Patent Publication Number: US-2006001163-A1

Title: Groundless flex circuit cable interconnect

Description:
FIELD  
      The present invention relates to integrated circuits, and more particularly, but not limited to, providing connections to integrated circuits through a groundless flex circuit cable.  
     BACKGROUND  
      Data transfer bit rates of processors are progressively increasing. In order to take advantage of these high rates, computer systems attempt to transmit signals along their buses and between system components at comparable rates.  
      Today&#39;s high data transfer rates of about 5-10 Gb/s and beyond are challenging conventional signal routing solutions, which may be constrained to a few Gb/s. Conventional signal routing solutions route signals between semiconductor packages through the printed circuit board. Signals transmitted from a driver package to a receiver package through the printed circuit board may experience a number of issues that may compromise the signal integrity. Some of these issues could include discontinuities and impedance mismatches between the packages and the printed circuit board, as well as dielectric and conductor losses through the signal path in the printed circuit board. These issues may unnecessarily constrain both the distance and the rates that the signals may be reliably transmitted.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which:  
       FIG. 1  illustrates a perspective view of a semiconductor package coupled to a flex circuit cable, in accordance with an embodiment of this invention;  
       FIG. 2  illustrates a cross-sectional view of a flexible interconnect bus of the flex circuit cable, in accordance with an embodiment of this invention;  
       FIG. 3  illustrates a transmitting signal trace pair coupling two semiconductor packages, in accordance with an embodiment of the present invention;  
       FIGS. 4   a - 4   d  illustrate various coupling embodiments using a flex circuit cable;  
       FIG. 5  illustrates a backplane arrangement utilizing flex circuit cables, in accordance with an embodiment of the present invention; and  
       FIG. 6  depicts a block diagram of a system including a flex circuit cable, in accordance with an embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION  
      A method, apparatus, and system for using a groundless flex circuit cable to interconnect semiconductor packages is disclosed herein. In the following detailed description, reference is made to the accompanying drawings that form a part hereof wherein like numerals designate like parts throughout. The drawings may show, by way of illustration, specific embodiments in which the invention may be practiced; however, it is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the embodiments of the present invention. It should also be noted that directions and references (e.g., top, bottom, etc.) may be used to facilitate the discussion of the drawings but are not intended to restrict the application of the embodiments of this invention. Therefore, the following detailed description is not to be taken in a limiting sense and the scope of the embodiments of the present invention are defined by the appended claims and their equivalents.  
       FIG. 1  illustrates a perspective view of a semiconductor package  104  coupled to a flex circuit cable  108 , in accordance with an embodiment of this invention. In this embodiment the semiconductor package  104  may include a die  112  electrically and mechanically coupled to a package substrate  116 . The flex circuit cable  108  may also be electrically and mechanically coupled to the package substrate  116  so that differential signals may be routed between the die  112  and the flex circuit cable  108  through the package substrate  116 . The flex circuit cable  108  may have a flexible interconnect bus enclosed by an insulating flexible casing material, e.g., rubber or plastic. The flexible interconnect bus may have pairs of signal traces without having a dedicated ground or power trace/plane. In one embodiment, the ground and power transfers supplied to the die  112  may be sent through the board  120 , thereby at least facilitating the use of the groundless flex circuit cable  108 . The flex circuit cable  108  could also include an end  110  to facilitate the coupling to the package substrate  116 .  
      The die  112  may include an integrated circuit formed in a piece of semiconductor material. Examples of such semiconductor material may include, but are not limited to silicon, silicon on sapphire, silicon germanium, and gallium arsenide. Examples of the die  112  may include, but are not limited to a processor (e.g., a central processing unit, a graphics processor, a digital signal processor, a network processor, etc.), an input/output device, a system on a chip (SOC), and volatile memory. In various embodiments, the semiconductor package  104  may also include more than one die attached to the package substrate  116 , for example, in a chipset configuration.  
      The package substrate  116  may be used for support, to interconnect multiple components, and/or to facilitate electrical connections with other components. The package substrate  116  may be made of one or more dielectric and/or ceramic layers. Electrically conductive paths, including a variety of traces and vias of different lengths, widths, and spacings, may be included in the package substrate  116 . These electrically conductive paths may be used to route the various signal, ground, and power paths to and from the die  112 . Electrically conductive paths may be coupled to both the flex circuit cable  108  and to a board  120 .  
