Abstract:
A thermal fuse includes a first contact surface connected to a top surface of a sensor and a bottom surface connected to a bottom surface of the sensor. The sensor includes a mixture of Sn and Zn. The distance between the top surface and the bottom surface of the sensor is sized to substantially limit Zn depletion in a center region of the sensor when a temperature of the sensor is below a melting temperature of the sensor. The center region of the sensor prevents the first contact surface and the second contact surface from separating when the temperature of the sensor is below the melting temperature, and the first contact surface and the second contact surface are configured to separate when the temperature of the center region of the sensor exceeds the melting temperature of the sensor.

Description:
BACKGROUND 
       [0001]    I. Field 
         [0002]    The present invention relates generally to electronic protection circuitry. More, specifically, the present invention relates to a thermal fuse. 
         [0003]    II. Background Details 
         [0004]    Protection circuits are often utilized in electronic circuits to isolate failed circuits from other circuits. For example, protection circuits may be utilized to prevent a cascade failure of circuit modules in an electronic automotive engine controller, or other damage. 
         [0005]    One type of protection circuit is a thermal fuse. A thermal fuse functions similar to a typical glass fuse. That is, under normal operating conditions the fuse behaves like a short circuit and during a fault condition the fuse behaves like an open circuit. Thermal fuses transition between these two modes of operation when the temperature of the thermal fuse exceeds an activation temperature. To facilitate these modes, thermal fuses may include a conduction element, such as a fusible wire, a set of metal contacts, or set of soldered metal contacts, that can switch from a conductive to a non-conductive state. The metal contacts are typically coupled to one another with a sensor that may be a form of solder. The sensor may correspond to a low melting point alloy that melts at a melting temperature that corresponds to the activation temperature of the thermal fuse. 
         [0006]    In operation, current flows through the thermal fuse. After the sensor reaches the specified activation temperature, the sensor may release the metal contacts, which changes the state of the thermal fuse from a closed state to an open state. This in turn prevents current from flowing through the thermal fuse. 
         [0007]    One disadvantage with existing thermal fuses is that because a sensor of a thermal fuse may deteriorate over time when utilized in high temperature environments, existing thermal fuses often have a limited life expectancy. For example, when a thermal fuse is utilized in high temperature environments, the melting point of the sensor may increase over time to a point where it is unable to prevent damage to other circuits. 
       SUMMARY 
       [0008]    In one aspect, a thermal fuse includes a first contact surface connected to a top surface of a sensor and a second contact surface connected to a bottom surface of the sensor. The sensor includes a mixture of tin (Sn) and zinc (Zn). The distance between the top surface and the bottom surface of the sensor is sized to substantially reduce the rate of Zn in a center region of the sensor when a temperature of the sensor is below a melting temperature of the sensor. The center region of the sensor prevents the first contact surface and the second contact surface from separating when the temperature of the sensor is below the melting temperature, and the first contact surface and the second contact surface are configured to separate when the temperature of the center region of the sensor exceeds the melting temperature of the sensor. 
         [0009]    In a second aspect, a thermal fuse includes a first contact surface connected to a top surface of a sensor and a second contact surface connected to a bottom surface of the sensor. The sensor includes a mixture of Sn and Zn. The first and second contact surfaces are made of an element that limits Zn migration out of the sensor and onto either the first or the second contact surface when a temperature of the sensor is below a melting temperature of the sensor. The first contact surface and the second contact surface are configured to separate when the temperature of the sensor exceeds the melting temperature. 
         [0010]    In a third aspect, a thermal fuse includes a first and a second contact surface. nickel (Ni) layers are deposited on the first and second contact surfaces and a sensor is disposed between the Ni layers. The sensor includes a mixture of Sn and Zn. The Ni layers are configured to substantially prevent Zn migration onto the first and second contact surfaces. The first contact surface and the second contact surface are configured to separate when the temperature of the sensor exceeds a melting temperature of the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is an exemplary thermal fuse configured to minimize Zn migration from a sensor. 
