Abstract:
An encapsulated device and method for making such encapsulated device containing a bladder disposed between a wall of the case and the encapsulant. The bladder defines a space devoid of encapsulant and contains a collapsible insert such as an open cell foam material, thereby allowing the unimpeded thermal expansion of the encapsulant. By reducing thermal expansion stresses on the encapsulated devices, the reliability of the encapsulated device is improved.

Description:
[0001]    This application is a continuation-in-part of co-pending and commonly assigned U.S. patent application Ser. No. 09/450,602 filed Nov. 30, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to the field of encapsulated components and to the manufacturing of such components. This invention relates more specifically to the field of electrical components that are encapsulated to improve their resistance to vibration and corrosive environments.  
           [0003]    Encapsulation is a process by which a relatively fragile component is surrounded by an encasing material which provides mechanical support to the component and which may seal the component from contact with the ambient environment. Solid state electrical devices are known to be susceptible to printed circuit board failures due to vibration loads and/or mechanical or electrical degradation caused by exposure to a corrosive environment. The assignee of the present invention provides components for the marine environment wherein high levels of vibration and/or corrosive atmospheres may be commonplace. It is known to encapsulate such components to improve their performance in the marine environment. Encapsulants commonly used in such applications include epoxy resin and urethane based products, and they are selected for their workability, mechanical strength and electrical insulating properties. As commonly practiced, the encapsulant is poured in a fluid state into a case containing electrical devices, then allowed to cure to form a solid mass encasing the electrical devices within the case.  
           [0004]    While the known process is effective to protect a component against vibration and environmental damage, failures may occur within the component due to mechanical damage caused by the thermal growth characteristics of the encapsulant. For example, it is known that such thermal growth may cause mechanical failure at the point where a conducting pin is soldered directly to a mating connector on a printed circuit board when the pin is fully constrained by being molded into a plastic case. Encapsulant disposed between the printed circuit board and the case is subject to thermal expansion and contraction. The thermal growth of the encapsulant may tend to move the printed circuit board away from the case. However, at the location of the soldered connection, the printed circuit board is maintained at a fixed distance from the case. The thermal expansion of the encapsulant may impose unacceptably high forces on the printed circuit board and/or the soldered connection. Encapsulated electrical components are known to have failed as the result of such differential thermal expansion.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    Thus, there is a particular need for an improved method for encapsulating a component to avoid failures resulting from the thermal growth of the encapsulant. There is also a particular need for an encapsulated component having a greater resistance to thermal growth failures.  
           [0006]    Described herein is a method for encapsulating components within a case, the method comprising the steps of: forming a bladder having an interior containing a collapsible insert; positioning the bladder within the case; installing at least one component within a case; depositing an encapsulant in a fluid state within the case and allowing the encapsulant to transform to a solid state; and providing a vent connection to the bladder interior so that the collapsible insert may compress and expand as the solid encapsulant expands and contracts. A product formed by such a process is also described herein, as well as a kit to be used for encapsulating a component by the described process.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which:  
         [0008]    [0008]FIG. 1 is a partial sectional view of a printed circuit board mounted within a case proximate an inflated bladder.  
         [0009]    [0009]FIG. 2 is a partial sectional view of the apparatus of FIG. 1 after being encased within an encapsulant.  
         [0010]    [0010]FIG. 3 is a partial sectional view of the apparatus of FIG. 2 wherein the bladder has been punctured by drilling a hole through the case.  
         [0011]    [0011]FIG. 4 is a top view of the bladder of FIGS.  1 - 3 .  
         [0012]    [0012]FIG. 5 is a partial sectional view of a printed circuit board mounted within a case above a bladder containing a collapsible insert and having a chimney portion extending above the level of the encapsulant for venting the interior of the bladder.  
         [0013]    Similar or identical components illustrated on successive drawings are identified with the same numeral in each drawing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    The applicants have discovered a method and apparatus for accommodating the thermal growth of an encapsulant disposed within a case. In the embodiment illustrated in FIGS.  1 - 3  an electrical printed circuit board is encapsulated within a case. The apparatus  10  includes a printed circuit board  12  housed within a case  14 . As is known in the art, case  14  may be a one piece injection molded plastic component or a metal component, a partial bottom wall of which is illustrated in FIG. 1. A plurality of electrical devices  16  are mounted to printed circuit board  12 . The electrical devices  16  may constitute all or part of a circuit, such as for example, circuitry necessary to perform the function of a power supply. Electrical connection to the circuit board  12  is provided by one or more conductive pins  18 . The pins  18  may be sealed within the wall of the case  14  as is known in the art of injection molding. Each pin is illustrated as being soldered to a connector  20  mounted on circuit board  12 , thereby providing both mechanical and electrical connections between the case  14  and the circuit board  12 . Circuit board  12  may also be supported mechanically by post  22  formed as an integral portion of the wall of case  14 . A screw  24  is shown attaching the circuit board  12  to post  22 . The circuit board  12  may also be secured directly to the case  14  by a screw or clamp, or may be secured by the action of a connector attached to the case.  
