Note: Descriptions are shown in the official language in which they were submitted.
25~
AMORPHOI~S TRANSITION METAL ALLOY,
TIIIN GOI-D C~ATED, ELECTRICAL CONTACT
BACKGROU~D ANI:) SUMMARY OF TEIE INVENTIC)N
The invention relates to an electrical
contact surface (such as an electrical switch
contact) having low contact resistance, relativeiy
low cost, acceptable solderability, and high
corrosion resistance. The electrical contact
surface, and method of formation thereof, according
to the present invention are particularly designed
to replace conventional electrical contact surfaces
wherein a gold layer of at least about 30
microinches thickness is applied over a crystalline
substrate~ .
In the electrical connector industry where
low voltage and/or current signals must be conducted
reliably, and in some situations where intermediate
voltage and/or current signals must be conducted,
gold is currently used to insure low contact
resistance, and thus effective conduction. Usually
the gold is applied over a crystalline base metal
such as copper, brass, or silver, with or without an
intermediate strike of nickel, and the thickness of
the gold layer is normally at least about 30 micro-
inches (e.g. 50-100 microinches). At a thickness of
50 microinches the cost of the gold layer is on the
order of-5 cents/cm2. It is the characteristic of
gold -- with such a thickness -- that it is porous
and thus through time the underlying material, or
its corrosion products, may migrate to the surface
of the gold and unacceptably raise the contact
resistance. Additionally~ certain organic materials
or sulfur compounds can polymerize on a gold surface
,~ ~,'~
2S9
and cause high contact resistance. Additionally,
gold has less than ideal solderability since it
dissolves in and embrittles some solder alloys.
According to the present invention an elec-
trical contact surface is provided which overcomesthe drawbacks associated with the conventional
electrical contact surfaces described above, so that
in low and intermediate voltage and/or current
signal situations effective conduction can be
obtained at less cost and over longer periods of
time. According to the present invention, an
electrical contact surface is provided comprising an
electrically conductive substrate (preferably a
met~l such as copper, bronze, brass, aluminum, or
silver) with an amorphous (as opposed to
crystalline) transition me~al alloy electrolytically
deposited thereon.
An "amorphous" alloy is one that has a
geometric or topological configuration of the atoms
forming the alloy that is different from crystalline
(i.e. non-crystalline). Typically, metal alloys are
crystalline. When X-ray diffraction tests are done
on crystalline materials, it can be seen that the
materials exhibit sharp peaks for the d-spacings
between planes in the ordered crystal structure, the
narrowness or width of these peaks relative to thick
height gives an indication of the size of the
crystals. For amorphous materials, there are no
particularly sharp peaks, the amorphous material
being characterized by lack of order in the atomic
structure. The exact nature of the amorphous
structures is not known, however there are a number
of theories which attempt to describe the configura-
tions of the atoms in amorphous material~. In this
regard attention is directed to an article entitled
~24~Z5i~
"Metallic Glasses" by Chaudhari et al, appearing in
Scientific American, Volume 242, No. 4, 1980, pages
9~-117~
The amorphous transition metal alloy
- s according to the invention preferably is a nickel-
phosphorus alloy, such as one having about 15-25
atomic percent phosphorus (preferably about 20
percent), with cobalt, or some other transition
metals, utilizable in addition to, or in place of,
the nickel~ Various materials may be added to the
plating bath to enhance the corrosion protection of
the electrical contact surface being formed, partic-
ularly advanta~eous materials being hexafluosilicate
(SiF~--), hexafluotitanate (TiF6--), or hexafluozir-
conate (ZrF6--) ions.
After the amorphous nickel alloy has been
electrolytically deposited on the substrate, the
amorphous nickel alloy is preferably coated with a
layer of gold. When the amorphous nickel alloy is
coated with a layer of gold of a given thickness,
the contactor that results exhibits superior
properties compared to conventional contactors
wherein the same thickness of gold is coated on a
crystalline metal alloy. For instance, a gold
thickness of less than 30 microinches over the
amorphous nickel alloy produces an electrical
contact structure according to the invention that is
equal to, or superior to, conventional contactors
wherein a coating of 50-100 microinches of gold is
provided. In fact, it is possible to obtain
entirely acceptable contactors even when the gold
coating is one microinch thick (at this thickness
the cost of the gold is only about 0.1 cents/cm2),
although a range of 5-15 microinches is preferred.
The electrical contact surface resulting has stable
2~
contact resistance both initially and after exposure
to a series ~f common atmospheric corrodants, and
initially (soon after production) when tested
pursuant to ASTM B667-80 has a contact resistance
- 5 less than 4 milliohms.
