Patent Publication Number: US-8976001-B2

Title: Protective device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic device, in particularly to a protective device capable of protecting electronic apparatus having it from damage by excessive current or excessive voltage. 
     2. Description of Related Art 
     In order to protect battery and battery charger from damage caused by excessive current or excessive voltage while charging is performed, a protective device is often put into the battery charger. Thus, when the excessive current or voltage is applied on the battery charger, the protective device can interrupt the circuit therein immediately and protect the battery and the electronic components in the battery charger. 
       FIG. 1  is a circuit diagram of a battery charger. There is a protective device  11  in the excessive current/voltage protective circuit  10  of the battery charger. The protective device  11  has two current fuses  111  and  112  arranged between the node A and node B. The current fuses  111  and  112  are made of low melting point metal and can be broken by excessive charging current passing therethrough. Consequently, the circuit between the node A and node B are interrupted and the battery  12  and the electronic elements in the battery charger can be protected. 
     Besides, the excessive current/voltage protective circuit  10  has an integrated circuit  13  for detecting excessive voltage. Once an excessive voltage is detected, the integrated circuit  13  will conduct a MOSFET  14  and the electrical current thus can be allowed to pass through path C. Then the heating member  113  of the protective device  11  generates heat for melting the current fuses  111  and  112  and a breakage is formed for protecting the battery  12  and the battery charger. 
     More specifically, as  FIG. 2  shows, the protective device  20  has a substrate  21 , two first electrodes  22  respectively formed at two opposite sides of the substrate  21 , and a low melting point metal layer  23  electrically connected across the two first electrodes  22 . A current path is formed from one of the first electrodes  22  to the low melting point metal layer  23  and then to the other one of the first electrodes  22 . So once excessive current enters either of the first electrodes  22 , the low melting point metal layer  23  will melt to break and form a breakage between the two first electrodes  22 . 
     As shown in  FIG. 2  and  FIG. 3 , the protective device  20  has two second electrodes  24  formed at another two opposite sides of the substrate  21 . The two second electrodes  24  each have an extending portion  241  extending under the low melting point metal layer  23 . A heating member  25  is formed between the two extending portions  241 . An insulating layer  27  is provided for covering the heating member  25  and the second electrodes  24 . Another current path is formed from one of the second electrodes  24  to the heating member  25  and then to the other one of the second electrodes  24 . Once current with excessive voltage enters either of the second electrodes  24  of this current path, the heating member  25  will generate heat for melting and breaking the low melting point metal  23  and form a breakage. In addition, the second electrode  24  at the right side of  FIG. 3  has a third electrode  242  and electrically extending to the low melting point metal layer  23 . 
     In order to rapidly break up the low melting point metal layer  23 , an appropriate amount of flux  26  is applied on the low melting point metal layer  23  for preventing oxidation occurred on the surface of the low melting point metal layer  23 . Besides, the flux  26  can remove the oxide layer formed on the low melting point metal layer  23  and help to increase the breaking thereof. The main composition of the flux  26  is rosin, which has a liquidus temperature as low as between 50 to 80 degrees Celsius. When the protective device  20  is being connected to a circuit board in a reflow soldering process, the high temperature over 200 degrees Celsius therein will immediately evaporate the flux or drive it to move away. Without the flux, the low melting point metal layer  23  will not easily be melted to break when an excessive current or voltage is applied on the protective device  20 , and the protective device  20  will fail to give any protection to the battery charger or the battery. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention is to provide a protective device for solving the above problem of the flux evaporating or moving away in the reflow soldering process. The protective device is capable of protecting the battery and the battery charger when excessive current or voltage is applied thereon. 
     For achieving the above objective, the protective device of the present invention includes a substrate, two first electrodes, a low-melting point metal layer and an assisting layer. The two first electrodes are respectively arranged at two opposite sides of the substrate. The low-melting point metal layer is arranged over the two first electrodes. The assisting layer is formed on the low-melting point metal layer. The liquidus temperature of the assisting layer is below the liquidus temperature of the low-melting point metal layer, and the liquidus temperature of the assisting layer is not below a predetermined temperature which is below the maximum working temperature of reflow soldering process by 25 degrees. 
