Abstract:
There is a memory module for use in conjunction with high speed, impedance-controlled buses. Each memory card may be a conventional printed circuit card with memory chips attached directly to the card. Alternately, high density memory modules assembled from pluggable sub-modules may be used. These sub-modules may be temporarily assembled for testing and/or burn-in. Bus terminations mounted directly on the memory card or the memory module eliminate the need for bus exit connections, allowing the freed up connection capacity to be used to address additional memory capacity on the module. An innovative pin-in-hole contact system is used both to connect sub-modules to the memory module and, optionally, to connect the memory module to a mother board or similar structure. A thermal control structure may be placed in the memory module to cool the increased number of memory chips to prevent excess heat build-up and ensure reliable memory operation.

Description:
RELATED PATENT APPLICATIONS 
     This application is related to U.S. Pat. No. 5,928,005, issued to Li et al. for SELF-ASSEMBLED LOW INSERTION FORCE CONNECTOR ASSEMBLY, and copending U.S. patent applications, Ser. No. 09/461,065, filed Dec. 14, 1999, pending and Ser. No. 09/461,064, filed Dec. 14, 1999, pending both filed concurrently herewith, all of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to high-density memory modules for computer applications and, more particularly, to high-density memory modules having impedance-controlled transmission line buses and, optionally, driver line terminators built into the memory modules. 
     BACKGROUND OF THE INVENTION 
     Modern, high speed digital computers and the sophisticated software running on them require ever-increasing amounts of volatile random access memory (RAM). As bus and clock speeds increase, the electrical drive requirements for servicing a number of memory devices become much more stringent than when slower memory was in use. 
     The operating speed of a memory system is largely determined by the electrical interconnections between the memory controller and the memory devices, or the bus. As the data rate increases, the signal propagation times through the interconnections are no longer negligible compared to the transition time of the signals. At high bus speeds, those interconnections behave as transmission line networks. The response characteristics of such transmission line networks define the maximum usable speed of the memory bus. 
     In the current generation of memory packaging technology, the amount of memory physically available on a single card or module is controlled by two factors: the capacity of the memory devices (chips) themselves and the number of electrical connections that physically may be made to the module. The number of cards or modules which may be daisy chained is dependent solely upon the capacity of the line drivers or receivers. To ensure fast memory cycle times, extremely shore, fast rise pulses are used. 
     For example, in conventional random access memory systems, because only one bit can exist on the bus during a certain time interval, the bus speed is mainly determined by the signal setup time of the bus. As a result, the highest data rate that such bus can currently achieve in PC memory systems is 266 Mbits per second. Usually, no impedance-matching termination is required or provided in such a conventional RAM system. 
     To achieve even higher bus speeds and, at the same time, allow for larger memory capacities, impedance controlled types of buses must be adopted. For example, RAMBUS® technology features a memory configuration wherein memory devices are disposed (packaged) on up to three RAMBUS Inline Memory Module (RIMM) cards all interconnected on a mother board by a high speed data bus. One or more termination components are placed on the mother board at the physical end of the bus. 
     In operation, address/data lines leave driver circuits on the mother board and enter a first RIMM card in the memory chain. These same address/data lines must leave the RIMM via a complete, second set of connections. This routing continues through a second and third RIMM module before the driver lines reach their terminations. This memory/bus configuration allows very fast transit signals to be transmitted between a memory controller and a data storing device over relatively long buses. These buses allow multiple bits to propagate simultaneously down each line of the bus, thereby achieving access data rates of 800 Mbits per second. Even higher bus rates appear feasible in the future. 
     One most important feature of such buses is that the effective impedance of the signal propagation paths is well controlled, and one end of the bus is terminated to the characteristic impedance of the bus in order to maintain signal fidelity and signal integrity. 
     In systems adopting such buses, the amplitude of the driving signals are generally much smaller than amplitudes of conventional digital signals. This is due to the limitation on the driving strength (dv/dt) of the devices. 
