BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to an electric contact member and a process for making
it. More particularly, the present invention relates to an electric contact member
comprising a contacting projection formed on a thin sheet of stainless steel, and
a process for making it.
2. Description of the Prior Art
[0002] Movable contacts of keyboard switches or cameras have heretofore been formed by punching
into predetermined sizes thin sheets of copper alloys such as phosphor bronze, nickel
silver and beryllium copper. Recently, stainless steel thin sheets have also been
proposed for use in these contact members.
[0003] In these electric contacts, the contacting sides are generally clad or deposited
with gold, and in order to ensure the contacting function, conical projections having
a diameter of 1 mm and a height of 0.15 mm are mechanically formed in the contacting
portions of these metal sheets.
[0004] Gold is excellent in the corrosion resistance and has a good electrical conductivity
and therefore, gold provides a contacting portion having a high reliability. However,
gold is expensive.
[0005] Accordingly, trials have been made to reduce the amount used of gold. In the early
stage, gold was clad in a thickness of 2 to 3 um, but at the present, gold is clad
or deposited in the form of a thin film having a thickness of up to about 1 µm or
gold is clad or deposited only on a stripe-like restricted contacting portion.
[0006] An electric contact member is a very important element in an electronic machine,
and is very broadly used in keyboard switches, computers, registers, telephone sets
and calculators. Namely, electric contact members are manufactured as large-quantity
parts and built in electronic machines.
[0007] The contacting portion is a most important part of an electric contact member, and
this contacting portion should be composed of a metal having a low contact resistance,
a high corrosion resistance and an excellent abrasion resistance. According to the
standard of the abrasion resistance test for keyboards, the metal of the contacting
portion should resist contacting under a pressing force of 20 g, which is repeated
10,000,000 times.
[0008] The minimum thickness of the gold film of the contacting portion, necessary for satisfying
this requirement, is about 1 µm, and if the thickness of the gold-film-is smaller
than about 1 µm, the substrate metal is exposed by abrasion of the gold film to cause
corrosion and drastically increase the contact resistance, with the result that transmission
of electric signals becomes inaccurate and the function of an electronic machine is
lost.
SUMMARY OF THE INVENTION
[0009] It is a primary object of the present invention to provide a cheap electric contact
member which comprises a contacting portion formed of a cheap metal and which has
good electrical conductivity and corrosion resistance and can resist the switching
operation repeated at a high frequency for a long time.
[0010] More specifically, in accordance with the present invention, there is provided an
electric contact member comprising a thin sheet of a stainless steel and a projection
of a tin-lead alloy solder formed on the surface of the thin sheet as a contacting
portion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Figs. 1 through 3 are sectional views showing the steps of the process for forming
an electric contact member according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] The stainless steel is a metal having a good corrosion resistance and an excellent
spring property, and the stainless steel is an effective material for an electric
contact member. However, the stainless steel is defective in that the platability
and solderability are poor.
[0013] The electric contact member of the present invention comprises a projection (contacting
portion) of a tin-lead alloy solder formed on a stainless steel thin sheet, soldering
of which has been considered difficult.
[0014] The solder constituting the contacting portion has the following composition:
Tin: 40 to 95% by weight
Lead: 5 to 60% by weight
Silver: 0 to 9.5% by weight
Antimony: 0 to 5% by weight
Bismuth: 0 to 5% by weight
[0015] Silver as one optional component of the solder exerts the function of increasing
the hardness. Antimony as another optional component of the solder makes a contribution
to enhancement of the dimension stability and bismuth as still another optional component
makes a contribution to increase of the hardness.
[0016] The process for preparing the electric contact member of the present invention will
now be described with reference to the accompanying drawings.
[0017] At first, a stainless steel hoop 1 is preliminarily printed with a resist paint 2,
as shown in Fig. 1, so that a small circular non-printed portion 3 having a diameter
of, for example, 1 mm is formed at a position where a contacting portion is to be
formed, and such non-printed portions 3 are arranged at predetermined intervals. It
is preferred that an ultraviolet ray-curable paint which is excellent in the chemical
resistance and the high-temperature resistance and can be dried in a moment be used
as the resist paint.