According to another aspect of the present
invention, a method of producing an electrical
contact surface is provided. The method comprises
the steps of providing a plating bath for electro-
lytically depositing an amorphous transition metalalloy on a ~nductive substrate, immersing the
substrate in the bath, and then subsequently coating
the amorphous electrolytically deposited alloy with
a flash of gold. Nickel chloride, cobalt carbonate,
and phosphor~us acid are preferred bath constitu-
ents. A number of bat~ additives can be provided to
influence contact resistance and corrosion protec-
tion in a positive way. Typical bath additives
include boric acid, hydroxyacetic acid, acetic acid,
B-alanine, succinic acid, surfactants of the
alkoxylated linear alcoholic class, SiF6-- ions,
TiF6-- ions and ZrF6-- ions. The bath temperature
conditions, and the current density at the cathode~
are maintained so that effective electrolytic
deposition takes place.
It is the primary object of the present
invention to provide for the production of
electrical contact surfaces that have low, stable
contact resistance over extended periods of time
even when subjected to corrosive conditions, and at
a relatively ~ow cost. This and other objects of
the invention will become clear from an inspection
of the detailed description of the invention and
from the appended claims.
~LZ4~2~
DETAILED DESCRIPTION
A deposition of an amorphous transition
metal alloy can be provided on a substrate by
- immersing the substrate (or a portion thereof) in a
5 plating bath. Amorphous transition metal alloys
have been found to have better corrosion resistance
than crystalline materials~ and a thinner coating of
gold over an amorphous transition metal alloy
produces a contactor having the same, or better,
properties than a contactor formed by a thicker
coating of gold over a crystalline material.
Further, acceptable contacts can be obtained,
according to the invention, with only a very thin
coating of gold.
Typical transition metal alloys that are
useful in forming electrical contact surfaces (such
as electrical switch contacts) according to the
invention are nickel and cobalt. Nickel is the
preferred transition metal since it has the least
~o cost for the most corrosion resistance, of suitable
transition metals. However, generally comparable,
and sometimes superior, results can be achieved
substituting cobalt, for all or part of the nickel,
in the plating bath.
An amorphous deposition of the nickel on
the conductive substrate (which preferably comprises
a metal such as copper, bronze, brass, aluminum, or
silver, or alloys thereof) is formed when
phosphorous acid is included in the plating bath,
and a relatively high percentage of phosphorus is
provided in the alloy that is formed. A phosphorus
concentration of at least about 12~, and preferably
of about 15-25 atomic percent is desired in order to
achieve good corrosion resistance and low contact
~4S2~
resistance. Most preferably the amorphous deposited
alloy has about 20 atomic percent phosphorus.
The bath temperature conditions, and the
current density at the cathode, are controlled in
- 5 order to maximize the corrosion resistance and
minimize the contact resistance. Typically current
density is about 50 amp./ft.2 - 2500 amp~/ft.2, with
a range of about 100-900 amp./ft.2 preferred.
Typical temperatures are 70-85C with 75-80~C
preferred. Temperature is not critical, but lower
temperature will have a tendency to increase the
preference of cobalt for nickel in the plating where
both are present in the bath.
Various additives may be provided in the
bath in order to positively affect the contact
resistance and corrosion resistance. When the bath
contains hexafluosilicate ions at a concentration of
about 0.1 molar to the solubility limit (with the
addition of small amount of HF to maintain
solubility if necessary) the overall corrosion
resistance of the amorphous nickel alloy may be
enhanced. A generally comparable enhancement of
corrosion resistance may also be obtained by
substituting TiF6-- or ZrF6-- ions for part or all
of the SiF6-- ions.
In a plating bath containing nickel or
cobalt ions and phosphorous acid any suitable anode
and cathode materials may be utilized. For instance
the anode can either be inert (platinized titanium,
platinum, or graphite), or can be of nickel (or like
~ransition metal to be deposited~. If TiF6--,
SiF6-- or ZrF~-- ions are included in the bath a
nickel or cobalt anode must be used. With an inert
anode additions of nickel or cobalt must be made
from time to time (preferably in the form of NiCO3
%~
or CoCO3) to maintain the nickel content. With the
nickel anode the content of nickel ion in the bath
tends to rise since each two electrons at the anode
cause the dissolution of about one nickel ion, while
at the cathode both nickel and phosphorus are being
reduced. A nickel and an inert ansde can be used
together such that each carries only a portion of
the current, and thus maintain a balanced bath with
regard to nickel. Phosphorous, in the form of
phosphorous or hypophosphorous acid, and preferably
in the form of phosphorous acid, must be added from
time to time -- irrespective of the anode
construction -- to maintain the proper bath balance,
although the proportion of phosphorous acid is not
critical and the bath balance can be maintained
rather easily.
After deposition of the nickel phosphorous
alloy, or the like, on a substrate, a coating of
gold is applied over the amorphous alloy.
