Patent Publication Number: US-9847202-B2

Title: SMD micro mixed fuse having thermal fuse function and method for manufacturing the same

Description:
TECHNICAL FIELD 
     The present invention relates to an SMD micro mixed fuse and, more particularly, to an SMD micro mixed fuse with a thermal fuse function that stably operates at high voltage surges and has a mixed structure including a heating electrode, a temperature, and a varistor to interrupt electrical current when heated to a specific temperature due to heat generated during operation of a micro fuse. Additionally, the present invention relates to a method for manufacturing the SMD micro mixed fuse with a thermal fuse function. 
     BACKGROUND ART 
     A micro fuse is a device to protect electronic elements by interrupting excessive electrical current in an electronic circuit. It is an essential component in an electronic device such as a mobile electronic device or a charger. With increasing use of mobile electronic devices and chargers therefor, surface mount device (SMD)-type micro fuses that can stably operate at high surges have been developed and used. 
     Categories of micro fuses include fast-acting fuses for overcurrent and time-lag fuses for inrush current or high surges. 
     A time-lag micro fuse is designed such that the length of a fuse wire (fusible element) is longer than that in a normal fuse. Korean Patent No. 10-1058946, which is issued to the present assignee and titled “Time-lag Micro Fuse with Multilayered Molding Layer and Method for Manufacturing the Same”, suggests a micro fuse including a fuse substrate, a fusible element connected to each terminal formed on the fuse substrate, and a molding layer formed on the entire surface of the fuse substrate to cover the fusible element. 
     Recently, with development of smart mobile electronic devices such as table computers and mobile telecommunication devices, there is the demand for electricity storage devices having high capacity. Therefore, there is a trend wherein high-capacity secondary batteries such as Li-ion and Li-polymer batteries are being developed. Along with such trends, there is also demand for development of a fuse having a thermal fuse function due to risk of fire or explosion during battery charging or discharging at high temperatures, in addition to the usual current fuse function that deals with overcurrent higher than rated current. 
     DISCLOSURE 
     Technical Problem 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a technology for reducing risk of fire or explosion attributable to overcurrent or to battery charging or discharging at high temperatures. Specifically, the present invention provides an SMD micro mixed fuse designed to stably operate at high voltage surges and to have a mixed structure including a heating electrode, a temperature fuse, and a varistor, and a method for manufacturing the same. The SMD micro mixed fuse according to the present invention has a thermal fuse function to interrupt electrical current according to heat generated during operation of a fuse. 
     Technical Solution 
     In order to accomplish the above objects, according to one aspect, there is provided an SMD micro mixed fuse with a thermal fuse function, including: a fuse substrate provided with at least one first electrode and at least one second electrode; a varistor layer formed on a front surface of the fuse substrate; a first contact terminal and a second contact terminal arranged on a front surface of the varistor layer, respectively at a first side and a second side of the varistor layer, and respectively connected to the at least one first electrode and the at least one second electrode; at least one thermal fuse that is not connected to the first and second contact terminals, is arranged on the front surface of the varistor layer, and is connected to the fuse substrate; and a fusible element that is not connected to the at least one thermal fuse but is wire-bonded to the first and second contact terminals. 
     In order to accomplish the above objects, according to another aspect, there is provided a method for manufacturing an SMD micro mixed fuse with a thermal fuse function, the method including: forming at least one first electrode and at least one second electrode on at least one fuse substrate; forming a varistor layer on a front surface of the fuse substrate; forming a first contact terminal and a second contact terminal that are arranged respectively at a first side and a second side of a front surface of the varistor layer and are respectively connected to the first electrode and the second electrode; forming a thermal fuse that is not connected to the first and second contact terminals, is arranged on the front surface of the varistor layer, and is connected to the fuse substrate; and forming a fusible element that is not connected to the thermal fuse but is wire-bonded to the first and second contact terminals. 
     Advantageous Effects 
     According to the present invention that relates to an SMD micro mixed fuse with a thermal fuse function and method for manufacturing the same, it is possible to enable stable operation of a micro fuse at high voltage surges, thereby prolonging the lifetime of a micro fuse and protecting an electronic circuit from abnormal current. 
     In addition, since the SMD micro mixed fuse according to the present invention has a structure in which a heating electrode, a thermistor thermal fuse, and a varistor are added to a micro fuse, the SMD micro mixed fuse can interrupt electrical current when heated to a specified temperature under occurrence of overcurrent. Accordingly, the SMD micro mixed fuse according to the present invention is immune to temperature changes, thereby stably operating at high temperatures. 
