Abstract:
A method or process of fabricating a solder cartridge and solder cartridge made according to the process is disclosed. The solder cartridge made according to the process provides a self-temperature regulating solder tip for an inductive current soldering station having improved heater quality and stability with reduced manufacturing costs.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    There are a number of different types of industrial and hobbyist soldering stations presently available. In this field, temperature control of the soldering tip is a critical function impacting the quality of the solder joint. Many types of solder materials are available that have different melt temperatures and physical properties to match to particular applications. Soldering also utilizes a number of different types of chemical formulations that aid the soldering process, including fluxes and cleaning agents. Due to the high temperatures, corrosive materials and different metals, the process of soldering is inherently destructive to the solder tip located at the most distal end of a solder cartridge attached to a solder station. Thus, a number of solder stations and solder handles have been offered with removable solder cartridges that can be swapped out to provide different shapes for specific types of solder tasks, as well as to ease replacement. 
         [0002]    For many applications, the size of the solder tip allows constructions having a heater element and temperature sensor located internally to the solder tip. The temperature sensor provides a feedback of the temperature of the tip to a solder station that can adjust the power delivery to the heater element to maintain a desired temperature level. One structure for providing a temperature sensor at the solder tip is described in U.S. Pat. No. 7,679,032, assigned to the assignee of the present invention, herein incorporated by reference. 
         [0003]    However, for some types of detail or fine solder work, the size of the tip is constrained and as a result the structural constraints do not allow the placement of a temperature sensor at the solder tip. For these types of requirements, temperature control of the solder tip may be provided by taking advantage of the Currie point of ferromagnetic materials to provide self-regulating temperature control using an inductive current heating method. Configurations of different inductive current self-regulating soldering tips, and the underlying principle of operation, are disclosed in expired U.S. Pat. Nos. 4,256,945 and 4,745,264. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is directed to method of forming the ferromagnetic heater element and configurations of various heater elements formed thereby allowing the fabrication of replaceable solder tips for use with an inductive current solder station to provide improved self-regulating temperature solder tips for a range of solder temperature applications. A preferred or exemplary embodiment is discussed in the context of forming the ferromagnetic heater element that may be used with various configurations of soldering iron tips and de-soldering nozzles. The ferromagnetic heater elements are formed using a punch press that stamps a flat sheet of ferromagnetic material forming a cap in the general configuration of tiny stove pipe top hat. The heater element is configured to be placed over and brazed to a copper core extending from the proximal side of the solder tip. An inductor coil is wound or placed around the heater element before being confined within an electromagnetic shield element. Specific self-regulated temperatures can be obtained by varying the percentage of nickel in the ferromagnetic material of the heater element, whereby all other components of the solder tip assembly are interchangeable. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0005]    The following drawings are not necessarily to scale, emphasis instead being placed generally upon illustrating the principles of the invention. The foregoing and other features and advantages of the present invention, as well as the invention itself, will be more fully understood from the following description of preferred and exemplary embodiments, when read together with the accompanying drawings, in which: 
           [0006]      FIG. 1  depicts a solder station, handle and a heater cartridge assembly having an exemplary solder tip; 
           [0007]      FIG. 2  is a partial cross sectional view of the distal portion of the heater cartridge assembly of  FIG. 1 ; 
           [0008]      FIG. 3  is an exploded view of the components of the distal end of the heater cartridge assembly; 
           [0009]      FIG. 4  is an exploded view of the components of the distal end of an alternative construction of a heater cartridge assembly; 
           [0010]      FIG. 5  is a temperature v. nickel content graph showing the Currie point of various iron-nickel compositions that may be used to form the ferromagnetic heater element depicted in  FIGS. 2 ,  3  and  4 ; 
           [0011]      FIG. 6  is a pair of temperature v. time graphs showing the operation and temperature recovery of a cartridge according to the present invention and a prior art cartridge. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  illustrates a soldering station  10  connected to a handle  12  for receiving a heater cartridge assembly  14  having an exemplary solder tip  16 . The cartridge assembly  14  may be removed and replaced with another cartridge assembly  14  as the solder tip  16  wears out or when a different solder tip configuration is better suited for a particular soldering operation. The solder station  10  provides a high frequency AC current to the heater cartridge assembly  14 . In a preferred embodiment, the solder station  10  provides a 13.56 MHz AC current to the heater cartridge assembly  14 . 
