Patent Publication Number: US-7591924-B2

Title: Apparatus for sealing flex circuits having heat sensitive circuit elements

Description:
This application is a divisional of U.S. application Ser. No. 11/050,303,filed on Feb. 3, 2005 now U.S. Pat. Ser. No. 7,128,801. 

   FIELD OF THE INVENTION 
   This invention is directed to flexible circuits, and, more particularly, to a method and apparatus for sealing circuit elements of a flexible circuit, including heat sensitive circuit elements, which are mounted on a substrate formed of liquid crystal polymer to protect them from exposure to moisture and contaminants. 
   BACKGROUND OF THE INVENTION 
   Flexible or “flex” circuits are used in a wide variety of applications where an electrical circuit must bend around corners or be flexed during operation. Flex circuits are thin, light weight, flexible and exhibit high routability. Traditionally, polyimide films have been used as substrates in the manufacture of flex circuits due to their good thermal stability and mechanical strength. Other properties of polyimide films, however, limit the speed or frequency at which electric components mounted thereto can operate. 
   Liquid crystal polymer (“LCP”) has been developed in recent years as a replacement for polyimide films in flex circuits. LCP is a thermoplastic aromatic polyester which is thermally stable, with an upper use temperature in excess of 250° C. and good inherent flame retardant properties. LCP films, in comparison to polyimide films, have about one-tenth of the moisture uptake and a lower coefficient of humidity expansion. Lower moisture absorption leads to higher frequency signal and data processing stability. Additionally, LCP films have a lower dielectric constant and a lower loss or dissipation factor over the functional frequency range of 1 kHz to 45 GHz, with negligible moisture effects, compared to polyimide films. 
   The fabrication of flex circuits with LCP films is expected to lead to their use in more demanding environments where moisture and other contaminants are prevalent. Particularly in such types of applications, the circuit elements applied to the LCP substrate of the flex circuit must be protected from damage. Soldermask coatings, which have been employed to provide protection from moisture and contaminants in polyimide films, have been considered for use with LCP substrates. Additionally, due to the thermoplastic nature of LCP, the application of an LCP film cover layer to an LCP substrate has been proposed as a means of encapsulating circuit elements. With respect to LCP cover layers, current practice is to employ an air knife or laser to create localized heating of the LCP cover layer and LCP substrate along the periphery of the flex circuit. A number of problems can arise from this approach. A failure of the seal between the cover layer and substrate at any point along the periphery of the flex circuit can expose all of the circuit elements to moisture, chemicals or other contaminants. If the air between the cover layer and substrate is not fully removed, pressurization of the flex circuit which would occur in underwater applications, for example, could compress such air and create a bubble potentially resulting in a rupture of the cover layer and/or substrate thus creating a failure of the entire circuit. 
   The melt temperature of LCP material is approximately 283° C., and soldermask coatings are also applied at relatively high temperatures. While a number of standard circuit elements are not affected by high temperatures, components such as micro-electrical-mechanical-system (“MEMS”) sensors, infra-red sensors and a variety of other circuit elements are temperature sensitive and can be damaged or destroyed upon exposure to elevated temperatures. There is a need for an efficient and dependable method and apparatus capable of individually sealing or encapsulating the electrical components of circuits which employ an LCP substrate, while protecting heat sensitive components of the circuit from damage due to the temperatures at which the sealing process is performed. 
   SUMMARY OF THE INVENTION 
   This invention is directed to a method and apparatus for affixing an LCP cover layer to a flex circuit consisting of circuit elements mounted to an LCP substrate, at least some of which are temperature sensitive, in order to individually protect the circuit elements from damage and/or reduced operational efficiency due to the presence of moisture, chemical and other contaminants. 
   In the presently preferred embodiment, the apparatus includes an iso-static press having a hollow interior connected to a source of oil or other liquid whose temperature can be accurately controlled and maintained. The oil is heated to a temperature in the range of approximately 283° C. to 320° C. and transferred from a reservoir into the interior of the press. The base of the press has a plate or membrane formed of a flexible material covered with a non-stick surface which does not adhere to LCP. 
