Patent Publication Number: US-8111126-B2

Title: Over-current protection device and manufacturing method thereof

Description:
BACKGROUND OF THE INVENTION 
     (A) Field of the Invention 
     The present invention relates to an over-current protection device and a manufacturing method thereof, and more particularly, to an SMD (surface mount device) over-current protection device with a positive temperature coefficient (PTC) characteristic. 
     (B) Description of the Related Art 
     The resistance of a positive temperature coefficient conductive material is sensitive to temperature variation and can be kept extremely low during normal operation so that the circuit can operate normally. However, if an over-current or an over-temperature event occurs, the resistance will immediately increase to a high resistance state (e.g., above 10 4  ohm.) Therefore, the over-current will be eliminated and the objective to protect the circuit device will be achieved. Consequently, PTC devices have been commonly integrated into various circuitries so as to prevent damage caused by over-current events. 
       FIG. 1  shows an over-current protection electrical apparatus  1  disclosed by U.S. Pat. No. 5,852,397. Metallic foil electrodes  11  are respectively attached to the upper surface and lower surface of a PTC material layer  10 . Next, the surfaces of the metallic foil electrodes  11  are etched to form long grooves  12  so that each of the metallic foil electrodes  11  is divided into two electrode portions of different sizes. Through holes are drilled on the left and right edges of the electrical apparatus  1 , and each of the through holes is respectively filled with a conductive rod  13  by plating. Therefore, all the electrode portions on a side are electrically connected to each other along a vertical direction. 
       FIG. 2  is a perspective diagram of an SMD electrical apparatus disclosed by R.O.C U.S. Pat. No. 415,624. The SMD electrical apparatus  2  has a PTC material layer  10 , and metallic foil layers  21  are respectively attached to the upper surface and lower surface of the PTC material layer  10 . Next, long slots are respectively formed on the left edge of the upper metallic foil layer  21  and the right edge of the lower metallic foil layer  21  by etching. Insulating films  22  are respectively coated on the upper and lower metallic foil layers  21 , and the slots are also filled with the insulating films  22 . Subsequently, metallic foil electrodes  23  are respectively attached to the upper and lower insulating films  22 . Similarly, an etching process is used to remove the middle portions of the metallic foil electrodes  23 , and symmetric left and right portions are left on two sides. 
     Through holes are formed on the left and right edges of the SMD electrical apparatus  2 . Conductive layers  24  are plated on the surfaces of the through holes so that the two left metallic foil electrodes  23  are connected to the lower metallic foil layer  21  and two right metallic foil electrodes  23  are connected to the upper metallic foil layer  21 . 
     The aforesaid prior arts all utilize methods similar to the manufacturing processes of printed circuit board, such as exposure, development, etching, drilling and plating. Therefore, not only do the prior arts require expensive equipment and complicated processes to manufacture, but also produce harmful etching liquids or plating liquids that pollute the environment. 
     In addition, regarding the electrical apparatus  1  and the SMD electrical apparatus  2 , the external electrodes and internal electrodes (or conductive rods and conductive layers) have smaller contact areas, so the electrical resistance is raised. Considering the ever-progressing requirements for reducing the size of electrical devices, the contact areas or the diameters of the through holes cannot be effectively reduced due to their inherent limitations. The prior arts are therefore not suitable for manufacturing miniature over-current protection devices. 
     Also, in the prior art, the two sides of the electrical apparatus  1  (the surface perpendicular to the upper and lower metallic foil layers, and the surface along the lengthwise direction of the main body) and the SMD electrical apparatus  2  have stacked layers exposed to the atmosphere. Consequently, moisture penetrates the PTC material layer and the upper and lower metallic foil layers so that the electrical reliability is affected. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is to provide an over-current protection device. The device has a simple structure, and its main body is completely covered. It is indeed an inexpensive, small-scaled, and reliable electrical device. 
     Another aspect of the present invention is to provide a method for manufacturing an over-current protection device. It utilizes easily implemented and low pollution manufacturing processes, so it can reduce the manufacturing cost and environmental pollution. 
     According to the aforesaid aspect, the present invention provides an over-current protection device. An over-current protection device comprises a PTC material layer, a first electrode layer, a second electrode layer, a first side electrode and a second side electrode. The PTC material layer is sandwiched between the first electrode layer and the second electrode layer. The first side electrode and the second side electrode are respectively disposed on two opposite side surfaces of the PTC material layer, and are respectively connected to the first electrode layer and the second electrode layer. Furthermore, the first side electrode and the second side electrode are respectively extended to four surfaces adjacent and perpendicular to the two side surfaces. 
     A body insulating layer is further disposed on the four surfaces of the device not covered by the first side electrode and the second side electrode. 
