Abstract:
The present invention is a magnetic flux controlling device in the form of a passive inductor which improves heat pattern control for induction heating of objects such as thin, flat bodies. The device comprises a magnetic core and at least one electrical conductor that together do not form a closed electrical loop. The device is located on the opposite side of the object to be heated from the induction coil and may be adjusted along the axis of the induction coil to manipulate the heat pattern in different zones of the part.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/003,815, filed Nov. 20, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to local induction heating of thin bodies, and more particularly to the increased power distribution control using a passive inductor located on the opposite side of a thin, flat body in relation to the inductor. 
       BACKGROUND OF THE INVENTION 
       [0003]    Induction heating is becoming a more popular technique for applications where very specific heating of small areas on thin, flat bodies is required. These applications include, but are not limited to, package sealing, joining of electrical and electronic components. In the past, these applications primarily utilized contact heating methods, such as a hot bar. However, due to demands for increased product quality, production rate, tool life and ability to use lower cost materials, induction heating has become a preferred method for new systems. 
         [0004]    Induction heating involves an induction coil, which can have many configurations, and also carries an alternating frequency current. This current generates an alternating magnetic field, which in turn, induces eddy currents in conductive bodies that are exposed to the alternating magnetic field. It also causes hysteretic heating of magnetic bodies exposed to the field. The distribution of eddy currents and hysteretic heating depends upon the shape of the induction coil, the level of alternating magnetic field, the shape of the conductive body, the orientation of the conductive body relative to the magnetic field and the electrical and magnetic properties of the body. 
         [0005]    One type of application, where induction heating is frequently used, is a thin, flat body. Prior art methods of induction heating for a thin, flat body are shown in  FIGS. 1A-1F ,  FIG. 2 , and  FIG. 3 . A typical induction heating system includes an induction coil or induction heating coil, generally shown at  10 , which includes individual coil windings  12  wound around an axis  44 . The coil windings  12  may or may not be surrounded by a concentrator  16 . A concentrator  16  is a device made of soft magnetic material and is used for concentrating a magnetic field, resulting in the heating of a part in a desired area. The induction coil  10  is placed in proximity to a thin, flat object or body  18 , which is substantially transparent to a magnetic field. The body  18  is shown in  FIGS. 1A-1F ,  FIG. 2 , and  FIG. 3  as a thin sheet and the induction coil  10  is used to provide localized heating of the sheet  18 . For local induction heating of thin, flat bodies such as the flat sheet  18 , higher efficiencies and better control are possible if the magnetic field generated by the induction coil  10  is transversal to the body  18 . 
         [0006]    Common styles of coils used for heating of thin, flat bodies  18  are shown in  FIGS. 1A-1F .  FIG. 1A  shows an induction coil  10  which can be of a cylindrical or linear nature and can be of a single turn or multi-turn type, and may or may not have a magnetic core. The induction coil  10  of  FIG. 1A  is referred to as a “hairpin” if the windings  12  extend into and out of the page.  FIG. 1B  shows an induction coil  10  similar to  FIG. 1A , which can be a hairpin, single turn or multi-turn inductor, but also incorporates a magnetic back-pad  24 .  FIG. 1C  shows an induction coil which is a multi-turn flat hairpin coil (or “pancake” coil if the coil windings  12  are cylindrical).  FIG. 1D  shows a split-n-return induction coil  10 , and  FIG. 1E  shows a vertical loop induction coil  10 , which can be used depending upon the desired heating area and space available.  FIG. 1F  shows a two-sided hairpin or round induction coil  10 ; the induction coil  10  of  FIG. 1F  is also referred to as a transverse flux inductor. The frequency used for these applications can range from 10 kHz to 2 MHz, with a preferred range of 50 kHz-1 MHz. 
         [0007]    To demonstrate the principle operation of an induction coil  10 , an Example is shown in  FIG. 2  of a small, two-turn cylindrical coil  10  with a magnetic core for heating of a circular area on a thin, flat body, such as foil  18 .  FIG. 2  shows the magnetic field lines  20  generated by the induction coil  10 . The magnetic field attenuates with distance from the magnetic core and windings  12 . Those skilled in the art will recognize that the maximum in power density will be at a radius significantly larger than the diameter of the windings  12 . 
         [0008]      FIG. 3  shows the magnetic field lines  22  for the same induction coil  10  located next to the same foil  18  when a magnetic back-pad  24  is used. The magnetic back-pad  24  attracts the magnetic field passing through the foil  18  and makes the magnetic field lines  22  more transverse to the foil  18 . It is clear from these lines  22  that the power density will be significantly more concentrated, leading to a smaller spot size. 
         [0009]    In many of these applications, there are significant savings that are realized by minimizing the zone of thermal influence from the coil  10 . These savings may result from better utilization of material (higher component density) or reduced scrap (extra area for bonding or material breakage due to thermal strain in semiconductors). In other cases, there are components adjacent to the desired heating area of the body  18  that if exposed to elevated temperatures or alternating magnetic fields will cause damage to the final product. 
