Organic thermistor device and method of producing same

Details Organic thermistor device and method of producing same Organic thermistor device and method of producing same

1999-09-28

2001-10-09

Easthom, Karl D. (Department: 2832)

Electrical resistors

Resistance value responsive to a condition

Current and/or voltage

C338S0220SD, C338S208000, C029S610100

Reexamination Certificate

active

06300861

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a surface-mountable thermistor device which may be used for protection against an overcurrent. More particularly, this invention relates to an organic thermistor device comprising a thermistor element made of an organic thermistor material, as well as a method of producing such organic thermistor devices.
Organic PTC (positive temperature coefficient) thermistors made of an organic thermistor material are coming to be used as circuit protection units for suppressing overcurrents. Such organic PTC thermistor devices make use of an organic thermistor material obtained by dispersing carbon or the like in a resin material such as polyethylene to provide a positive temperature characteristic (PTC characteristic). They are generally produced, as shown in
FIG. 6
, by forming surface electrodes
52
a
and
52
b
by pressing a metallic foil of nickel or copper on both upper and lower surfaces of a thermistor body
51
of an organic thermistor material shaped in a planar form and then forming outer electrodes
53
a
and
53
b
by plating or sputtering. Alternatively, an organic thermistor device may be formed, as shown in
FIG. 7
, by using an electrically insulating material
54
such as an insulating resin to cover exposed parts such as the thermistor body
51
and the surface electrodes
52
a
and
52
b,
leaving only the outer electrodes
53
a
and
53
b
exposed.
An organic thermistor device, as described above, may be surface-mounted, as shown in
FIG. 8
, by electrically and mechanically connecting the outer electrodes
53
a
and
53
b
to wiring electrodes (or “lands”)
56
on a printed circuit board
55
by a solder reflowing method through a solder fillet
57
.
In the case of a PTC thermistor device for protecting a circuit from an overcurrent situation, its resistance value at normal temperatures is desired to be 0.1&OHgr; or less such that a voltage drop in the PTC thermistor device during the use of the circuit can be avoided. If the specific resistance, the thickness and the cross-sectional area of the PTC thermistor body
51
are &rgr;, T and S, respectively, the resistance value of the PTC thermistor device is given by &rgr;T/S.
If an organic PTC material is to be used for the PTC thermistor device, the fact at the present time is that it is difficult to make the specific resistance equal to or less than 0.5 &OHgr;cm if this PTC thermistor material must also have the required electrical characteristics when its resistance value changes suddenly under a high-temperature condition. Accordingly, if it is attempted to use such an organic PTC thermistor material to produce an organic PTC thermistor device with resistance value equal to or less than 0.1&OHgr; at normal temperatures, the result will be a structure as shown in
FIG. 7
having surface electrodes
52
a
and
52
b
formed on both upper and lower surfaces of a planar thermistor body
51
made of an organic thermistor material by pressing a metallic foil of nickel or copper.
Even if a PTC thermistor device is produced in a form as shown in
FIG. 7
with surface electrodes on both upper and lower surfaces of the thermistor body, the thickness of the thermistor body
51
must be made very small and its cross-sectional area large in order to make its resistance value at normal temperatures equal to or less than 0.1&OHgr;. With prior art organic PTC thermistor devices, therefore, the dimensions of the thermistor body
51
were, for example, 4.5 mm (length)×3.2 mm (width)×0.3 mm (thickness).
Although it is an essential requirement for a PTC thermistor device to reduce the resistance value at normal temperatures, this requirement could be satisfied with the prior art technology only by reducing the thickness of the thermistor body and increasing its cross-sectional area (or its planar area). As a result, the planar dimensions of the product remained large and a large space was required for its surface-mounting. Secondly, a relatively large amount of organic thermistor material will be used for the production and this gives rise to an increased production cost. Thirdly, if the thermistor body is very thin, it is likely to become twisted or bent after being mounted. Fourthly, if a large amount of the organic thermistor material is used between the pair of outer electrodes, the action time of the PTC thermistor device becomes long and there may arise situations where a sufficient protective characteristic against overcurrents cannot be obtained and the circuit element to be protected may break before the PTC thermistor device can act.
An attempt may be made to introduce inner electrodes into the PTC thermistor body by stacking organic PTC sheets with an electrode formed thereon, but the layer-forming process including steps of making thinner organic PTC sheets, forming conductors to serve as inner electrodes and stacking up the sheets one on top of another tends to increase the production cost as a whole. Thus, the price of the product will increase significantly and hence such a method is not a practical solution to the problem.
SUMMARY OF THE INVENTION
It is therefore an object of this invention, in view of the problems described above, to provide a compact organic thermistor device which has a small resistance value at normal temperatures and is both reliable and economically advantageous.
Another object of this invention is to provide a method of producing such organic thermistor devices.
An organic thermistor device embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a thermistor body made of an organic thermistor material, a pair of mutually facing outer electrodes formed on end parts of this thermistor body, and metallic wires extending inside and through the thermistor body transversely to the direction in which the outer electrodes face each other. With an organic thermistor device thus structured, the resistance value at normal temperatures can be significantly reduced. If metallic wires with specific resistance of about 1 &OHgr;cm are buried inside an organic thermistor material, it is equivalent to reducing the distance between the pair of outer electrodes because the specific resistance of the metallic wires is negligible and is much smaller than that of the organic thermistor material. If both exposed surfaces of the thermistor body not covered by the outer electrodes and exposed end surfaces of the metallic wires not covered by the thermistor body are all covered by an electrically insulating layer, the reliability of the product is further improved because, when it is mounted to a circuit board, short-circuiting between the various components of the device and other electronic components and wires on the circuit board can be prevented. Where there are a plurality of wires passed through the organic thermistor body, it is preferred that they extend approximately parallel to one another because the volume ratio of the metallic wires inside the thermistor body can be kept high.
To produce organic thermistor devices of the invention as characterized above, an elongated wire-containing member is formed by molding an organic thermistor material by covering metallic wires so as to extend longitudinally through the thermistor body, and a pair of longitudinally elongated outer electrodes is formed on mutually opposite sides of the external peripheral surface of this wire-containing member. Such outer electrodes may be formed by entirely covering the wire-containing member as formed above with an electrically insulating material and then removing portions of it from a pair of longitudinally continuous external peripheral surface areas of the wire-containing member. The outer electrodes are thereafter formed on this pair of longitudinally continuous portions of the external peripheral surface of the wire-containing member from which the insulating material has been removed. The wire-containing member is then cut transversely to the direction of its elongation at specified positions to b

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