     Especially, the varistor made of a suitable material eliminates transient waveforms of irregular voltages or currents, thereby contributing to stable operation and increased lifetime of a micro fuse, which improves operation reliability of a micro fuse. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating an SMD micro mixed fuse with a thermal fuse function, according to one embodiment of the present invention; 
         FIG. 2  is a cross-sectional view taken along line I-I′ of  FIG. 1 ; 
         FIG. 3  is a circuitry diagram illustrating the SMD micro mixed fuse with a thermal fuse function, according to the embodiment of the present invention; 
         FIG. 4  is a flowchart showing a method of manufacturing an SMD micro mixed fuse with a thermal fuse function, according to another embodiment of the present invention; 
         FIG. 5  is a layout of a base substrate used for mass production of SMD micro mixed fuses; and 
         FIG. 6  is a perspective view illustrating a thermal profile of an SMD micro fuse that is obtained via simulation using ANSYS simulation software. 
     
    
    
     MODE FOR INVENTION 
     Reference will now be made in detail to various embodiments of the present invention, specific examples of which are illustrated in the accompanying drawings and described below, since the embodiments of the present invention can be variously modified in many different forms. While the present invention will be described in conjunction with exemplary embodiments thereof, it is to be understood that the present description is not intended to limit the present invention to those exemplary embodiments. On the contrary, the present invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments that may be included within the spirit and scope of the present invention as defined by the appended claims. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a perspective view illustrating an SMD micro mixed fuse with a thermal fuse function, according to one embodiment of the present invention; and  FIG. 2  is a cross-sectional view taken along line I-I′ of  FIG. 1 .  FIG. 3  is a circuitry diagram illustrating the SMD micro mixed fuse with a thermal fuse function, according to the embodiment of the present invention. 
     The SMD micro mixed fuse with a thermal fuse function illustrated in  FIGS. 1 to 3  includes: a fuse substrate  2  provided with at least one first electrode  3  and at least second electrode  4 ; a varistor layer  8  formed over a front surface of the fuse substrate  2 ; a first contact terminal  10  and a second contact terminal  11  that are arranged on a front surface of the varistor layer  8  respectively at a first side and a second side of the fuse substrate  2  and that are respectively connected to the at least one first electrode  3  and the at least one second substrate  4 ; at least one thermal fuse  12  that is arranged on the front surface of the varistor layer  8  without being connected to the first or second contact terminals  10 ,  11  and that is connected to the fuse substrate  2  through a plurality of contact holes that extends through the variator layer  8 ; and a fusible element  14  that is wire-bonded to the first and second connection terminals  10  and  11 , without being connected to the at least one thermal fuse  12 . 
     In addition, the SMD micro mixed fuse with a thermal fuse function may further include: at least one heating electrode  6  that is provided between the front surface of the fuse substrate  2  and a rear surface of the varistor layer  8 , without being connected to the at least one first electrode  3  and the at least one second electrode  4 ; and a molding layer  16  that entirely covers the fuse substrate  2  so that the first and second contact terminals  10  and  11 , the fusible element  14 , and the at least one thermal fuse  12  are covered by the molding layer  16 . The at least one thermal fuse  12  may be electrically connected to the at least one heating electrode  6  through any one of the plurality of contact holes that extends through the varistor layer  8 . 
     The fuse substrate  2  may be made a FR4 PCB having high thermal resistance. The fuse substrate  2  is provided with at least one first electrode  3  and at least one second electrode  4  at respective opposing sides (first side and second side opposite to each other) of the fuse substrate  2 . The first and second electrodes  3  and  4  can be connected to external terminals. 
     The at least one first electrode  3  and the at least one second electrode  4  are attached to or provided to surround some portions of the first and second sides of the fuse substrate  2 . Alternatively, they may be formed as plugs provided in through holes formed in the fuse substrate  2  of  FIG. 2 . 