     All of the above mentioned factors make the reliable operation of such memory buses very dependent upon controlling the impedance of the interconnections along the bus. Any impedance mismatches along the signal transmission path result in signal degradations which, in turn, may lead to errors in data transmissions. At the same time, maintenance of accurate timing among all the signal bits and clocks is also extremely critical to reliable data transmission. For this reason, minimizing signal-to-clock delay difference (data to clock skew) is another important requirement for such buses. 
     Prior art memory system designs generally consist of a memory controller, a clock driver and bus terminations all mounted on the mother board with up to three memory slots between the controller and the termination. The data signals must pass through every module and also through a total of six edge connectors before they reach the termination. Because of their design, current edge connectors introduce impedance mismatches and crosstalk which degrade signal quality and therefore limit the performance of the signal channels. 
     The inclusion of the terminations on the memory modules themselves also provides several types of performance improvement. First, because only a single set of contacts need be used (i.e., there is no need to have the bus lines exit the module), the additional contact capacity may be devoted to addressing capability for even greater amounts of memory on a single card or module. For example, current RAMBUS® technology RIMM cards support only eight or sixteen memory chips with a total capacity of no more than 256 Mbits per card. The memory modules of the present invention, however, can hold up to 32 chips per card, thereby providing a double to quadruple improvement in memory capacity. By eliminating essentially half of the required contacts, an even greater number of chips (e.g., 64 chips) may be packaged on a single card. 
     Total bus path length is significantly reduced because more memory may be placed on a single card physically much closer to the driver circuits than has heretofore been possible. Even more improvement is obtained because the extra passage of signals through exit contacts is eliminated. Also eliminated is that portion of the bus path between the memory modules and the external terminator resistors of the prior art. 
     In addition, this inventive design may reduce the design complexity and manufacturing cost of the mother board. For memory systems having one to three memory modules, using a terminated module as the last module helps to achieve maximum system performance. 
     The present invention also allows integration of all of the memory chips that a channel can have onto a single, terminated module, which leads to better system integrity and lower cost. The inventive, self-terminated module needs only half of the I/O connections of a conventional module of the prior art. Using a conventional prior art connector on a module, two channels of memory can be integrated onto one module, which yields increased bandwidth and double the memory capacity. 
     When the inventive, self-terminating memory modules are combined with innovative pin/hole interconnection technology, densities are achieved that are much higher than have been possible heretofore. This allows far more memory to be packaged on a single memory module or card. This means that more memory capacity may be deployed closer to the line drivers/receivers, thereby reducing path lengths, especially when a memory module is self terminated. 
     Thermal management structures may be included on the high-density, self-terminated memory modules. These structures are described in detail in copending U.S. patent application, Ser. No. 09/461,065, filed concurrently herewith. 
     It is, therefore, an object of the invention to provide a compact, high density memory card with up to 64 memory chips supported thereon. 
     It is an additional object of the invention to provide a high density memory module utilizing a novel high density connector technology. 
     It is another object of the invention to provide a high density memory module with bus terminations provided on the memory module itself. 
     It is a still further object of the invention to provide a high density memory module which can operatively reduce data path lengths, thereby helping ease driver electrical requirements in a high speed digital computer or the like. 
     It is another object of the invention to provide a high density memory module made from detachable sub-modules which may be temporarily connected together for testing and/or burn-in and then, optionally, attached by soldering, electrically adhering, or performing a similar process. 
     It is yet another object of the invention to provide a single, high-density memory module containing all of the memory capacity supported by a single bus channel or dual bus channels. 