           [0012]      FIG. 2  illustrates the effects of Zn migration on the composition of a sensor. 
           [0013]      FIG. 3  illustrates a second embodiment of a sensor configuration for minimizing Zn migration from a sensor. 
           [0014]      FIG. 4A  is a schematic representation of a circuit that includes a thermal fuse in a closed state. 
           [0015]      FIG. 4B  is a schematic representation of a circuit that includes a thermal fuse in an open state. 
           [0016]      FIG. 5A  illustrates a second exemplary thermal fuse in a closed state. 
           [0017]      FIG. 5B  illustrates the second exemplary thermal fuse in an open state. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    To overcome the problems described above, various thermal fuse configurations are disclosed. The thermal fuses include sensors configured to minimize Zn migration so that the activation temperature of the sensor is maintained when the thermal fuse is utilized in a high temperature environment. 
         [0019]      FIG. 1  is an exemplary thermal fuse  100 . The thermal fuse  100  includes a spring bar  105 , a sensor  110 , a first substrate  115  and a second substrate  117 . 
         [0020]    The spring bar  105  may include a first end  109 , a curved section  112 , and a second end  107 . The first end  109  of the spring bar  105  includes a contact surface  109   a  configured to adhere to a top surface  110   a  of the sensor  110 . The second end  107  of the spring bar  105  is fastened to the second substrate  117 . For example, the second end  107  may be soldered, spot welded, and/or riveted to the second substrate  117 . The spring bar  105  may be made from a conductive material, such as a metal or alloy. The spring bar  105  may have elastic characteristics that enable the spring bar  105  to open in a spring-like manner when the temperature of the thermal fuse  100  reaches an activation temperature. For example, the activation temperature may be about 199° C. 
         [0021]    The sensor  110  has a width across an X axis, a thickness along a Y axis, a top surface  110   a , and a bottom surface  110   b . The top surface  110   a  of the sensor  110  is configured to adhere to the contact surface  109   a  on the first end  109  of the spring bar  105 . The bottom surface  110   b  is configured to adhere to the first substrate  115 . In one implementation, the sensor  110  may be made of an alloy that is in a solid state below a melting temperature of the alloy. When the temperature of the alloy rises above the melting temperature, the sensor  110  may melt or lose its resilience. The melting temperature may correspond to the activation temperature of the thermal fuse  100 . For example, in automotive applications the activation temperature of the thermal fuse  100  may be about 199° C. In one implementation, the sensor  110  may be configured to have a melting temperature of about 199° C. 
         [0022]    In some implementations, the sensor  110  may be a form of solder and may include a mixture of Sn and Zn. The solder may include other elements. For example, the solder may include mixtures of Sn/Zn/bismuth (Bi), Sn/Zn/aluminum (Al), Sn/Zn/indium (In), Sn/Zn/gallium (Ga), Sn/Zn/In/Bi, and Sn/Zn/silver (Ag). The ratio of the Sn to Zn may be 91 parts Sn to 9 parts Zn by weight. The alloy formed from the combination of Sn and Zn has a melting point of about 199° C. 
         [0023]    It can be shown that Zn in the sensor  110  tends to migrate out of the sensor  110  and onto the contact surface  110   a  and substrate  115  at a rate that is dependent on temperature of the sensor  110 , the humidity surrounding the sensor  110 , the composition of the contact surfaces that contact the sensor  110 , and the thickness of the sensor  110 . When Zn migrates out of the sensor  110 , the ratio of Sn to Zn may increase in certain regions as shown in  FIG. 2   
         [0024]      FIG. 2  illustrates the effects of Zn migration on the composition of the sensor  110 . Referring to  FIG. 2 , the sensor  110  includes outer regions  205  and a center region  207 . In the center region  207 , the ratio of Sn to Zn remains relatively unchanged over time and temperature. For example, the ratio of the Sn to Zn may be 91 parts Sn to 9 parts Zn by weight. In the outer regions  205 , the ratio of Sn to Zn may increase. It can be shown that the melting point of the sensor  110  in the outer regions  205  is higher than the melting point in the center region  207  because of the increased concentration of Sn in the outer regions  205 . This change in composition of the sensor  110  changes the overall characteristics of the sensor  110 . If too much Zn is allowed to migrate out of the sensor  110 , then the effective activation temperature or melting point of the sensor  110  may exceed the original activation temperature. For example, the activation temperature of the sensor  110  may initially be 199° C., but over time during operation in high temperature environments, the activation temperature of the sensor  110  may increase to a temperature in excess of 217° C., which is the temperature at which bonding pads in a field-effect-transistor FET may melt. If the activation temperature of the sensor  110  were to rise above the temperature at which bonding pads in the FET may melt, the thermal fuse may not be able to activate before damage to or detachment of the FET occurs. 
