Patent Publication Number: US-2006019551-A1

Title: Method of manufacturing a product having encapsulated electronic components

Description:
The present invention relates to the inexpensive manufacture of products having electronic components encapsulated in plastic housings.  
     BACKGROUND OF THE INVENTION  
      Products such as fuel level detectors of the type mounted within the fuel tanks of automobiles or other vehicles incorporate electronic components, which must be protected against the corrosive effects of the fuel stored within the tank. To protect the electronic components from corrosion, it is desirable to embed the electronic components into a molded plastic housing or the like.  
      The most inexpensive method of making a plastic housing is to injection mold a thermoplastic material in which plastic pellets are heated under pressure until they become a viscous liquid and then inject the viscous liquid into a mold having an inner cavity which is complementary to the shape of the housing to be formed. The problem with injection molding, however, is that the injected plastic is under high pressure and at high temperatures which are harmful or destructive to various electronic components and therefore it is not common to encapsulate electronic components in a thermoplastic material using an injection molding process. In the transfer mold process, however, a thermoset material, in which a resin and a hardener chemically react is utilized. This process does not require that the mold be preheated to temperatures that are destructive to electronic components or require that the liquefied plastic be subjected to intense pressure and is therefore suitable for encapsulating electronic components. The thermoset chemicals used in a transfer molding process, however, are considerably more expensive than the thermoplastic chemicals used in an injection molding process, and as a result, a product requiring a housing made of a thermoset plastic can be considerably more expensive to manufacture than a thermoplastic housing made with an injection molding process.  
      Although a thermoplastic and injection molding are unsuitable for encapsulating electronic components, the process is suitable for making moveable plastic parts, such as the parts of a fuel level detector to detect the level of a fuel in a fuel tank. It would be desirable, therefore, to provide a method whereby a product having mechanical and electrical components can be manufactured at a minimal expense.  
     SUMMARY OF THE INVENTION  
      Briefly, the present invention is embodied in a method of manufacturing a product having at least one electronic component. The method is best suited for manufacturing a product having a mechanically functional enclosure, such as the housing of a fuel level sensor. In accordance with the invention, a first mold is provided for forming a first housing that incorporates the mechanically functional features of the device. The first housing, which also has a cavity therein, is formed by injection molding a thermoplastic material into the first mold. Thereafter, a second mold is provided for forming a second housing in which the electronic components of the device are inserted for encapsulating. A thermoset material is then injected into the second mold in a transfer molding process to form a second housing. The second housing is inserted into the cavity of the first housing.  
      Preferably the first mold for forming the first housing includes an inner mass that is complementary in shape to at least a portion of the outer shape of the second housing such that the first housing is formed with a cavity therein that is complementary to the shape to a portion of the second housing. Prior to injecting material into the first mold at least one lead is inserted therein. In the preferred embodiment, a lead frame that includes including a plurality of leads is inserted into the first mold. A thermoplastic material is then injected into the first mold and allowed to harden, after which the first housing is removed. At least one electronic component is thereafter attached to the lead or leads retained in the first housing.  
      A second mold is then provided for forming a second housing. The second mold is adapted to receive the first housing including the lead frame molded therein. Upon receipt of the first housing into the second mold, the electronic components connected to the leads of the lead frame will be positioned within the inner opening defined within the second mold. A transfer molding process is used in which a thermoset material is injected into the inner opening of the second mold to form a second housing. Upon removal of the completed part from the second mold, the electronic components will be embedded in a second housing and the second housing will be, in turn, bonded into the cavity of the first housing.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A better understanding of the present invention will be had after a reading of the following detailed description taken in conjunction with the drawings wherein:  
       FIG. 1  is a cross-sectional view of a fuel tank having therein a fuel level detector manufactured in accordance with the present invention;  
       FIG. 2  is an isometric view of the fuel level sensor shown in  FIG. 1 ;  
       FIG. 3  is an exploded view of the fuel level sensor as shown in  FIG. 2 ;  
       FIG. 4  is a top-elevational view of the fuel level sensor shown in  FIG. 2 ;  
       FIG. 5  is a cross-sectional view of the fuel level sensor as shown in  FIG. 4  taken through line  5 - 5  thereof;  
       FIG. 6  is a bottom view of the rotor of the fuel level sensor shown in  FIG. 2 ;  
       FIG. 7  is an isometric view of the rotor shown in  FIG. 6 ;  
       FIG. 8  is a front elevational view of the housing for the fuel level sensor shown in  FIG. 2 ;  
       FIG. 9  is a side elevational view of the housing shown in  FIG. 8 ;  
       FIG. 10  is a top view of a lead frame for insertion in the housing in  FIG. 8 ;  
       FIG. 11  is an enlarged isometric view of an inner housing portion of the housing shown in  FIG. 8 ;  
       FIG. 12  is a cross sectional view of the inner housing shown in  FIG. 11 ;  
       FIG. 13  is an enlarged cross-sectional view of a first and a second mold used to form an outer housing portion for the housing shown in  FIG. 8 ;  
       FIG. 14  is a front elevational view of one of the molds depicted in  FIG. 13  showing the contour of the cavity therein;  
       FIG. 15  is a front elevational view of the second mold depicted in  FIG. 13  showing the contour of the cavity therein;  
       FIG. 16  is a front elevational view of the outer housing after being removed from the molds shown in  FIG. 13 ;  
       FIG. 17  is a cross sectional view of a third and fourth molds used to form an inner housing portion shown in  FIG. 11 , and  
       FIG. 18  is a block diagram of the method of making the housing shown in  FIG. 16 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT  
      Referring to  FIG. 1 , to measure the fuel level  10  of fuel  12  in a tank  14  of a motor vehicle, not shown, a fuel level sensor  16  is provided that includes a stationary housing  18  and a rotor  20 . The rotor  20  is rotated with respect to the housing by the movement of a float  24  attached to the rotor by means of a rod  26 . Where the fuel level sensor  16  includes electronic components, the electronic components of the sensor  16  will deteriorate rapidly when exposed to the corrosive effects of the fuel  12  in the tank  14  unless those electronic components are encapsulated in a plastic housing or the like.  
