CA2092807A1

CA2092807A1 - Self-regulating ptc devices having shaped laminar conductive terminals

Info

Publication number: CA2092807A1
Application number: CA002092807A
Authority: CA
Inventors: Chi-Ming Chan; Shou-Mean Fang
Original assignee: Individual
Current assignee: Raychem Corp
Priority date: 1990-09-28
Filing date: 1991-09-26
Publication date: 1992-03-29
Also published as: EP0551384A4; HK1006888A1; DE69124256T2; WO1992006477A1; ATE147882T1; JPH06501817A; JP3260750B2; EP0551384B1; US5089801A; EP0551384A1; DE69124256D1

Abstract

An electrical device (1) in which a conductive terminal (3, 9) is physically and electrically attached to a laminar resistive element (19) which has a first periphery by means of a laminar conductive element (21, 23). The three layers are positioned in such a way that the periphery of the conductive element does not extend beyond the first periphery and at least a part of the periphery of the conductive terminal lies within the first periphery. In a preferred embodiment the conductive element (21, 23) is solder and the periphery of the conductive terminal is shaped in such a way that no excess solder bridges from one laminar surface of the resistive element (19) to the other. Devices of the invention are useful as circuit protection devices.

Description

WO 92/0~7 1 ~ '(lr! PCTtUS9l/07041 SEhF-REGULATI~G ~C P~ylCE~_ ~aYl~G S~APE~ ~AMIX
CONDU~IV~ ~ER~

~ GROVND QF T~E INVENTIQ~
Field of the Invention :.
This invention relates to electrical devices and methods fGr making them, particularly electrical devices which are suitable for use as circuit protection devices.

Introduction to the Invention Electrical devices for use in protecting against over-15 voltage or over-temperature conditions in a circuit are ;
well known. Such circuit protection devices frequently comprise materials which exhibit a positive temperature coefficient of resistance, i.e. PTC behavior, and thus act to shut down a circuit if conditions are unsafe by increasing in resistance by orders of magnitude from a normal, low temperature value. Devices of this type may comprise an inorganic material, e.g. BaTiO3, or a conductive polymer composition. For any material, the time required for the device to switch into its high resistance state, i.e. to "trip", is a function of the resistivity of the material, the geometry of the device, and the thermal environment. It is generally preferred that the resistance of the device at 23C be as low as possible in order to contribute as little resistance as possible to the circuit during normal, low,temperature opera~ion. For most low voltage applications, i.e. 60 volts or less, devices of planar geometry are preferred. Such planar devices comprise a laminar resistive element which is electrically connected to two laminar electrodes. For a material of a given resistivity, planar devices of specified area will have the lowest resistance when the distance between the electrodes, i.e the current path length, is the smallest.
Therefore, thin devices are preferred and result in lower . ,, . ~ ; , ................. ~ -;: --:., ~ ,, ~ . .:. . : , . , :
: -~. : , , . . .: : :

W092/064~7 ~ 3 ~ 2 PCT/US91/070~1 resistances, lower materials requirements, and smaller "real estate" requlrements for a printed circuit board.

There are problems, however, with thin laminar devices. When the device trips into its high resistance state, heat is generated by I2R heating Because of the relatively small thermal mass of a thin device, it tends to dissipate the heat rapldly and to trip rapidly. Such rapid tripping is not desirable for all applications. For example, when a device is designed to protect a motor used to raise or lower a window, the device heats as the motor operates. It is necessary that the window be fully opened or closed before the device heats sufficiently to cause it to trip. Therefore, a relatively long trip time is needed when compared to many conventional applications. One technique to increase the trip time is to increase the thermal mass by electrically and physically attaching elements of high thermal mass, e.g. metal terminal plates, to the device. 'The most common technique for connecting the thermal elements to the laminar device is to solder them into position, e.g. by applying a solder paste to either the thermal element or the laminar device or both, heating the solder paste to cause it to flow, and then cooling the solder to attach the thermal element to the laminar device. If there is excess solder, during the reflow operation, it may~be forced out from under the thermal element and bridge across the resistive element from one laminar electrode to the other. As a result, during operation there will be failure of the device as an electrical short dccurs.
~: ...
S~MMARY OF TH~ INV~N~ION