      The semiconductor package  104  may be connected to the board  120  in order to interconnect multiple components such as, e.g., other semiconductor packages, high-power resistors, mechanical switches, and capacitors, which are not readily placed onto the package substrate  116 . The semiconductor package  104  may be mounted directly onto the board  120  by connectors  124 , which may be, for example, solder balls or pin/socket connectors. In various embodiments the semiconductor package  104  may include a land grid array (LGA) package, a micro pin grid array (mPGA) package, a pin grid array (PGA), a ball grid array (BGA), and the like. The board  120  may represent, for example, a carrier, a printed circuit board (PCB), a printed circuit card (PCC), a backplane, or a motherboard. Board materials could include, but are not limited to, ceramic (thick-filmed, co-fired, or thin-filmed), plastic, and glass.  
      As signals travel between the board  120  and the die  112  through the connectors  124  they may experience discontinuities that result in impedance mismatches. These mismatches may cause reflections that could limit the data transfer bit rates that may be reliably transmitted.  
      Impedance mismatches may also limit the distance that the signals may travel through board traces. Signals may experience deterioration as they travel through the board  120 , which may have a relatively high electrical loss tangent compared to the flex circuit cable  108 . For example, in one embodiment, the board  120  may be a flame retardant 4-printed circuit board (FR4-PCB), which may have an electrical loss tangent around 0.021-0.025, while the flex circuit cable  108  may include a dielectric made of, for example, polytetrafluoroethylene (PTFE), polyamide, a liquid crystal polymer, and the like. Such a flex circuit cable  108  may have electrical loss tangents less than 0.002, or so, which may allow for signals to travel longer distances through board traces at increased data transfer bit rates as compared to signals traveling through the board  120 .  
      This signal deterioration may increase proportionally with the data transfer bit rates at which the signals are transmitted. Therefore, in one embodiment, higher speed signals may be routed to/from the die  112  through the flex circuit cable  108 , while relatively lower speed signals may be routed through the board  120  where they may experience less significant deterioration.  
      In one embodiment, the end  110  of the flex circuit cable  108  may be electrically coupled to the topside of the package substrate  116  by, for example, a hat bar solder or a gold bump-pad. In this embodiment, the signals transmitted between the flex circuit cable  108  and the die  112  may only have to travel through the package substrate  116  without going through the connectors  124  into the board  120 . This embodiment could thereby result in a reduction of parasitic impedance and/or wave reflection back along the signal line due to the discontinuities of the signal path from the die  112  to the board  120 . While the electrical coupling may also serve to mechanically couple the end  110  to the package substrate  116 , various embodiments may reinforce the mechanical coupling with other connectors, such as one or more pins placed in the corners of the end  110 .  
       FIG. 2  illustrates a cross-sectional view of a flex interconnect bus  200  of the flex circuit cable  108  that may be used to route differential signals to and/or from the die  112 , in accordance with an embodiment of the present invention. The flex interconnect bus  200 , as shown, may include two signal trace pairs  204  and  208  separated by a flexible packaging material, such as a dielectric material  210  without having a ground trace or plane. The signal trace pairs  204  and  208  may be arranged as a transmitting line and receiving line to allow for the respective transmitting and receiving of differential signals to and from the die  112 . Various embodiments may have any number of signal trace pairs.  
       FIG. 3  illustrates the transmitting signal trace pair  204  coupling two semiconductor packages, in accordance with an embodiment of the present invention. In particular this embodiment may include a driver package  304  coupled to transmit differential signals to a receiver package  308  through the signal trace pair  204 . Semiconductor packages of various embodiments may include facilities to accommodate both the transmission and reception of differential signals.  
      In this embodiment, signal traces  316  and  320  may have V+ voltage and a V− voltage, respectfully. The receiver package  308  may be sensitive to a differential signal level on the two traces  316  and  320  and compare the differential signal level to a threshold level to determine a binary state of the transmitted signal. Because the voltages on traces  316  and  320  are referenced to one another, the receiver package  308  does not need a separate reference voltage for the differential signals. The differential signals sent on signal trace pair  204  may be resistant to noise, as any noise present may have similar effect on both traces  316  and  320  and appear as common-mode voltage at the receiver package  308 . This arrangement may also enable relatively low voltage applications, as differential signaling may be relatively insensitive to absolute voltage levels compared to single-ended signaling.  
      Differential signaling, especially in low voltage applications, may provide a number of additional desirable characteristics compared to single-ended signaling including, but not limited to, reduced electromagnetic interference, improvements in switching speeds, and reduction in power consumption. The above characteristics may at least facilitate the use of a groundless flex circuit cable for transmission of differential signals over distances at high data transfer bit rates. An example of differential signaling is low voltage differential signaling (LVDS), which may use, for example, a 500 mV differential signal at 1.2V.  
