Abstract:
Mechanisms for interconnecting and distributing signals and power between PCBs are provided. A first PCB having land grid arrays (LGAs) and a first wiring layer designed for interconnect components on the first PCB, and a second wiring layer for connecting the components to a second PCB, are provided. The second PCB has opposed parallel first and second surfaces, the first surface having a LGA. A wiring layer designed to interconnect components on the second PCB, and a layer for interconnecting the components on the second PCB with the components on the first PCB, are provided. A first interposer couples to a LGA of a first surface of the first PCB and connects a component to the first PCB. A second interposer is sandwiched between and couples to a LGA of a second surface of the first PCB and to the LGA of the first surface of the second PCB.

Description:
This application is a divisional of application Ser. No. 12/579,051, filed Oct. 14, 2009, status pending. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to printed circuit boards used in large computing systems, and in particular to a motherboard assembly having stacked land grid array (LGA) connectors for interconnecting and distributing signals and power therein. 
     2. Description of the Related Art 
     The electronics industry widely uses electrical connectors. In many computers and other electronic circuit structures, an electronic module or chip, such as a central processor unit (CPU), memory module, application-specific integrated circuit, or other integrated circuits having a module substrate, connects to a printed wiring board using an electronic connector. Printed wiring boards are also known as printed circuit boards (PCBs) and etched wiring boards. A printed circuit board is commonly referred to as a printed circuit board assembly (PCBA) when it is populated with one or more electronic modules. When a PCBA is used as the central PCB in a complex electronic system, it is commonly referred to as a motherboard. To connect an electronic module to a PCB or motherboard, a plurality of individual electrical contacts on the base of the electronic module must connect to a plurality of corresponding individual electrical contacts on the PCB/motherboard. 
     When an LGA connector connects the electronic module to the PCB, this set of contacts is known as land grid arrays (LGAs). A LGA is a type of surface-mount packaging where there are no pins on the module. Rather, in place of pins are pads of gold-plated copper, for example, that couple to pads on the PCB (e.g., motherboard). Rather than permanently soldering the electronic module contacts to the LGA site, it is desirable to use LGA connectors that allow the user to install and remove the electronic module to/from the LGA site. LGA connectors provide the user with the flexibility to upgrade or replace electronic modules during the manufacturing cycle and in the field. LGA connectors are also known in the art as sockets, interconnects, interposers, carriers, and button board assemblies. In general, LGA connectors provide electrical connections between two parallel electrical substrates in computing equipment through the use of an interposer. Typically, one of these substrates is a PCB (e.g., motherboard) and the other is an electronic module, which may, for example, have either a ceramic or organic laminate substrate. 
     PCBs typically include multiple conductive layers laminated with insulating plastic there between. The conductive layers are typically reserved for power, power return, ground, and signals. The layers reserved for signals contain etchings to form “traces” that conduct the signals. The layers reserved for power, power return and ground are typically referred to as “power planes”, “power return planes”, and “ground planes”. The conductive layers connect together by drilling holes called vias and then plating each via with a conductor to form a plated-through-hole (PTH). 
     In large symmetric multiprocessing (SMP) computer systems, it is advantageous to package as much of the system as possible within a single rack drawer. To maximize component density as a function of printed circuit board density, it is desirable to package an entire system on a single printed circuit motherboard.  FIG. 1  is a top perspective view of motherboard  100  having many modules, including high power modules  102 , such as CPUs, associated memory modules  104 , as well as “concentrator” modules  108  such as hubs  1 - 8 . Unfortunately, the ability to manufacture a large enough motherboard that is capable of placing all such modules shown on motherboard  100  is limited by the existing standard tooling available by printed circuit board manufacturers. Generally, the larger the PCB, the smaller the yield because there are more risk sits on the PCB. At some point, the cost to manufacture a PBC of a certain size or larger becomes prohibitive. For example, a vendor with large panel capability can manufacture PCBs with active areas up to a dimension of 30.35″ by 22.87″, but no larger. Because motherboard  100  requires a dimension larger than this, PBC vendors are not tooled to manufacture motherboard  100 . It is simply too large. 