         [0010]    Properly designed two-sided inductors, such as the example shown in  FIG. 1F , can provide better control and higher efficiencies than inductors that heat from only one-side due to mutual inductance of the windings  12  on opposite sides of the part to be heated. The drawback of two-sided inductors is that they require electrical and/or water connections between the two inductor halves. This can be impractical in many in-line processing systems. 
         [0011]    In cases where two-sided inductors are not practical, one of the varieties of one-sided inductors shown in  FIGS. 1A-1E  may be used. Heat pattern control in one-sided inductors is accomplished through variation of the geometry of the windings  12 , magnetic flux controller material or geometry variation, and in some cases with the use of the magnetic back pad  24  shown in  FIG. 1B , placed selectively on the opposite side of the sheet  18 . The magnetic flux controller on the opposite side of the sheet  18  helps to increase the component of magnetic field transverse to the surface of the sheet  18 , and increases heating in the area with a concentrator relative to adjacent zones. This method is considered to be the best available technology for local heating and joining where two-sided inductors are not possible. One of the drawbacks to this method is that the control achieved with this method does not match that for the two-sided inductors. 
         [0012]    In view of the foregoing, there exists a need to provide a device or devices for magnetic field control on the backside of the work piece (or magnetic body) that would provide improved heat pattern control for one-sided heating inductors while at the same time approaching the performance of two-sided inductors, without requiring an electrical connection. 
       SUMMARY OF THE INVENTION 
       [0013]    The above and other objects are achieved by the present invention, which is a passive inductor located on the opposite side of a part in relation to the induction coil. The part may be a thin, flat body, with the induction coil on one side, and the passive inductor on the other. One embodiment of the present invention is a passive inductor which provides magnetic flux control from the opposite side of a part as the induction coil. The passive inductor is a magnetic flux controlling device, which contains a magnetic core and at least one conductor that do not form a closed electrical loop. 
         [0014]    Another embodiment of the present invention is a magnetic flux controlling device having a passive inductor for generating induction heating of an object. The passive inductor includes at least one electrical conductor operable for generating a desired heating area on the object. The electrical conductor may optionally include a magnetic flux concentrator which also generates a desired heating area on the object. The electrical conductor does not form a closed electrical loop. 
         [0015]    Operation of the present invention, areas of applicability and provided effects will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1A  is a schematic sectional view of a first type of induction coil used for heating a thin flat, body; 
           [0017]      FIG. 1B  is a schematic sectional view of a second type of induction coil used for heating a thin flat, body; 
           [0018]      FIG. 1C  is a schematic sectional view of a third type of induction coil used for heating a thin flat, body; 
           [0019]      FIG. 1D  is a schematic sectional view of a fourth type of induction coil used for heating a thin flat, body; 
           [0020]      FIG. 1E  is a schematic sectional view of a fifth type of induction coil used for heating a thin flat, body; 
           [0021]      FIG. 1F  is a schematic sectional view of sixth type of induction coil used for heating a thin flat, body; 
           [0022]      FIG. 2  is a sectional side view of a type of induction coil and magnetic field generated by the induction coil used for local heating of a small spot on a thin, flat body; 
           [0023]      FIG. 3  is a sectional side view of a type of induction coil used with a magnetic back-pad to provide local heating of a small spot on a thin, flat body; 
           [0024]      FIG. 4  is a sectional side view if an induction coil used with a passive inductor for providing local heating of a small spot on a strip, according to the present invention; 
           [0025]      FIG. 5  is a first graphical comparison of relative power density distribution for prior art induction coils and a passive induction system, according to the present invention; and 
           [0026]      FIG. 6  is a second graphical comparison of relative power density distribution for prior art induction coils and a passive induction system, according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0028]    A passive induction system according to the present invention is shown in  FIG. 4 , generally at  26 . The passive induction system  26  of the present invention also includes an induction coil  10  which includes induction coil windings  12 , which work in a similar fashion to the prior art embodiments described above. However, also included is a second inductor, or passive inductor, generally shown at  28 , which, in relation to the inductor  10 , is positioned on the opposite side of the sheet  18 . The passive inductor  28  may be used with or without a magnetic flux concentrator, and does not form a closed electrical loop. In a preferred embodiment, the passive inductor  28  has a conductor  30  which is optionally surrounded by a magnetic flux concentrator in the form of a core  32 . The core  32  may or may not be used, depending upon the amount of heat desired. The core  32  is made up of any soft magnetic material such as ferrites, soft magnetic composite materials (such as Fluxtrol® brand material, available from Fluxtrol Inc., Auburn Hills, Mich.), insulated lamination, or combinations of these. In some cases, soft magnetic alloys may be used, depending upon the frequency used for heating. The conductor  30  has a front side  36  and a back side  38 , and is made of any electrically conductive material, and is preferably a non-magnetic conductor. By way of explanation and not limitation, the conductor  30  may be made up of aluminum, copper, silver, or brass, or combinations of these. The core  32  surrounding the conductor  30  has a first flux surface  40  and a second flux surface  42 . The passive inductor  28  of the present invention operates by redirecting the magnetic field from the induction coil windings  12 . 