The tendency for an electrical short to form between the terminals can be decreased by increasing the distance from one terminal to the other. This can be accomplished by using a laminar device which has a larger distance and ~ ~ .. . .. . . , . , , ,. ,,, ., .-WO 92/06477 PCT~US91/07041 , ~` ~ 3 ~, thus a longer current path between the electrodes, i.e. a greater thickness. To maintain a low resistance with a greater thickness, however, the resistivity of the material ; for a device of this type would need to be very low, often 5 lower than is commercially feasible. Alternatively, ~ ; '!
reservoirs for excess solder can be provided by moving the terminals away from the edge of a device and/or by c~tting holes in the terminals. However, this procedure may not be desirable ~ecause those sections of the device not in contact with the terminal are subject to undesirable thermal effects, e.g. formation of a hot zone or an area of higher current concentration, which can result in device failure. In addition, it is undesirable to have a significant portion of a laminar device exposed due to the ~-possibility of mechanical damage to the device. We have now discovered that if the edges of the terminals of a device are notched or otherwise indented, and the indentations on one of the terminals are staggered with respect to the indentations on the other terminal, this has relatively little thermal effect on the device, but yet provides reservoirs along the periphery of the device into which excess solder or cther conductive paste can flow.
- This prevents the solder from flowing over the edge of the ~ -'l resistive element to cause a solder bridge. The staggered -~
indentations can be present around part or all of the periphery of the device. ~-In a first aspect, this invention relates to an electrical device which comprises ~1) a laminar resistive element which (a) is composed of a first material having a first resistivity at 23C, and lb) has a first periphery; ~

: , , {J ,J, ~ ~ 7 4 : ~

(2) a laminar conductive element which :, (a) is secured to a face of the resistive element, !.
~.
(b) is composed of a second material having a -~`
second resistivity at 23C which is ! substantially lower than the first ~ resistivity, and 1 0 , (c) has a second periphery wnich does not ~ ~
extend beyond the first periphery; and i:: :
'.~

(3) a conductive terminal comprising a laminar portion which (a) is secured to a face of the conductive element remote from the resistive element, (b) is composed of a third material having a third resistivity at 23C which is substantially lower than the first . resistivity, and (c) has a third peripher~ at least a part of which lies within the first periphery.

In a second aspect, this invention relates to a ;
circuit protection device which comprises .~ ;~
(l) a laminar resistive element which (a) is composed of a conductive polymer composition and .
(b) has a first periphery; ~:~

' , ::; - , , , . ,.,. . , . . : .: , ~ . , . .,, . . . :
:, ::, :. :. ,. -, ~., , . : . . ., : . ~ ,. : , , -:.,: : : . ~ ~ . ... , - , . . .

W092/06477 PCT/USgttO~041 ") 3 ~ ~ O i :
(2) two laminar electrodes which are attached to opposite surfaces of the resistive elemen~, each of which electrodes ~, (a) is composed of metal, and (b) has a periphery which coincides with the first periphery; :~

~3~ two conductive elements, eacll of which :::

(a) comprises solder, (b) has a second periphery which does not extend beyond the first periphery, and :~

(c) is attached to a face of one of the laminar electrodes remote from the resistive ;
element; and (4) two conductive terminals, each of which ~ ~"
(a) is composed of metal, (b) has a third periphery, at least a part of which lies within the first periphery, (c) is attached to a laminar surface of one of the conductive elements remote from one of th,e laminar electrodes.

In a third aspect, this invention relates to a method ~:
of making a an electrical device of the second aspect, which method comprises : 35 (1) providing a laminar resistive element which WO 92/0~77 P~T/US91/~7041 0 ~ii 6 ~a) is composed of a conductive polymer composition, ;

(b) has a first periphery, and (c) is attached on opposite surfaces to two laminar electrodes, each of which (i~ is ::
composed of metal and ~ii) has a periphery ~:
~ which coincides with the first periphery;
';
(2) applying a conductive paste to a face of each of ~-the laminar electrodes;
' ~:;
~3) positio~ing onto the conductive paste in a ~:
selected position a conductive terminal which (a) is composed of metal, and "
(b) has a third periphery which has an irregular shape comprising notches, wherein at least a part of the third periphery lies within the first periphery; and ~::

~4) attaching the terminals to the electrodes so ~:
.. . .
that excess conductive paste is forced from `~
under each.terminal into the notches.

