Abstract:
A method and apparatus is disclosed for affixing a cover layer formed of liquid crystal polymer to a flex circuit consisting of circuit elements mounted to a liquid crystal polymer substrate in order to encapsulate the circuit elements between the cover layer and substrate to protect them from exposure to moisture and contaminants.

Description:
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 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. 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 effectively encapsulating circuit elements. There is a need, however, for an efficient and dependable method and apparatus to perform such an encapsulation operation. 
   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 in order to protect the circuit elements from damage and/or reduced operational efficiency due to the presence of moisture and 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 tank 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 a support such that the circuit elements are exposed. An LCP cover layer is then placed atop the flex circuit, after which time 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 them to the underlying LCP substrate of the flex circuit. The temperature and pressure applied by the press is sufficient to cause the LCP cover layer and substrate to “flow” or melt to a limited extent and thus adhere together forming a secure bond so that the circuit elements between the two are substantially encapsulated. 

   
     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; and 
       FIG. 2  is a block diagram illustrating the operation of the apparatus shown in  FIG. 1 . 
   

   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. 
   As discussed in more detail below, 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 325° C. A first reservoir  24  having heating elements (not shown) containing a valve  32 , 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. 
   System Operation 
   As discussed above, the method and apparatus  10  of this invention are designed to provide a means for 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 “flow” 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. 
   The apparatus  10  is operated by a commercially available controller  56  as schematically depicted in the flow diagram of  FIG. 2 . Initially, oil or other fluid within the first reservoir  24  is brought up to a temperature in the range of 283° C. to 325° C. by activating heating elements (not shown) therein. The controller  56  is operative to activate the heating elements via a signal input through lead  58 , 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  60  and  62  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  64  and  66 , respectively. The heated oil flows into 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 recirculation line  42  via a signal input through line  68  to allow the 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. 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 on the order of 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  70 . As a result, ambient temperature oil is recirculated within the press  12  to reduce its temperature. 
   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. 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.