      Referring to  FIGS. 2 through 7 , the rotor  20  of the fuel level sensor  10  includes a tubular sleeve  28  having an end wall  30 , and extending from the tubular sleeve  28  is an elongate arm  32  to which the rod  26  is attached by a plurality of pairs of opposing claws  34 ,  36 . Extending axially through the transverse end wall  30  is an axial hole  38 , and extending through that hole  38  is an elongate pin  48 . Mounted to the inner surface of the end wall  30  are a pair of permanent magnets  40 ,  42  with the north poles  40 N,  42 N thereof oriented at approximately ninety degrees to each other through an arc that extends around the axis of the sleeve  28 .  
      Referring to  FIGS. 8 through 11 , the housing  18  has a body  50  that includes cylindrical sleeve  82  having a cylindrical inner wall  84  and a coaxial cylindrical outer wall  85 . Extending from diametrically opposing portions of the cylindrical outer wall  85  of the body  50  are portions of a generally rectangular frame  86  having embedded therein a metal lead frame  88 . The lead frame  88  includes a plurality of leads of which three are depicted, namely leads  90 ,  92 , and  94 . Lead  90  is a ground lead and includes a generally planar contact pad  98  having a transverse hole  100  therethrough through which a portion of the pin  48  extends. Leads  92  and  94  connect to the electronic components of the fuel level sensor  16  as is further described below and have distal ends  102 ,  104  respectively for attachment to such components.  
      For the purposes of assembling the parts, the leads  90 ,  92 ,  94  are retained together by a tie bar  96 , which connects the outer ends of the leads  90 ,  92 ,  94 . After the housing  18  is fully assembled, as further provided below, the tie bar  96  can be broken free of the distal ends of the leads  90 ,  92 ,  94  leaving the leads spaced from one another so as not to be electrically connected to one another.  
      The body  50  includes attachment portions, such as holes in the frame  86  or tabs on the frame  86  or the cylindrical sleeve, not shown, for attaching the fuel level sensor  16  to the inner surface of a fuel tank  14 . Within the tubular sleeve  82  is a generally cylindrical inner housing  106  having a planar forward surface  108 , and extending axially through the center of the tubular inner housing  106 , is an axially hole  114  for receiving the portion of the pin  48  extending axially from the sleeve  28  of the rotor  20 . When the parts are assembled, as shown in  FIGS. 2, 4  and  5 , the pin  48  retains the cylindrical sleeve  28  of the rotor  20  in axial alignment with the cylindrical sleeve  82  of the housing  18 .  
      Referring to  FIGS. 3, 11  and  12 , the inner housing  106  has extending axially from the forward surface  108  a projection  116  containing a Hall effect sensor  118 . The Hall effect sensor  118  has contacts  120 ,  122 , which are electrically connected, to the inner ends  102 ,  104  of leads  92 ,  94  by solderings  95 ,  96 , or any other suitable means for electrically joining the parts. Accordingly, the leads  92 ,  94  are electrically connected to the Hall effect sensor  18  embedded in the projection  116  of the inner housing  106 .  
      Referring to  FIGS. 1, 2  and  3 , when the rotor  16  is rotatably joined to the housing  18  by the pin  48 , the projection  116  is oriented to extend between the north poles  40 N,  42 N of the magnets  40 ,  42  on the rotor  20 . Accordingly, upward movement of the float  24  will move the Hall effect  118  towards the north pole of one of the magnets  40 ,  42  and downward movement of the float  24  will move the Hall effect sensor  118  towards the north pole of the second of the magnets  40 ,  42 . When the leads  92 ,  94  are connected into appropriate circuits for measuring the voltage or the current provided from the Hall effect sensor  118 , the output form the Hall effect sensor  118  will be related to the volume of fuel  12  in the tank  14 .  