         [0025]    To overcome the problems of the Zn migration, in some implementations, the overall thickness of the sensor  110  along the Y axis is increased so that the effective activation temperature of the sensor  110  remains essentially unchanged over the design life of the thermal fuse. For example, the design life of a thermal fuse operating in an automotive engine compartment environment may be about 10 years. The design life of the thermal fuse may be increased or decreased by changing the thickness of the sensor  110 . For example, increasing the thickness may increase the design life and decreasing the thickness may decrease the design life. It can be shown that if the thickness T  215  from the top surface  110   a  and the bottom surface  110   b  of the sensor  110  to a center line  210  of the sensor  110  that extends along the X axis is about 0.10 mm (0.004 inch), giving a total thickness from the top surface  110   a  to the bottom surface  110   b  of about 0.20 mm (0.008 inch), the ratio of Sn to Zn in the center region  207  of the sensor  110  remains generally unchanged over temperature, humidity, and composition of the surfaces that contact the sensor  110 . Therefore, the sensor  110  activation temperature will remain essentially unchanged over the design life when operated in a high temperature environment. 
         [0026]    It may be shown that the Zn tends to migrate onto contact surfaces in contact with the sensor  110  until the contact surfaces become saturated with Zn. To maintain a given ratio over the design life of the thermal fuse, in some implementations excess Zn may be added to the sensor  110  to compensate for the Zn migration onto the contact surfaces. 
         [0027]    In other implementations, Zn migration out of the sensor  110  may be minimized by making the surfaces that contact the sensor  110  from a material that includes Ni, gold (Au), aluminum (Al), palladium (Pd), and/or Zn, or other similar material. For example, referring to  FIG. 1 , the contact surface  109   a  of the first end  109  of the spring bar  105  and the substrate  115  may be made of a material that includes Ni, Au, Al, Pd, and/or Zn. 
         [0028]      FIG. 3  illustrates another sensor configuration  300  for minimizing Zn migration from a sensor  310 . Shown in the configuration  300  are a sensor  310 , layers  305 , which may be Ni, and contact surfaces  302 . In some implementations, the sensor  310  may include an alloy comprising Sn and Zn as described above. The ratio of the Sn to Zn may be 91 parts Sn to 9 parts Zn by weight. Layers  305  may be referred to hereinafter as first layer and second layer. 
         [0029]    The contact surfaces  302  may correspond to the contact surface  109   a  on the first end  109  of the spring bar  105 , and also the substrate  115  shown in  FIG. 1 . 
         [0030]    The layer  305  may be deposited or disposed between the contact surfaces  302  and the sensor  310 . It can be shown that a sufficiently pore free and uniform layer of Ni deposited in between the contact surfaces  302  and the sensor  310  will minimize Zn migration from the sensor  310 . In some implementations, a sufficiently pore free and uniform layer of Ni may be achieved when the thickness T  307  of the layer  305  is about 0.0023 mm (0.000090 inch) or greater. 