         [0015]    A bladder  26  is positioned within the case  14 , and may be attached to a surface of the case by an adhesive  28 . Bladder  26  is a hollow structure defined by a flexible wall member which takes a predetermined shape when inflated to an internal pressure that is higher than the ambient external pressure. The bladder may be formed of a flexible, airtight material, such as polyvinylchloride (PVC). Other materials of construction for the bladder  26  may be selected for ease of manufacturing, electrical insulation properties, resistance to heat, and/or compatibility with the encapsulant material to be used. In the embodiment of FIG. 1, bladder  26  consists of two layers of PVC material sealed on their respective edges  28  by a thermal sealing process. Bladder  26  is also illustrated as having a sealed hole  27  formed in one location for fitting around post  22 . Such a hole  27  may be seen more clearly in FIG. 4 which is a top view of bladder  26 . FIG. 4 also illustrates the location of a section A-A which is the sectional view of the bladder  26  seen in FIGS.  1 - 3 . Bladder  26  may be formed in any desired shape, and it preferably will conform to the geometry of at least a portion of the interior of case  14 . The shape of bladder  26  is selected to match the shape of a desired space within case  14  devoid of encapsulant, as will be described more fully below. The interior of bladder  26  may be filled with air or other fluid so that the bladder takes a predetermined shape when inflated. The bladder  26  may be inflated prior to being installed within the case  14 , or for certain applications, it may be installed in a deflated state and inflated once it is in position within the case  14 .  
         [0016]    [0016]FIG. 2 illustrates the apparatus  10  of FIG. 1 after encapsulant  30  has been deposited in the case  14 . Prior to depositing the encapsulant  30 , the printed circuit board  12 , electrical devices  16  and case  14  may be preheated to a predetermined elevated temperature, such as about  85  degrees Centigrade, for a period of time sufficient to remove moisture from the components including the printed circuit board  12 . Encapsulant  30  in a fluid state is then poured into the case  14  to a predetermined level. In certain embodiments, it may be necessary to tilt the case  14  and enclosed components while introducing the encapsulant  30  in order to avoid the entrapment of air under the circuit board  12 , thereby ensuring the complete encapsulation of all of the devices  16 . In the embodiment of FIGS.  1 - 3 , bladder  26  is positioned so that it does not contact any portion of the printed circuit board  12 . This insures that the encapsulant  30  will fully encase the electrical devices  16  located on the side of the circuit board  12  proximate the bladder  26 . Encapsulant  30  is allowed to cure to transform to a solid state with the bladder  26  in its inflated condition, thereby forming a space devoid of encapsulant  30  at a desired location within the case  14 .  
         [0017]    [0017]FIG. 3 illustrates the apparatus  10  of FIGS.  1 - 2  with a hole  32  having been drilled through case  14 , thereby causing a puncture  34  in bladder  26 . The puncture  34  allows the interior of bladder  26  to be in fluid communication with and at pressure equilibrium with the ambient environment of the apparatus  10 . As encapsulant  30  grows due to an increase in temperature, the space defined by the bladder  26  which is devoid of encapsulant  30  may decrease to accommodate the thermal expansion of the encapsulant  30 . Without the puncture  34 , the deformation of encapsulant  30  and resulting decrease in volume of the space devoid of encapsulant may result in an undesirable increase in pressure in bladder  26 , thereby negating the desired affect of providing space for the unimpeded thermal growth of encapsulant  30 . Because the encapsulant  30  is free to grow into the space defined by bladder  26 , the stresses generated in the printed circuit board and attached structures are reduced. The material of bladder  26  is selected so that it remains flexible during the operation of the apparatus  10  and so that it may remain within the apparatus  10  throughout its operating life without detrimental effect.  
         [0018]    In lieu of drilling a hole  32  or otherwise penetrating the case  14  in order to form puncture  34 , the puncture  34  may be formed by cutting off a portion of bladder  6  extending above the top level of encapsulant  30 . Alternatively, a valve may be attached to the wall of bladder  26  and made accessible outside the area of the encapsulant  30 . Once the encapsulant  30  has transformed to a solid state, the valve may be opened to provide a fluid communication path between the interior of the bladder  26  and the ambient environment.  
         [0019]    The bladder  26  and encapsulant  30  may be supplied together with a fully assembled apparatus  10 , or they may be supplied separately as a kit for installation subsequent to the assembly of the circuit board  12  and case  14 . Such a kit may include a bladder  26  shaped to fit within the case  14 , along with a supply of encapsulant  30 . The kit may also include a supply of adhesive  28 , such as a tube of liquid adhesive, for securing the bladder  26  in its proper position while the encapsulant  30  is poured into case  14 .  