     Alternatively, to generate an assembly having a large enough surface area, but which can be manufactured using standing tooling, motherboard  100  may be divided into two coplanar printed circuit boards and connected using a connector, such as a right angle-to-right angle or coplanar connector. Coplanar PBCs mean two or more boards lying in the same plane.  FIG. 2  is a top view perspective of an illustrative conventional PCBA  200  having high power modules  202  positioned on PCB  204 , and concentrator modules  208  positioned on PCB  210 . Right angle interconnect  206  connects coplanar PCBs  204  and  210  such that they function as a single motherboard. Because PCBs  204  and  210  are each much smaller in terms of X,Y dimension than motherboard  100 , they may be manufactured using standard large panel tooling. Unfortunately, however, the system drawer (not shown) housing PCBA  200  must grow to accommodate the width of right angle interconnect  206  as an addition to the card surface area. Moreover, the use of interconnect  206  may degrade signal integrity because signals traveling between PCBs  204  and  206  must pass through it. 
     It should therefore be apparent that a need exists for an enhanced mechanism for interconnecting and distributing signals and power between coplanar boards in large systems that consume minimal additional surface area. 
     SUMMARY 
     A computing system, method, and motherboard assembly are described for interconnecting and distributing signals and power between co-planar boards that function as a single motherboard. The motherboard assembly includes a multilayered first printed circuit board having opposed parallel first and second surfaces, each having at least one land grid array (LGA) disposed thereon. The assembly further includes at least two wiring layers (Y and Z) designed to only electrically interconnect components on or within the first PCB, and at least two wiring layers (X and W) designed to only electrically connect the components on the first PCB to a multilayered second PCB. The multilayered second PCB has opposed parallel first and second surfaces, the first surface having at least one LGA disposed thereon. It further includes at least one wiring layer (V) designed to only electrically interconnect components on or within the second PCB, and at least two layers (X and W) designed to only electrically interconnect the components on the second PCB with the components on the first PCB. A first LGA interposer couples to the LGA disposed on the first surface of the first PCB, and electrically connects at least one component to the first PCB. A second LGA interposer is sandwiched between and couples to the LGA disposed on the second surface of the first PCB and to the LGA disposed on the first surface of the second PCB. It electrically connects the first PCB to components on the second PCB, such that the first and second PCB function as a single motherboard. The first and second LGA interposers route only signals on the X and W wiring layers between the first and second PCBs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view perspective of a conventional motherboard  100  having many modules disposed thereon. 
         FIG. 2  is a top view perspective of a conventional PCBA having two PCBs connected using a right angle interconnect. 
         FIG. 3  illustrates a top view perspective of a motherboard assembly having two co-planar PCBs electrically connected using double stacked LGAs according to an embodiment of the present invention. 
         FIG. 4  illustrates a cross sectional view of the motherboard assembly shown in  FIG. 3  at the LGA sites. 
         FIG. 5  illustrates the wiring required to interconnect modules of motherboard assembly  300 . 
         FIG. 6  illustrates a top view perspective of vias and pads used in a PBC according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the preferred embodiments of the present invention, a computer system and motherboard assembly are described for interconnecting and distributing signals and power between co-planar boards that function as a single motherboard.  FIG. 3  illustrates a top view perspective of motherboard assembly  300 , which may be used in a computing system, comprising two co-planar PCBs  302  and  308  electrically interconnected using double stacked LGAs  306 . In one embodiment, PCB  302 , which in this embodiment is the “bottom” board, has multiple LGAs on its “top” surface to facilitate electrical communication with high power modules  304  (such as central processing units (“CPUs”)  1  through  8 ) and associated memory modules  320 , and also in region  318  to facilitate communication with PCB  308 . In a second embodiment, PCB  302  is alternatively divided into four separate co-planar PCBs  310 ,  312 ,  314 , and  316 . CPUs  1 - 2 , a portion of associated memory modules  320 , and two LGAs  306  position on the top surface of PCB  310 , CPUs  3 - 4 , a portion of associated memory modules  320 , and two LGAs  306  position on the top surface of PCB  312 , and so on. One skilled in the art will recognize that other combinations of PCBs could be used as well. 