         [0031]    To further enhance the characteristics of the sensor  310  the various embodiments described above may be combined. For example, the thickness from the top surface and bottom surface of the sensor  310  to a center line of the sensor  310  that extends along the X axis of the sensor may be configured to be about 0.10 mm (0.004 inch) or greater, i.e. giving a total thickness from the top surface to the bottom surface of the sensor of 0.20 mm (0.008 inch) or greater, as described above. In addition or alternatively, the layer  305  of the sensor  310  may be made from a material that includes Ni, Au, Al, Pd and/or Zn. For example, if Ni is used as layer  305  having a thickness T 307  of about 0.0023 mm (0.000090 inch) in combination with a total sensor thickness of 0.20 mm, Zn migration out of the sensor  310  can be reduced so that the activation temperature of the sensor  310  remains generally unchanged over the design life of the thermal fuse when operated in a high temperature environment. 
         [0032]    The implementations described above, therefore, overcome the problem of operating a thermal fuse in a high ambient temperature environment by providing a sensor  310  with an activation temperature that remains generally unchanged in high ambient temperature environments. This enables the manufacture of thermal fuses suitable for high temperature environments, such as an engine compartment of an automobile. 
         [0033]      FIG. 4A  is a schematic representation of a circuit  400  that includes a thermal fuse  405  with one or more of the properties described above. Shown are a thermal fuse  405 , a power source  420 , a switching device  423 , a power control circuit  407 , and a load  425 . The thermal fuse  405  is connected in-between and in series with the power source  420  and a first terminal of the switching device  423 . A second terminal of the switching device  423  may be driven by the power control circuit  407 . A third terminal of the switching device  423  may be connected to the load  425 . 
         [0034]    The switching device  423  may correspond to a field-effect-transistor (FET) or other semiconductor switching device. For example, the first, second, and third terminals may correspond to the drain, gate, and source, respectively, of a FET. The power control circuit  407  may correspond to a circuit operable to regulate voltage and/or current delivered to the load  425 . The power control circuit  407  may generate a pulse pattern or other signal that causes the switching device  443  to “open” and “close” and therefore output, via the third terminal, an average DC voltage. The load  425  may include one or more passive and/or active circuit components. For example, the load  425  may include resistors, capacitor, inductors, semiconductor circuits and transistors. The load  425  may include other devices. 
         [0035]    The thermal fuse  405  may correspond to the thermal fuse  100  of  FIG. 1 . When the ambient temperature surrounding the thermal fuse  405  is below the activation temperature of the thermal fuse  405 , the thermal fuse remains in a closed state and current flows from the power source  420 , through the thermal fuse  405 , and to the load  425 . For example, in some implementations, when the ambient temperature is below about 199° C., the thermal fuse  405  remains in a closed state and current flows through the thermal fuse  405 . 
         [0036]      FIG. 4B , illustrates a thermal fuse in an environment where the ambient temperature of the circuit  400  exceeds the activation temperature of the thermal fuse  405 . Under these conditions, the sensor in the thermal fuse  405  may begin to lose its resilience. For example, the sensor in the thermal fuse  405  may begin to change from a solid state to a liquid state. When this occurs, the sensor begins to lose its ability to adhere to the contact surfaces, such as the contact surface  109   a  ( FIG. 1 ) of the second end  109  ( FIG. 1 ) of the spring bar  105  ( FIG. 1 ), and also the first substrate  115  ( FIG. 1 ). In this state, elastic energy stored in the spring bar  105  causes the spring bar  105  to separate from the first substrate  115 , which places the thermal fuse  405  in an open electrical state effectively disconnecting the load  425  from the power source  420 . The thermal fuse is, therefore, capable of protecting circuits that operate in high temperature environments for extended periods of time such as in an engine compartment of an automobile. 
         [0037]    While the thermal fuse and the method for using the thermal fuse have been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the claims of the application. For example, one of ordinary skill will appreciate that the thickness of the sensor may be increased. Other contact surface materials that do not absorb Zn may be utilized. A material, other than Ni, that limits Zn migration may be deposited on the contact surfaces. Furthermore, the solutions described may be combined. 