BRI~F DESCRIPTION OF T~E DRAWING

Figure l shows a plan view of a device of the invention; and .
Figure 2 shows a cross-sectional view of the device of ;
Figure l along line 2~2 :~

,.

, . ,: . . ,. -: . . . , : . . .
,.. .. . : . .: ,' ',: ': : , '. .

,' :.: : : -: ,.- . . .. : .' ., ,- : ' ' ;. - : ': : ' : . . .:

WO 92/0$477 PCTtUS91/07041 7 ~ fJ ~ ~J 7 D~aIL~ P~SCRIPTIO~ QF T~ V~TIO~

The electrical devlce of the invention comprises a laminar resistive element which rnay be of any shape, e.g.
rectangular, round, or square, and which has a first periphery, i.e. the maximum distance around the edge (the perimeter) of the element. The resistive element is composed of a first material having a first resistivity at 23C. Sui,table materials include inorganic compositions such as BaTiO3, and conductive polymer compositions. Such conductive polymer compositions comprise a particulate conductive filler which is dispersed or otherwise distributed in a polymeric component~ The polymeric component may be an organic polymer, preferably a crystalline organic polymer, an amorphous thermoplastic polymer, an elastomer, or a blend comprising one or more of ~.
these. Suitable crystalline polymers include polymers of one or more olefins, particularly polyethylene; copolymers of at least one olefin and at least one monomer copolymerisable therewith, such as ethylene/acrylic acid, ethylene/ethyl acrylate, and ethylene/vinyl acetate copolymers, melt-shapeable fluoropolymers such as ~-polyvinylidene fluoride; and blends of two or more such crystalline polymers. Dispersed or distributed in the polymeric component is a particulate conductive filler which may be, for example, carbon black, graphite, metal, metal oxide, particulate conductive polymer, or a combination of these. The quantity of conductive filler needed is based on the required resistivity of the first material which depends on the desired application and the geometry of the electrical device. When, as is preferred, the device functions as a circuit protection device, a resistance at 23C of 0.00l to l00 ohms is usually required. For this type of application, when the first material is a conductive polymer composition, the resistivity at 23C is 0.00l to l000 ohm-cm, preferably ~: -: ,...
,, : . ,, :.
-......... - :. . . . .
, ~ :: ,.: . : , : .

W092/0~77 ~ J ~ PCT/US~J/07041 0.005 to 500 ohm-cm, particularly 0.01 ~o 100 ohm-cm, e.g.
0.1 to 25 ohm-cm.

When the first material comprises a conductive polymer composition, additional components such as inert fillers, antioxidants, chemical crosslinking agents, stabilizers, or dispersing agents may be present.

For ~any applications, it i5 desirable that the first material exhibit PTC behavior. The term "PTC behavior" is used in this specification to denote a composition or an electrical device which has an Rl4 value of at least 2.5 and/or an R1co value of at least 10, and it is particularly preferred that the composition should have an R30 value of 15 at least 6, where R14 is the ratio of the resistivities at ;
the end and the beginning of a 1~C temperature range, R
is the ratio of the resistivities at the end and the ;:
beginning of a 100C range, and R30 is the ratio of the ;
resistivitles at the end and the beginning of a 30C range.
When the first material is a conductive polymer composition which exhibits PTC behavior, crystalline organic polymers are preferred. Suitable conductive polymer compositions may be found in U.S. Patent Nos. 4,237,441 (van Konynenburg et al), 4,304,987 (van Konynenburg), 4,388,607 (Toy et al), 4,514,620 (Cheng et al), 4,534,889 (van ~onynenburg et al), `-4,545,926 (Fouts et al), 4,560,498 ~Horsma et al~, 4,591,700 (Sopory), 4,729,417 ~Au et: al), 9,774,024 (Deep et al), 4,910,389 ~Sherman et al), and 5,049,850 ~Evans).

