Patent Publication Number: US-2007107837-A1

Title: Process for making high count multi-layered circuits

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a utility patent application based on provisional application entitled “PROCESS FOR MAKING HIGH COUNT MULTI-LAYERED CIRCUITS” and having Ser. No. 60/597,021, filed Nov. 4, 2005, which is a continuation-in-part patent application of utility application entitled “METHOD AND APPARATUS FOR FORMING MULTI-LAYERED CIRCUITS USING LIQUID CRYSTALLINE POLYMERS” and having Ser. No. 11/187,220, filed Jul. 22, 2005, all of which are herein incorporated in their entirety. 
    
    
     FIELD OF INVENTION  
      This invention relates, generally, to a method and apparatus for forming multi-layered circuits and, in particular, to a method and apparatus for forming high layer count circuits comprising liquid crystalline polymer insulating or dielectric layers.  
     BACKGROUND OF THE INVENTION  
      Multi-layered circuit boards are typically fabricated from layers of distinct circuit patterns separated by insulating material such as thin dielectric layers and interconnected by vias, or holes, that are drilled through the circuit board and plated with metal. It is desirable to interconnect integrated circuit packages and discrete electronic devices in highly dense assemblies to reduce signal paths and overall size.  
      However, the number of layers comprising multi-layered circuit boards becomes limited due to the increased non-uniformity in thickness and high fluid flow as more layers are added. During the processing of some exemplary circuit boards, layers of liquid crystalline polymer with copper on both sides are stacked up and then laminated together using a mechanical/electrical press, a mechanical hot oil press, or a mechanical hot steam press. Vias are then made by drilling holes in the multi-layered board and the interior surface of the vias are plated with metal to connect the distinct circuit patterns of the different layers. When the multilayer stack produces more liquid when being pressed into a laminated package, features or circuit tracers can shift and, as a result, not line up. In addition, when pressure and heat are applied to the multi-layer stack by the mechanical/electrical press, more pressure gets applied to the center of the multi-layer stack than the outer perimeter of the stack. This non-uniform pressure distribution results in a multi-layer stack having a non-uniform thickness and conductor layer feature shifting. In addition, it becomes difficult to drill and plate the interior surfaces of the vias within the non-uniformity of the multi-layer stack. This non-uniformity also becomes a performance issue at high frequencies.  
      Controlling or limiting the pressure applied to the multi-layer stack during the lamination cycle would decrease shifting of the layer features and would enable the production of a multi-layer stack having a precise uniform thickness. A multi-layer stack having reduced feature shifting and a uniform thickness enables more precise processing for the remaining processing steps including the creation of vias within the multi-layer stack having more uniform depths and diameters and, as a result, more uniform plating of the interior of the vias.  
      Current methods for fabricating multi-layer stacks of liquid crystalline polymers with copper foil are not capable of limiting feature shifting in the various layers and/or controlling or limiting the pressure applied to the multi-layer stacks during lamination. As a result, the number of layers that can be laminated to form a multi-layer circuit is limited in order to avoid feature shifting and non-uniformity in the thickness of the multi-layer stack.  
      Accordingly, there is a need for a method and apparatus for producing a high layer count, multi-layer circuit board having reduced feature shifting and a uniform thickness in order to provide a structure for supporting and interconnecting a high density of electronic devices.  
     SUMMARY OF THE INVENTION  
      In general, the present invention provides a method and apparatus for forming high layer count, multi-layered circuits. The present invention is particularly useful for forming high layer count, multi-layered circuits comprising liquid crystalline polymer layers. The method and apparatus of the present invention function to control and/or limit the pressure applied to a multi-layer material (product) stack during lamination while fabricating multi-layer circuits.  
      In accordance with one aspect of the present invention, an apparatus for forming multi-layered circuits is provided which includes a press having top and bottom platens capable of applying pressure to a material stack located between the platens and a fixture positioned between the platens having an opening therein in which to position the material stack. The fixture functions to limit or control the pressure applied to the material stack which in turn results in a laminated material stack having a uniform thickness.  
      In accordance with a further aspect of the invention, a top caul plate is positioned between the top platen and the fixture and a bottom caul plate is positioned between the bottom platen and the fixture. In addition, a top separator plate may be positioned between the top caul plate and the fixture and a bottom separator plate may be positioned between the bottom caul plate and the fixture.  