     SUMMARY OF THE INVENTION 
     The present invention provides a memory module with bus terminations mounted directly on the memory module for use with high speed, impedance-controlled memory buses. The memory module may be formed on a conventional printed circuit card with unpacked or packed memory chips attached directly to the memory module. Alternately, high density memory modules assembled from pluggable sub-modules or daughter cards may be used. These sub-modules may be temporarily assembled for testing and/or burn-in. Using memory modules with bus terminations mounted directly on the module improves the signal quality and integrity and therefore enhances the performances the systems. Such designs may also eliminate the need for bus exit connections, thereby allowing the freed-up connection capacity to be used to address additional memory capacity on the module. Thermal control structure may be included to maintain the high-density memory modules within a reliable range of operating temperatures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when taken in conjunction with the detailed description thereof and in which: 
     FIG. 1 is schematic representation of a multi-card memory arrangement of the prior art with bus termination on the mother board; 
     FIG.  2   a  is a front, plan view of a prior art memory card as shown in FIG. 1; 
     FIG.  2   b  is a top view of the prior art memory card shown in FIG.  2   a;    
     FIG.  2   c  is a side view of the prior art memory card shown in FIG.  2   a;    
     FIG. 3 is a schematic representation of a memory arrangement of the present invention with bus termination on the last memory module; 
     FIG. 4 is a schematic representation of a multi-card memory arrangement of the present invention with bus termination on a memory card; 
     FIG.  5   a  is a front, plan view of the self-terminating memory card of the invention as shown in FIG. 4; 
     FIG.  5   b  is a top view of the self-terminating memory card of the invention shown in FIG. 4; 
     FIG.  5   c  is an end view of the self-terminating memory card shown in FIG. 4; 
     FIG. 6 is a schematic representation of an alternate embodiment of the inventive memory card using daughter cards; 
     FIG.  7   a  is a front, plan view of the self-terminating memory card of the invention shown in FIG. 6; 
     FIG.  7   b  is a top view of the self-terminating memory card shown in FIG. 6; 
     FIG.  7   c  is front, schematic view of the daughter card shown in FIGS.  7   a  and  7   b;  and 
     FIG. 8 is a front, schematic view of the self-terminating memory card adapted to support two memory channels. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally speaking, the present invention is a high density memory card or module made either from bare memory chips or conventional memory chip packages mounted on memory cards or on demountable memory sub-modules (daughter cards). These memory cards or modules have built-in bus terminations and, optionally, thermal management structures. 
     Referring first to FIG. 1, there is shown a schematic representation of a multi-card (three-card) memory system  10  of the prior art. Conventional two-slot and three-slot boards required terminations on the mother board  12 , which were needed even when all slots were not used. Of course, in that situation, signal quality could be proportionally degraded by the module connectors which provide signal paths between memory modules and circuitry on the mother board. 
     A portion of a mother board  12  is shown with support circuitry needed for implementation of the RAMBUS memory system. A Direct RAMBUS Clock Generator (DRCG) circuit  14 , and a master device  16  containing a Direct RAMBUS ASIC Cell (RAC)  18  are implemented on mother board  12 . The RAMBUS channel  20  connects RAC  18  to a first memory socket  22 . Socket  22  is physically connected to mother board  12 . RAMBUS channel  20  connections are generally made by internal printed wiring traces (not shown). First socket  22  generally has a plurality of spring-loaded contacts designed to engage mating contact pads on a first RIMM card  24 . 
     In the RAMBUS architecture, generally 184 contacts are provided on each memory module. RAMBUS channel  20  enters RIMM card  24  at a bus entry region  26  and is then connected to a number of individual memory devices  28  attached to RIMM card  24 . The RAMBUS channel next exits RIMM card  24  via a RAMBUS channel exit region  30 , and passes from first RIMM card  24  back to mother board  12 . Additional printed wiring traces carry the RAMBUS channel  20  to a second socket  31  also on mother board  12 . Second socket  31  holds a second RIMM card  35 . 
     A RAMBUS channel entry portion  32 , a series of memory devices  28  and a RAMBUS channel exit portion  34  make up second RIMM card  35 . In a similar fashion, a third socket  36 , third RIMM card  38 , RAMBUS channel entry portion  40 , and RAMBUS channel exit portion  42  make up the third RAMBUS memory card. RAMBUS channel  20  reaches terminations  44  at the end of the circuitous routing of the bus  20 . 
     Terminations components, such as resistors, blocking capacitors and/or decoupling capacitors  44  are also placed on mother board  12 . All RAMBUS channel  20  signals must pass through three sockets  22 ,  31 ,  36  and across three RIMM cards  24 ,  35 ,  38  before reaching terminations  44 . The driver requirements to reliably access memory devices  28  on the three RIMM cards  24 ,  35 ,  38  are stringent. Signal degradation occurs along the path of RAMBUS channel  20 , especially at connectors  22 ,  31 ,  36 . In addition, valuable “real estate” is consumed on the mother board  12  itself. 