A laminar conductive element is secured to the surface of the resistive element so that there is physical and electrical contact between the two elements. The conductive element is composed of a second material having a second resistivity at 23C which is substantially lower than the first resistivity. In this application, when a material is said to have a resistivity which is substantially lower than the first material, it means that .

:;::, . . ~ . . . .. . ., . ., . . ., W092/0~7~ PCT/US9l/07041 the resistivity is at least 10 times less, preferably at least 50 times less, partlcularly at least 100 tirnes less than the resistivity of the first material at 23C. The conductive element has a second periphery which does not extend beyond the first periphery, i.e. it may be entirely within the first periphery or may coincide with the first periphery. It is also preferred that the second material be thermally conductive in order to enhance the flow of heat from~the resistive element to the conductive terminal.
10 Many conductive materials may be used for the conductive -~
element, e.g. conductive inks, conductive pastes, or conductive epoxies. For many applications, however, it is preferred that the second material is solder, which can be ; ~ ;
readily applied, attached to and form an electrical connection between the laminar resistive element and the conductive terminal. The appropriate type of solder .
depends on the properties of the material comprising the resistive element. For example, a tin eutectic solder which can be melted and reflowed at a relatively low temperature, is suitable for use with conductive polymer resistive elements comprising polyethylene. Other, higher melting solders, such as silver-based solders, may be used with resistive elements comprising higher melting po~ymers or inorganic materials. When solder is used, it may be applied to either the surface of the resistive element (or any attached electrode) or the surface of the conductive terminal, or both, preferabl~ in the form of solder paste.
The composite is then heated in a solder reflow furnace (which may be an infrared oven, a hot air oven, or a vapor phase reflow oven,) to melt and reflow the solder. After the solder is cooled, a bond is formed between the various elements.

The conductive terminal comprises a laminar portion which is secured to a face of the conductive element remote from the resistive element and which is composed of a third material having a third resistivity at 23C which is : :: , , : : : : . :: :: ~:: . ~

W092~06477 PCT/US91/07041 r``~ '1 10 ~:
s stantially lower than the first resistlvity. The conductive terminal is preferably a laminar metal sheet which comprises one or more metal layers, although for some applications, il may be a metal mesh or screen, a fabric containing a metal fiber, or a layer formed from a conductive ink. When the terminal is a metal sheet, the type of metal depends on the thermal, e:lectrical, environmental, ar.d cost requirements for the devlce.
Different~ layers may be present in order to meet di~ferent requirements and it is often preferred that the interior surface layer, i.e. that surface in contact with the conductive element, and the exterior surface layer be different. For example, devices which are to be soldered to a circuit board or to another component may require an exterior surface layer of copper, brass, or tin, while devices to be welded would require copper rather than tin which would contaminate the welding electrodes. For de~ices to be used in a corrosive environment, nickel may be suitable. A preferred terminal comprises copper-coated steel. It is also possible that the shape and/or texture of the interior and exterior surface layers be different in order to meet different requirements. Thus one surface may be an electrodeposited layer which comprises nodules suitable for enhanced adhesion to the conductive element and the other surface may have "fins" for improved convection of heat from the device.

The thickness of the terminal is also affected by the thermal requirements of the device. In general, it is preferred that the~terminal have a thickness of at least 0.002 inch (0.005 cm), preferably at least 0.005 inch (0.0127 cm), particularly at least 0.010 inch (0.025 cm), especially at least 0.015 inch ~0.038 cm), e.g. 0.020 inch ~0.051 cm), but that it have a thickness of less than 0.100 inch ~0.254 cm), preferably less than 0.080 inch ~0.203 cm), in order to prevent the restriction of any necessary expansion of the resistive element. If fins are present on ~-: . ~ ': . .

,:. , . . : , . , : . ., .: , . .

WO 92/06477 PCTtUS91/07041 ll ~.;.J ~

one surface of the terminal, the thickness does not include the height of the fin.