   The flex circuit is placed on the surface of the thermally conductive top plate of a support such that the circuit elements are exposed. A thermal insulating compound may be placed over each temperature sensitive circuit element for added thermal protection. The support includes a housing formed with a cavity within which a number of conduits are mounted each having an upper end communicating with a channel formed in the underside of the top plate. Cooling fluid from a source is circulated through selected conduits, i.e. those which are located beneath the temperature sensitive circuit elements of the flex circuit, to provide localized cooling of such elements during the lamination process. Other conduits, which are not located near the temperature sensitive circuit elements, may be supplied with a heating fluid to raise the temperature of the top plate of the support in selected areas and thus assist with the lamination process. 
   With the flex circuit in place on the top plate of the support, and the conduits within the support receiving cooling fluid or heating fluid to cool or heat selected areas of the top plate, an LCP cover layer is placed atop the flex circuit and the press is activated to move into contact with the cover layer. The flexible membrane at the base of the press is capable of substantially conforming to the shape of the circuit elements, thus urging the LCP cover layer around each of them individually to the underlying LCP substrate of the flex circuit. The temperature and pressure applied by the press, and the elevated temperature of selected areas of the support top plate, are sufficient to cause the LCP cover layer and substrate to “relax” or melt to a limited extent and thus adhere together forming a secure bond so that the circuit elements are individually encapsulated between the two layers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure, operation and advantages of the presently preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic, perspective view of the apparatus of this invention; 
       FIG. 2  is a block diagram illustrating the operation of the press of the apparatus shown in  FIG. 1 ; 
       FIG. 3  is a perspective view of the support for the flex circuit with a portion cut away to show the conduits mounted within a cavity in the support; 
       FIG. 4  is a schematic view of one conduit of the support connected by 3-way valves to a hot fluid source and a cold fluid source, wherein cooling fluid is being supplied to the conduit; 
       FIG. 1A  is an enlarged view of the encircled portion of  FIG. 1 ; 
       FIG. 5  is a view similar to  FIG. 4 , except with hot fluid being supplied to the conduit; 
       FIG. 6  is a schematic, block diagram depicting the structure for supplying hot fluid and cooling fluid to the conduit; and 
       FIG. 7  is a cross sectional view of the connection between a conduit and the top plate of the support. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the Figs., the apparatus  10  of this invention is schematically illustrated. The apparatus  10  includes an iso-static press  12  having a housing  14  formed with a hollow interior. The base of the housing  14  mounts a flexible membrane  16  having an exposed surface coated with Teflon® or other release agent which will not stick to LCP, and an inside surface coated with a hydrophobic film. Preferably, the flexible membrane  16  is formed of high density polyethylene, butyl rubber, ethylene propylene diene monomer rubber or a similar material. 
   The press  12  is operative to apply heat and pressure against a cover layer  18  which overlies a flex circuit  20  placed upon a support  22 . In the presently preferred embodiment, the press  12  is heated by the introduction into its hollow interior of heated oil or a similar fluid whose temperature can be relatively accurately controlled and maintained within the range of about 283° C. to 320° C. A first reservoir  24  having heating elements (not shown) is connected by a supply line  26  to a manifold  28 . A pump  30  and valve  32  are located in the supply line  26 , between the first reservoir  24  and manifold  28 , as shown. The manifold  28 , in turn, is connected by an input line  34  to one port at the top of the press  12 , and by an output line  38  to a second port. A recirculation line  42 , containing a valve  32 , is connected between the manifold  28  and the top of the first reservoir  24 . 
   In view of the relatively high temperature obtained by the press  12  during operation, it is advantageous to provide a cooling capability to step the temperature down. To that end, a second reservoir  44  is provided which contains the same fluid as first reservoir  24  except at ambient temperature. The bottom of second reservoir  44  is connected by a line  46  to the manifold  28 , and a recirculation line  48  connects the manifold  28  to the top of the second reservoir  44 . A pump  30  and valve  32  are located in the line  46  between the second reservoir  44  and manifold  28 , and a valve  32  is mounted in the recirculation line  48 . 
   The press  12  is moved with respect to the support  22  by a number of pneumatic or hydraulic pistons  50  which are mounted at equal intervals along the top surface of the press  12 . Conventionally, the pistons  50  are independently actuated by a source of air or fluid (not shown) to ensure that the press  12  applies uniform pressure to the cover layer  18  and flex circuit  20  over the entire surface area of the flexible membrane  16 . The detailed construction of the press  12  forms no part of this invention, and is therefore not discussed further herein. 
