Stock material or miscellaneous articles – All metal or with adjacent metals – Foil or filament smaller than 6 mils
Reexamination Certificate
2002-09-20
2004-03-09
La Villa, Michael (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Foil or filament smaller than 6 mils
C428S687000, C428S674000, C428S685000
Reexamination Certificate
active
06703143
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a metal foil excellent in electrical contact stability, used as the material of a metal disc spring for a keypad, a tactile switch, etc. of an electronic device, in a cable terminal board of an electric signal transmission system or the like.
BACKGROUND ART
Keypads for cellular phones and tactile switches for various kinds of electronic devices, wherein miniaturization and weight reduction are advancing to the extreme, must provide a feeling of a sharp click when they are pushed. Therefore, for keypads and tact switches, thin and electrically conductive metal disc springs, which have the function of switching a circuit by making them directly contact with or detach from a metal pattern formed on a printed circuit board or a switch substrate, are used.
The main function of keypads and tactile switches used for cellular phones and various kinds of electronic devices is to transmit signals to microprocessors, digital circuits or the like contained in the devices. When a circuit is closed by pushing a switch having poor electrical contact stability, a voltage fluctuation beyond the threshold values of an Hi potential and an Lo potential occurs owing to the fluctuation of resistance or the like at the contact portion, and this leads to an incorrect recognition as to how many times the switch has been pushed. In other words, it is an essential requirement of a switch of this type to have a very small voltage alteration when it is closed.
Screw type cable terminal boards have so far been widely used for electric signal transmission systems but, for the ease of cable connection, pressure contact type terminal boards using a springy metal ribbon are being used. When the electrical contact stability of this kind of terminal is hindered, noise is generated, leading to various kinds of malfunctions.
Phosphor bronze or SUS 301 stainless steel, which are highly springy, have conventionally been used as the materials of a metal spring for an electrical contact. However, the problem here is that, when a disc spring formed of a foil produced from such a metal is used in a switch, sufficient electrical contact stability is not secured. Therefore, a foil produced from such a metal has been used after being plated with a metal such as gold, silver and nickel.
One of known technologies attempting to solve the problem is a “Stainless Steel Contact Material for Electronic Components and Method of Producing the Same” disclosed in Japanese Unexamined Patent Publication No. S63-137193. The publication discloses a method of producing a stainless steel contact material for electronic components excellent in corrosion resistance, wear resistance and springiness by plating the surface of a stainless steel substrate, sequentially, with Ni, Cu, Ni and Au alloy, so that each plating has a prescribed thickness.
According to the publication, the method is specifically as follows. Firstly, for the purpose of improving the adhesiveness of the subsequent Cu plating, a Ni plating layer 0.05 to 0.5 &mgr;m in thickness is formed on the surface of a stainless steel substrate, by electrolysis, using an electrolyte containing NiCl
2
and free HCl and using the stainless steel substrate as the cathode. Secondly, for the purpose of obtaining electrical conductivity, a Cu plating layer 2 to 50 &mgr;m in thickness is formed thereon, by electrolysis, using an electrolyte containing CuSO
4
and free H
2
SO
4
. Thirdly, for the purpose of protecting the Cu layer, a Ni plating layer 0.1 to 5 &mgr;m in thickness is formed thereon, by electrolysis, using an electrolyte containing NiSO
4
and NiCl
2
.
Finally, for the purpose of improving wear resistance and corrosion resistance of a contact portion, an Au alloy plating layer 0.05 to 1 &mgr;m in thickness is formed, by electrolysis, using an electrolyte containing KAu(CN)
2
and salt of metals, except Au, such as Ni and Co with sulfamic acid.
However, the method according to the publication requires complicated manufacturing processes as described above and, thus, is costly. Therefore, it is considered desirable to apply the plating only to a local contact portion.
DISCLOSURE OF THE INVENTION
As explained above, when a springy metal material is used as an electrical contact material without surface treatment, the electrical stability of a contact point is hindered. As a common countermeasure, plating with a metal, such as gold, silver and nickel, which hardly forms an oxide film has so far been applied, but this inevitably leads to the drawback of a high material cost.
The object of the present invention is to provide a metal foil excellent in electrical contact stability, which does not require plating and is capable of reducing cost by eliminating plating processes.
Phosphor bronze or stainless steel is used as a springy metal for the switches but an oxide film forms naturally on the surface of the metal when the metal is exposed to the atmosphere and, as a result, contact resistance is generated. In addition, since electrical contact has so far been established by pressing a smooth metal surface to a counterpart metal surface, the density of contact points per unit area has been low, the state of the contacts with an oxide film between has been disturbed by slight vibrations and so on, and, as a result, a large and rapid fluctuation of the contact resistance has been generated and the electrical contact stability has been hindered.
In view of the above situation and based on the facts that the formation of a very thin oxide film on the surface of a metal material was a natural phenomenon which was difficult to control and that the electrical contact stability could be improved by increasing the pressing force even when an oxide film was formed, the present inventors have found that good electrical contact stability could be obtained even under a light pressing force by increasing the density of contact points per unit area and that, to realize this, the ratio between the average interval of concavities and convexities in surface roughness and the arithmetic average of roughness Ra was important. The present invention was established based on these findings.
The gist of the present invention for solving the above problem is as follows:
(1) A metal foil excellent in electrical contact stability characterized by having a surface roughness wherein the ratio Ra/Sm between the arithmetic average of roughness Ra and the average interval Sm of concavities and convexities is 0.001 or more, which average values are obtained by measuring the roughness in the direction in which the average interval Sm of the concavities and convexities on the surface is smallest.
(2) A metal foil excellent in electrical contact stability according to the item (1), characterized in that the material of the metal foil is stainless steel or phosphor bronze.
BEST MODE FOR CARRYING OUT THE INVENTION
A metal foil 0.3 mm or less in thickness which is cold tempered to provide springiness is generally used as a material for electrical contacts. A keypad or a tactile switch is constructed by cutting out components into a disc shape from the metal foil material, forming them into a concave shape like a dish, then placing them on circuit patterns of a copper foil printed on a substrate plated with gold, silver and/or nickel, so that a circuit is closed when each of the disc-shaped components is pressed, and thereafter, mounting resin covers, rubber parts and casing components thereon for the purpose of insulating operator's fingers from the electrically conductive portions and making the touch feel smooth.
In case of cable terminals too, the connecting method is changing from the conventional screw type to a press-in, ribbon-shaped spring type, considering the convenience of connecting work. In either of the electrical contacts, electrical contact stability is one of the essential factors for suppressing noise generation from a contact portion and correctly transmitting electric signals to other components. In particular, in the field of electronic devices, where
Kihira Hiroshi
Kikuchi Masao
Kenyon & Kenyon
La Villa Michael
Nippon Steel Corporation
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