      In accordance with yet a further aspect of the invention, the material stack may be enclosed so that a vacuum can be applied to the bag during lamination. Other means of vacuum such as an enclosed vacuum press will work as well. A thermocouple is inserted into the material stack before the vacuum is applied and before lamination. A number of release sheets and other materials may be used to enclose the material stack. In one exemplary embodiment of enclosing the material stack, a first release sheet is positioned on top of the bottom separator plate, a bagging material is positioned on top of the first release sheet, a breather material is positioned on top of the bagging material, a second release sheet is positioned between the breather material and the fixture containing a material stack, a sealant tape is applied around at least half of the perimeter of the bagging material and underneath and around the thermocouple and also around copper tubing positioned along the perimeter of the bagging material where the copper tubing is connected to a disconnect, a third release sheet is positioned on top of the material stack, and half of the perimeter of the bagging material is folded over the sealant tape and pressure is applied to the sealant tape to seal the bagging material thereby producing a vacuum enclosure containing the breather material, the fixture, and the material stack.  
      In accordance with yet a further aspect of the present invention, at least one pin may be inserted into the fixture to hold the fixture in place during lamination of the material stack.  
      The present invention also provides a method for forming multi-layered circuits which includes the steps of providing a press having top and bottom platens, positioning a fixture having an opening therein between the top and bottom platens, positioning a material stack within the opening in the fixture, and applying pressure to the material stack by applying pressure to the top and bottom platens.  
      In accordance with a further aspect of the method of the present invention, the step of positioning a fixture having an opening therein includes the step of first creating the fixture so that it has an opening with a desired shape, size, and depth depending upon the end lamination product. The method of the present invention may also include the step of placing the fixture and the material stack within a vacuum enclosure and applying a vacuum to the vacuum enclosure before the step of applying pressure to the material stack.  
      In accordance with yet another aspect of the method of the present invention, heat may be applied to the top and bottom platens during the step of applying pressure to the material stack. In addition, the method may include positioning a top caul plate between the top platen and the fixture and a bottom caul plate between the bottom platen and the fixture. The method of the present invention may also include positioning a top separator plate between the top caul plate and the fixture and a bottom separator plate between the bottom caul plate and the fixture.  
      In accordance with still a further aspect of the invention, a fixture is provided for placement between top and bottom platens in a press where the fixture includes an opening in which to position a material stack for lamination. The fixture may also include a slot connecting the opening in the fixture to the exterior of the fixture so that the slot can retain connection means for connecting the material stack to a thermocouple.  
      The present invention is directed to using materials in the general category of high frequency, high speed materials in the process and apparatus described in the patent application entitled “Method and Apparatus for Forming Multi-Layered Circuits using Liquid Crystalline Polymers” (attached hereto and herein incorporated by reference). The following are the primary manufacturers of high frequency, high speed materials:  
      Rogers: RT/duroid, RO3000, RO4000, R/flex 3800/3900 series  
      Arlon: DiClad, CuClad, AR, AD, Isoclad, CLTE, 25N/FR series  
      Taconic: TLY, TLX, TLC, TLE, RF, TSM, CER and HyRelex series  
      Nelco/Neltec: N4000, N5000, N7000, N8000, N9000 series  
      Isola: IS400, IS500, IS600, G200, FR406, FR408, DE100, Duramid, P95, P96 series  
      Polyclad: Getek, LD-600, GI-100, HF-500, GI-700, FR-200 and FR-300 series  
      Hitachi Chemical: MCL-E, MCL-LX, MCL-HD, MCL-BE series  
      Nippon Steel Chemical: Espanex series  
      Chukoh: CGC, CGK, CGH, CGA, CGF, CGS, CGP series  
      High Performance, High Speed materials are made in several ways from different sets of materials.  
      The first class is PTFE or Teflon, which comprises the best electrical performance available at high frequency. These materials are typically blends of PTFE and E glass or S glass reinforcement (woven or random), sometimes including various types of ceramic and/or other fillers to tailor electrical or mechanical properties.  
      The second and largest set of high performance materials is comprised of blends containing any number of thermoset resins filled with ceramic or other fillers to enhance the electrical or mechanical performance, usually with E or S woven glass reinforcement. The types of resins used can include epoxies, various butadienes and other hydrocarbon resin systems, CE (cyanate ester), polyimide, BT (bismaleimide triazine), PPE (polyphenylene ether), PPO (polyphenylene oxide), either alone or in blends to suit the desired electrical/mechanical properties.  
      A third class of materials would include Liquid Crystal Polymer based materials. Even though we are using thin Type II LCP based materials for the flex portion of the rigid-flex part, these same materials can be used for the rigid portion of the rigid-flex as well, making a homogeneous LCP rigid-flex board. Type II LCP are primarily used today, although higher melt point Type III LCP materials are being vigorously developed and would be quickly adopted for use in this type of application.  
      Any of these materials may be used in the method and apparatus for forming high layer count multi-layered circuits as described in the attached pending patent application. The material stack ( 24 ) may comprise a high number of insulating layers comprised of any of the high frequency, high speed materials described above wherein each of the insulating layers has patterned circuit features in attached conductive foils contained thereon. (See paragraphs 30-33 of the detailed description of the attached pending patent application).  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will hereinafter be described in conjunction with the appended drawing figures, wherein like numerals denote like elements, and:  
       FIG. 1  is a perspective view of the apparatus of the present invention for forming multi-layer circuits before applying pressure to the material stack;  
       FIG. 2  is a perspective view of the apparatus of the present invention for forming multi-layer circuits after applying pressure to the material stack;  
       FIG. 3  is an exploded view of an exemplary embodiment of a material stack prior to loading it into the opening of the fixture;  
       FIG. 4  is a top plan view of the material stack positioned within the opening of the fixture prior to applying pressure to the material stack;  
       FIG. 5  is a cross-sectional view of the apparatus of the present invention before applying pressure to the material stack;  
       FIG. 6  is a cross-sectional view of the apparatus of the present invention after applying pressure to the material stack;  
       FIG. 7  is a cross-sectional schematic showing an exemplary embodiment of a portion of the apparatus of the present invention;  
       FIG. 8  is a schematic showing an exemplary embodiment of the present invention in which the fixture and material stack are enclosed within a vacuum bag prior to applying pressure to the material stack;  
       FIG. 9  is a flowchart depicting an exemplary embodiment of the method of the present invention for forming multi-layered circuits;  
       FIG. 10  is a flowchart depicting another exemplary embodiment of the method of the present invention for forming multi-layered circuits;  
       FIG. 11  is a flowchart depicting an exemplary method for custom creating the fixture of the present invention;  
       FIG. 12  is a flowchart depicting an exemplary process for completing the production of a multi-layered circuit in accordance with the present invention; and  
       FIG. 13  is a flowchart depicting an exemplary process for desmear via preparation in accordance with the present invention prior to plating the inside of the vias. 
    
    
     DETAILED DESCRIPTION  
      Methods and apparatus in accordance with the present invention generally provide a method and apparatus for forming high layer count multi-layered circuits comprising liquid crystalline polymer (LCP) insulating layers where there is minimal feature shifting of the layers and uniform thickness of the high layer count multi-layer circuit. The subject invention is specifically directed to a press having top and bottom platens capable of applying pressure to a material stack located between the platens and a fixture positioned between the platens where the fixture contains an opening in which to position the material stack before applying pressure. It should be understood by those skilled in the art that any type of press may be used in accordance with the invention. For example, the press may be a mechanical/electrical press, a mechanical hot oil press, a mechanical hot steam press, or any other type of press that is capable of applying pressure to a material stack.  
       FIGS. 1 and 2  show perspective views of an exemplary embodiment of the apparatus  10  of the present invention for forming high count multi-layer circuits both before and after the apparatus is used to apply pressure to a material stack, i.e. a stack of liquid crystalline polymer insulating layers each having patterned circuit features in the attached conductive foil contained thereon. Apparatus  10  generally includes a press  12  having a top platen  14  and a bottom platen  16 , a top caul plate  18 , a bottom caul plate  20 , and a fixture  22  having an opening therein for placement of a material stack  24 . As previously stated, the material stack  24  may comprise a high number of liquid crystalline polymer insulating layers each having patterned circuits in the attached foil features. In addition, a liquid crystalline polymer adhesive layer may be placed between each of the liquid crystalline polymer layers to create the material stack which is then pressed and laminated to form a high layer multi-level circuit.  
      As shown in  FIG. 1 , material stack  24  is positioned within the opening of fixture  22 , and extends in height above the height of fixture  22 , prior to applying pressure to material stack  24 . During lamination (when pressure is applied to the material stack  24  by press  12 ), fixture  22  functions to limit the lamination pressure. Limiting lamination pressure on material stack  24  with fixture  22  during lamination provides a uniform pressure which in turn provides a laminated package having a uniform thickness and minimal shifting of features on the various layers comprising the material stack.  FIG. 2  shows the apparatus of the present invention and material stack  24  after applying uniform pressure to material stack  24  using fixture  22 . Material stack  24  is now a laminated package having a thickness or height equal to or less than the thickness or height of fixture  22 .  
      Lamination of the material stack is done by bonding the layers of the material stack with heat and pressure. It should be noted that platens  14  and  16  also provide a heat source for elevating the temperature of the layers in the material stack. When using liquid crystalline polymer layers in material stack  24 , lamination temperatures and pressures are selected to bond the layers together and the temperature used is less than the temperature at which the liquid crystalline polymer layers and any conductive layer (such as copper) deteriorate. Lamination may be performed with heated rolls or presses, used in combination with the fixture, to bond the layers in the material stack.  
      Turning now to  FIG. 3 , an exploded view of an exemplary embodiment of a material stack is shown prior to loading it into the opening of fixture  22 . The material stack shown in  FIG. 3  includes alternating layers of liquid crystalline polymer  26  and adhesive  28 . In one exemplary embodiment of the invention, the fixture thickness is 0.097 inches. When using a sixteen layer stack having a thickness of about 0.992 inches like that depicted in  FIG. 3 , the laminated package that is produced from applying pressure to the material stack contained within the fixture results in a package having a uniform thickness of about 0.0376 inches.  
       FIG. 4  shows a top plan view of material stack  24  positioned within the opening of fixture  22  prior to applying pressure to material stack  24 . Fixture  22  includes an opening  23  and is preferably made of a metal material other than aluminum, and more preferably a hard metal material such as stainless steel, copper, titanium, or any other metal material that can withstand a temperature of 550° F. In the exemplary embodiment of fixture  22  shown in  FIG. 4 , fixture  22  comprises a rectangular shape having opening  23  with a slot  30  connecting opening  23  with the exterior of fixture  22 . Slot  30  is for receiving and retaining a thermocouple wire from a thermocouple which is used to attach the thermocouple to a material stack  24  contained within opening  23  of fixture  22 . In the exemplary embodiment shown in  FIG. 4 , there is a clearance of 0.5 to 1.0 inches between the fixture  22  and the material stack  24 .  
       FIGS. 5 and 6  show cross-sectional views of the fixture  22  and material stack  24  after being prepared for the application of heat and pressure. These figures will be discussed in more detail after describing the stack material preparation and lay-up with the fixture and the bagging of the prepared stack materials, both of which are carried out prior to lamination of the material stack.  
      A cross-sectional schematic showing an exemplary embodiment of a portion of the apparatus of the present invention is shown in  FIG. 7 . A product stack is stacked for lamination by first placing a second bottom caul plate  36  (i.e. a different caul plate than that described with reference to  FIG. 1 ) on the lay up surface (the surface which is being used to prepare the stack). A first bottom separator plate  38  is then placed on top of the second bottom caul plate  36 . The fixture  22  (See  FIG. 1 ) is then placed on the first bottom separator plate  38  and a few pins are inserted into the fixture, second bottom caul plate  36 , and first bottom separator plate  38  to hold the fixture in place. The product layers are then positioned within the opening  23  of the fixture  22  (See  FIG. 4 ). The lamination book  40  shown in  FIG. 7  includes the fixture and the product layers. The product layers may be liquid crystalline polymer layers which may alternate with adhesive layers as shown in  FIG. 3 . A thermocouple wire  39  is then installed close to the middle of the product layers (See  FIG. 8 ) and attached with Kapton tape. High temperature thermocouple wire is used and it is not placed near any part of the circuitry of the board. A second top caul plate  42  is then placed over the product layers and a first bottom separator plate  44  is then placed on the second top caul plate  42 . Any remaining tooling pins are then installed to ensure that the fixture, second top and bottom caul plates  42  and  38 , and first top and bottom separator plates  36  and  44  are seated in place. Second top and bottom caul plates  42  and  38  and first top and bottom separator plates  44  and  36  are preferably comprised of any strong metal material that can withstand temperature of 550° F. and be able to maintain its shape and form.  
      Next, the stack material prepared and set up in accordance with the preceding paragraph is placed in a vacuum enclosure. First, a bagging material  46  and a breather material  48  are cut such that they are approximately six inches wider than the second top and bottom caul plates  42  and  38  and the first top and bottom separator plates  44  and  36 . In one exemplary embodiment, the bagging material  46  and breather material  48  for a twelve inch by eighteen inch stack should be eighteen inches wide and forty-two inches long. A skived Teflon release sheet  50  is cut three inches larger than the second top and bottom caul plates  42  and  38  and the first top and bottom separator plates  44  and  36 . In one exemplary embodiment, the skived Teflon release sheet  50  should be cut to fifteen inches by twenty-one inches where the second top and bottom caul plates  42  and  38  and the first top and bottom separator plates  44  and  36  are twelve inches by eighteen inches.  
      The skived Teflon release sheet  50  is placed on a second bottom separator plate  52  and the bagging material  46  is placed on the skived Teflon release sheet  50 . Breather material  48  is placed on bagging material  46  and a second skived Teflon release sheet  54  is placed on the breather material  48 . A sealant tape  56  (See  FIG. 8 ) is applied to the bagging material  46  near the edge of the bagging material  46  and under the thermocouple wire  39 . The sealant tape  56  is wrapped around the thermocouple wire  39  aligning the sealant tape  56  with the sealant tape  56  on the edge of the bagging material  46 . Sealant tape  56  is then wrapped around a copper tubing  58  approximately four inches from the end of a quick disconnect  60  (See  FIG. 8 ). The copper tubing  58  is positioned along side of the lamination book  40  aligning sealant tape  56  on the edge of the bagging material  46  with the sealant tape  56  wrapped around the copper tubing  58 .  
      The third skived Teflon release sheet  62  is placed on top of first top separator plate  44  and the flaps of the breather material  48  and bagging material  46  are then folded over the lamination book  40  and the third skived Teflon release sheet  62 . The bagging material  46  is then aligned evenly and sealant tape  56  is used to close the bag. A fourth skived Teflon release sheet  64  is placed over the top of the bag and a second top separator plate  66  is placed over the fourth skived Teflon release sheet  64 . The entire prepared, stacked, and bagged assembly as described in the preceding paragraphs is then loaded into the press  12  between top and bottom caul plates  18  and  20  and top and bottom platens  14  and  16  shown in  FIG. 1 . A vacuum is applied to the sealed bag containing the material stack before applying the press.  
      Returning to  FIGS. 5 and 6 , the material stack  24  is shown having a thickness greater than the fixture  22  prior to lamination in  FIG. 5  and in  FIG. 6  the material stack  24  has a thickness equal to or less than the fixture  22  after lamination.  
      Turning now to  FIG. 9 , a flowchart  70  is shown which depicts an exemplary embodiment of the method of the present invention for forming high count multi-layered circuits. In step  71 , a press or at least one roller is provided which is capable of applying pressure and heat to a material stack. Next, in step  72 , a fixture having an opening is positioned within the press or under a roller and in step  73  a material stack is placed within the opening in the fixture. Pressure is applied to the material stack in step  74  to laminate the material stack and the process ends in step  75 . Heat may also be applied along with applying the pressure.  
       FIG. 10  is a flow chart  80  depicting another exemplary embodiment of the method of the present invention for forming high count multi-layered circuits. In step  81 , a fixture is created having an opening therein in which to place a material stack. In step  82 , a press is provided for applying pressure and heat to the material stack and in step  84  the fixture created in step  81  is positioned within the press. Optional steps  83  and  85  may also occur in which a pair of caul plates (step  83 ) and separator plates (step  85 ) are placed between platens in the press and the fixture is then positioned between the separator plates of the press in step  84 . In step  86 , a material stack is positioned within the opening contained within the fixture. Next, in step  87 , the fixture and the material stack are placed in a vacuum bag and a vacuum is applied in step  88 . Pressure is then applied in step  89  or, alternatively, heat and pressure are applied in step  90  in order to laminate the material stack. The process then ends in step  91 .  
      Turning now to  FIG. 11 , a flow chart  100  is shown which depicts an exemplary method for custom creating a fixture in accordance with the present invention. In step  102 , the customer presents their specifications for a high count multi-layer circuit and customer service takes in those specifications in step  104 . In step  106 , a contract is reviewed in which the customers specs are contained and product engineering begins in step  108  to meet product specifications communicated by the customer. Prints and customer specs are again checked in step  110 . If the prints and customer specs are not okay, these problems are discussed with the customer in step  111 . After customer problems are discussed in step  111 , results of those discussions are then forwarded to customer service in step  104  to begin the process once again. If, in step  110 , the prints and customer specs are acceptable, then further planning and generation of traveler documents (reference documents) are created in step  112 . Data is then released to the cam in step  114  and the customer data is checked in step  116 . Document control takes place in step  118  and an inquiry as to the acceptability of the data is undertaken in step  120 . If there are data problems, customer service is contacted in step  121  and problems are again discussed with the customer in step  111 . If no data problems are present in step  120 , working tools are developed in step  122 . As part of that process, a photo plot is undertaken in step  124  and an artwork inspection is performed in step  126 . Image problems are encountered in step  128  and, if there are image problems, there is a return to step  122  in which work tools are again developed. If no image problems exist in step  128 , silver T/U diazo tooling occurs in step  130  and a traveler document (reference document) is released in step  140 . Alternatively, working tools are developed by first developing programming in step  132 , drilling and routing in step  134 , obtaining drilling/routing approval in step  136 , and then inquiring as to whether there are any first article problems in step  138 . If there are first article problems in step  138 , there is a return to step  132  to redevelop programming. If there are no first article problems in step  138 , a traveler document (reference document) is released in step  140 . After the traveler document is released in step  140 , document control is performed in step  142  and the document is released to production in step  144 . If problems or changes occur during production, traveler change notices and other standard operating procedures may be created in step  146  before further planning and generation of a traveler document (reference document) is generated in step  112 .  
       FIG. 12  is a flow chart  200  depicting an exemplary process for completing the production of a high count multi-layered circuit in accordance with the present invention. After lamination in step  202 , drilling takes place in step  204  and deburring takes place in step  206 . Desmear hole prep takes place in step  208 , electroless copper is applied in step  210  and flash copper plating takes place in step  212 .  FIG. 13  is a flow chart  208  depicting an exemplary process for desmear via hole preparation in accordance with the present invention prior to plating the inside of the vias. In step  302 , the laminated panel is dipped in a permanganate solution which comprises 55-65 grams per liter of potassium permanganate in a 5-7% sodium hydroxide solution. The panel is dipped for five minutes at a minimum of 175° F. In step  304  the panel undergoes a water rinse in ambient water temperature for approximately 2-3 minutes. In step  306 , another water rinse is conducted at ambient water temperature for 5-7 minutes. The panel is then neutralized in step  308  in a neutralizer solution containing 9-11% neutralizer and 8-10% sulfuric acid. The panel is dipped in the neutralizer for approximately four minutes at a temperature of 105° F. The panel then undergoes a water rinse at ambient temperature for 3-5 minutes in step  310  and another water rinse at ambient water temperature for 3-5 minutes in step  312 . After step  312 , the electroless copper step  210  is conducted as previously described in relation to  FIG. 12 .  
      It will be understood that the foregoing description is of preferred exemplary embodiments of the invention and that the invention is not limited to the specific forms shown or described herein. Various modifications may be made in the design, arrangement, and type of elements disclosed herein, as well as the steps of making and using the invention without departing from the scope of the invention as expressed in the appended claims.