     In another aspect, the present invention also provides a protective device, which includes a substrate, a low-melting point metal layer, an assisting layer, a bridging structure and a heating member. The low-melting point metal layer is arranged over the substrate. The assisting layer is formed on the low-melting point metal layer. The bridging structure crosses the low melting point metal layer. The heating member is arranged on the substrate. The liquidus temperature of the assisting layer is below the liquidus temperature of the low-melting point metal layer, and the liquidus temperature of the assisting layer is not below a predetermined temperature, the predetermined temperature is below the maximum working temperature of reflow soldering process by 25 degrees. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a conventional battery charger; 
         FIG. 2  is a cross sectional view of a conventional protective device; 
         FIG. 3  is a top view of a conventional protective device; 
         FIG. 4  is a cross sectional view of a protective device according to the first embodiment of the present invention; 
         FIG. 5  is a top view of the protective device according to the first embodiment of the present invention; 
         FIG. 6  is another cross sectional view of the protective device according to the first embodiment of the present invention; 
         FIG. 7  is a bottom view of the protective device according to the first embodiment of the present invention; 
         FIG. 8  is a top view of the protective device according to another example of the first embodiment of the present invention; 
         FIG. 9  is a cross sectional view of the protective device according to another example of the first embodiment of the present invention; and 
         FIG. 10  is a cross sectional view of the protective device according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detailed description of the present invention will be made with reference to the accompanying drawings. 
       FIG. 4  is a cross sectional view showing a protective device according to the first embodiment of the present invention. In this embodiment, the protective device  30  is a surface mount type electronic device, which can be mounted to a circuit board by a reflow soldering process. The protective device  30  includes a substrate  31  made of insulating material and having a shape of rectangular plate. Specifically, the materials for the substrate can be inorganic material including ceramic like aluminum oxide, zirconium dioxide, silicon nitride, aluminum nitride and boron nitride, or can be plastic, glass or epoxy. In practical use, the inorganic material is preferred. 
     Above the substrate  31 , the protective device  30  includes two first electrodes  32  respectively arranged at two opposite sides of the substrate  31 , a third electrode  33  extending between the two first electrodes  32  and a low melting point metal layer  34  arranged over the first electrodes  32  and the third electrode  33 . The low melting point metal layer  34  is soldered onto the first electrodes  32  and the third electrode  33  with a solder material and thus forms an electrical connection with the first electrodes  32  and the third electrode  33 . The materials for the low melting point metal layer  34  include tin-lead alloy, tin-silver-lead alloy, tin-indium-bismuth-lead alloy, tin-antimony alloy, tin-silver-copper alloy. 
       FIG. 5  is an upper view of the protective device  30 . The third electrode  33  laterally extends along the substrate  31  and is substantially of dumbbell shape. The low melting point metal layer  34  covers the middle portion of the third electrode  33  and the two opposite ends of the third electrode  33  are exposed. As  FIG. 5  and  FIG. 6  show, the protective device  30  further includes a bridging structure  35  located over the low melting point metal layer  34 . The bridging structure  35  is connected to the two ends of the third electrode  33 , and crosses o-ver the low melting point metal layer  34 . The materials for the bridging structure  35  can be gold, silver, nickel, tin, silver-copper alloy, nickel-copper alloy, tin-nickel-copper alloy, tin-nickel alloy. The connection between the bridging structure  35  and the two exposed ends of the third electrode  33  can be made by soldering, arc welding, ultrasonic welding, laser welding, and thermal pressure welding. 
     In addition, the protective device  30  further includes an assisting layer  36  located between the bridging structure  35  and the low melting point metal layer  34 . Preferably, the assisting layer  36  in its molten phase has good wettability with respect to the bridging structure  35  and is miscible with the low melting point metal layer  34 . So the assisting layer  36  can help the molten low melting point metal layer  34  remain between the bridging structure  35  and the third electrode  33 , and help the low melting point metal layer  34  melted to break. In manufacturing, the assisting layer  36  is formed by first dispensing liquid material between the bridging structure  35  and the low melting point metal layer  34  and then solidifying the liquid material. Because of having good flowability in its molten phase, the assisting layer  36  is formed through capillary action into a fan shape between the bridging structure  35  and the low melting point metal layer  34 . 