   As discussed above, the method and apparatus  10  of this invention are designed to provide a means for individually encapsulating circuit elements to protect them from moisture and contaminants. The flex circuit  20  consists of a substrate  52  formed of LCP—upon which a number of circuit elements  54  are mounted. The cover layer  18  is also formed of LCP, which, because of its thermoplastic nature, will “relax” or begin to melt at a temperature of about 283° C. By placing the cover layer  18  over the flex circuit  20  and applying heat and pressure, the cover layer  18  and substrate  52  adhere to one another with a secure bond and entirely enclose the circuit elements  54  between them. 
   At least some of the circuit elements  54  are temperature sensitive and would be damaged by exposure to heat on the order of the melt temperature of the LCP layers. The support  22  is constructed to provide localized cooling of the temperature sensitive circuit elements  54 , and to generate heat in other areas of the substrate  52  which raises its temperature to assist with the lamination or encapsulation process. With reference now to  FIGS. 3-7 , the support  22  includes a side wall  56  defining an internal cavity  58  which is closed by a top plate  60  and a bottom plate  62 . The top plate  60  is preferably formed of a highly thermally conductive material such as aluminum silica carbide. A number of channels  64  are machined or otherwise formed in the underside of the top plate  60 , each of which mounts a conduit  66 . See  FIG. 7 . The conduits  66  are depicted in the Figs. as a pair of side-by-side pipes  68  and  70  each having an upper end received within a channel  64  in the top plate  60  so that fluid can pass between the two. It should be understood, however, that the conduit  66  may be a unitary structure formed with an internal wall so as to define two separate flow paths, for purposes to become apparent below. 
   An array of conduits  66  is carried within the cavity  58  of the support  22  atop the bottom plate  62  and extending beneath substantially the entire surface area of the top plate  60 . Structure is provided to transmit either cold fluid or hot fluid into each conduit  66 , depending on the position of the temperature sensitive circuit elements  54  resting on the top plate  60  of the support  22 , so that such fluid produces localized heating or cooling of the top plate  60 . As best seen in  FIGS. 4-6 , this structure includes a cold fluid source  72  connected to a pump  74 , which, in turn, is connected to a distribution manifold  76  and, a hot fluid source  78  connected by a pump  80  to a second distribution manifold  82 . While the “hot” fluid source  78  is shown as a separate reservoir in the Figs., it is contemplated that the first reservoir  24  supplying heated fluid to the press  12  may be employed to transmit hot fluid to the conduits  66 , if desired. 
   As described more fully below in connection with a discussion of the operation of the apparatus  10  of this invention, the distribution manifold  76  connected to the cold fluid source  72  transmits such fluid to number of 3-way valves  84 . Each 3-way valve  84 , in turn, is connected to the inlet of the pipe  70  of a conduit  66 . Similarly, the distribution manifold  82  receiving hot fluid from source  78  is connected to a number of 3-way valves  86 , each of which connects to the inlet of a pipe  68  of a conduit  66 . Consequently, depending upon the operative position of the 3-way valves  84  and  86 , either hot or cold fluid can be circulated through each conduit  66  to obtain localized heating or cooling of the top plate  60  in the area located immediately above such conduit  66 . 
   The operation of the press  12  of this invention is described initially below, followed by a discussion of the localized cooling and heating provided by the support  22 . 
   The apparatus  10  is operated by a commercially available controller  56  as schematically depicted in the block diagrams of  FIGS. 2 and 6 . Initially, oil or other fluid within the first reservoir  24  is brought up to a temperature in the range of 283° C. to 320° C. by activating heating elements (not shown) therein. The controller  56  is operative to activate the heating elements via a signal input through lead  88 , or they may be independently activated by a switch (not shown) located at the first reservoir  24 . The controller  56  then inputs signals through leads  90  and  92  to start the pump  30  and open valve  32 , respectively, thus initiating the flow of heated oil out of the first reservoir  24 . When it is desired to heat the press  12  in preparation for circuit encapsulation, the controller  56  deactivates the pump  30  and valve  32  in line  46  from second reservoir  44  by signals input through leads  94  and  96 , respectively. The heated oil flows to the press  12  through the manifold  28  and into the input line  34  leading into the interior of the press  12 . Preferably, the temperature of the heated oil within the press  12  is controlled and maintained by continuously recirculating it from the first reservoir  24  through the manifold  28  and input line  34  into the press  12 , and then out of the press  12  through the output line  38  and manifold  28  to the recirculation line  42  connecting the manifold  28  to the first reservoir  24 . The controller  56  opens the valve  32  within the recirculation line  42  via a signal through the line  98  to allow heated oil to pass from the manifold  28  into the first reservoir  24 . 
