wherein electrical circuit 36 is closed, as shown in FIG. 4. An 
example of a suitable electrical conductor 56 is an electrical conductor 
formed of a copper alloy which has been electroplated with a nickel layer 
and a gold layer. 
Advancement of electrical conductor 56 to close electrical circuit 36 and 
retraction of electrical conductor 56, illustrated by arrow 60, to open 
electrical circuit 36 causes electrical conductor 56 to move across 
contact layers 44,46. The coefficient of friction of contact layers 44,46 
is significantly lower than the noble metal component of the electrically 
conductive composition forming contact layers 44,46. Therefore, movement 
of electrical conductor 56 across contact layers 44,46 to open or close 
electrical circuit 36 results in significantly less wear of contact layers 
44,46 than would occur if contact layers 44,46 were formed of only the 
noble metal component of the electrically conductive composition. 
EQUIVALENTS 
Those skilled in the art will recognize, or be able to ascertain using no 
more than routine experimentation, many equivalents to specific 
embodiments of the invention described specifically herein. Such 
equivalents are intended to be encompassed in the scope of the following 
claims.