         [0013]    The components of the distal portion of the heater cartridge assembly  14  are described with reference to  FIG. 2  which provides a partial cross sectional view of the distal portion of the heater cartridge assembly  14  and  FIG. 3  which provides an exploded view of the components of the distal end of the heater cartridge assembly  14 . The solder tip  16  has a distal end-face  18  that as depicted defines a double sided flat solder face configuration. It should be appreciated that this specific configuration of the shape of the solder face is exemplary and other shapes including pointed, circular, bent tip or flat iron faces may be incorporated. The proximal end  20  of the solder tip  16  has a projecting core  22  having a generally cylindrical shape. The solder tip  16  is preferably fabricated from cast or machined copper, sintered copper, copper alloy, silver or a silver alloy with the distal end face  18  being coated with a thin layer of iron, iron alloy, sintered iron, nickel and cobalt or alloys of two or more of these materials. 
         [0014]    As shown in  FIGS. 2 and 3 , a heater element  24  is configured to fit over the entirety of the projecting core  22  of the solder tip  16 . The heater element  24  has a small flange  26 , a central cylinder section  28  and a flat end  30 . The heater element  24  is configured to fit snuggly over and be brazed onto the projecting core  22  of the solder tip  16 . A coil winding  32 , having lead wires  36  and  38 , is either wrapped around the heater element  24  or formed separately and then inserted over the heater element  24 . A shield  40  is then installed over the coil winding  32  and the core  22  as well as a portion of the solder tip  16 , and together with the heater element  24 , forms a magnetic barrier enclosing the coil winding  32  to provide containment for the electromagnetic field produced by the coil winding  32 . The shield  40  may thus include a first cylindrical section  42 , a tapered section  44  and a second cylindrical section  46 . The solder tip  16 , heater element  24 , coil winding  32  and shield  40  when combined form a tip assembly. The proximal end  20  of the tip assembly is inserted into the distal end of a cylindrical sleeve  48  ( FIG. 1 ) with the lead wires  36  and  38  extending axially there through to a connector assembly  50  at the proximal end of sleeve  48 . The proximal end of sleeve  48  includes a connector assembly  50  of known design intended to be inserted into an axial opening within the handle  12 , and provide electrical contacts for connecting the coil winding  42  to the power source of solder station  10 . 
         [0015]    The alternative construction of a heater cartridge assembly  114  is depicted in  FIG. 4 . The heater cartridge assembly  114  has an identical construction for the solder tip  16  with the projecting core  22 , the heater element  24  and coil winding  32 . The primary difference is the shape of the shield  140 , which is a cylinder of constant diameter as opposed to having the stepped configuration of the shield  40  of the heater cartridge assembly  14  of  FIGS. 2 and 3 . The constant diameter cylindrical shield  140  is simpler to fabricate and install over the coil winding  32 , without degrading the stray electromagnetic shield performance. 
         [0016]    The solder tip  16  may be machined or formed by casting or sintering copper, a copper alloy, silver or a silver alloy and then the exposed distal tip is coated for example with iron or an iron alloy by a known process such as plating, sintering or vapor deposition. 
         [0017]    The heater element  24  is formed from a sheet of iron-nickel alloy material that is processed by forming the net shape in a punch press. In an exemplary embodiment, a sheet of iron-nickel alloy material having a thickness of 0.12 mm is punch pressed to form the heater element  24  having a wall thickness of 0.1 mm. As a result of the punch press forming process, the magnetic properties of the iron-nickel alloy material are degraded. The magnetic properties are preferably restored by a heat treating or annealing process. The annealing process is carried out in a hydrogen atmosphere oven wherein the heater element  24  is heated to a temperature between 1050 to 1200 degrees centigrade (1050-1200° C.) for at least twenty minutes and up to about four hours, then gradually cooled. The heater element  24  preferably has a wall thickness in the range of from 0.05 mm to 0.15 mm, which provides optimal temperature control when powered by the 13.56 MHz power supply. 
         [0018]    The temperature of the solder tip  16  is controlled by the Currie point temperature of the iron-nickel alloy of the heater element  24  when excited by the coil winding  32  energized by the solder station.  FIG. 5  provides a temperature v. nickel content graph showing the Currie point of various iron-nickel compositions that may be used to form the ferromagnetic heater element  24  shown in  FIG. 3  and  FIG. 4 . With the benefit of the information provided in the graph of  FIG. 5 , a variety of self-regulated temperature heater cartridge assemblies  14  for a single solder station may be formed by varying the nickel content of the iron-nickel alloy of the heater element  24 . Accordingly, for the preferred range of solder tip temperatures, the nickel content by weight percentage for desired heater temperatures is provided in the flowing chart: 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 % nickel 
                 Curie Temperature 
                 Curie Temperature 
               
