Surface mountable electrical device

1. Field of the Invention

This invention relates to a surface mountable electrical device, more particularly to a surface mountable electrical device that can serve as a circuit protection device, such as a fuse.

2. Description of the Related Art

FIG. 1 illustrates a conventional surface mountable electrical device (PTC thermistor) which includes a plurality of stacked PTC sheets 4 that have a positive thermal coefficient characteristic, a plurality of metal foil sheets 21 , 23 alternately laminated with the PTC sheets 4 , a first electrode layer 22 formed on an upper surface of the device, a second electrode layer 24 formed on a bottom surface of the device, a conductive first side layer 51 interconnecting the first electrode layer 22 and one of the metal foil sheets 23 , and a conductive second side layer 52 interconnecting the second electrode layer 24 and another one of the metal foil sheets 21 . Commercially available metal foil sheets, which are made from copper, normally have been surface treated at one side face so as to be attachable to a contacting surface of a PTC sheet. However, the other side face of each of the metal foil sheets is not surface treated and is relatively smooth. As a result, the non-surface-treated side face of each of the aforesaid metal foil sheets 21 , 23 is in poor contact with the respective PTC sheet 4 , and thus tends to peel from the respective PTC sheet 4 .

In order to overcome the aforementioned drawback, U.S. Pat. No. 6,157,289 disclosed a surface mountable electrical device (see FIG. 2) that includes a metal foil sheet 11 , which is sandwiched between two stacked PTC sheets 14 and that is surface treated by being plated with nickel layers 12 on opposite side faces thereof so as to roughen the side faces and so as to permit secure attachment of the same to contacting surfaces of the PTC sheets 14 . However, the extra processing step of forming the nickel layers 12 significantly increases the cost of manufacture. Moreover, the metal foil sheet 11 is required to be formed with grooves along cutting lines by etching or die press prior to being laminated with the PTC sheets 14 for facilitating formation of pieces of the surface mountable electrical device in a subsequent cutting step.

Therefore, the object of the present invention is to provide a surface mountable electrical device that is capable of overcoming the aforementioned drawbacks.

According to the present invention, a surface mountable electrical device comprises: a laminate body having top and bottom faces and opposite first and second side faces transverse to and interconnecting the top and bottom faces, the laminate body including a plurality of stacked PTC sheets which have a positive thermal coefficient characteristic, two overlapping metal foil sheets sandwiched between each two adjacent ones of the PTC sheets and having contact ends that extend to and that are exposed from the first side face, a conductive first electrode layer formed on the top face, a conductive second electrode layer formed on the top face and spaced apart from the first electrode layer, and a conductive third electrode layer formed on the bottom face, the two adjacent ones of the PTC sheets having contact faces with inlaid portions respectively inlaid with the metal foil sheets, and non-inlaid portions bonded to each other; and at least a conductive transverse layer formed on the first side face and interconnecting the first and third electrode layers and the contact ends of the metal foil sheets.

In drawings which illustrate embodiments of the invention,

FIG. 1 is a cross-sectional view of a conventional surface mountable electrical device;

FIG. 2 is a perspective partly sectioned view of another conventional surface mountable electrical device;

FIG. 3 is a perspective view of a first preferred embodiment of a surface mountable electrical device of this invention;

FIG. 4 is a sectional view of the first preferred embodiment, taken along line IV—IV in FIG. 3;

FIG. 5 is a perspective view of a second preferred embodiment of a surface mountable electrical device of this invention;

FIG. 6 is a perspective view of a third preferred embodiment of a surface mountable electrical device of this invention;

FIG. 7 is a sectional view of the third preferred embodiment, taken along line VII—VII in FIG. 6;

FIG. 8 is a perspective view of a fourth preferred embodiment of a surface mountable electrical device of this invention; and

FIG. 9 is a sectional view of the fourth preferred embodiment, taken along line VII—VII in FIG. 6 .