[0018] The non-printed portions 3 may be spot-like small circules arranged regularly as
described above or they may be stripes.
[0019] A special preliminary treatment is carried out so that soldering becomes possible
on the non-printed portions 3 distributed on the surface of the stainless steel hoop.
[0020] As is seen from its name, a special passivated film is formed on the surface of the
stainless steel and this film prevents oxidation and controls occurrence of rusting.
However, plating or soldering is impossible because of the presence of this film.
Accordingly, in order to render soldering possible, it is first of all necessary to
remove the passivated film.
[0021] In the present invention, only a passivated film is removed by alkali decreasing
and activating treatments of the surface of the stainless steel sheet. The alkali
degreasing treatment may be carried out according to the conventional method using
a commercially available alkali degreasing solution. According to one preferred embodiment,
the stainless steel sheet is dipped in a dilute solution of an alkali degreasing solution
at an elevated temperature and electrolytic degreasing is then carried out in a dilute
aqueous alkali solution.
[0022] After the alkali degreasing treatment, the stainless steel sheet is subjected to
the activating treatment. It is preferred that this activating treatment be carried
out in two stages. Namely, the activating treatment comprises the first step of dipping
in an activating solution and the subsequent step of cathodic electrolysis in a cathodic
electrolytic solution.
[0023] The activating solution used in the first activating treatment is preferably an aqueous
mixed acid solution containing, based on the weight of the solution, (i) 3 to 20%
by weight of hydrochloric acid, (ii) 2 to 30% by weight of sulfuric acid, (iii) 0.1
to 5% by weight of a nonionic, cationic or ampholytic surface active agent and (iv)
0.1 to 20% by weight of 2-pyrrolidone or its N-alkyl derivative such as N-erhyl-2-pyrrolidone
or N-methyl-2-pyrrolidone.
[0024] The first activation treatment may be carried out by dipping the stainless steel
sheet in the activating solution at a normal temperature for 30 seconds to 7 minutes,
preferably under irradiation with ultrasonic waves.
[0025] The activated stainlees steel sheet is then subjected to the cathodic electrolytic
activation. The cathodic electrolytic activation solution used is preferably an aqueous
mixed acid solution containing, based on the weight of the solution, (i) 5 to 20%
by weight of phosphoric acid, (ii) 2 to 10% by weight of nitric acid, (iii) 0.1 to
5% by weight of a nonionic cationic or ampholytic surface active agent and (iv) 0.1
to 20% by weight of 2-pyrrolidone or its N-alkyl derivative.
[0026] At the cathodic electrolytic activation step, the electrolysis may be carried out
at a normal temperature to about 65°C at a cathode current density of 1 to 7 A/dm
for 30 seconds to 5 minutes by using a platinum-coated titanium anode and the stainless
steel sheet as the cathode.
[0027] The stainless steel sheet which has been subjected to the activating treatment is
then preferably subjected to nickel plating. This nickel plating is performed according
to the conventional electrolytic plating method using nickel salts such as nickel
sulfate, nickel sulfamate and nickel chloride. The nickel electroplating may be carried
out at a temperature of 45 to 65°C in an electroplating solution at a cathode current
density of 4 to 12 A/dm
2 for 10 seconds to 2 minutes by using the stainless steel sheet as the cathode and
a nickel plate as the anode.
[0028] It is preferred that the amount deposited of nickel be so small that a color tone
intermediate between the color tone before plating and the color tone of nickel is
produced on the surface. Supposing that a uniform film of nickel is formed by plating
(actually, no uniform film of nickel is formed because the amount deposited of nickel
is very small), the amount of nickel is ordinarily controlled so that a film having
a 0 0 thickness of 100 to 1000 A, preferably 300 to 700 A, most preferably about 500
A, is formed.