The conductive terminal has a third periphery, at least a part or which lies within the first periphery. For many applications, it is preferred that the majority of the third periphery, i.e. at least 50~ of the third periphery, ;~
lie within the first periphery. Portions of the third peripher~ which lie outside the first periphery may be used for making electrical contact from the electrical device to a circuit board or an electrical lead. Therefore, it is common that "tabs" for welding or otherwise connecting the terminal to a source of electrical power, extend beyond the first periphery. The third periphery, which is equivalent to the edge of the conductive terminal, may be shaped in any way which is suitable for the application and which will allow the terminal to be positioned correctly with respect to the resistive element and the conductive element. It is generally desired that the amount of metal in contact with the conductive element be maximized in order to maximize the thermal mass of the device and minimize any areas of high current concentration which result from a nonuniform contact between the terminal an~
the resistive element (or the electrode attached to the resistive element). Therefore, an effective conductive terminal can be prepared by removing only a small quantity of material from at least part of the edge of the terminal or by positioning the terminal only slightly away from the edge of the resistive element. A suitable terminal can be prepared from a metal sheet which initially has the same dimensions as the resistive element, but which is then treated around at least a part of its edge to remove material in either a regular or irregular pattern. A metal foil can be stamped or die-cut into a suitable shape. Fo_ many applications, at least lO~, preferably at least 20%, particularly at least 30% of the terminal edge is notched or indented. In a preferred embodiment, as shown in Figure :: ~ .: .-.:. . : , . , , :. . . . . . .. .. .

:` . i: ' '` ' ' ' : :' ' ' .: , , ' . . ., ' : . .' ~ . . ::. ' ' , ::: ~.: , . . .' : , . ' .. '. . - ' ' , . .. ' :::: ' ' '. . . ' . : - . - ' WO 92t0~77 PCTlUS91/07041 ~"J~) J 12 1, material is removed from the edge of the terminal in a regular pattern except in the region of the tab to form rectangular notches. In other embodiments, the pattern at the edge may be scalloped, or otherwise indented. ~hen, as is preferred, the device comprises two conductive terminals, one secured to a conductive element on each laminar face of the resistive element, it is preferred that the first and the second conductive terrninals have the same shape but~that they each be secured to the conductive element in a selected position. The selected positioning allows the first and the second conductive terminals to be oriented in such a way that when the device is notionally cut into slices in a direction normal to the laminar face of the resistive element, each slice is in contact with at 15 least one conductive terminal over the entire surface of -;
one face. This positioning ensures that the resistive element is in contact with at least one conductive terminal for every slice for the maximum thermal mass. In some designs, one or more vent holes are present in the conductive terminal in order to provide a site for excess solder to flow during the refIow process. If there are two conductive terminals and each contains a vent hole, the vent holes are positioned offset from one another. To avoid adverse thermal effects, the vent holes comprise less than 20%, preferably less than 15%, particularly less than 10~ of the surface area of the conductive terminal.

While it is possible to attach the conductive element directly to the resistive element, for most applications it is preferred that,the conductive element be attached to a laminar electrode which itself is attached to the resistive element. The laminar electrode is composed of a fourth material having a fourth resistivity at 23C which is substantially lower than the first resistivity. The fourth material is generally a laminar metal foil such as copper or nickel, particularly an electrodeposited metal foil which has a nodular surface for enhanced adhesion to a :.. :: , .. . ... . .. .
.:. , . . , , ~ . . , . - : , : , . . . . .
. . , .:
.. :~ ~, , . . ,~ . ;, ;,. , . , . : . . . .

W092/0~7~ PCT/US91/07041 ~ -13 ~ J^i conductive polymer or other substrate. Electrodes of this type, and devices comprisi~ them, are described in ~.S.
Patent Nos. 4,689,475 ~Matthiesen) and 4,800,253 (Kleiner et al). Alternatively, the laminar electrode may comprise a conductive ink, a conductive epo.Yy, or a metal layer deposited by flame-spray techniques or vacuum deposition.
When two laminar electrodes are secured to the two laminar faces of a resistive element, an electrical device is formed. ~Devices of this type are disclosed in U.S. Patent Nos. 4,238,812 (Middleman et al), 9,255,798 ~Simon), 4,272,471 (Walker), 9,315,237 (Middleman et al), 4,317,027 (Middleman et al), 4,330,703 (Horsma et al), 4,426,633 (Taylor), 4,475,138 (Middleman et al), 4,472,417 (Au et al), ~,780,598 (Fahey et al), 4,845,838 (Jacobs et al), 4,907,340 (Fang et al), and 4,924,074 (Fang et al).