     When the protective device  30  is practically mounted to a circuit board through reflow soldering process, the assisting layer  36  will remain between the bridging structure  35  and the low melting point metal layer  34  and will not be evaporated or driven to move like conventional flux. Therefore, when an excessive voltage or current is applied, the assisting layer  36  can help the low melting point metal layer  34  precisely and stably melted to break. 
     Besides, it should be noticed that the liquidus temperature of the assisting layer  36  is below the liquidus temperature of the low-melting point metal layer  34 . However, if the assisting layer  36  has too low a liquidus temperature, the assisting layer  36  will be easily miscible with the low melting point metal layer  34  through reflow soldering process, and thus changes the value of both the liquidus temperature and resistance of the low melting point metal layer  34 . Consequently, it causes the melting stability of the protective device to become worse. Therefore, the liquidus temperature of the assisting layer  36  is needed to be set within a specifically preferable range. Thus, the liquidus temperature of the assisting layer  36  should be not below a predetermined temperature. The predetermined temperature is below the maximum working temperature of reflow soldering process by 25 degrees Celsius. Preferably, the liquidus temperature of the assisting layer  36  is not below the maximum working temperature of reflow soldering process. The composition of the assisting layer  36  is determined according to the composition of the low melting point metal layer  34 . In this embodiment, since the composition of the low melting point metal layer  34  includes tin, the composition of the assisting layer  36  can accordingly include tin for obtaining better miscibility with the low melting point metal layer  34  and helping the low melting point metal layer  34  melted. For illustration, the assisting layer  36  can be tin-silver alloy, tin-lead alloy, tin-silver-copper alloy, tin-antimony alloy or tin-lead-antimony alloy. It should be mentioned that the better miscibility may be obtained by other ways without having similar compositions as above described. 
     As  FIG. 4  and  FIG. 7  show, the protective device  30  includes a heating member  37  located at the lower surface of the substrate  31 , and two second electrodes  38  respectively arranged at another two opposite sides of the substrate  31 . The two second electrodes  38  each have an extending portion  381  extending along the lower surface of the substrate  31  and electrically connected with the heating member  37 . One of the second electrodes  38  is electrically connected with the third electrode  33 . Besides, the protective device  30  further includes an insulating layer  39  covering the heating member  37  and the extending portions  381 . 
     In the above mentioned first embodiment, the bridging structure  35  is provided so as to fix the assisting layer  36  between the bridging structure  35  and the low melting point metal layer  34 . In another embodiment, as  FIG. 8  and  FIG. 9  show, the assisting layer  36  can be directly applied on the low melting point metal layer  34  without forming the bridging structure  35  in advance. Since the composition of the assisting layer  36  is determined according to the composition of the low melting point metal layer  34 , which implies that both of them have similar composition. With the similar composition, the assisting layer  36  can be effectively fixed onto the low melting point metal layer  34  and will not be evaporated or driven to move like conventional flux. When an excessive voltage or current is applied, the assisting layer  36  can help the low melting point metal layer  34  precisely and stably melted to break. 
     Besides, the assisting layer  36  can help additionally added flux fixing on the low melting point metal layer  34 . The assisting layer  36  only should be put above the third electrode  33  but needs not to cover the entire low melting point metal layer  34 . The material for the assisting layer  36  can include tin, silver, copper or alloy thereof. Conventional soldering tin paste with or without flux can also be adopted as the assisting layer  36 . 
       FIG. 10  shows a protective device  30  according to the second embodiment of the present invention. The difference with respect to the first embodiment is that in present embodiment, the heating member  37 ′, the extending portion  381 ′ of the second electrode  38 , and the insulating layer  39 ′ are arranged on the upper surface of the substrate  31  and under the third electrode  33 . More specifically, the heating member  37 ′ is located between the upper surface of the substrate  31  and the third electrode  33 . The insulating layer  39 ′ is located between the heating member  37 ′ and the third electrode  33 . The extending portions  381 ′ extend along the upper surface of the substrate  31  and electrically connect with the heating member  37 ′. 
     Although the present invention has been described with reference to the foregoing preferred embodiments, it will be understood that the invention is not limited to the details thereof. Various equivalent variations and modifications can still occur to those skilled in this art in view of the teachings of the present invention. Thus, all such variations and equivalent modifications are also embraced within the scope of the invention as defined in the appended claims.