     PCB  308 , which in this embodiment is the “top” board, positions various other modules thereon, such as hubs  1 - 8 , and includes functionality such as PCI Express interconnects  1 - 18  and other links  324 . Other or different modules and functionality—collectively referred to as “components”—may be positioned/incorporated on PCB  308  as well. PCB  308  is referred to as the “top” board because it overlaps PCB  302  (or alternatively PCBs  310 - 316 ) at region  318 , which is the location of a plurality of double stacked LGAs  306 . Double stacked LGAs  306  electrically interconnect signals and power between PCBs  302  and  308 , such that they function as a single motherboard. In region  318 , PCB  308  has LGAs disposed on its “top” surface to electrically interconnect with concentrator modules  322  (e.g., hubs  1 - 8 ), and LGAs disposed on its “bottom” surface (which is parallel to the “top” surface) to electrically interconnect with PCB  302 . As such, the board-to-module interconnect area overlaps the board-to-board interconnect area in region  318 . For large SMP computing systems, the density of placed components can be maximized without requiring the development of a printed circuit motherboard beyond the tooled capabilities of printed circuit board vendors. 
       FIG. 4  illustrates a cross sectional view of motherboard assembly  300  in a portion of region  318 , which contains double stacked LGAs  306 . For ease in explanation, only one concentrator module  322  is shown. Motherboard assembly  300  includes heat sink  402  (not shown in  FIG. 3 ) for providing heat transfer functions, pressure plate  404  (not shown in  FIG. 3 ) for applying compression to the stacked components, concentrator module  322  (which illustratively may be one of hubs  1 - 8  shown in  FIG. 3 ), LGA interposers  408  and  412 , PCBs  302  and  308 , and stiffener  416 . 
     Concentrator module  322  has electrically conductive LGA pads (not shown) disposed on its bottom surface, at  406 , for electrically connecting to PCB  308  via LGA interposer  408 . To do so, LGA interposer  408  also has: i) electrically conductive pads disposed on its top surface, at  406 , for coupling (mating) to the conductive pads of module  322 , and ii) a bottom surface, parallel to its top surface, having conductive pads, at  410 , for coupling to an LGA  306  disposed on the top surface of PCB  308 . The electrically conductive pads disposed on the top and bottom surfaces of LGA interposer  408  connect via copper contacts (not shown) disposed within LGA interposer  408 . Typically, an LGA interposer, such as LGA interposers  408  and  412 , is a molded insulator or thin polymer insulator. A plurality of LGA contacts are press-fit into holes drilled in the molded or thin polymer insulator. These LGA contacts may be, for example, of a press-fit design, surface mount design, and/or friction fit design (e.g., waded wire buttons or molded metal filled elastomer contacts). 
     As previously described, PCB  308  is the “top” board, and has gold plated conductive LGA pads disposed on both its top surface at  410  and parallel opposing bottom surface at  414  (described in more detail in  FIG. 6 ) for coupling (mating) with LGA pads on LGA interposers  408  and  412 , respectively. PCB  302  is the “bottom” board and also has gold plated conductive LGA pads on its top surface, at  418 , to electrically couple with LGA pads on the bottom surface of LGA interposer  412 . Accordingly, LGA interposer  412  is sandwiched between PCB  308  and PCB  302  to electrically interconnect them. Further, PCB  308  is sandwiched between module  322  and PCB  302 , with LGA interposers  408  and  412  providing the respective connectivity. The average static forces through LGA interposers  408  and  412  are substantially the same and are set by conventional load screw and springs (not shown). The flatness of each PCB  308  and  302  divides between LGA interposers  408  and  412  such that the contact load distribution is no worse than expected with a conventional single LGA. Stiffener  416  positions to the bottom surface of PCB  302  to provide rigidity support, and is preferably a metal or steel plate. 