     The varistor layer  8  covers the entire area of a front surface of the fuse substrate  2  as well as the first and second electrodes  3  and  4 . The varistor layer  8  allows passage of current therethrough only at a specified voltage or higher according to the composition thereof, thereby interrupting a specified voltage surge. Accordingly, the varistor layer  8  can protect against a specified voltage surge. In other words, when current surges so that a high voltage is applied to the varistor layer  8 , the resistance value of the variator layer  8  decreases to allow passage of high electrical current, thereby interrupting a specified surge voltage. Accordingly, the varistor layer  8  can protect an electronic circuit from a surge voltage. The varistor layer  8  may be made of a SiC-based or ZnO-based material. Alternatively, the varistor layer  8  may be made of a mixed material including SiC or Zno as a main substance and conductive silicon, carbon complex, or oxide as an auxiliary substance. Specifically, it is possible to control conduction of electrical current of a specified level according to a composition ratio of the content of metal oxide (or ceramic) with respect to the content of SiC or ZnO or a dimension ratio (for example, 3:1 (60 μm:20 μm)). The varistor layer  8  made of a material having an optimal composition ratio or structure eliminates excessive transient waveforms of irregular voltages or currents, thereby contributing to stable operation or increased lifetime of a micro fuse. When the varistor layer  8  is formed to cover the entire front surface of the fuse substrate  2  as well as the first and second electrodes  3  and  4 , the contact area and volume can be increased, which increases stability in operation of a micro fuse. 
     The first and second contact terminals  10  and  11  are arranged at portions of the front surface of the varistor layer  8 , which are near the first and second sides of the varistor layer  8 , respectively. The first and second contact terminals  10  and  11  are electrically connected to the at least one first electrode  3  and the at least one second electrodes  4 , respectively. Specifically, the first and second contact terminals  10  and  11  are made of copper (Cu), aluminum (Al), silver (Ag), gold (Au), or an alloy of copper and aluminum through a patterning process. The first and second contact terminals  10  and  11  are arranged at portions of the front surface of the varistor layer  8 , which are near the first and second sides of the varistor layer  8 , respectively. In addition, the first and second contact terminals  10  and  11  are electrically connected to the at least one first electrode  3  and the at least one second electrode  4 , respectively and to the fuse substrate  2  through contact holes formed in the varistor layer  8 . 
     The at least one thermal fuse  12  is arranged on the front surface of the varistor layer  8  and is not connected to the first and second contact terminals  10  and  11 . The at least one thermal fuse  12  is connected to the fuse substrate  2  through at least any one of the plurality of contact holes formed in the varistor layer  8 . Since the at least one thermal fuse  12  is in contact with and fixed to the varistor layer  8  through any one of the plurality of contact holes, heat conductivity and current control efficiency can be increased. 
     The thermal fuse  12  is fabricated made by sintering a metal oxide. The thermal fuse  12  has electrical characteristics in which its electrical resistance is variable. The thermal fuse  12  may be a binary system or a ternary system so that it is formed by mixing powder of two or three transition metal oxides of manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe), and copper (Co). 
     As illustrated in  FIGS. 2 and 3 , the at least one thermal fuse  12  may include at least one heating electrode  6 . The at least one heating electrode  6  is formed on a portion of the front surface of the fuse substrate  2  and is not connected to the first and second electrodes  3  and  4 . The varistor layer  8  is arranged to cover the heating electrode  6  and is formed through deposition and curing processes. The at least one thermal fuse  12  is patterned to be electrically connected to the at least one heating electrode  6  through at least one of the plurality of contact holes that extends through the varistor layer  8 . Since the at least one thermal fuse  12 , the varistor layer  8 , and the heating electrode  6  are connected and fixed to each other through the at least one contact hole, heat conductivity, which depends on a contact area between elements, and control efficiency for current conduction can be increased. 
     The at least one heating electrode  6  is heated by heat generated during operation of a micro fuse. When the heating element  6  is heated, its resistance, volume, and coefficient of thermal expansion change. The thermal fuse  12  that is directly connected to the at least one heating electrode  6  and the varistor layer  8  controls current conduction according to the heating degree of the heating electrode  6 . 
     The at least one fusible element  14  is not connected to the at least thermal fuse  12  but is electrically connected to the first and second contact terminals  10  and  11  through a wire bonding method. The at least one fusible element  14  is made of a material that has similar electrical conductivity to the first and second contact terminals  10  and  11 . The at least one fusible element  14  is formed to be connected to the first and second contact terminals  10  and  11  through a ball wire bonding method and made of a metal selected from the group consisting of silver, copper, gold, aluminum, and alloys thereof, or of a material plated with any of those metals. The at least one fusible element  14  is used to connect independent patterns to each other and also functions as a fusible body that can safely protect a circuit from abnormal inrush current. 