   With the press  12  at the appropriate temperature, the encapsulation process can proceed. The flex circuit  16  is positioned on the support  22  so that the circuit elements  54  on the LCP substrate  52  are exposed. In the presently preferred embodiment, a thermal insulating compound  99  such as Aerogel or a silica based material is placed over the top of each temperature sensitive circuit element  54 . See  FIG. 1A . The compound  99  is effective to assist in preventing thermal damage to the upper portion of such circuit elements during the encapsulation operation. The LCP cover layer  18  is then placed atop the substrate  52  and circuit elements  54 . The controller  56  operates the pistons  50  causing the press  12  to move toward the support  22 . Upon engagement of the flexible membrane  16  at the bottom of the press  12  with the cover layer  18 , at a uniform pressure up to 200 psi, the flexible plate  16  substantially conforms to the shape of the circuit elements  54  beneath. In turn, the cover layer  18  is forced around the circuit elements  54  into contact with substrate  52 . The press  12  is maintained in this position for a period of time sufficient to heat both the LCP cover layer  18  and LCP substrate  52  to a melt temperature of at least 283° C., but not more than about 320° C., causing them to bond to one another and thus encapsulate the circuit elements  54  between the two. 
   After completing one or more encapsulation procedures, the temperature of the press  12  may be stepped down by circulating comparatively cool, ambient temperature oil into the press  12  from the second reservoir  44 . The controller  56  is operative to deactivate the pump  30  and close valve  32  within line  26  connected to the first reservoir  24 , while activating pump  30  and opening valve  32  within the line  46  connected to the second reservoir  44 . The controller  56  closes the valve  32  within the recirculation line  42 , and then opens the valve  32  within the recirculation line  48  extending from the manifold  28  to the second reservoir  44  by inputting a signal to such valve  32  through a line  100 . As a result, ambient temperature oil is recirculated within the press  12  to reduce its temperature. 
   As noted above, a thermal insulating compound  99  is placed on the top surface of the temperature sensitive circuit elements  54  prior to the encapsulation operation to aid in the protection of them from the heat of the press  12 . For additional thermal protection, it is desirable to provide localized cooling of the top plate  60  of support  22  in those areas located immediately beneath the circuit elements  54 . Additionally, the encapsulation process may be enhanced by heating areas of the top plate  60  which are spaced from the thermally sensitive circuit elements  54 . The support  22  provides such localized heating and cooling as follows. 
   For purposes of the present discussion, and with reference initially to  FIGS. 4 and 6 , a single conduit  66  including pipes  68  and  70  is shown. It should be understood that the following discussion describing the manner in which cold fluid or hot fluid is supplied to conduit  66  applies equally to all of the other conduits  66  mounted within the support  22 . One of the 3-way valves  84  is connected to the inlet of pipe  70  of conduit  66  by a line  102 , and one of the 3-way valves  86  is connected to the inlet of pipe  68  of conduit  66  by a line  104 . The 3-way valve  84  is also connected to the hot fluid source  78  through a line  106 , and the 3-way valve  86  is connected to the cold fluid source  72  by a line  108 . In order to circulate cold fluid through the conduit  66 , and into contact with the underside of the top plate  60  immediately above the conduit  66 , the controller  56  inputs a signal through line  110  to activate the pump  74  so that it begins pumping cold fluid from the source  72  into the distribution manifold  76 . The operation of pump  80  is governed by the controller  56  via signals input through line  112 , as described below in connection with a discussion of  FIG. 5 . The cold fluid passes from the pump  74  into the distribution manifold  76  and then to the inlet of each of the 3-way valves  84 . It should be understood that the three valves  84  and three valves  86  shown in  FIG. 6  are for purposes of illustration, and there could be essentially any number of valves  84 ,  86  depending on how many conduits  66  are employed in the support  22 . 