[0029] The stainless steel sheet which has been subjected to the activating treatment may
be electroplated with gold, silver or palladium or an alloy thereof. If the stainless
steel sheet is subjected to the nickel electroplating as mentioned above, the nickel-electroplated
stainless steel sheet is immediately electroplated with gold, silver or palladium
or an alloy thereof. The electroplating procedure may be conventional. Usually, the
electroplating is carried out at a temperature of 20 to 65°C at a cathode current
density of 0.5 to 18 A/dm
2 for one second to two minutes by using the stainless steel sheet as the cathode and
a nickel plate as the anode. The electroplating of gold may be carried out preferably
by using an electroplating solution containing about 80 to 140 g/ℓ of citric acid,
about 80 to 140 g/¿ of sodium citrate, about 15 to 40 g/ℓ of nickel sulfamate and
about 4 to 10 g/1 of potassium gold cyanide and maintained at a temperature of 40
to 65°C. The electroplating of silver or palladium is preferably carried out at a
temperature of 20 to 30°C.
[0030] The amount deposited of gold, silver, palladium or an alloy thereof is important
in the present invention. Namely, the amount deposited is so small that a color tone
intermediate between the color tone before plating and the color tone of gold, silver,
palladium or an alloy thereof is produced on the plated surface. Supposing that a
uniform film of the metal or alloy is formed by plating, the amount deposited of the
metal or alloy is controlled so that the thickness of the gold or palladium film is
30 to 500 Å, preferably 50 to 300 Å, and the thickness of the silver film is 50 to
2000 Å, preferably 100 to 1000 Å (actually, no uniform film is formed but the metal
or alloy is deposited in the form of spots).
[0031] When the thus-obtained stainless steel hoop 1 which has been printed with the resist
paint 2 and on which the underground plating 4 for soldering has been formed is passed
through a bath of a soldering melt set at a temperature of 230 to 250°C, conical projections
5 of a solder having a diameter of 1 mm and a height of 0.12 to 0.15 mm are continuously
formed on the surface of the stainless steel hoop 1 in a moment, as shown in Fig.
3, whereby the contacting portion of the electric contact member of the present invention
is formed.
[0032] According to the intended use, the resist paint 2 is removed from the thus-obtained
stainless steel plate to expose the texture of the stainless steel sheet 1 and the
sheet is then subjected to the punching operation. When the insulating function of
this resist paint is utilized, the paint is not peeled but the stainless steel sheet
1 is directly subjected to the punching operation and built as a contacting portion
into an electronic machine.
[0033] According to the above-mentioned process, contacting portions of a solder can be
easily formed in an optional shape and arrangement on the stainless steel sheet by
changing the printing pattern.
[0034] Since the contacting portion of the electric contact member obtained by punching
the stainless steel sheet having projections of a solder is formed of a cheap alloy
solder, according to the present invention, the cost of electric contacts such as
keyboard contacts or movable contacts of cameras, which are used in large quantities
in the field of electronic industry, can be greatly reduced.
[0035] The present invention will now be described in detail with reference to the following
examples.
Example 1
[0036] A stainless steel hoop of SUS-304 having a thickness of 0.1 mm, a width of 10 mm
and a length of 1200 m was subjected to partial soldering for forming contacting portions
through the following steps.
(1) Masking Printing Step
[0037] The stainless steel hoop was mask-printed with an ultraviolet ray-curable resist
paint so that circular non-printed portions having a diameter of 1.5 mm were formed
at intervals of 14 mm in the central portion of the stainless steel hoop, and the
back surface was entirely printed.
(2) Alkali Electrolytic Degreasing Step
[0038] A commercially available alkali degreasing solution was heated at 70 to 80°C in a
stainless steel tank, and the stainless steel hoop was passed through the degreasing
solution in the tank in succession to effect the primary degreasing. Then, in the
alkali degreasing solution maintained at 40 to 60°C, direct current electrolytic degreasing
was carried out by applying a voltage of 6 volts and using a stainless steel sheet
as the anode and the above-mentioned stainless steel hoop as the cathode.
(3) First Activation Treatment Step
[0039] Then, the stainless steel hoop was passed through an activating solution formed by
adding 0.2% by weight of a nonionic surface active agent such as polyethylene glycol
alkyl ether or polyethylene glycol fatty acid ester or an amphoteric surface active
agent and 0.1% by weight of an amine type anti-corrosive agent (Armohiboo 28 supplied
by Lion-Armar Co.) to a mixed acid comprising 20% by volume of hydrochloric acid (35%
solution), 10% by volume of sulfuric acid (85% solution), 10% by weight of citric
acid (powder), 1% by volume of acetic acid (90% solution), 5% by volume of nitric
acid (68% solution) and 5% by weight of N-methyl-2-pyrrolidone under irradiation with
ultrasonic waves of 600 W, whereby the oxides and impurities were removed from the
non-printed portions of the stainless steel hoop.