     In other words, since the at least one fusible element  14 , the at least one thermal fuse  12  including the at least one heating electrode  6 , and the varistor layer  8  are in contact with and are fixed to each other through at least one contact hole, heat conduction efficiency and current conduction control efficiency that depend on the contact area can be increased. 
     The molding layer  16  covers the entire surface of the fuse substrate  2  as well as the first and second contact terminals  10  and  11 , the fusible element  14 , and the at least one thermal fuse  12 . The molding layer  16  is formed by coating photoimageable solder resist mask (PSR) ink on the surface of the fuse substrate  2  to be a predetermined thickness in order to prevent the fusible element  14  from being contaminated by impurities or to prevent the fusible element  14  from being damaged by external impact. In this case, it is preferable that the molding layer  16  surrounds the fusible element  14  for protection and safety for the fusible element  14 . The PSR ink is preferably coated through a screen printing method. That is, a printing mask (not shown) having an opening that corresponds to the fuse substrate  2  is first prepared. Next, PSR ink is applied to the printing mask so that the PSR ink can be coated on the fuse substrate through the opening. In this way, a predetermined thickness of PSR ink is coated on the fuse substrate  2  and then cured. As a result, the molding layer  16  is formed. 
       FIG. 4  is a flowchart illustrating a method for manufacturing an SMD micro mixed fuse with a thermal fuse function. 
     With reference to  FIG. 4 , at a base substrate preparation step ST 1 , a base substrate  20  in which a plurality of fuse substrates  2  are defined to be arranged in rows and columns is prepared. With reference to  FIG. 5  that illustrates the structure of the base substrate  20 , multiple fuse substrates  2  are defined to be arranged at regular intervals in the base substrate  20 . For example, 60 rows×60 columns of fuse substrates are defined. 
     At a fuse substrate formation step ST 2 , FR4 PCB having high thermal resistance is divided into fuse substrates  2 . At least one first electrode  3  and at least one second electrode  4  are formed at portions of first and second sides of each fuse substrate  2 , respectively. The at least one first electrode  3  and the at least one second electrode  4  are formed to be attached to or to surround the portions of the first and second sides of the fuse substrate  2 . Alternatively, the at least one first electrode  3  and the at least one second electrode  4  may be formed to extend through through-holes in the fuse substrate  2  as illustrated in  FIG. 2 . 
     At a heating electrode formation step ST 3 , at least one heating electrode  6  is patterned on a front surface of each fuse substrate  2 . At this point, the heating electrode  6  is formed not to be connected to the first and second electrodes  3  and  4 . The at least one heating electrode  6  may be made of the same material as the first and second electrodes  3  and  4 , through the same patterning process as the first and second electrodes  3  and  4 . That is, the at least one heating electrode, and the first and second electrodes  3  and  4  may be simultaneously formed through a patterning process in which light exposure and etching are sequentially performed using at least one mask. 
       FIG. 6  illustrates  3 D modeling of an SMD micro mixed fuse that is a simulation result obtained using ANSYS software. Thermal characteristics at overcurrent show that temperature is higher toward the center of the SMD micro mixed fuse than it is toward the edges. Accordingly, it is preferable that the at least one heating electrode  6  be formed at a center portion of each fuse substrate  2  and not connected to the first and second electrodes  3  and  4 . 
     At a varistor layer formation step ST 4 , a varistor layer  8  is formed to cover the entire front surface of each fuse substrate  2  as well as the first and second electrodes  3  and  4 . The varistor layer  8  is formed by depositing a SiC-based material, a ZnO-based material, or a combination of SiC-based material film and ZnO-based material film, and patterning the deposited film. For example, in each varistor layer  8 , a ratio of the SiC-based material to the ZnO-based material may be 3:1 (for example, 60 μm:20 μm). In the patterning process for the varistor layer  8 , a plurality of contact holes is also formed. 
     At a contact terminal and thermal fuse formation method ST 5 , a first contact terminal  10  and a second contact terminal  11  are formed on a front surface of the varistor layer  8  so as to be connected to the first electrode  3  and the second electrode  4 , respectively, through the contact holes. The first and second contact terminals  10  and  11  are formed by depositing and patterning copper (Cu), aluminum (Al), silver (Ag), or gold (Au). Alternatively, the first and second contact terminals  10  and  11  may be formed by depositing and patterning an alloy of those metals. Since the first and second contact terminals  10  and  11  are fixed to the first and second electrodes  3  and  4 , and the fuse substrate  2  through the contact holes, current conduction paths are formed and the structure of the varistor layer  8  can be securely fixed. 