      Referring also to  FIG. 1 , in one embodiment, providing a groundless flex circuit cable  108  may allow for the end  110  to accommodate more signal (as opposed to ground) connections. Additionally, the flex circuit cable  108  may be a single layer, which may facilitate the flex circuit cable  108  maintaining a low-loss characteristic (e.g., having an electrical loss tangent less than 0.01). Not providing grounds for the signals transmitted through the flex circuit cable  108  could also allow for smaller package substrate  116  and/or board  120  dimensions as the amount of electrical pathways may be reduced in each. Such a reduction in dimensions may translate into cost and resource savings for embodiments of the present invention.  
       FIG. 4   a  illustrates a cross-sectional view of semiconductor packages coupled to one another in accordance with an embodiment of the present invention. In particular, this embodiment may include a semiconductor package  402  coupled to a board  406  and electrically coupled to a semiconductor package  410  through a flex circuit cable  414 , as shown. In one embodiment the two ends of the flex circuit cable  414  are coupled to the semiconductor packages  402  and  410  through respective package substrates  416  and  418 . The semiconductor packages  402  and  410 , the board  406 , and the flex circuit cable  414  may be similar to like components described with reference to  FIGS. 1 and 2 .  
      In one embodiment, differential signals may be routed between the semiconductor packages  402  and  410  through the flex circuit cable  414 . In one embodiment, relatively low-speed input/output (I/O) signals may be routed between the semiconductor packages  402  and  410  through signal traces  422  in the board  406 , while high-speed I/O signals are routed through the flex circuit cable  414 . Having the high-speed I/O signals routed over the flex circuit cable  414 , which may have a lower electrical loss tangent than the board  406 , may allow for the semiconductor packages  402  and  410  to be further away from one another, without sacrificing signal integrity. In one embodiment, ground and/or power may be respectively provided to the semiconductor packages  402  and  410  from the board  406 . In this embodiment, the flex circuit cable  414  may be detached from the board  406 , which may provide, among other things, additional space for mounting components on the surface of the board  406  beneath the flex circuit cable  414 .  
      As depicted, both the semiconductor packages  402  and  410  are on the same board  406 ; however, this may not be the case in other embodiments. That is, various embodiments may include a flex circuit cable coupling semiconductor packages residing on different boards.  
       FIG. 4   b  illustrates another embodiment using the flex circuit cable  414  to couple the semiconductor package  402  with the semiconductor package  410 . In this embodiment, the first end of the flex circuit cable  414  is coupled to the semiconductor package  402  at the package substrate  416 ; however, the second end of the flex circuit cable  414  is coupled to the board  406 . In this embodiment the board  406  may include signal traces  426  to respectively correspond with the signal traces of the flex circuit cable  414 . The board signal traces  426  may be used to route the high-speed differential signals to and from the semiconductor package  410 .  
      In one embodiment, the flex circuit cable  414  may be coupled to the board  406  relatively near the semiconductor package  410  so that significant signal loss does not occur due to the signals traveling through the board  406 . The desired proximity between the semiconductor package  410  and the point where the flex circuit cable  414  is coupled to the board  406  may be determined for a particular embodiment.  
       FIG. 4   c  illustrates an embodiment using the flex circuit cable  414  to couple the semiconductor package  402  with the semiconductor package  410  through board signal traces. In this embodiment, the flex circuit cable  414  may be coupled to the board  406  at both ends. The board  406 , in turn, may have signal traces  426  and  430  corresponding to the signal traces of the flex circuit cable  414  to couple the respective semiconductor packages  410  and  402  to the flex circuit cable  414 . Similar to above embodiment, the flex circuit cable  414  may be coupled to the board  406  relatively near the semiconductor packages  402  and  410  so that significant signal loss does not occur due to the signals traveling through the board  406 .  
       FIG. 4   d  illustrates an embodiment having the flex circuit cable  414  being coupled with the board  406 . In this embodiment the differential signals may travel to and/or from the semiconductor packages  402  and  410 . The flex circuit cable  414  may be attached to or embedded in the board  406 . In an embodiment where the flex circuit cable  414  has a lower loss characteristic, the differential signals may not experience the same degree of loss traveling between the semiconductor packages  402  and  410  as they would if the signal traces were internal to the board  406 .  
       FIG. 5  illustrates a backplane arrangement  500  utilizing flex circuit cables to couple components together, in accordance with an embodiment of the present invention. In this embodiment a backplane  504 , which may be a circuit board, may receive cards  508  and  512  in card slots  516  and  520 , respectively. Cards  508  and  512  may also be referred to as line cards or blades, and these terms as used herein are intended to be synonymous. In one embodiment, these cards  508  and  512  may also be circuit boards and may be designed to mate with the backplane  504  to provide modular extensibility. In various embodiments, these cards  508  and  512  may provide additional logic, I/O modules, memory, etc., to the backplane arrangement  500 . The backplane  504  may be adapted to accommodate any number of cards depending on the scalability of the particular embodiment. The backplane  504  may include a central processing node  524 , e.g., a local area network (LAN) controller, to control data traffic to/from the cards  508  and  512 .  