     The wiring layers in PCBs  302  and  308  required to interconnect the various component impact the thickness of PCBs  302  and  308 .  FIG. 5  illustrates the wiring layers required to interconnect components of motherboard assembly  300 . Referring to  FIGS. 3 and 5 , the wiring layers for transmitting signals between concentrator modules  322  are denoted by ‘Y’ layers, and between concentrator modules  322  and other concentrator modules and functionality (collectively referred to as components)  324  are denoted by ‘Z’ layers, both of which reside only in the top board (i.e., PCB  308 ). Accordingly, because layers Y and Z are not included in the bottom board (i.e., PCB  302 ), signals on those layers are not routed through double stacked LGAs  306 . Similarly, the wiring layers required to connect high power modules  304  to other components (e.g., memory modules  320 ) positioned on the bottom board (i.e., PCB  302 ) are denoted as ‘V’ layers, which also need not be routed through double stacked LGAs  306  because V layers are not included in the top board (i.e., PCB  308 ). The wiring layers required to transmit signals between high power modules  304  on the bottom board (i.e., PCB  302 ) and: i) the concentrator modules (e.g., hubs  1 - 8 ) on the top board (i.e., PCB  308 ) are denoted by ‘X’ layers; and ii) other concentrator modules and functionality  324  on the top board (i.e., PCB  308 ) are denoted by ‘W’ layers. Signals on layers X and W are transmitted between PCBs  302  and  308  using the double stacked LGAs  306 . 
     The wiring layers in PCBs  302  and  308  required to interconnect the various component impact the thickness of PCBs  302  and  308 .  FIG. 5  illustrates the wiring layers required to interconnect components of motherboard assembly  300 . Referring to  FIGS. 3 and 5 , the wiring layers for transmitting signals between concentrator modules  322  are denoted by ‘Y’ layers, and between concentrator modules  322  and other concentrator modules and functionality (collectively referred to as components)  324  are denoted by ‘Z’ layers, both of which reside only in the top board (i.e., PCB  308 ). Accordingly, because layers Y and Z are not included in the bottom board (i.e., PCB  302 ), signals on those layers are not routed through double stacked LGAs  306 . Similarly, the wiring layers required to connect high power modules  304  to other components (e.g., memory modules  320 ) positioned on the bottom board (i.e., PCB  302 ) are denoted as ‘V’ layers, which also need not be routed through double stacked LGAs  306  because V layers are not included in the top board (i.e., PBC  308 ). The wiring layers required to transmit signals between high power modules  304  on the bottom board (i.e., PCB  302 ) and: i) the concentrator modules (e.g., hubs  1 - 8 ) on the top board (i.e., PCB  308 ) are denoted by ‘X’ layers; and ii) other concentrator modules and functionality  324  on the top board (i.e., PCB  308 ) are denoted by ‘W’ layers. Signals on layers X and W are transmitted between PBCs  302  and  308  using the double stacked LGAs  306 . 
     As can be seen, the number of layers in the bottom board (i.e., PCB  302 ) and top board (PCB  308 ) may be reduced, while a conventional single motherboard used in this manner would require more layers. The yield of those smaller boards will be improved due to the significant reduction in risk sites compared with a single printed circuit board. 
       FIG. 6  illustrates a top view perspective of the top surface of PCB  308  at region  318  in  FIG. 3 , which shows a representative LGA  306  disposed on PCB  308 . This view also illustrates the opposing LGA  306  on the bottom surface of PCB  308  at region  318 , which is parallel and opposed to the top surface. LGA  306  includes a plurality of electrically conductive pads  602  having, with the exception of pads  608 , holes or vias  604  drilled into PCB  308  to electrically connect top and bottom opposed pads  602  together at each x, y location. This allows module  322 , for example, to electrically connect to PCB  302  through pairs of pads  602  disposed on both surfaces of PCB  308 . On the other hand, at each pad  608  on the top surface of PCB  308 , a “dogbone” trace may be made from such top pad to via  606 . Similarly, at each pad  608  on the corresponding opposed bottom surface of PCB  308 , another “dogbone” trace may be made from such bottom pad to via  610 . Vias  606  and  610  do not overlap and may terminate at different layers within PCB  308 , such that opposed top and bottom pad pairs  606  do not electrically connect through PCB  308 . This enables module  322 , for example, to electrically interconnect to a particular layer within PCB  308  at a particular upper pad  606 , while at the opposed bottom pad  606 , PCB  302  may electrically interconnect to a different layer within PCB  308 . 
     In summary, the present embodiment interconnects components via two coplanar PCBs, functioning as a single motherboard, using double stacked LGA interposers. One skilled in the art will appreciate that many variations are possible within the scope of the present embodiment. Thus, while the present invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that these and other changes in form and detail may be made therein without departing from the spirit and scope of the present invention.