     Next, at least one thermal fuse  12  is formed, through a patterning process, on the front surface of the varistor layer  8 . At this point, the thermal fuse  12  is formed not to be connected to the first and second contact terminals  10  and  11 . The thermal fuse  12  is formed by sintering a metal oxide. The thermal fuse  12  has electrical characteristics in which its electrical resistance varies according to temperature. The thermal fuse  12  may be a binary metal oxide or a ternary metal oxide. The thermal fuse  12  may be formed by mixing powder of two or three kinds of transition metal oxides of manganese, nickel, cobalt, iron, copper, and so on. The thermal fuse  12  is connected to the fuse substrate  2  through any one of the plurality of contact holes that extends through the varistor layer  8 . 
     At a fusible element formation step ST 6 , at least one fusible element  14  is formed to be connected the first and second contact terminals  10  and  11  in a wire bonding manner. The fusible element  14  is formed not to be connected to the at least one thermal fuse  12 . The fusible element  14  is made of a material that has similar electrical conductivity to the first and second contact terminals  10  and  11  and extends in the same direction as the first and second contact terminals  10  and  11 . The fusible element  14  is connected to the first and second contact terminals  10  and  11  through a ball wire bonding method and is made of a metal selected from the group consisting of silver, copper, gold, aluminum, and alloys thereof, or of a material plated with any of those metals. 
     At an insulation step ST 7  for fusible element and fuse substrate, a molding layer  16  is formed to cover the fuse substrate  2  as well as the first and second contact terminals  10  and  11 , the fusible element  14 , and the at least one thermal fuse  12 . The molding layer  16  is formed by coating a predetermined thickness of photoimageable solder resist mask ink (PSR ink) on the fuse substrate  2 . It is preferable that the molding layer  16  surrounds the fusible element  14  for protection and safety of the fusible element  14 . Coating of the PSR ink is performed through a screen printing process. That is, first, a printing mask (not shown) having an opening that corresponds to the shape of the fuse substrate  2  is prepared. Next, the PSR ink is coated on the fuse substrate  2  through the opening of the printing mask. As a result, the PSR ink is coated on the fuse mask  2  to be the predetermined thickness. Next, the coated PSR ink is cured to form the molding layer  16 . 
     At a mass production step ST 8 , a plurality of SMD micro mixed fuses formed in one PCB used as a base substrate is cut at regular intervals using a high speed blade. Through this process, the plurality of SMD micro mixed fuses is separated from each other. That is, the SMD micro mixed fuses are mass-produced. With reference to  FIG. 5  that illustrates a base substrate and SMD micro mixed fuses, a plurality of SMD micro mixed fuses is arranged at regular intervals in the base substrate. 
     The SMD micro mixed fuse with a thermal fuse function and the method for manufacturing the same according to the embodiment of the present invention increases the lifetime of a micro fuse by enabling stable operation at high voltage surges and protects an electronic circuit from abnormal current. 
     In addition, since a heating electrode, a thermistor thermal fuse, and a varistor are incorporated into an SMD micro fuse, electrical current can be interrupted when the SMD micro fuse is heated to a predetermined temperature under occurrence of overcurrent. Therefore, it is possible to provide a micro fuse that safely interrupts electrical current according to temperature changes. 
     The varistor made of a suitable material eliminates excessive transient waveforms of irregular voltages and currents and contributes to stable operation of a micro fuse. Accordingly, the varistor maximally increases the lifetime of a micro fuse, which results in improvement in operation reliability of an SMD micro fuse. 
     Although preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 
     INDUSTRIAL APPLICABILITY 
     According to the described embodiments of the present invention, the SMD micro mixed fuse with a thermal fuse function, which has the various advantages and technical features described above, and the method for manufacturing method for the same increase the lifetime of a micro fuse by enabling stable operation of the micro fuse at high voltage surges and protects an electronic circuit from abnormal current. 
     In addition, since a heating electrode, a thermistor thermal fuse, and a varistor are incorporated into an SMD micro fuse, electrical current can be interrupted when the SMD micro fuse is heated to a predetermined temperature under occurrence of overcurrent. Therefore, it is possible to provide a micro fuse that safely interrupts electrical current according to temperature changes.