      Conventional backplane arrangements may be geometrically constrained due to limitations on routing high-speed signals through the printed circuit boards over distance, as discussed above. As a result of these limitations, conventional backplane arrangements have limited the distance that the cards can be placed from a central processing node, and therefore the backplane trace length, to about 20 inches or less.  
      In the embodiment depicted by  FIG. 5 , the card slot  520  may be coupled to the central processing node  524  through a flex circuit cable  528 . In various embodiments, high-speed signals may be routed between the central processing node  524  and the card  512  through the flex circuit cable  528 , while low-speed signals may be routed through traces in the backplane  504 . In this manner, the distance  532  that the card  512  may be placed from the central processing node  524 , and therefore the length of the backplane traces, may be greater than 20 inches if need be.  
      Each card  508  and  512  may include one or more semiconductor packages  536  to facilitate the particular functionality of the card. In one embodiment, a semiconductor package  536  on card  508  may be coupled to a semiconductor package  536  and card  512  with a flex circuit cable  540 . In some embodiments, one or more of the semiconductor packages  536  may be coupled to the central processing node  524  or the backplane  504  with a flex circuit cable.  
      In various embodiments, either of the flex circuit cables  528  or  540  may be used independently or in combination with other flex circuit cables coupling together various components.  
      Various embodiments may employ any combination of the above coupling configurations with flex circuit cables as well as others consistent with the scope of this invention.  
       FIG. 6  shows an example of a system  600  suitable for employing a flex circuit cable in accordance with an embodiment of this invention. In one embodiment a processing node  604  (e.g., one or more central processing units) could be coupled to a hub module  608  that may act as an intermediary between the processing node  604  and the other components. The processing node  604  could include a control unit, an arithmetic logic unit, and memory (e.g., registers, caches, RAM, and ROM) as well as various temporary buffers and other logic. In various embodiments the processing node  604  may be an application specific integrated circuit (ASIC), stacked or multichip modules, a digital signal processor, a blade processor, a network processor, etc.  
      In one embodiment the hub module  608  may arbitrate data and processing requests between the processing node  604  and a graphics processor  612 , memory  616 , mass storage device  620 , and/or other input/output (I/O) modules  624 . The hub module  608  may include, for example, a memory bridge, an I/O bridge, and/or a switch as either integrated or discrete components to facilitate the flow of data and processing requests. In one embodiment, the hub module  608  may include logic to allow for peer-to-peer communication without sending the traffic to the processing node  604 . Examples of the memory  616  include but are not limited to static random access memory (SRAM) and dynamic random access memory (DRAM). Examples of the mass storage device  620  include but are not limited to a hard disk drive, a compact disk drive (CD), a digital versatile disk drive (DVD), and so forth. Examples of the I/O modules  624  include but are not limited to a keyboard, cursor control devices, a display, a network interface, and so forth. The network interface may be adapted to couple to networks having a variety of topologies, protocols, and architectures. In various embodiments, one or more of the I/O modules  624  may be coupled to the hub module through a peripheral component interconnect (PCI) slot.  
      The components of system  600  may be coupled to one another as shown by being disposed on the same or different boards. Examples of the boards may include but are not limited to, line cards, expansion boards, motherboards, and backplanes. In various embodiments, a flex circuit cable, similar to the flex circuit cable described with reference to the above embodiments, may be used to interface with one or more of the components of system  600  using differential signals. In an embodiment, the flex circuit cable may be used to couple the hub module  608  to the graphics processor  612  and/or one or more of the I/O modules  604 . In various embodiments, the flex interconnect bus of the flex circuit cable may comprise at least a portion of a peripheral interconnect express (PCI Express) link between components.  
      In various embodiments, the system  600  may be a wireless mobile phone, a personal digital assistant, a tablet PC, a notebook PC, a set-top box, an audio/video controller, a networking router, a networking switch, a workstation, and a server.  
      Thus, it can be seen from the above descriptions, a novel approach for a groundless flex circuit cable to facilitate connections with semiconductor packages has been described.  
      Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the above embodiments without departing from the scope of the present invention. Those with skill in the art will readily appreciate that the present invention may be implemented in a wide variety of embodiments. This application is intended to cover any adaptations or variations of the embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.