(4) Cathodic Electrolytic Activation Step
[0040] A solution formed by adding 0.2% by weight of the same nonionic or amphoteric surface
active agent as described above and 0.1% by weight of the same anti-corrosive agent
as described above to a mixed acid comprising 10% by volume of phosphoric acid (85%
solution), 10% by volume of sulfuric acid (85% solution), 10% by volume of nitric
acid (70% solution), 5% by weight of citric acid (powder), 1% by volume of acetic
acid (90% solution) and 5% by weight of N-methyl-2-pyrrolidone was heated at 60°C,
and the stainless steel hoop was passed through the solution by applying a voltage
of 4 volts between the stainless steel hoop as the cathode and a platinum-deposited
titanium plate as the anode, whereby the non-printed portions of the stainless steel
hoop were activated.
(5) Gold Plating Step
[0041] Gold plating was carried out for 3 seconds in a plating solution comprising 120 g/1
citric acid, 120 g/ℓ of sodium citrate, 30 g/ℓ of nickel sulfamate and 8 g/l of potassium
gold cyanide at a current density of 10 to 3 A/dm
2 at a plating solution temperature of 35°C by using the stainless steel hoop as the
cathode and a platinum-plated titanium plate as the anode.
[0042] A gold-nickel alloy layer having a thickness of 0.01 µm was formed as an underground
plating layer for soldering on the non-printing portions of the stainless steel hoop.
(6) Soldering Step
[0043] A solder comprising 60% of tin, 36% of lead, 2% of silver and 2% of antimony was
melted at 230°C in a soldering tank, and the stainless steel hoop was passed through
the soldering tank for an immersion time of 3 seconds, whereby a stainless steel hoop
having conical projections of the solder having a bottom diameter of 1.5 mm and a
central height of 0.12 mm, as contacting portions arranged continuously, was obtained.
Example 2
[0044] A stainless steel hoop of SUS-304 having a thickness of 0.15 mm, a width of 15 mm
and a length of 7CO mm was subjected to partial soldering for forming contacting portions
through the following steps.
(1) Pressing Step
[0045] The stainless steel hoop was subjected to the pressing operation using a mold so
that recesses having a width of 2 mm, a length of 5 mm and a depth of 0.1 mm were
formed at intervals of 10 mm in the central portion of the stainless steel hoop.
(2) Masking Printing Step
[0046] The stainless steel hoop from the step (1) was continuously mask-printed with an
ultraviolet ray-curable resist paint so that only the inner faces of the recesses
having a width of 2 m and a length of 5 mm were formed into non-printed portions,
and the back surface of the stainless steel hoop was entirely printed.
(3) Alkali Electrolytic Degreasing Step
[0047] The alkali electric degreasing treatment was carried out in the same manner as described
in Example 1.
(4) First Activation Treatment Step
[0048] The first activation treatment was carried out in the same manner as described in
Example 1.
(5) Cathodic Electrolytic Activation Treatment Step
[0049] The cathodic electrolytic activation treatment was carried out in the same manner
as described in Example 1.
(6) Nickel Plating Step
[0050] Flash plating was carried out at a current density of 6 A/dm
2 for 15 seconds at a plating solution temperature of 50°C in a plating solution comprising
300 g/ℓ of nickel sulfate, 40 g/ℓ of nickel chloride and 30 g/ℓ of boric acid by using
the stainless steel hoop as the cathode and a nickel plate as the anode.
(7) Gold Nickel Alloy Plating step
[0051] Plating was carried out at a current density of 10 to 3 A/dm
2 for 2 seconds at a plating solution temperature of 45°C in a plating solutiom comprising
120 g/t of citric acid, 120 g/t of sodium citrate, 30 g/ℓ cf nickel sulfamate and
8 g/ℓ of potassium gold cyanide by using the stainless steel hoop as the cathode and
a platinum-deposited titanium sheet as the anode.
