Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
1999-07-06
2001-09-18
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C358S302000, C358S302000, C358S302000, C358S302000, C358S296000
Reexamination Certificate
active
06292088
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrical devices.
INTRODUCTION TO THE INVENTION
Application Ser. No. 08/121,717 discloses a variety of improved devices (and methods of making such devices) which comprise a laminar electrical element, preferably a PTC resistive element composed of a conductive polymer, sandwiched between two laminar electrodes. These improved devices include a transverse conductive member (often referred to as a cross-conductor) which passes through the electrical element and is connected to one of the electrodes but not to the other. Preferably the device comprises a first laminar electrode which is connected to the cross-conductor; a second laminar electrode which is not connected to the cross-conductor; and an additional laminar conductive member which is (i) connected to the cross-conductor, (ii) secured to the same face of the electrical element as the second electrode, and (iii) spaced apart from the second electrode. The additional conductive member and the second electrode are preferably formed by removing a strip from a laminar conductive member, thus dividing the laminar conductive member into two parts.
These improved devices are particularly useful for installation, e.g. onto a printed circuit board, by soldered connections to the second electrode and the additional conductive member. For such installation, the additional conductive member and/or the second electrode are preferably provided with an outer layer of solder. As disclosed in Ser. No. 08/121,717, when the devices are made by dividing up an appropriately treated laminate comprising many devices, the preferred methods of preparation result in the surface of the first electrode also carrying an outer layer of the same solder. The layers of solder on the additional conductive member and on the first electrode can also serve to improve the current-carrying capacity of (or even to create) the cross-conductor, by flowing into the aperture during the connection process.
SUMMARY OF THE INVENTION
We have found that during installation of these devices containing solder layers, particularly their installation on printed circuit boards, there is a danger that melting of the solder layers will not only make the desired connections, but will also create short circuits between the electrodes. These short circuits can be created by solder flowing across the gap between the additional conductive member and the second electrode, and/or by solder flowing between the electrodes. We have also found that if the outer surface of the first (upper) electrode is completely covered by a layer of solder which melts during installation of the device, this makes it impossible to provide the device with permanent markings which will identify the device after installation.
We have found, in accordance with the present invention, that the problems caused by solder flow during installation can be mitigated or solved by the use of masking and/or separating materials which are applied to the device to provide permanent or temporary members which (a) ensure that solder layers to be used in the connection process are formed only in desired locations and/or (b) during installation of the device, prevent (or at least hinder) solder flow which results in short circuits between the electrodes, and/or (c) provide a convenient, permanent location for identification marks on the device. As discussed in detail below, the masking or separating materials preferably applied to an assembly which is later separated into a plurality of individual devices.
In a first aspect, the present invention provides an electrical device which has a reduced tendency to suffer from short circuits caused by solder flow during installation and which comprises
(1) a laminar PTC resistive element which has a first face and second face;
(2) a first laminar electrode which has (i) an inner face which contacts the first face of the PTC element and (ii) an outer face;
(3) a second laminar electrode which has (i) an inner face which contacts the second face of the PTC element and (ii) an outer face;
(4) an additional laminar conductive member which
(a) has (i) an inner face which contacts the second face of the PTC element and (ii) an outer face, and
(b) is spaced apart from the second electrode;
the PTC element, the first electrode and the additional conductive member defining an aperture which runs between the first electrode and the additional conductive member, through the PTC element;
(5) a transverse conductive member which
(a) is composed of metal,
(b) lies within the aperture, and
(c) is physically and electrically connected to the first electrode and the additional conductive member;
(6) a first layer of solder which is secured to the outer face of the additional conductive member;
(7) a second layer of solder which is secured to the outer face of the second electrode; and
(8) a separation member which
(a) is composed of a solid, non-conductive material,
(b) lies between the first and second layers of solder, and
(c) remains solid at temperatures at which the layers of solder are molten.
The separation member prevents the first and second layers of solder from flowing to create a short circuit between the electrodes when the layers of solder are heated to temperatures at which they are molten during installation of the device, e.g. on a printed circuit board.
In a second aspect, the present invention provides an electrical device which overcomes the problem that permanent markings cannot be made on a device whose entire upper surface is covered by a layer of a solder which melts when the device is installed. The devices of the second aspect of the invention comprise
(1) a laminar PTC resistive element which has a first face and second face;
(2) a first laminar electrode which has (i) an inner face which contacts the first face of the PTC element and (ii) an outer face;
(3) a second laminar electrode which has (i) an inner ace which contacts the second face of the PTC element and (ii) an outer face;
(4) an additional laminar conductive member which
(a) has (i) an inner face which contacts the second face of the PTC element and (ii) an outer face, and
(b) is spaced apart from the second electrode;
the PTC element, the first electrode and the additional conductive member defining an aperture which runs between the first electrode and the additional conductive member, through the PTC element;
(5) a transverse conductive member which
(a) is composed of metal,
(b) lies within the aperture, and
(c) is physically and electrically connected to the first electrode and the additional conductive member;
(6) a first layer of solder which is secured to the outer face of the additional conductive member;
(7) a second layer of solder which is secured to the outer face of the second electrode;
(8) a third layer of solder which is secured to the outer face of the first electrode around the transverse conductive member; and
(9) a masking member which
(a) is composed of a solid material, and
(b) is secured to the outer face of the first electrode adjacent to the third layer of solder.
In one embodiment of the second aspect of the invention, the masking member can be one which remains in place after the device has been installed and which
(a) extends so that the second and third layers of solder do not overlap (when viewing the device at right angles to its principal plane), and/or
(b) carries identification marks.
The masking member can be composed of a non-conductive material or a conductive material, e.g. a solder having a melting point substantially higher than the solder in the first, second and third layers of solder.
In another embodiment of the second aspect of the invention, the masking member is stripped off the first electrode before the device is installed. In this case also, the masking member can extend so that the second and third layers of solder do not overlap. After the masking member has been stripped off, identification marks can, if desired, be placed on the exposed surface of the first electrode, or on a metallic layer plated thereo
Fang Shou-Mean
Zhang Michael
Easthom Karl D.
Gerstne Marguerite E.
Tyco Electronics Corporation
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