Electricity: circuit makers and breakers – Multiple circuit control – Pivoted contact
Reexamination Certificate
1999-04-07
2001-06-19
Friedhofer, Michael (Department: 2332)
Electricity: circuit makers and breakers
Multiple circuit control
Pivoted contact
C029S622000, C200S268000, C200S269000, C200S292000
Reexamination Certificate
active
06248964
ABSTRACT:
FEDERALLY-FUNDED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of circuit elements that are made with thick film fabrication techniques. More specifically, the invention relates to a thick film switching element that is well-adapted for use as an encoder contact element. The invention also relates to a method of fabricating such an element.
Switching elements in the form of encoder contact elements are commonplace in many electrical and electronic products. A typical prior art encoder contact element comprises a pattern of conductive areas or contacts printed, etched, or deposited on an insulative substrate, usually along a circular or arcuate track, whereby the contacts are sequentially contacted by a conductive wiper so as to produce an encoded electrical signal. For example, the track may comprise alternating conductive contacts and nonconductive areas so as to produce a square wave as the wiper is moved along the track.
There have been many techniques and materials used in the past to fabricate switching elements that are configured for use as encoder contact elements. One technique that has been used is the deposition or printing of thick film conductive inks, such as cermet inks, to form the contacts. This construction produces contacts that are elevated above the surface of the insulative substrate. Consequently, as the wiper passes repeatedly along the encoder track, both the wiper and the contacts are subject to relatively rapid wear, decreasing the lifetime (in terms of rotational cycles) of the encoder. Thus, where durability (long lifetime or a large number of rotational cycles) is desired, thick film cermet inks may not meet performance criteria.
Another technique is described in U.S. Pat. No. 5,702,653-Riley. In the Riley '653 patent, the contacts are formed by fusing a high-temperature glass frit to a non-conductive substrate. A cermet layer having a low temperature glass matrix is applied in the desired encoder pattern to the surface of the frit, and then fired so as to sink into the frit. This results in the cermet forming contacts that are sunk into the frit such that the thickness of the contacts is approximately equal to the original thickness of the frit layer. In actual practice, as described in the '653 patent, the contacts extend above the surface of the frit layer by a distance of about 4 to 10 microns. A similar technique is disclosed in U.S. Pat. No. 5,169,465-Riley.
While the fabrication methods of the aforementioned patents (U.S. Pat. No. 5,702,653 and U.S. Pat. No. 5,169,465) provide improved wear characteristics over conventional thick film fabrication methods, they are limited to encoder devices employing substrate materials that can withstand the relatively high firing temperatures involved (i.e., above the melting point of the glass frit). Thus, while suitable for ceramic and metal substrates, these techniques cannot be used to make encoders that include substrates formed from typical printed circuit (“PC”) board materials, such as fiberglass-filled polymeric resins.
Thus, there has been an heretofore unmet need for an encoder element formed on a PC board substrate that provides good wear-resistance with a low contact profile. There has also been a need for method of fabricating such an element.
SUMMARY OF THE INVENTION
Broadly, a first aspect of the present invention is a novel switching element, comprising a polymeric resin substrate, a conductive metal track formed on a first major surface of the substrate, and a thick film dielectric insulator material screen printed on the track in a preselected pattern of nonconductive areas, whereby a desired pattern of conductive contacts is formed by the metal areas left exposed between the nonconductive areas. A preferred embodiment is an encoder comprising two or more annular contact arrays or encoder elements concentrically surrounding a central collector contact in a “bull's-eye” pattern. Each of the encoder elements and the collector contact comprises a copper base layer of about 25 to 40 microns in thickness, which is plated first with a nickel layer of a thickness of approximately 1 to 2 microns, and then a gold layer of a thickness of at least about 0.25 microns. The insulative material is, in the preferred embodiment, a mixture of a difunctional monomer (e.g., diallyl isophthalate), a glycol ether ester (e.g., diethylene glycol monobutyl ether acetate), an inorganic filler (e.g., BaSO
4
), and an acetate surface modifier (e.g., an ethyl acrylate/ethylhexylacrylate copolymer), with a small amount of organic pigment. The insulator material is preferably applied to a thickness of about 10 to 30 microns.
Conductive traces are formed on the opposite major surface of the substrate to provide the necessary electrical connections between the conductive elements on the first major surface and terminal pins that are installed in the substrate for installation into an electrical circuit. Each of the traces comprises a copper base layer plated with nickel and gold in the aforementioned thicknesses.
In another aspect, the present invention is a method of fabricating a switching element, comprising, in the preferred embodiment, the steps of:
(1) Providing a substrate comprising a layer of fiberglass-filled polymeric resin sandwiched between first and second copper layers, each of which is approximately 25 to 40 microns thick;
(2) Using conventional PC board fabrication techniques, (a) forming the first copper layer into a central collector contact base layer and at least one conductive track base layer concentrically surrounding the collector contact base layer, and (b) forming the second copper layer into a plurality of conductive trace base layers;
(3) Plating the collector contact base layer, the conductive track base layer or layers, and the conductive trace base layers with a first plating layer of nickel and with a second plating layer of gold, to form a central collector contact, at least one conductive track, and a plurality of conductive traces; and
(4) Screen printing a thick film insulative material onto the conductive track or tracks in a preselected pattern of insulative areas, whereby a desired pattern of conductive contacts is formed by the metal areas left exposed between the insulative areas.
In the preferred embodiment, the materials and thicknesses are as described above.
As will be better appreciated from the detailed description that follows, the present invention provides a wear-resistant, low profile contact pattern that is specifically well-suited for fabrication on PC board material. The resulting contacts are particularly useful in encoder applications, because of the extended lifetime that can be obtained. It will be readily understood, however, that the novel concept of forming a pattern of conductive contacts by screen printing a pattern of thick film insulative material onto a conductive metal track or layer can be applied to a wide variety of switching devices, and is not limited to encoders.
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Bailey Craig R.
Green Geary
Riley Richard E.
Waszak Stephen J.
Bourns Inc.
Friedhofer Michael
Klein & Szerkeres LLP
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