Abstract:
An overmolded electronic assembly having an electromagnetic interference shield, in the form of a thin metal film or foil, coupled to the top of or within an overmolded body. The shield effectively reduces the amount of electromagnetic interference (“EMI”) emissions from penetrating within the assembly to the circuit board without substantially increasing the cost of the unit. Thus, an electronic assembly having improved vibration, moisture, and EMI emission resistance is achieved as compared with traditional overmolded or metal assemblies. Further, because the shield can be formed on the electronic assembly in one continuous processing step, a substantial savings in time and cost for the manufacturing process is also realized.

Description:
TECHNICAL FIELD 
     The present invention relates generally to electronic systems and more particularly to an electromagnetic interference shield for overmolded packaging of an electronic assembly. 
     BACKGROUND OF THE INVENTION 
     Circuit boards with semiconductor devices such as flip chips must often be protected from the environment in which the board is employed. A widely practiced method is to enclose such circuit boards in an assembly that includes a pair of case halves that must be assembled together by hand to form an enclosure that supports the circuit board within. Connectors secured to one of the case halves provide for electrical interconnection to the circuit board. Sealing elements are also typically required to exclude moisture from the enclosure. Finally, fasteners are required to secure the assembly together. Such assembly processes are labor intensive, and the resulting package must be tested for leakage to ensure the package was properly assembled. 
     To simplify the design, an overmolded electronic assembly that is compatible with automated assembly methods has been used. The overmolded assembly, typically comprised of plastic or epoxy type material, includes a heat conductive member, such as a heat sinking backplate, in thermal contact with one or more of the circuit devices mounted to the circuit board. The overmolded body encloses the circuit board and the circuit device with the heat conductive member, such that the overmolded body and heat-conductive member form a moisture-impermeable seal around the circuit board and circuit device. The overmolded body has a connector housing integrally formed in its outer surface. The overmolded body is non-metallic and is typically formed of a plastic or epoxy-type material and provides a secure environment against vibration and shock. One example of an overmolded electronic assembly is described in U.S. Pat. No. 6,180,045 to Brandenburg et al., which is herein incorporated by reference. 
     One problem with overmolded electronic assemblies is that the non-metallic topside of the assembly cannot shield electromagnetic interference (“EMI”) emissions as can typical sheet metal or die cast metallic housings. 
     From the above, it can be appreciated that an electronic assembly that combines the simplified processing and improved moisture and vibration resistance of an overmolded body assembly with improved EMI emissions resistance would be highly desirous. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an overmolded electronic assembly having a thin metal shield, in the form of a film or foil, coupled to a top portion of the overmolded module. The thin metal shield effectively reduces the amount of EMI emissions from penetrating within the assembly to the circuit board without substantially increasing the cost of the unit. It also prevents EMI radiated emissions generated inside the assembly from escaping outside of the enclosure. Thus, an electronic assembly having improved vibration, moisture, and EMI emission and radiation resistance is achieved as compared with traditional overmolded or metal assemblies. Further, because the metal shield can be formed on the electronic assembly in one process step, a substantial savings in time and cost for the manufacturing process can be realized. 
     The method for manufacturing the electronic assembly can be performed in three distinct ways. In one method, the metal shield is preformed as a film to a desired shape and placed in a mold cavity just prior to molding. A vacuum holds the metal film preform in place during the molding process. The metal film is then adhered to a top surface of the overmolded body of the overmolded electronic assembly. In another method, the roll of metal foil is incorporated into a film assisted molding equipment (“FAME”) mold press. A modified FAME process is used to incorporate the metal film within the overmolded body of the overmolded electronic assembly. In a third method, the overmolded assembly is first formed and then a thermal or kinetic spray coating of metal film is applied to the outside of the overmolded assembly. 
     Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 shows an overmolded electronic assembly having a thin metal EMI shield according to a first embodiment of the present invention; 
     FIG. 2 is an exploded view of FIG. 1; 
     FIG. 3 shows and exploded view of an insert molded metal EMI shield prior to overmolding according to a second embodiment of the present invention; 
     FIG. 4 is a cross-sectional view of FIG. 1 taken along line  4 — 4 ; 
     FIG. 5 illustrates a preferred thin metal film arrangement for the embodiment of FIG. 1; 
     FIGS. 6-13 illustrates one preferred method of making the overmolded housing having a thin metal EMI shield as shown in FIG. 3; 
     FIG. 14 is a perspective view of another preferred method of making the overmolded housing having a thin metal EMI shield as shown in FIG. 3; and 
     FIG. 15 is a perspective view of another preferred method for making the overmolded housing of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1,  2  and  4  show an overmolded electronic assembly  20  enclosing a circuit board  22  in accordance with a preferred embodiment of the present invention. The circuit board  22  may be laminate printed wiring board (PWB), or any other material known in the art. Shown mounted to one surface of the circuit board  22  in FIG. 4 are several integrated circuit packages, or IC packages  26 , that dissipate heat. The packages  26  will typically be flip chips, though the invention is applicable to essentially any surface-mount through hole mounted device. The circuit board  22  has a pin retainer  36  includes I/O (input/output) connector pins  24  electrically interconnected with the circuit board  22  in any suitable manner. In addition to the circuit board  22 , the assembly  20  includes a heatsink  28  formed of a conductive material, such as metal or a metal-filled plastic. The heatsink  28  has a number of pedestals  30  in thermal contact with the IC packages  26  such that the heat is conducted from the packages  26  and into the heatsink  28  for subsequent dissipation to the surrounding environment. 