         [0029]      FIG. 4  shows the magnetic field lines  34  generated by a passive induction system  26  according to the present invention. The magnetic field is drawn to the central core  32  of the passive inductor  28 , in a similar manner as the magnetic back-pad  24  as described above. However, as the passive inductor  28  is moved towards the inductor  10 , the magnetic field cannot pass through the conductor  30  itself, the magnetic field is redirected either through the central magnetic core  32 , or away from the front side  36 . The magnetic field that flows through magnetic core  32  via the flux surfaces  40 , 42 , then flows through the core  32  on the back side  38  of the conductor  30  and returns on the outside of the conductor  30  via the flux surfaces  40 , 42 . This increases the transversal component of the magnetic field in the desired heating area and reduces this component in the area directly under the face of the conductor  30  in the passive inductor  28 . This leads to an increased gradient in the power density in cross-section compared to the use of a magnetic back-pad  24  alone. 
         [0030]      FIG. 5  is a comparison of the power density along the length of the sheet  18  for the inductor  10  only, inductor  10  with magnetic back-pad  24 , and inductor  10  with passive inductor  28 . It is clear that there is a drastic improvement with both the magnetic back-pad  24  and passive inductor  28  being used with the induction coil  10  compared to the induction coil  10  alone. To better appreciate the advantages of the passive inductor  28  working in combination with the induction coil  10  compared to the magnetic back-pad  24  working in combination with coil  10 ,  FIG. 6  shows a comparison of only these two cases up to a radius of three millimeters. The peak for the passive inductor  28  is at a significantly smaller radius compared to the magnetic back-pad  24  alone. In addition, the power density at radii past the peak values is significantly lower for the passive inductor  28  compared to the magnetic back-pad  24  alone. This will lead to a smaller heating spot size from the passive inductor  28 . 
         [0031]    Further heat pattern control is also possible in the length or depth of the coil  10  and part by adjusting the passive inductor  28  component dimensions. An example would be to heat several zones on a flat, linear surface simultaneously, such as the thin, flat body  18 , without heating the areas in between. This could be accomplished by using several conductors  30 , and removing the magnetic core  32  in the areas where heating was undesirable and bringing the conductors  30  closer together. Without the core  32 , and with a very small space for magnetic flux to flow through between the conductors  30 , the magnetic resistance of the path for the magnetic field would be increased and the heating would be subsequently decreased. 
         [0032]    The passive induction system of the present invention is useful for providing localized heating in applications such as precise control soldering. The passive induction system of the present invention may be used for heating environments, connecting electrical components to circuit boards, localized heating in packaging applications, thin layer silicon soldering, or the like. As mentioned above, the thin, flat sheet  18  is substantially transparent to a magnetic field. The sheet  18  in one embodiment may be 150 μm or less. The thickness of the sheet  18  with which the subject invention is effective is selected based on the electrical reference depth, or skin depth. The electrical reference depth, “δ,” (Greek letter “Delta”) is a reference value which depends on material properties and frequency, but does not account for body size and shape. For non-uniform materials, δ is calculated usually for properties on the body surface. Reference depth, δ, is directly proportional to root square of material resistivity, “ρ” (Greek letter “Roh”), and inversely proportional to root square of relative magnetic permeability “μ” (Greek letter “Mu”), and current frequency. 
         [0033]    The equation for the reference depth is: δ=k√(ρ/fμ) 
         [0034]    In this equation, “f” is the frequency, and “k” is a constant. The units for these values are shown below: 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 System 
                 ρ 
                 f 
                 δ 
                 k 
               
               
                   
                   
               
             
             
               
                   
                 Metric 
                 mkOhm-cm 
                 kHz 
                 Millimeters 
                 1.6 
               
               
                   
                 English 
                 mkOhm-inch 
                 kHz 
                 Inches 
                 0.1 
               
               
                   
                   
               
             
          
         
       
     
         [0035]    The thickness of the sheet  18  is generally one reference depth or less, and is typically substantially one-third of a reference depth or less, and preferably is one-fifth of a reference depth or less. A passive induction system according to the present invention may be used for performing soldering operations on a sheet  18  having a larger thickness, depending upon the frequency of the current flowing through the induction coil  10 . 
         [0036]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. For example, variations in cooling methods and methods of fixturing of the passive inductor of the present invention are not to be regarded as a departure from the spirit and scope of the invention.