   As best seen in  FIG. 4 , flow of cold fluid or heated fluid into each individual conduit  66  is dependent on the operation of a valve pair, i.e. one of the 3-way valves  84  and one of the 3-way valves  86 . Once the position of the thermally sensitive circuit elements  54  on the top plate  60  of support  22  is determined, the group(s) of conduits  66 , or individual conduits  66 , which are located immediately beneath such areas are identified. The controller  56  is operative to introduce cold fluid into such conduit(s)  66  as follows. A signal is input to 3-way valve  84  though a line  114  which opens a path through 3-way valve  84  to the line  102  connected to the inlet of pipe  70 , but closes the flow path to line  106  which connects such valve  84  to the hot fluid source  78 . At the same time, the controller  56  inputs a signal through line  116  to the 3-way valve  86  connected to the pipe  68  associated with that conduit  66 . The 3-way valve  86  is operated to permit the flow of fluid from pipe  68  into the line  108  connecting 3-way valve  86  to the cold fluid source  72 , while closing the flow path through 3-way valve  86  from the distribution manifold  82 . Consequently, cold fluid from the source  72  is pumped via pump  74  through the distribution manifold  76 , to the 3-way valve  84 , and then into the pipe  70  of conduit  66  via line  102 . The cold fluid is directed by pipe  70  into the channel  64  formed on the underside of the top plate  60  where it contacts and reduces the temperature of a discrete area of the top plate  60  beneath one or more temperature sensitive circuit elements  54 . See also  FIG. 7 . The cooling fluid passes through the channel  64  and enters the pipe  68  of conduit  66  from which it is discharged into the line  104  leading to the 3-way valve  86 . The 3-way valve  86  passes the cold fluid into line  108  where it is transmitted back to the cold fluid source  72 . The cold fluid is continuously recirculated along the above-described flow path, as depicted by arrows  118  in  FIG. 4 , throughout the encapsulation process to assist in protection of the temperature sensitive circuit elements  54  from thermal damage. 
   Other areas of the flex circuit  20  mount circuit elements  54  which are not affected by the temperature of the encapsulation process. In these areas, it is desirable to locally heat the top plate  60  of the support  22  to assist the press  12  with the encapsulation process. The same valve arrangement described in  FIG. 4  is employed to deliver hot fluid to each individual conduit  66 . With reference to FIGS.  5  and  6 , in order to obtain a flow of hot fluid into the conduit  66  the controller  56  inputs a signal though line  112  to start the pump  80  connected to the hot fluid source  78 . The hot fluid passes through the distribution manifold  82  to the inlet of each 3-way valve  86 . The controller  56  inputs a signal through line  116  causing selected 3-way valves  86  to accept the flow of hot fluid from the distribution manifold  82  while closing the flow path from valve(s)  86  into line  108 . The hot fluid passes through the 3-way valve  86  into line  104  which connects to the inlet of pipe  68  of conduit  66 . At the same time, the controller  56  operates selected 3-way valves  84  to open a flow path through such valve(s)  84  from the line  102  connected to the pipe  70  of conduit  66  into line  106  extending between the valve(s)  84  and hot fluid source  78 . The inlet of 3-way valve  84  connected to the distribution manifold  76  is closed. As a result, hot fluid from the source  78  and distribution manifold  82  passes through the 3-way valve  86  into the pipe  68  of conduit  66  via line  104 , and moves along a channel  64  at the underside of top plate  60  thus locally heating the top plate  60  in that immediate area. The hot fluid enters the pipe  70  of conduit  66  from the channel  64  and is transmitted to the line  102 . After passing through the 3-way valve  84 , the hot fluid is returned to the source  78  through the line  106 . The hot fluid is preferably continuously recirculated in the direction of arrows  120  in  FIG. 5  throughout the encapsulation process. 
   The support  22  of this invention therefore provides localized heating and cooling of those areas of its top plate  60  where enhanced heat for encapsulation, or additional cooling to protect temperature sensitive circuit elements  54 , is desired. Depending upon how the valve pair  84 ,  86  associated with each conduit  66  is operated, as described above, localized heating or cooling can be provided by each individual conduit  66  or groups of conduits  66 , as needed. This allows for the efficient encapsulation of LCP circuits  20  of essentially any configuration, one after the other. The flow of hot or cold fluid through any given conduit  66  can be readily reversed by the controller  56 , thus permitting the temperature of the area of the top plate  60  immediately above to be rapidly cooled if it was previously heated, or vice versa. 
   While the invention has been described with reference to a preferred embodiment, it should be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. 
   For example, the views in  FIGS. 4 and 5  depict lines  106  and  108  extending from the 3-way valves  84  and  86 , respectively, directly to the respective fluid sources  78  and  72 . It should be understood that additional distribution manifolds (not shown) could be provided between these lines  106 ,  108  and the sources  72  and  78 , if desired. 
   Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.