     An overmolded housing assembly  32  encases the circuit board  22 , contacting the upper surface of the circuit board  22  (opposite the IC packages  26 ) and underfilling the circuit board  22 , and therefore contacting the heatsink  28  and the lower surface of the board  22 . In this manner, the overmolded housing assembly  32  forms a moisture-impermeable seal around the circuit board  22  and its IC packages  26 , as best seen in FIG.  4 . The overmolded housing assembly  32  includes a pair of integrally formed connector housings  38  into which the pins  24  project, such that the pins  24  and connector housings  38  form a pair of connectors on the exterior of the electronic assembly  20 . 
     As best shown in FIG. 4, an EMI shield  50  is coupled to a top portion  44  of an overmolded body  33  to form the overmolded housing assembly  32 . The EMI shield  50  functions to absorb and block electromagnetic interference (“EMI”) energy that may be emitted by or absorbed by the circuit board  22 . The EMI shield  50  may consist of any conductive material. Preferably, as shown in FIG. 4, the EMI shield  50  comprises a thin metal film such as an aluminum foil film. In another preferred embodiment, the shield  50  comprises a spray coating of either tin, zinc, a mixture of tin and antimony, or a mixture of silicon and bronze. 
     Referring to FIG. 5, the EMI shield  50  preferably extends over the entire top portion  44  of the overmolded body  33  and is irreversibly or reversibly attached to a rail portion  70  of the heatsink  28 . This allows the EMI shield  50  to be grounded to the heatsink  28 , which provides additional electromagnetic interference shielding compared to non-grounded versions. To irreversibly attach, the EMI shield  50  is secured to the rail portion  70  by using an adhesive or by welding. To reversibly attach, a mechanical interlocking feature (not shown) could be added to the rail portion  70  that is capable of reversibly coupling the EMI shield  50  to the heatsink  28 . 
     Two methods for making the overmolded housing assembly  32  having the EMI shield  50  coupled to the top surface  44  of the overmolded body  33  or coupled within the overmolded body  33  near the top surface  44  are described below in FIGS. 6-13 and FIG. 14 respectively below. A method for making the overmolded housing assembly having a sprayed thermal metal EMI shield  50  is shown in FIG. 15 below. 
     In a second embodiment, as shown in FIG. 3, an electronic assembly  120  having the thin EMI shield  150  formed as a thin metal stamping. The EMI shield  150  includes integrally-formed locking appendages  142  that interlock with the heatsink  128 , and biasing members  144  that apply pressure to the near surface of the circuit board  122  to promote thermal contact between the IC packages and the pedestals  130  on the heatsink  128 . The assembly  120  includes a circuit board  122  with connector pins  124 , IC packages (not shown) and pin retainers  136 , a heatsink  128  with pedestals  130 . The locking appendages  142  and biasing members  144  can be any suitable elements, such as the resilient cantilevered springs shown in FIG.  3 . However, it is foreseeable that other elements could be used for these purposes, such as elastomeric pads on the lower surface of EMI shield  150  to contact the circuit board  122 . While not shown, the EMI shield  150 , after attachment to the heatsink  128 , is subsequently overmolded in a manner similar to FIGS. 1,  2  and  4  to form an overmolded electronic assembly having the EMI shield  150 . 
     Referring now to FIGS. 6 through 13, one method of making the overmolded housing assemblies  20  having the EMI shield  50  located along a top surface  44  of the overmolded body  33  is depicted. First, as shown in FIG. 6, the EMI shield  50 , in the form of a thin metal foil, is unrolled from a creel  202  and feathered through a top chase  204  and a bottom chase  206  of a film assisted molding equipment, or FAME mold  208 . Next, as shown in FIG. 7, a vacuum is introduced through the vacuum slots  212  of the top chase  204  that pulls the EMI shield  50  through suction to a cavity face  214  contained on the bottom surface of the top chase  204 . In FIG. 8, the heatsink  28  and the rest of the components of the assembly  20  are set to the bottom chase  206 . This may be accomplished using an automated system or manually. 