Preferably this is accomplished by providing an
electrodeposit that is applied for a controlled time
at a controlled current density. The thickness of
the gold coating is determined by the desired end
properties of the contactor produced. Within wide
ranges, whatever the thickness of the gold coating
on the amorphous alloy, the contactor that results
can be expected to have enhanced properties compared
to contactors formed by the same thickness of gold
coating over a crystalline material. In fact,
acceptable electric contacts can be produced even
when the thickness of the gold coating is about 1
microinch. Preferably the gold coating is in the
range of 5-30 microinches, and more preferably 5-15
microinches~
2~i~
The gold used for the coating preferably is
hard gold, although soft gold is also practical
although usually with somewhat less desirable
_ results. The thickness of the amorphous transition
- 5 metal alloy on the substrate is not particularly
critical. It merely need be thick enough to achieve
the desired results according to the invention. A
preferred thickness is in the range of about 50
microinches - 150 microinches. Ranges of 25
1~ microinches - 1000 microinches are practical.
In the typical manufacture of an electrical
contact according to the invention (which may be of
a wide variety of forms, such as edge card connec-
tors, contact leaf springs, rigid electrical switch
contact structures, etc.)~ desirably, the elec-
trically conductive substrate is formed into the
desired final contact shape. It is then immersed in
a cleaner, and then deionized water, and then a
dilute hydrochloric acid solution. Then it is
placed in the Ni-Co-P plating bath and after plating
it is rinsed is deionized water. Then the gold
plating is provided thereon in any conventional way,
such as in a gold plating bath maintained at about
30-35C with a current density of about
10 amp./ft.2. After the gold plating is applied it
is again immersed in deionized water.
Alternatively, for many electrical contact
shapes (such as edge card connectors), it is
possible to plate blanks or coupons first, and only
after they have been plated and a g31d strike
applied are they formed into the desired shape.
The following are examples of the practice
of the invention:
" ~2~5~9
~xample 1
A plating bath was formed with the
following composition:
_ .75 M/l NiC12 6H20
.25 M/l NiC03
1.25 M/l H3P03
An electrically conductive substrate was
immersed in the bath, which was maintained at a
temperature of about 80C, and with a current
density at the cathode of about 150 ma/cm2. When
removed from the bath, the substrate had an
amorphous nickel-phosphorus alloy thereon. A one
(1) microinch ~trike of gold was provided on the
amorphous alloy. The electrical contact surface
that resulted had a contact resistance that was
substantially as low as a similar substrate with a
50 microinch or greater c02ting of gold, the contact
resistance was stable over time, and as stable in
corrosive environments (such as when subjected to
20 the S02 test -- 100 percent relative humidity and 1
percent concentration of sulfur dioxide, room
temperature, over forty hours --, and the mixed gas
test -- the same conditions as the S02 test only
adding 1 percent nitrogen dioxide and 1 percent
chlorine). The electrical contact surface formed
was much less e~pensive than the conventional one,
and had better solderability characteristics.
Exam~ple 2
In this example, the bath composition,
3~ temperature, and current density characteristics
were substantially the same as in example 1. After
the substrate with an amorphous nickel-phosphorus
1~
alloy was removed from the bath, an approximately 10
microinch strike of gold was provided on the
amorphous alloy. The electrical contact surface
that resulted had contact resistance, and other
properties, equal, or superior to, an electrical
contact surface formed utilizing similar materials
in crystalline form, and with a 50 microinch coatin~
of gold.
Example 3
The bath composition in this example was as
follows:
.2 M/l NiC12 .~ (H20
.8 M/l NiS04 6 (H20)
.5 M/l H~R03
.5 M/l H3~04
The bath temperature conditions, current
density, and like parameters, were substantially the
same as for example 1, and after deposition of the
amorphous nickel-phosphorus alloy on the substrate a
1 microinch flash of gold was applied. By testing,
the electrical contact surface formed was found to
have acceptable contact resistance (i.e. less than 4
milliohms when tested according to ASTM B667-80) and
corrosion resistance, although it was not as good as
the electrical contact surface produced in example
1.
Example 4
The bath composition for this example was
as follows:
.2~
11
.88 M/1 NiC12 6H20
.25 M/l MiCO3
1O25 M/1 H3PO3
.4 M/1 ~3BO3
- 5 .2 M/l Acetic acid
.1 M/l CoCO3
The bath temperature was maintained at
about 75~C, with a current density at the cathode of
about 200 ma/cm2. An analysis of the plating
resulting from immersion of the substrate in this
bath showed bulk values (in atomic percent) of 6.8
~4%.cobalt, 0.6 oxygen, 73O3 percen~ nickel, and
19.3 percent phosphorus. A 1 microinch strike of
gold was provided on the amorphous alloy~ The
electrical contact surface formed had low contact
resistance and hiqh corrosion resistance, and was an
excellent substitute for conventional electrical
contact surfaces of gold about S0 microinches thick
(or thicker) applied over a crystalline base metal.