         [0038]    In addition to these modifications, many other modifications may be made to adapt a particular situation or material to the teachings without departing from the scope of the claims. For example, the sensor may be adapted to operate in the thermal fuse of  FIG. 5A . 
         [0039]      FIG. 5A  illustrates a second exemplary thermal  500  fuse in a closed state. The thermal fuse  500  includes first and second end structures  545  and  546 , a middle structure  505 , first and second sensors  510  and  511 , and a spring  515 . The first end, middle, and second end structures ( 545 ,  505  and  546 ) may be made of any conductive material, such as copper, aluminum or other metal, or a conductive alloy. The first and second end structures  545  and  546  are separated from one another so that no current may flow directly between the first and second end structures  545  and  546 . The first and second end structures  545  and  546  each include a first end  545   a  and  546   a  and a second end  545   b  and  546   b . The first end  545   a  and  546   a  of each structure includes a contact surface configured to adhere to a bottom surface  510   a  and  511   a  of the first and second sensor  510  and  511 , respectively. 
         [0040]    The second end  545   b  and  546   b  of the first and second end structures  545  and  546 , respectively, is configured to adhere to a substrate  560  or a printed circuit board pad. 
         [0041]    The middle structure  505  is configured to bridge the first and second end structures  545  and  546  and includes a pair of contact surfaces  505   a . Each contact surface  505   a  is configured to adhere to a top surface  510   b  and  511   b  of the first and second sensor  510  and  511 , respectively. 
         [0042]    The first and second sensors  510  and  511  may correspond to the sensor  110  described above. For example, the sensors  510  and  511  have a width across an X axis and a thickness along a Y axis. The sensors  510  and  511  may be made of an alloy that is in a solid state below a melting temperature of the alloy. The sensors  510  and  511  may melt or lose their resilience above the melting temperature. The melting temperature may correspond to the activation temperature of the thermal fuse  500 . 
         [0043]    The spring  515  may be generally cylindrically shaped and may include a spiral round elastic material such as metal, an alloy, plastic, or other elastic material. The spring  515  may be positioned over the first and second end structures  545  and  546  and below the middle structure  505 . 
         [0044]    In operation, the thermal fuse  500  may be connected in-between and in series with a power source and a load, such as the power source  420  and load  425  shown in  FIG. 4A . When the ambient temperature surrounding the thermal fuse  500  is below the activation temperature of the thermal fuse, the thermal fuse remains in closed stated and current flows through the thermal fuse and into the circuit. For example, current may flow from the first end structure  545 , through a first sensor  510 , into the middle structure  505 , through a second sensor  511 , and into the second end structure  546 . During this mode of operation, the spring  515  is held in a compressed state between the middle structure  505  and the first and second end structures  545  and  546 . 
         [0045]    When the ambient temperature around the thermal fuse  500  exceeds the activation temperature of the thermal fuse  500 , the sensors  510  and  511  may begin to lose their resilience. Under these conditions, the sensors  510  and  511  may lose their ability to adhere to the contact surfaces on the first and second end structures  545  and  546  and the middle structure  505 , respectively. After this occurs, energy stored in the spring  515  forces the middle structure  505  to separate from the first and second end structures  545  and  546 , as shown in  FIG. 5B . Current stops flowing through the thermal fuse  500  after the middle structure  505  separates from the first and second structures  545  and  546 . 
         [0046]    In addition to these modifications, yet other modifications may be made. For example, the thermal fuses described above may be configured to be placed on a circuit board or substrate via a reflow processes. For example, a retaining wire (not shown) may be configured to secure the thermal fuse to prevent premature activation during the reflow process, as described in U.S. patent application Ser. No. 12/383,560 (Matthiesen et al.), filed Mar. 24, 2009, and U.S. patent application Ser. No. 12/383,595 (Galla et al.), filed Mar. 24, 2009, which are hereby incorporated by reference in their entirety. Therefore, it is intended that thermal fuse and method for using the thermal fuse are not to be limited to the particular embodiments disclosed, but to any embodiments that fall within the scope of the claims.