     The present invention provides a method for manufacturing an over-current protection device, which comprises the steps of: providing a PTC material layer; respectively forming a first electrode layer and a second electrode layer on upper and lower surfaces of the PTC material layer; cutting the multilayer structure of the PTC material layer, the first electrode layer and the second electrode layer into a plurality of main body units; and dipping each of the main body units into a conductive material to form a pair of opposite side electrodes. 
     The present invention further comprises a step of disposing a body insulating layer on the surfaces of the device not covered by the first side electrode and the second side electrode. 
     The present invention further comprises a step of disposing a solderable metal on the side electrodes by rolling plating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objectives and advantages of the present invention will become apparent upon reading the following description and upon reference to the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a conventional over-current protection device; 
         FIG. 2  is a schematic diagram of another conventional over-current protection device; and 
         FIGS. 3A to 3G  are schematic diagrams illustrating the manufacturing steps of an over-current protection device in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 3A to 3G  are schematic diagrams illustrating the manufacturing steps of an over-current protection device in accordance with the present invention. A PTC material layer (or substrate)  31  is provided. In this embodiment, a polymer PTC material is employed. Next, a first electrode layer  32  and a second electrode layer  33  are respectively formed on the upper surface  311  and lower surface  312  of the PTC material layer  31  by printing, as shown in  FIG. 3B . 
     The upper surface  311  and lower surface  312  of the PTC material layer  31  can be given rough treatment such as shotblasting or grinding so as to make the interfaces between the PTC material layer  31  and both the upper surface  311  and lower surface  312  have excellent bonding ability. The first electrode layer  32  and second electrode layer  33  are vertically disposed in a staggered manner. That is, the first electrode layer  32  and second electrode layer  33  respectively have a plurality of strip areas with a constant interval, and the strip areas do not align with each other vertically. The present invention can directly define the patterns of the first electrode layer  32  and second electrode layer  33  on the PTC material layer  31  by screen printing. In contrast, the prior arts utilize a photolithography process to define the patterns of the copper foils. The present invention is easily implemented, and has low cost on process. The material of the first electrode layer  32  and second electrode layer  33  is Au, Ag, Pt, Cu, Ni, carbon-type conductive material, or the mixture of the aforesaid several materials. 
     As shown in  FIG. 3C , the stacked layer structure of the PTC material layer  31 , the first electrode layer  32 , and the second electrode layer  33  are finished. The structure can be divided into a plurality of main body units  39  along mesh cutting lines, as shown in  FIG. 3C . 
       FIG. 3D  shows a schematic diagram of the main body unit  39 . The left sides of the first electrode layer  32  and the PTC material layer  31  are approximately aligned with each other. The right side of the first electrode layer  32  does not extend far enough to align with the right side of the PTC material layer  31 . However, the right side of the second electrode layer  33  and the PTC material layer  31  are approximately aligned with each other. The left side of the second electrode layer  33  does not extend far enough to align with the left side of the PTC material layer  31 . 
     Each of the main body units  39  is dipped in a conductive material such as silver or copper to create two opposing electrodes: a first side electrode  36  and a second side electrode  37 . The first side electrode  36  and the second side electrode  37  are disposed on opposite side surfaces of the main body unit  39 , and are respectively connected to the first electrode layer  32  and second electrode layer  33 . The first side electrode  36  and the second side electrode  37  extend their margins from two opposite side surfaces to four surfaces which are adjacent to the side surfaces and mutually perpendicular to each other. Therefore, each of the side electrodes with five surfaces is formed. Compared with the side electrode with three surfaces of the prior arts, the over-current protection device of the present invention is more easily implemented in a subsequent SMD process. 
     Referring to  FIG. 3E , a body insulating layer  38  covers the main body unit  39 . The portion of the first electrode layer  32  adjacent to the first side electrode  36  is not covered by the body insulating layer  38 , and the portion of the second electrode layer  33  adjacent to the second side electrode  37  is not covered by the body insulating layer  38 . Because the body insulating layer  38  can protect the main body unit  39  from moisture, the reliability of the over-current protection device  30  is improved. 
     In order to improve the solderability of the first side electrode  36  and second side electrode  37  during subsequent SMD process, a solderable material is deposited on the surfaces by rolling plating. Therefore, the over-current protection device  30  has a preferably solderable first side electrode  36 ′ and second side electrode  37 ′, as shown in  FIG. 3G . Ni, Sn, and Sn—Pb alloy are suitable for the solderable material. 
     In view of the descriptions of the aforesaid embodiments, the present invention includes many amendments and variations to these embodiments. Therefore, it is necessary to further refer to the scope of the following claims. In addition to the aforesaid detailed descriptions, the present invention can be widely applied to various other embodiments. The above-described embodiments of the present invention are intended to be illustrative only. Those skilled in the art may devise numerous alternative embodiments without departing from the scope of the following claims.