      The chemicals in the fuel for motor vehicles are extremely corrosive and damaging to electronic components and therefore it is desirable that the Hall effect sensor  118 , including the contacts  120 ,  122  and the solder joints  95 ,  96  be hermetically embedded in a suitable plastic inner housing  106 . The electronics that make up the Hall effect sensor  118  are easily damaged by excessive heat and pressure applied to a mold, as would be needed to form the inner housing  106  from a thermoplastic using injection molding techniques. It is therefore not desirable to use injection molding to form the inner housing  106 . It is customary to embed, and hermetically seal, electronic components in plastic parts made from a thermoset plastic using transfer molding techniques and therefore, the inner housing  106 , as shown if  FIGS. 11 and 12 , is manufactured using a transfer molding process. Thermoset plastic, including the resin and hardener from which the plastic is made, is considerably more expensive than a thermoplastic material made with an injection molding. Specifically, the materials used in transfer molding cost approximately $4.00 to $5.00 per pound, whereas the chemicals used in injection molding cost between $1.00 and $1.25 per pound. The cost of making the housing  18  to which the rotor  20  is attached, therefore, can be greatly reduced by manufacturing the housing body  50  from a thermoplastic material using injection molding techniques.  
      Referring to  FIGS. 13 through 16 , and  18 , the housing body  50  is manufactured in a pair of opposing metal molds  130 ,  132  having cavities  134 ,  136  therein respectively. The cavity  134  of mold  130  includes a generally annular groove  137  defined by cylindrical wall  138 , which is generally complimentary to the outer wall  85  of the tubular portion  82 , and co-axial with the cylindrical wall  138 , a second cylindrical wall  140  complimentary to cylindrical inner wall  84  of sleeve  82 . The depth of groove  137  is equal to the distance of the lead frame  88  from one wall  141  of the housing body  50 , as will be apparent when taken into consideration with mold  132 . The cavity  136  of mold  132  also has an annular groove  142  defined by walls  144  and  146  complimentary to portions of cylindrical walls  85 ,  86 , and the depth of groove  142  is determined by the distance of the lead frame  88  from the opposing wall  147  of the housing body  50 .  
      The molds  130 ,  132  have mating surfaces  148 ,  150  respectively which when in contact with each other form a seal allowing a thermoplastic material to be injected through suitable runners  152 ,  154  extending from a hopper, not shown, and pressure chamber, not shown, in which pellets of thermoplastic material, not shown, are melted. In the immediate proximity of mating surface  148 , cavity  134  has an enlarged indentation  160  that is complementary in shape to the perimeter to at least a portion of the lead frame  88  such that the lead frame  88  may be positioned in the indentation  160  before the mating surfaces  148 ,  150  are engaged to each other.  
      To manufacture the housing body  50 , a lead frame  88  is positioned in the indentation  160  of mold  130  and the molds  130 ,  132  are assembled to each other with the mating surfaces  138 ,  140  forming a seal to retain liquefied thermoplastic material within the cavities  134 ,  136 . Thereafter, liquefied plastic thermoplastic material is injected through the runners  152 ,  154  and into the cavities  134 ,  136 . The liquefied thermoplastic is then permitted to cool after which a formed housing body  50 , as depicted in  FIG. 16 , is removed from the molds  130 ,  132 .  
      Referring to  FIGS. 12, 17  and  18 , to form the inner housing  106  within the tubular portion  82  of the housing body  50  third and fourth molds  164 ,  166  having inner cavities  168 ,  170  therein respectively, and having opposing mating surfaces  172 ,  174 , are provided. Mold  164  includes a portion of the cavity  168  adapted to receive the cylindrical outer wall  85  of the sleeve  82  such that the sleeve  82  of the housing body  50  is insertable into the cavity  168  of the third mold body  164 . The cavity  170  of the fourth mold body  166  is complementary in shape to the outer surface of the projection  116  extending from the forward surface  108  of the inner housing  106 . The third and fourth mold bodies  164 ,  166  further have feed lines  180 ,  182  through which a liquefied thermoset material, including a suitable resin and hardener into the cavities  168 ,  170  thereof.  
      To form the inner housing  106 , the contacts  120 ,  122  of a Hall effect sensor  118  are first soldered, forming solderings  95 ,  96 , to connect the contact ends  102 ,  104  of leads  92 ,  94  thereto. Thereafter, the tubular sleeve  82  of the housing body  50  is assembled into the cavity  168  of the third mold  164  with the Hall effect sensor  118  extending into the portion of the cavity  170  of the fourth mold  166 . The mating surfaces  172 ,  174  of the third and fourth molds  162 ,  168  are then engaged with each other after which the resin and hardener of a thermoset material is injected through the feeder lines  180 ,  182  until the cavities  168 ,  170  are filled with material thereby encasing the Hall effect sensor  118 . After the plastic hardens a completed housing  18  is removed from the molds  164 ,  166 .  
      While the present invention has been described with respect to a single embodiment, it will be appreciated that many modifications and variations may be made without departing from the true spirit and scope of the invention. It is therefore the intent of the appended claims to cover all such modifications and variations which fall within the true spirit and scope of the invention.