The laminar electrode lies between the resistive element and the conductive element and is secured to both the resistive element and the conductive element. It has a fourth periphery, at least a part of which substantially follows at least a part of the first periphery. For many applications, the fourth periphery does not extend beyond the first periphery, and it is preferred that the fourth periphery coincide with the first periphery.
Devices of the invention can be prepared by a method of the invention in which a conductive material such as a solder paste or conductive epoxy is applied to a laminar surface of the resistive element. A conductive terminal is then positioned onlthe conductive material in a selected position so that at least part of the third periphery of the conductive terminal lies within the first periphery.
For many applications, it is desirable that the selected position be such that, when two conductive terminals are present, there is no section of the first periphery which is not in contact with at least one conductive terminal.
The conductive terminal is then electrically and physically ; , :. ,~ , . . . .

'.' ' ' ' ' ' ' , " ' ' ' '' , ' ` "

WO 92/~6477 PCTJUS9t/07041 attached to the resistive element, e.g. by reflowing and cooling the solder or curing the epoxy.

For some applications, it may be desirable to make the solder flow in a nonuniform manner in order to direct any excess solder into specific reservoirs. Under these conditions, the conductive terminal may be prepared with an indented or notched edge in only a specific region of the periphery~. Alternatively, for conductive terminals which ;
comprise a tab for electrical connection, a reservoir or channel at the point where the tab contacts the conductive terminal may be desirable.
: `
The invention is illustrated by the drawing in which Figure 1 shows a plan view of an electrical device 1. A
first terminal 3 comprises an electrical tab 5 to which electrical connections can be made. Also visible is an electrical tab 7 from the second terminal 9. The edge 11 of the first terminal 3 is irregularly shaped and has rectangular notches 13 cut into it. When the conductive layer comprises solder, any excess will be forced out from underneath first terminal 3 into the space created by the rectangular notches 13, thus avoiding solder bridging. The cut-out portions of the notches 13 reveal a laminar 25 electrode 15 which is attached-to resistive element 19 (not ~
visible). Also shown is a vent hole 14 though which excess ~;
solder can flow. ~otted lines indicate the edge 12 of the second terminal 9 which lies underneath first terminal 3.

Figure 2 shows a cross-section of an electrical device 1 taken along line 2-2 of Figure 1. Resistive element 19 comprises a conductive polymer composition which is attached to laminar electrodes 15,17. Conductive solder paste layers 21,23 physically and electrically attach the laminar electrodes 15,17 to first and second conductive terminals 3,9.
' ~`,~ .
' W~ 92/0~77 PCT/US91/07041 1 5 2~ 2 ~ rj7 The invention is illustrated by the following example.

~a~le 1 A conductive polymer composition is prepared by preblending 48.6% by weight high density polyethylene (Petrothene~ LB832, available from USI) with 51.4% by weight carbon blac~ (Raven~ 430, available from Columbian Chemicals~. The blend is mixed in a Banbury~ mixer, and the resulting composition is extruded through a 2. 5 inch (6.35 cm~ extruder to form a sheet with a thickness of 0.010 inch (0.025 cm). The sheet ls laminated on each side ;
with 0.001 inch (0.0025 cm) thick electrodeposited nickel foil (available from Fukuda) and the laminate is irradiated to a dose of 10 Mrad using a 4.5 MeV electron beam. Chips with dimensions of 0.39 x 0.79 inch ~1 x 2 cm) are cut rrom the irradiated sheet.

Copper-plated steel with a thickness of 0.020 inch ~0.051 cm) is cut into pieces shaped as shown in Figure 1 to form conductive terminals. Each terminal has a maximum dimension of 0.98 inch (2.5 cm) including the 0.20 x 0.20 inch (0.5 x 0.5 cm) tab for making electrical connection to the circuit, and a length of 0.39 inch (1 cm). Each edge of the terminal, excluding the tab, is cut to form rectangular-shaped notches with dimensions of 0.070 inch (0.178 cm) x 0.021 inch (0.059 cm).