               
                 (by weight) 
                 ° F. 
                 ° C. 
               
               
                   
               
             
             
               
                 32% 
                 500° F. 
                 260° C. 
               
               
                 36% 
                 536° F. 
                 280° C. 
               
               
                 39% 
                 644° F. 
                 340° C. 
               
               
                 42% 
                 716° F. 
                 380° C. 
               
               
                 45% 
                 824° F. 
                 440° C. 
               
               
                 46% 
                 860° F. 
                 460° C. 
               
               
                 48% 
                 900° F. 
                 480° C. 
               
               
                 50% 
                 968° F. 
                 520° C. 
               
               
                 52% 
                 1004° F.  
                 540° C. 
               
               
                   
               
             
          
         
       
     
         [0019]    As noted above, the coil winding  32  may be formed on the heater element  24  or pre-wound and then inserted over the heater element  24  forming an inductor. In preferred embodiments, the coil winding  32  has 13.5 to 18.5 turns in 2 layers formed from an insulated silver, copper or nickel plated copper (NPC) wire having a wire diameter of between about 0.15 mm and 0.25 mm and preferable about 0.2 mm. The resulting coil winding  32  has an impedance (Z) in the range of between 15 ohms and 30 ohms when excited by a 5 MHz AC current at room temperature. 
         [0020]    The shield  40  and shield  140  are preferably made from iron or an iron alloy and have a total thickness in the range of from about 0.03 mm to 0.15 mm. The shield  40  may be formed as a pair of cylinders joined in the central portion or it may be formed from a thin sheet that is wrapped around the proximal end of the solder tip  16 . The cylindrical sleeve  48  is preferably a cylinder formed from a thermally non-conductive material such as stainless steel. 
         [0021]    The construction of the tip assembly for the heater cartridge assembly  14  provided herein provides an optimal heat transfer to the distal end of the solder tip  16 . The high frequency AC current applied to the coil winding  32  and the resulting rapidly oscillating magnetic field induced in the heater element  24  causes eddy currents to flow and joule heating. Brazing the heater element  24  to the integrally formed projecting core  22  of the solder tip  16  creates a large surface area for heat transfer from the heater element  24  to the projecting core  22 , and being formed of a high thermal conductivity copper or copper alloy material, the solder tip  16  is uniformly and efficiently heated to the Currie point temperature of the ferromagnetic material from which the heater element  24  is formed. 
         [0022]    The construction of the tip assembly for the heater cartridge assembly  14  provided herein is also beneficial in providing lower manufacturing costs with the ability to change the self-regulated tip temperature by proper selection of the material for the heater element  24 , with the constructions of the solder tip  16 , coil winding  32  and shields  40  or  140  being consistent across an entire spectrum of tip temperatures. Further, stamping the heater element  24  from a sheet of material provides enhanced quality control for the shape and thickness of the ferromagnetic material forming the heater element  24  while also being less expensive as compared to a core formed separate from the tip that has a ferromagnetic coating adhered to the core. Finally, the structures of the solder tip  16 , heater element  24 , coil winding  32  and shield  40  when combined as described herein allow precise control over the placement of the coil winding  32  on the heater element  24 , and constraint of the coil winding  32  by the tapered section  44  of the shield  40 , which increases the uniformity of the manufacturing process whereby the resulting tip temperature is consistent among heater cartridge assemblies  14  having the same material forming the heater element  24 . 
         [0023]    To illustrate the enhanced performance of the solder cartridges according to the present invention,  FIG. 6  presents a pair of temperature v. time graphs showing the operation and temperature recovery of the solder cartridge according to the present invention and a prior art solder cartridge. For these graphs, a solder cartridge according to the present invention was compared to a Metcal and Oki International “SmartHeat” solder cartridge as described at www.okinternational.com and available from Oki International located at 12151 Monarch Street, Garden Grove, Calif., U.S.A. The Metcal solder cartridge had a double sided flat solder face configuration at the distal end generally identical to that depicted in  FIGS. 1-3  herein. In the Metcal cartridge, the solder tip had a proximal cavity into which a core including a ferromagnetic coating was inserted and then surrounded by an excitation coil. 
         [0024]    In the pair of temperature v. time comparison graphs of  FIG. 6 , the performance of the Metcal solder cartridge is depicted in the top graph and the performance of the solder cartridge according to the present invention is depicted in the bottom graph. In each graph, the solder tip is allowed to heat to its design temperature and then used for an identical sequence of soldering tasks. As shown in the graphs, the solder cartridge of the present invention exhibits a faster temperature recovery profile for each of the solder tasks. 
         [0025]    Those skilled in the art will readily appreciate that the disclosure herein is meant to be exemplary and actual parameters depend upon the specific application for which the process and materials of the present invention are used. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto; the invention may be practiced otherwise than as specifically described.