Example 5
A member of platings were produced on elec-
trically conductive substrates to produce platings
having about 20% phosphorous, X% cobalt, and 80 X%
nickel, utili2ing the constituents indicated in the
following table:
~Z452~9
12
¦ Bath
Constituent X=5~ ¦ X=10~ ¦ X=15%
_ I NiC12 1~75M I o75M I~75M
NiCO3 1.22M ¦ .175M ¦.13M
5 I CoCO3 ¦. 03M ¦ ~ 075M ¦ ~12~ ¦
¦ H3PO3 ¦1. 25M ¦ 1~25M ¦1~25~.
H3PO4 ¦. 20M I . 20M ¦. 20.''1
Platin~ was accomplished at 75-78C using a
hard anode (e.g. platinum or platinized titanium)
and a current density of about 100 amp./ft. 2~
The sum of the nickel plus cobalt is one
mole/liter in each for~ulation, and CoCO3 i5 the
source of all the cobalt in each of the formula-
tions. Therefore, the Co+2/Ni+2 ratio in the bathis M/l CoCO3/(1-M/1 CoCO3). The Co/Ni ratio in the
plating is ~Co(80-%Co). The relationship between
Co+2/Ni+2 in the bath and Co/Ni in the plate is:
Co 2/Ni 2 Co/NiCo+2/Ni+2 in plate
in bath in plateCo/Ni in bath
(1)oO31 ~0~7 2.2
t2)~081 ~143 1~8
(3)~136 ~231 1~7
It is evident that the cobalt is being
2~ plated preferentially to the nickel and that at low
cobalt levels this preference is slightly greater.
Operating at lower temperature will make the
preference (Co/Ni in plating~ greater, as will
`` ~Z~2S~
13
operating at higher current density. Further, these
formulations produce lower than nominal amounts of
~obalt when they are new, i.e. for the first lO0
amp-minutes~liter the baths will produce only ca.
- 5 1/2 to 2/3 the desired cobalt content in the
plating.
The plating, formed as actual electrical
connectors, having 10~ Co were coated with ~, lO, or
15 microinches of hard gold, or 5 microinche~ soft
gold, and subjected to durability cycling utilizing
conventional techniques, and exposure in a BCL Class
III environment, and utilizin~ the same material on
both the PC boards and the connector openings. The
following results were achieved, wherein
Rc= the contact resistance and
a~ a measure of the deviations of the individual
contact values from their average
TABLE 1. 70Z Ni - 10X Co - 20~ P
~DGE CARD CONNECTORS
Plating
(Microinches~ear Befcre ~fter
of Gold)Cycles Cycling Cycling 4 days
R,. a Rc a Rc a
Hard 10015.4 1.70 16.0 1.59 17.7 1.75
2510 Hard 10011.0 0.92 10.2 0.63 14.0 9.46
Hart 10011.0 1.25 11.0 0.92 10.8 1.06
So~t 10015.4 1.49 21.3 6.81 27.2 19.3
5 Soft 25013.8 2.311~.9 3.5835.1 20.2
5 So~t 500 13.6 2.24 27.1 16.834.1 32.8
305 Hard 100 13.9 1.42 13.9 1.6813.0 1.80
5 Hard 250 15.3 1.34 14.2 2.0515.0 1.63
5 Hart 50013.2 1.3115.1 2.4138.8 29.1
~ 5Z59
14
TABLE 1. (Cont.)
Plating
(Microi~che~
~ of C~ld) 10 days 15 t~y~ 2~ Disturbed
- 5 ~; a Rc a ~; a a~ a
5 Hard 17.0 2.0120.26~3069.1104. 17.8 3.94
10 Hard 18.8 6.0311.31.4813.08.3E 12.7 7.64
15 Hard 11.3 1.0114.07.2616.41.5~ 11.7 0.64
5 Soft 23.2 13.435.722.523.413.t 21.3 4.33
105 Soft 21.6 8.56~6.7102.610287.~ 15.3 0.94
5 Soft 2~5 1~.84Q.840.051.341.7 38.8 37.
5 Hard 13.G 1.9430.635.116.87.64 13.3 1.70
5 Hard 16.3 1.6318.7~.1615.72.92 31.1 17.9
5 Hard 25.0 25.523.821.815.51.77 15.4 2.18
These results indicate improved performance
of the electrical connectors according to the
invention compared to a 50 microinch plating of hard
gold over conventional sulfamate nickel.
While the invention has been herein shown
and described in what is presently conceived to be
the most practical and preferred embodiment thereof,
it will be apparent to those of ordinary skill in
the art that many modifications may be made thereof
within the scope of the invention, which scope is to
be accorded the broadest interpretation of the
appended claims so as to encompass all equivalent
structures and methods.