FIGS. 3 and 4 illustrate a first embodiment of a surface mountable electrical device of this invention. The surface mountable electrical device includes: a laminate body 10 having top and bottom faces 61 , 62 and opposite first and second side faces 63 , 64 transverse to and interconnecting the top and bottom faces 61 , 62 , the laminate body 10 including a plurality of stacked PTC sheets 100 which have a positive thermal coefficient characteristic, two overlapping first metal foil sheets 71 sandwiched between each two adjacent ones of the PTC sheets 100 and having contact ends 711 that extend to and that are exposed from the first side face 63 , a conductive first electrode layer 81 formed on the top face 61 , a conductive second electrode layer 82 formed on the top face 61 and spaced apart from the first electrode layer 81 , and a conductive third electrode layer 83 formed on the bottom face 62 ; and two conductive first transverse layers 91 formed on the first side face 63 and interconnecting the first and third electrode layers 81 , 83 and the contact ends 711 of the first metal foil sheets 71 . Each two adjacent ones of the PTC sheets 100 have contact faces with first inlaid portions 102 respectively inlaid with the first metal foil sheets 71 , and non-inlaid portions 103 bonded to each other. The laminate body 10 is formed by thermally pressing the PTC sheets 100 , the first metal foil sheets 71 , and the first, second and third electrode layers 81 , 82 , 83 so as to permit the PTC sheets 100 to be inlaid with the first metal foil sheets 71 and the non-inlaid portions 103 of the contact faces of the PTC sheets 100 to be bonded together. The first transverse layers 91 can be formed by conventional plating techniques.

The laminate body 1 can further include a conductive fourth electrode layer 84 that is formed on the bottom face 62 and that is spaced apart from the third electrode layer 83 . Two conductive second transverse layers 92 are formed on the second, side face 64 , interconnect the second and fourth electrode layers 82 , 84 , and are spaced apart from the first metal foil sheets 71 by the non-inlaid portions 103 of the contact faces of the PTC sheets 100 .

FIG. 5 illustrates a second embodiment of the surface mountable electrical device of this invention, which has a configuration similar to that of the previous embodiment shown in FIG. 3, except that an adhesive layer 200 is coated on the non-inlaid portions 103 of the contact faces of each two adjacent ones of the PTC sheets 100 so as to enhance the bonding strength therebetween.

FIGS. 6 and 7 illustrate a third embodiment of the surface mountable electrical device of this invention, which has a configuration similar to that of the previous embodiment shown in FIG. 3, except that two overlapping second metal foil sheets 72 are sandwiched between each two adjacent ones of the PTC sheets 100 and have contact ends 721 that extend to and that are exposed from the second side face 64 so as to be connected to the second transverse layers 92 . The contact faces of each two adjacent ones of the PTC sheets 100 further have second inlaid portions 104 that are respectively inlaid with the second metal foil sheets 72 . The first and second metal foil sheets 71 , 72 are spaced apart by the non-inlaid portions 103 of the contact faces of the PTC sheets 100 .

The first and second metal foil sheets 71 , 72 are commercially available, and each of which has one side face that is surface treated and that is attached securely to the inlaid portion 102 of the contact face of a respective one of the PTC sheets 100 , and an opposite side face that is not surface treated and that is in contact with an adjacent one of the first and second metal foil sheets 71 , 72 . Because they are made of the same material, the two overlapping first or second metal foil sheets 71 , 72 are in better surface contact as compared to that between the PTC sheet and the metal foil utilized in the prior art (see FIG. 1 ). Moreover, because of the non-inlaid portions 103 of the contact faces of the PTC sheets 100 , each two adjacent ones of the PTC sheets 100 can be bonded together via thermal-pressing techniques, thereby eliminating the drawbacks as encountered in the prior art (see FIGS. 1 and 2 ).

The bonding strength between each two adjacent ones of the PTC sheets 100 is dependent on the contacting area therebetween. FIGS. 8 and 9 illustrate a fourth embodiment of the surface mountable electrical device of this invention, which has a configuration similar to that of the first embodiment shown in FIG. 3, except that the contact face of each PTC sheet 100 further has additional non-inlaid portions 104 , 105 so as to increase the contacting area between each two adjacent ones of the PTC sheets 100 and so as to enhance the bonding strength therebetween.

With the invention thus explained, it is apparent that various modifications and variations can be made without departing from the spirit of the present invention. It is therefore intended that the invention be limited only as recited in the appended claims.