[0052] A gold-nickel alloy plating layer having a thickness of 0.007 um was thus formed
as an undercoating layer for soldering in the non-printed portions of the stainless
steel hoop.
(8) Paint-Peelling Step
[0053] The stainless steel hoop was passed through a solution comprising 40% of methylene
dichloride and 3% of formic acid and maintained at 35°C for an immersion time of 30
seconds, whereby the ultraviolet ray-curable resist paint was peeld.
(9) Soldering Step
[0054] A solder comprising 88% of tin, 2% of silver, 10% of lead and 2% of antimony was
melted at 240°C in a soldering tank, and the stainless steel hoop was passed through
the soldering tank for an immersion time of 3 seconds, whereby a stainless steel hoop
having contacting portions having a thickness of 0.16 to 0.2 mm, which were continuously
formed only on the recesses having a width of 2 mm, a length of 5 mm and a depth of
0.1 mm, was obtained.
[0055] The projecting contacting portions of the silver-incorporated alloy solder of the
stainless steel hoop prepared through the above-mentioned steps had excellent properties
described below.
(A) Electrical Conductivity
[0056] The electrical conductivity of the contacting poriton of the silver-incorporated
alloy solder corresponded to about 14% of the electric conductivity of copper, which
was substantially equal to the electric conductivity of iron corresponding to 14.8%
of the electrical conductivity of copper. Thus, it was confirmed that the contact
portion could be effectively used as an electric contact.
(B) Hardness
[0057] The contacting portion had a Vickers hardness of about 42, which is higher than the
Vickers hardness of pure gold (25) but lower than the Vickers hardness of a gold-nickel
alloy plating (about 120). Although the allowable thickness of the gold-nickel plating
layer is about 1 um, the solder layer of the present invention can be applied in a
thickness of 30 to 100 µm to the contacting portion, whereby the contacting portion
which is excellent in the abrasion resistance over the conventional gold-clad or gold-plated
contacting portion can be formed.
1. An electric contact member comprising a thin sheet of a stainless steel and a projection
of a tin-lead alloy solder formed on the surface of the thin sheet as a contacting
portion.
2. An electric contact member according to claim 1, wherein said electric contact
member is made by cutting a thin stailess steel sheet having projections of the tin-lead
alloy solder in the form of spots.
3. An electric contact member according to claim 1, wherein said electric contact
member is made by cutting a thin stainless steel sheet having projections of the tin-lead
alloy solder in the form of stripes.
4. An electric contact member according to claim 1, wherein an ultra-thin plating
layer of a metal selected from the group consisting of gold, silver and palladium,
or an alloy selected from the group consisting of gold-, silver- or palladium-containing
alloys is formed between the stainless steel sheet and the tin-lead type solder projection.
5. A process for making an electric contact member comprising a thin sheet of a stainless
steel and a projection of a tin-lead alloy solder formed on the surface of the thin
sheet as a contacting portion, which comprises the steps of:
printing a stainless steel sheet with a resist paint having a desired pattern,
electroplating the stainless steel sheet wiht a metal selected from the group consisting
of gold, silver and palladium, and alloys of these metals,
dipping the electroplated stainless steel sheet in a bath of a tin-lead alloy solder
melt whereby projections of the tin-lead type solder in the form of spots or stripes
are formed on the stainless steel sheet, and then
cutting the stailess steel sheet into electric contact members.
6. A process according to calim 5, wherein the electroplating of the resist-formed
stainless steel sheet is carried out to an extent such that the resulting plated stainless
steel exhibits a color tone intermediate between the color tone of the unelectroplated
stainless steel and the color tone of the gold, silver, palladium or alloy thereof
used.
7. A process according to the claim 5, wherein, prior to the electroplating with gold,
silver, palladium or their alloys, the resist-formed stainless steel sheet is electroplated
with nickel.
8. A process according to claim 7, wherein the electroplating with nickel is carried
out to an extent such that the resulting nickel-plating stainless steel exhibits a
color tone intermediate between the color tone of the unelectroplated stainless steel
and the color tone of nickel.