     Next, in FIG. 9, the bottom chase  206  is clamped to the top chase  204  at a press tonnage of approximately 80 tons. In FIG. 10, a resinous material  216  in the form of a mini tablet is forced upward by a plunger  217  under pressure of into the cavity  218  defined between the bottom chase  206  and EMI shield  50 . As shown in FIG. 11, the resinous  216  material is cured to the EMI shield  50  to form the overmolding body  33 . As is understood by those of skill in the art, the shape of the cavity  218  dictates the shape of the overmolded body  33 . Thus the overmolded body  33  and  133  may be formed in the same manner by simply changing the shape of the cavity  218 . 
     The amount of pressure, curing time and curing temperature of the overmolded body  33  is dependent upon numerous factors, most notably the type of resinous material  216  used. For an epoxy type material used in making the overmolded body  33  similar to those shown in FIGS. 1-4, for example, a pressure of approximately 500 pounds per square inch and a molding temperature maintained at approximately 165 degrees Celsius for about 2 minutes is necessary to form the overmolded body  33 . 
     In FIG. 12, the bottom chase  206  is opened and the overmolding housing assembly  32  containing the EMI shield  50  along the top surface  44  of the overmolded body  33  is ejected from the top chase  204  of the FAME mold  28  using ejector pins (not shown). Finally, in FIG. 13, the EMI shield  50  is indexed and the process repeated to form the next overmolded body  33  containing the EMI shield  50 . In a preferred embodiment, the process of FIGS. 6-13 for forming the overmolded housing assembly  32  takes approximately two and one-half minutes. 
     FIG. 14 illustrates another preferred process for making the overmolded housing assemblies  32  of FIGS. 1,  2  and  4 . The method similar to that of FIGS. 6-13, however the vacuum forming of FIG. 7 is replaced by FIG. 14, in which the preformed EMI shield  50  is inserted into the mold cavity  218  of a FAME mold  208  and held stationary using vacuum assist. The resinous material  216  is then injected into the cavity  218  to surround the EMI shield  50 . The resinous material  216  is then cured under similar molding conditions to those shown in FIG. 10 above, thereby forming an overmolded body  33  having the EMI shield  50  surrounded by the resinous component  216 , as compared with on a top surface  44  of the overmolded body  33  as in the embodiments of FIGS. 1-4. 
     In another preferred process for making the overmolded assembly  32 , the overmolded body  33  is first made without an EMI coating by the method discussed in U.S. Pat. No. 6,180,045 to Brandenburg et al., which is herein incorporated by reference. The EMI shield is added as a EMI shield  50  to the top surface  44  of the overmolded body  33  by a spray process to form the overmolded assembly  32 . This is shown in FIG.  15 . 
     Referring now to FIG. 15, the metal EMI shield  50  is applied as a spray from a spray gun to the top surface  44  of the overmolded body  32  to form the overmolded housing assembly  33 . The spray gun can be a thermal spray gun, shown as  300  in FIG. 15, or a kinetic spray gun. Examples of thermal spray guns include a flame sprayer, an electric arc sprayer, a plasma-arc sprayer, or any other type of thermal sprayer that can apply a liquid metal film that is known in the art. 
     The raw metal material  304  preferably comprises metals such as tin or zinc or alloys such as tin/antimony or silicon/bronze. The raw metal material  304  is fed into the thermal spray gun  300  by conventional methods. The thermal spray gun  300  has a thermal heat source  308  that melts the raw metal material  304 . The material  304  is then dispersed from the gun  300  as molten droplets  310  and is applied to the top surface  44  of the overmolded body  33  to form a thin coating. The thin coating cools and dries to the top surface  44  to form the EMI shield  50 . The EMI shield  50  thickness is preferably about between 0.002 and 0.004 inches thick, and more preferably about 0.003 inches thick, which is sufficient to obtain adequate EMI protection for the circuit board  22 . Typically, the top surface  44  is roughened prior to application of the EMI shield  50  to increase adhesion. 
     The present invention offers many advantages over previous overmolded and EMI shielding applications. First and foremost, the present invention provides an overmolded packaging for a circuit board that provides a secure environment from moisture, vibration and shock as well as protection from harmful EMI emissions. Because the overmolded body  33  having an EMI shield can be formed in one continuous processing step, substantial savings in terms of time and cost in manufacturing are realized. Also, the EMI shield  50  as shown in FIGS. 1-4 offers an inexpensive method for absorbing and blocking electromagnetic interference energy that may be emitted by or absorbed by the circuit board  22 . 
     While the invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings.