Patent Publication Number: US-8119922-B2

Title: Dual cavity, high-heat dissipating printed wiring board assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 11/567,906, filed on Dec. 7, 2006, now U.S. Pat. No. 7,444,737, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention is directed to printed wiring board assemblies and more particularly, to a high-heat dissipating RF antenna assembly using a printed wiring board. 
     2. Related Art 
     The constant demand for faster communication systems and more sensitive radar systems is pushing the phased array industry to move designs to accommodate higher power levels and higher frequencies. This demand makes it difficult to produce phased arrays inexpensively and compactly. For low-power, low-frequency applications, Chip-on-Board (COB) technology provides an affordable solution. However, for high-power phased arrays, which are critical components of high-speed long-range communications and state of art radar applications, COB technology can no longer provide a satisfactory solution due to its limited power handling capability. The limiting factor for highly sensitive radars and long-range communications antenna systems is their higher power output requirements. Typically, to deliver the required power, traditional antenna systems rely on using exotic ceramic materials and complex multi-part assemblies. These materials are also expensive and require long manufacturing lead times. 
     Therefore, what is needed is an apparatus and associated method that allows the manufacture of low cost high-heat dissipating phased array assembly. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a method for manufacturing dual cavity, high-heat dissipating printed wiring board assembly is provided. This method for manufacturing the printed wiring board comprises: providing a first laminate stackup and a second laminate stackup, having a plurality of conductive and dielectric layers, including vias within each stackup, cutouts and alignment holes through each stackup; attaching a heatsink and standoffs to the first stackup; routing out an anisotropic conductive film (ACF) sheet; placing and aligning the ACF sheet over the lower stackup; inverting and assembling upper stackup onto lower stackup; and curing the ACF to join the stackups together. 
     In another aspect of the present invention, a structure for a dual cavity, high-heat dissipating printed wiring board assembly is provided. The structure for the printed wiring board assembly comprises: an upper and a lower laminate stackup, each having a first side and a second side, and at least one cutout area, wherein the cutout area of the upper stackup extending from the first side to the second side of the stackup; an anisotropic material layer disposed between the two stackups providing mechanical and electrical connection between the stackups; and a heatsink laminated to the upper stackup. 
     This brief summary has been provided so that the nature of the disclosure may be understood quickly. A more complete understanding of the invention may be obtained by reference to the following detailed description of embodiments thereof in connection with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features and other features of the present disclosure will now be described with reference to the drawings. In the drawings, the same components have the same reference numerals. The illustrated embodiment is intended to illustrate, but not to limit the invention. The drawings include the following Figures: 
         FIG. 1  is a simplified cross sectional view of a conventional COB package for an RF antenna application; 
         FIG. 2  shows a simplified cross sectional view of a dual cavity high heat dissipating printed wiring board assembly, in accordance with an embodiment of the present disclosure; 
         FIG. 3A  shows a simplified cross sectional view of a unit cell of a RF antenna assembly using a dual cavity high heat dissipating printed wiring board, in accordance with an embodiment of the present disclosure; 
         FIGS. 3B and 3C  show a simplified bottom view of an upper and top view of a lower board of a 16 cell RF antenna assembly, in accordance with an embodiment of the present disclosure; and 
         FIG. 4  is a flowchart showing a method of producing a dual cavity high heat dissipating printed wiring board assembly in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     The following definitions are provided as they are typically (but not exclusively) used in relation to printed wiring board technology, referred to in various aspects of the present disclosure. 
     A “stackup” is an arrangement of various layers of conductive and dielectric materials stacked together to form a multi-layer board. 
     A “laminate” is a structure used to unite conductive and dielectric layers together. 
     A “honeycomb” is a circular cell configuration including the antenna radiating aperture, which in the present application permits RF transmission from the printed wiring board to free-space while also acting as the front face of the antenna enclosure. 
     To facilitate an understanding of the embodiments of the present disclosure, the general architecture and the process of making a printed wiring board assembly suitable for RF antenna applications are described. The specific architecture and process of the present invention are described with reference to the general architecture and process. 
       FIG. 1  is a simplified cross sectional view of module  100 , having a traditional printed wiring board (PWB)  102  with a COB structure  104 , a heat sink  110 , and a seal ring  112 . As shown in  FIG. 1 , module  100  combined together with heat sink  110  and seal ring  112  defines an enclosed space  111 . Seal ring  112  is coupled to PWB  102  around a periphery of ICs or chipset  106 . Heatsink  110  is attached to seal ring  112  to enclose module  100  and create enclosed space  111 . It should be understood that each individual module  100  is sealed, even when combined with other modules to create large PWBs containing multiple sites. 
     A number of ICs or chipsets  106 , such as ASICs and a MMICs are enclosed in enclosed space  111  between PWB  102  and heat sink  110  and encircled by seal ring  112 . In this example, ICs  106  are disposed horizontally on PWB  102  alongside one another inside enclosed space  111 . The ICs  106  are electrically coupled together using PWB  102  and a plurality of wirebonds  114 . Generally, module  100  is not suitable for high power applications, because its heat evacuation capability is limited, primarily since ICs  106  are not in direct contact with heatsink  110 . 
       FIG. 2  shows a cross sectional view of a dual cavity high heat dissipating PWB assembly  200  (hereinafter “assembly  200 ”) in accordance with an embodiment of the present disclosure. In this embodiment, assembly  200  includes a first PWB assembly  202  and a second PWB assembly  206  formed to define an enclosed space  213 . As described below, enclosed space  213  results from the joining of the two fully assembled PWB assemblies  202  and  206 , using a conductive epoxy  204 , such as Anisotropic Conductive Film sheet (hereinafter “ACF  204 ”). ACF sheet  204  provides a connection for RF and DC signals, without the need for other interconnect means, such as wirebonds, mechanical interconnects or solder balls. 
     In one embodiment, first PWB assembly  202  is designed to provide superior heat dissipation for high power ICs. In this embodiment, first PWB assembly  202  includes; a multilayer stackup  203 , a heatsink  208 , and a standoff  210 . Stackup  203  includes a cutout portion  220  and has heatsink  208  laminated on its top side. Standoff  210  is disposed inside cutout portion  220  and is attached to heatsink  208 . Standoff  210  may be used to elevate high power ICs, such as IC  212 , to bonding-pedestal  218  and provides a heat conduction path from IC  212  to heatsink  208 . 
     In one embodiment, the structure of second PWB assembly  206  includes a traditional COB structure, the nature of which is known in the art. In this embodiment, ICs  214  and  216  are disposed horizontally alongside one another in enclosed cutout portion  222 . 
     As understood from  FIG. 2 , first PWB assembly  202  and second PWB assembly  206  are joined such that cutout portions  220  and  222  are combined to define and create enclosed space  213 , in which ICs  212 ,  214  and  216  are disposed.  FIGS. 3A ,  3 B and  3 C show three different views of a high power RF antenna assembly  300  (hereinafter “assembly  300 ”).  FIG. 3A  shows a cross sectional view of a unit area of assembly  300 .  FIG. 3B  shows the bottom side&#39;s view of an upper printed wiring board assembly  302 B and  FIG. 3C  shows the top side&#39;s view of a lower printed wiring board assembly  302 C. 
     Assembly  300  includes, precut ACF sheet  304  interposed between fully populated assembly  300 B ( FIG. 3B ) and fully populated assembly  300 C ( FIG. 3C ). ACF sheet  304  joins assemblies  300 B and  300 C together mechanically as well as electrically. ACF sheet  304  provides RF and DC signal connection without the need for other interconnection means, such as wirebonds, mechanical interconnects or solder balls. 
     Assembly  300  further includes a plurality of antenna aperture structures  330  imbedded in a honeycomb structure  328 , and attached to a bottom side of assembly  300 C. In one embodiment, a pair of alignment holes  332  and alignment pins (not shown), are disposed in at least one corner of assembly  300  to ensure a proper corresponding alignment of the assemblies. 
     Assembly  300 B ( FIG. 3B ) includes; multilayer stackup  302 , heatsink  308 , standoff  310  and IC  312 , such as a high power MMIC. Stackup  302  includes cutout portion  321  and has heatsink  308  laminated on a first side  309 . A standoff  310  is disposed inside cutout portion  321  and is coupled to heatsink  308 . Standoff  310 , which may be made from any heat conductive material, such as Molybdenum, elevates IC  312  to wirebonding-pedestal  318  and provides heat conduction from IC  312  to heatsink  308 . IC  312  is wirebonded to stackup  302  at wirebonding-pedestal  318 . 
     Disposed on the bottom side of stackup  302  are arrays of transition points  334  and  336 , which provide transition points for RF signals and DC supplies, respectively. Transition points  334  and  336  are the locations where AFC sheet  304  establishes electrical contact between assemblies  300 B and  300 C. 
     Assembly  300 C ( FIG. 3C ) includes a multilayer stackup  306 , and a plurality of ICs  314  and  316 , such as ASICs and MMICs, respectively. Stackup  306  has cutout portion  320  in which ICs  314  and  316  may be positioned. For example, at cutout portion  320  ICs  314   316  may be physically coupled to stackup  306  and electrically connected to it with, for example, wirebonds. Disposed on the top side of stackup  306  are arrays of transition points  334  and  336 . 
     In one embodiment, multilayer stackups  302  and  306  may be constructed from double-sided RF grade laminate material. 
       FIG. 4  is a flowchart, illustrating a method  400  for producing a dual cavity, high-heat dissipating PWB assembly, according to one aspect of the present disclosure. 
     Referring now to  FIGS. 3A and 4 , in step S 402 , multilayer stackups  302  and  304  are provided. Stackups  302  and  304  are pre-routed in a predefined area to form cutout portions  320  and  321 . 
     In step S 404 , heatsink  308  is laminated onto stackup  302  and standoff  310  is attached to heatsink  308 . 
     In step S 406 , stackups  302  and  306  are populated with ICs  312 ,  314  and  316 . In one embodiment, ICs  314  and  316  are positioned within cutout portion  320  of stackup  306 , while IC  312  is positioned on standoff  310 . 
     In step S 408 , ACF sheet  304  is routed to remove portions of ACF sheet  304 , which overlay and correspond to cutout portions  320  and  321  and alignment holes  332 . 
     In step S 410 , alignment pins (not shown) are inserted into assembly  300 C in alignment holes  332 . ACF sheet  304  is then aligned and placed over assembly  300 C ( FIG. 3C ). 
     In step S 412 , assembly  300 B ( FIG. 3B ) is inverted, aligned, and deposited onto the ACF covered assembly  300 C. The combination of assembly  300 B and assembly  300 C to form assembly  300  define and create enclosed space  319 . As a result of the combination, ICs  312 ,  314  and  316  are positioned and enclosed within enclosed space  319 . 
     In step S 414 , assemblies  300 B and  300 C are pressed together, while ACF sheet  304  is cured under pressure at a predetermined temperature. 
     In one embodiment, thereafter, a plurality of antenna apertures  330  are imbedded in a honeycomb structure  328  and are attached to a bottom side of assembly  300 C to form assembly  300 . 
     The embodiments disclosed provide a packaging structure and method, which allows for a higher operating frequency, and higher power output. Packaging density may be doubled, while assembly complexity and manufacturing costs may be reduced and system reliability may be enhanced. The packaging structure also allows for building panel sized PWB assemblies. 
     Although the present technology has been described with reference to specific embodiments, these embodiments are illustrative only and not limiting. Many other applications and embodiments of the present disclosure will be apparent in light of this disclosure and the following claims.