Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-10-27
2003-11-25
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C338S0220SD, C029S612000, C029S621100
Reexamination Certificate
active
06651315
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrical devices.
2. Introduction to the Invention
Many electrical devices comprise two laminar electrodes and, sandwiched between them, an electrical element which may be a conductor, e.g. a resistive element, as for example in a resistor or a varistor, or a non-conductor, as for example in a capacitor. Particularly useful devices of this type are circuit protection devices which comprise a laminate of two laminar electrodes and, sandwiched between the electrodes, a laminar resistive element which exhibits PTC behavior. The resistive element may be composed of conductive polymer (this term being used to denote a composition comprising a polymer and, dispersed, or otherwise distributed, therein, a particulate conductive filler) or a ceramic, e.g. a doped barium titanate. When a conductive polymer is used, such devices are generally prepared by stamping (or otherwise cutting) a plurality of the devices out of a laminate of a sheet of the conductive polymer between two metal foils. When a ceramic is used, such devices are usually prepared by applying liquid electrode material to the major surfaces of a preformed laminar resistive element, and solidifying the liquid electrode material.
The products of such processes can sometimes be used without the addition of electrical leads, for example by installation between two spring-loaded terminals. In most cases, however, an electrical lead must be secured to each of the laminar electrodes, so that the device can be connected to other components of a circuit, e.g. mounted on a circuit board. The addition of leads is an additional expense and usually involves heating (e.g. during soldering or welding) which can cause damage, particularly to conductive polymer elements. The latter problem is particularly severe when a conductive polymer is heated a second time when the leads are connected to other circuit elements, in particular when the leads are connected to a printed circuit board by a soldering process. A further problem which can arise when such devices are to be mounted on a printed circuit board is that they protrude further from the board than is desirable.
SUMMARY OF THE INVENTION
We have now realized, in accordance with the present invention, that when at least one of the laminar electrodes of an electrical device as described above is to be connected to an electrical conductor on an insulating substrate, in particular a printed circuit board, then by appropriate modification of the device and/or of the configuration of the electrical conductor on the substrate, the connection can be made either without the need for a lead (or other connecting member), or with the aid of a connecting member which is electrically connected to the electrode in the same step as it is electrically connected to the conductor on the substrate. We have also realized, in accordance with the present invention, that such modification can also be extremely valuable for electrical devices which are connected via elastically deformed terminals. The invention will be described herein chiefly by reference by circuit protection devices comprising a laminar PTC resistive element sandwiched between, and preferably contacted directed by, two laminar electrodes, but it is also applicable to other devices comprising two laminar electrodes and another type of laminar element sandwiched between them.
Typically, the devices of the invention have, in addition to a main portion having a normal configuration (typically a simple geometric shape such as a rectangle or a circle), at least one connection leg which extends away from the main portion and which comprises an extension of the PTC resistive element and an extension of one of the electrodes secured to the PTC element. The connection leg can, but preferably does not, also contain an extension of the other electrode.
In one use of such modified devices, the device is mounted on a circuit board by inserting the end of the connection leg into an aperture in the board and soldering the electrode to a metal conductor which is secured to the board and which leads to the aperture. If the connection leg does not include extensions of both electrodes, then the solder connection can be carried out in conventional ways. If the leg does include extensions of both electrodes in the region of the aperture, then care must be taken to ensure that the solder connects only the desired electrode to the metal conductor. For this purpose, the metal conductor can lead to one side only of the aperture and preferably can be substantially narrower than the aperture and the face of the connection leg to which it is soldered. A circuit board having a metal conductor and an aperture of this kind is believed to be novel per se and as such forms a part of the present invention. Generally, a device which is to be mounted in this way will be modified so that it has two connection legs of the kind described, extending in the same direction from the main portion of the device, so that the legs can be mounted in adjacent apertures in the same circuit board. The legs preferably have distal portions which are inserted into the apertures in the board, and intermediate stand-off portions which cannot pass through the apertures and which ensure that the main part of the device is spaced apart from the board.
As discussed above (and in greater detail below), when the device is to be mounted in apertures in a circuit board, the connection leg can comprise extensions of both electrodes; preferably, however, the connection leg includes a bridge portion which extends across the full width of the leg and which includes only one of the electrodes, so that the device can be soldered to a conventional circuit board in the conventional way. When the connection leg is of the latter kind, the device can be mounted in the apertures of a circuit board as outlined above, or can be mounted flat on top of a circuit board.
To mount the device flat on top of a circuit board, the device is placed on the substrate with the electrode which extends into the connection leg on top. The bottom electrode is connected directly to a conductor on the board. The top electrode is connected to another conductor on the board by means of a connection member secured to the top electrode and extending downwards below the lower face of the PTC (or other) element. In a particularly preferred embodiment, the connection member is a transverse conductive member which passes through the PTC (or other) element; such a transverse member is often referred to herein as a “cross-conductor”. The connection member is preferably electrically connected to the upper electrode before it is electrically connected to the conductor on the board; alternatively, both electrical connections can be made simultaneously. If there are two connection legs, one containing an extension of one of the electrodes only, and the other containing an extension of the other of the electrodes only, the device can be placed on the board with either electrode on top, and if the device is symmetrical, the connections to the device and the subsequent operation of the device can be the same.
The novel devices of the invention can be made by securing electrodes of appropriate shapes to resistive elements of the desired final shape; or by securing electrode precursors of appropriate shapes to resistive elements which are larger than the desired final shape, and then dividing the assembly into a plurality of devices of the desired final shape or shapes; or by preparing a plurality of devices of the desired final shape or shapes by division of a simple laminate of constant cross-section and, if desired or necessary, and before or after the division, removing unwanted portions of one or both of the electrodes. Such removal can be effected for example by milling or by etching. Preferably such removal of unwanted portions of the electrodes removes little or none of the PTC resistive element, which provides desirable physical strength to the connection leg. Pre
Chan Chi-Ming
Chandler Daniel
Fang Shou-Mean
Graves Gregory A.
Siden Dennis
Arbes Carl J.
Trinh Minh
Tyco Electronics Corporation
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