         [0020]    [0020]FIG. 5 illustrates a cross-sectional view of an engine controller  40  for a marine propulsion apparatus wherein a bladder  26  is formed to contain a collapsible insert  36 . Circuit board  12  supports and interconnects a plurality of electrical devices  16  functional as part of a control system for a marine engine (not shown). The type, quantity and interconnection of such electrical devices  16  necessary to achieve such functionality are well known in the art and may take any of many embodiments. Bladder  26  is positioned within case  14  proximate the bottom  42  of the case  14  and may be affixed in its location by an adhesive  28 . Bladder  26  is positioned between the circuit board  12  and the case  14  to define a space devoid of encapsulant  30 . In this embodiment, there is no need to inflate bladder  26  since the collapsible insert  36  acts to maintain the bladder in an expanded condition during the pouring of the liquid encapsulant  30  into the case  14 .  
         [0021]    Flexible insert  36  may be a material having a resistance to crushing sufficient to support bladder  26  in an expanded condition during the pouring of encapsulant  30  against the bladder  26 , and a compressibility sufficient to allow for the expansion and contraction of encapsulant  30  after it has hardened. In one embodiment, insert  36  is a section of 2-pound open cell polyester foam, such as is provided by Federal Foam Technologies, Inc. of New Richmond, Wis. The thickness of the insert  36  may be selected as a function of the expected expansion and contraction of the encapsulant  30  over the expected temperature range, and as a function of the allowable deflection in the circuit board  12  that may be caused by such expansion and contraction. In one embodiment a ¼ inch thick insert was used to create a space devoid of encapsulant that otherwise would have been ¾ inch thick had it been completely filled with encapsulant.  
         [0022]    Bladder  26  is formed to have an opening  44 , and an end of insert  36  may extend therefrom. Opening  44  functions as a vent to allow air to flow out of and into the bladder  26  as the encapsulant  30  expands and contracts during temperature changes. The opening  44  may be formed to be part of a chimney portion  46  of the bladder  26 . The chimney portion  46  extends upward away from the bottom  36  of case  14  from a bend  48  to the vent opening  44  located above a top level  50  of encapsulant  30 . Opening  44  may be formed in the bladder  26  during its original construction by removing a sealed edge portion of the chimney  46  or by not sealing a portion of the perimeter of the bladder  26 . It is important that the collapsible insert be free to expand the bladder  26  to an expanded condition prior to the step of pouring the liquid encapsulant  30  into the case  14 . One method of manufacturing the bladder  26  is to sandwich a layer of open foam material between two layers of PVC material and to sonically seal weld the edges of the PVC material together. The edges of the foam material are spaced away from the edges of the PVC material to be welded in order to ensure that a good seal is created. The bladder may be pressed flat during the welding process to improve the quality control of the welding process. Unless there is a path for air to enter the bladder after the welding process, the two layers of PVC material will remain in a collapsed state. In this case, it is important for the vent opening  44  to be formed prior to the step of pouring the liquid encapsulant  30  into the case  14  in order to ensure that the collapsible insert  26  can expand the bladder  26 . Preferably, insert  36  extends through bend  48  to prevent the bladder sides from collapsing and creating a seal preventing the flow of air into and out of the interior of the bladder  26 . This is particularly important when the bladder will be exposed to an elevated temperature in order to remove moisture before the encapsulation process, since the elevated temperature may act to cause a welding of the opposed sides of the bladder within the bend  48 .  
         [0023]    It is possible to use the embodiment of FIG. 5 without a separate bladder material and with only the collapsible insert  36  defining the space devoid of encapsulant. The selection of materials for such an application must consider any possible flow of the encapsulant  30  into the collapsible insert  36  prior to the hardening of the encapsulant  30 . For example, if an open cell foam material is used for the insert  36 , the size and material of the cells and the viscosity of the liquid encapsulant will determine the rate of flow of the encapsulant into the cells. Any such migration of encapsulant prior to its solidification would, of course, decrease the compressibility of the insert  36  and should be accounted for in the selection of the thickness of the insert. It is also possible to cover the insert  36  with a barrier material that is impervious to the liquid encapsulant to prevent this undesirable migration of encapsulant into the insert  26 . Such barrier materials may include, for example, a PVC plastic or a common kitchen cooking wrap. One or both sides of the insert  36  may need to be covered, depending upon the specific application. Federal Foam Technologies, Inc. of New Richmond, Wis. provides an open cell foam product laminated with a flexible polyester film that may be useful in such applications.  
         [0024]    While the preferred embodiments of the present invention have been shown and described herein, such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. For example, the embodiment illustrated in FIGS.  1 - 3  is for an electrical printed circuit board component. Other embodiments may include, for example, discreet electrical components, mechanical devices, sensors, or fragile containers, etc. mounted in a case. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.