An electrical device is prepared by positioning a first terminal in a fixture, depositing solder paste comprising a tin (Sn 63) eutectic solder on the exposed laminar surface of the terminal, positioning a conductive polymer chip onto the solder paste, depositing solder paste on the exposed laminar surface of the conductive polymer chip, and positioning a second terminal 180 out of phase with the first terminal onto the solder paste. The resulting device has tabs for electrical connection at ,,, .,.~, . ~, .

~ , , . . : :
~; ' : ; : : ' . ~:, . ~ . , , W092/0~477 ~CT/~S91/07041 ~ S~ l 16 opposite sides of the device. The fi~ture is then passed through an oven to heat and reflow the solder, and to attach the terminals to the chip. The mold is cooled and the completed device is removed. No solder "bridges" are observed.

Claims

What is claimed is:

1. An electrical device which comprises (1) a laminar resistive element which (a) is composed of a first material having a first resistivity at 23°C, and (b) has a first periphery;

(2) a laminar conductive element which (a) is secured to a face of the resistive element, (b) is composed of a second material having a second resistivity at 23°C which is substantially lower than the first resistivity, and (c) has a second periphery which does not extend beyond the first periphery; and (3) a conductive terminal comprising a laminar portion which (a) is secured to a face of the conductive element remote from the resistive element, (b) is composed of a third material having a third resistivity at 23°C which is substantially lower than the first resistivity, and (c) has a third periphery at least a part of which lies within the first periphery.

2. A device according to claim 1 which further includes (4) a laminar electrode which (a) is composed of a fourth material having a fourth resistivity at 23°C which is substantially lower than the first resistivity, (b) lies between the resistive element and the conductive element and is secured to the resistive element and the conductive element, and (c) has a fourth periphery at least a part of which substantially follows at least a part of the first periphery.

3. A device according to claim 1 or 2 wherein the fourth periphery coincides with the first periphery.

4. A device according to claim 1, 2, or 3 wherein the first material comprises (a) a conductive polymer which exhibits PTC behavior, or (b) an inorganic composition.

5. A circuit protection device according to claim 2 which comprises (1) a laminar resistive element which is composed of a conductive polymer composition which exhibits PTC behavior;

(2) two laminar electrodes which are attached to opposite surfaces of the resistive element, each of which electrodes (a) is composed of metal, and (b) has a periphery which coincides with the first periphery;

(3) two conductive elements, each of which (a) comprises solder, (b) has a second periphery which does not extend beyond the first periphery, and (c) is attached to a face of one of the laminar electrodes remote from the resistive element; and (4) two conductive terminals, each of which (a) is composed of metal, (b) has a third periphery, at least a part of which lies within the first periphery, and (c) is attached to a laminar surface of one of the conductive elements remote from one of the laminar electrodes.

6. A device according to claim 5 wherein the two conductive terminals are positioned in a staggered configuration so that at least part of the periphery of one terminal lies within at least part of the periphery of the second terminal.

7. A device according to claim 6 wherein (a) substantially all of the third periphery of each of the conductive terminals has an irregular shape which comprises notches cut into the third periphery, and (b) the notches of the first terminal are staggered with the notches of the second terminal.

8. A device according to claim 2 or 5 wherein the or each electrode comprises a metal sheet with an electrodeposited metal surface, which surface is in contact with the resistive element.

9. A method of making an electrical device according to claim 5, which method comprises (1) providing a laminar resistive element which (a) is composed of a conductive polymer composition, (b) has a first periphery, and (c) is attached on opposite surfaces to two laminar electrodes, each of which (i) is composed of metal and (ii) has a periphery which coincides with the first periphery;

(2) applying a conductive paste to a face of each of the laminar electrodes;

(3) positioning onto the conductive paste in a selected position a conductive terminal which (a) is composed of metal, and (b) has a third periphery which has an irregular shape comprising notches, wherein at least a part of the third periphery lies within the first periphery; and (4) attaching the terminals to the electrodes so that excess conductive paste is forced from under each terminal into the notches.

10. A method according to claim 9 wherein the conductive paste comprises a solder, and the terminal is attached to the resistive element by means of heating and then solidifying the solder.