     Referring now to FIGS.  2   a,    2   b,  and  2   c,  there are shown front, top and side views, respectively, of the typical prior art RIMM card  24  shown in FIG.  1 . Memory devices  28  are arranged in four vertical rows of four devices each; however, other physical arrangements could also be used. RAMBUS channel  20  (FIG. 1) enters card  24  at a bus entry region  26  and is then connected to memory devices  28 . The RAMBUS channel  20  next exits RIMM card  24 , via a RAMBUS channel exit region  30 , and passes out of first RIMM card  24  and back to mother board  12  (FIG.  1 ). 
     FIG.  2   b  is a top view of the RIMM card of FIG.  2   a  showing memory devices  28  disposed on both sides of RIMM card  24 . 
     FIG.  2   c  is a side view of the RIMM card of FIG.  2   a  also showing memory devices  28  disposed on RIMM card  24 . 
     Referring now to FIG. 3, there is shown a schematic, representation of a memory system  10  with the inventive memory module. A portion of a mother board  12  is again shown with support circuitry needed for implementation of the RAMBUS memory system. A Direct RAMBUS Clock Generator (DRCG) circuit  14 , and a master device  16  containing a Direct RAMBUS ASIC Cell (RAC)  18  are implemented on mother board  12  in the same manner as in the prior art memory card implementation shown in FIG.  1  and discussed hereinabove. 
     The RAMBUS channel  20  connects RAC  18  to a memory socket  22 . Socket  22  is physically connected to mother board  12  and RAMBUS channel  20  connections are generally made by printed wiring traces (not shown) on one or both surfaces of motherboard  12 . Alternately, an internal printed wiring trace (not shown) can be used for connections. Socket  22  generally has a plurality of spring-loaded contacts designed to engage mating contact pads on a RIMM card  24 . 
     RAMBUS channel  20  enters RIMM card  24  at a bus entry region  26  and is then connected to a number of individual memory devices  28  attached to RIMM card  24 . The RAMBUS channel  20  then exits RIMM card  24 , via a RAMBUS channel exit region  30  and passes out of the first RIMM card  24  and back to mother board  12 . Additional printed wiring traces carry the RAMBUS channel  20  to a second socket  31  also on mother board  12 . Second socket  31  holds second RIMM card  35 . Similarly, the RAMBUS channel  20  exits the second conventional RIMM card and is connected to the last RIMM card  54 , which also contains bus termination  52 . Unlike the prior art, terminations  52  are mounted directly on card  54 , thereby eliminating the need for an exit portion of RAMBUS channel  30  (FIG.  1 ). The complete, additional set of contacts is thereby eliminated, allowing their use for addressing additional memory capacity or the like. 
     Referring now to FIG. 4, there is shown a schematic representation of the inventive memory card system  50 . A portion of a mother board  12  is again shown with support circuitry needed for implementation of the RAMBUS memory system. A Direct RAMBUS Clock Generator (DRCG) circuit  14 , and a master device  16  containing a Direct RAMBUS® ASIC Cell (RAC)  18  are implemented on mother board  12  in the same manner as in the prior art memory card implementation shown in FIG.  1  and discussed hereinabove. 
     The RAMBUS channel  20  connects RAC  18  to a memory socket  22 . Socket  22  is physically connected to mother board  12  and RAMBUS channel  20  connections are generally made by printed wiring traces (not shown) on one or both surfaces of mother board  12 . Alternately, an internal printed wiring trace (not shown) can be used for connections. Socket  22  generally has a plurality of spring-loaded contacts designed to engage mating contact pads on a RIMM card  54 . 
     RAMBUS channel  20  enters RIMM card  54  at a bus entry region  56  and is then connected to a number of individual memory devices  28  attached to RIMM card  54 . Unlike the prior art, terminations  52  are mounted directly on card  54 , thereby eliminating the need for an exit portion of RAMBUS channel  30  (FIG.  1 ). The complete, additional set of contacts is also eliminated. 
     Referring now also to FIGS.  5   a,    5   b  and  5   c,  there are shown front, top and side views, respectively, of a RIMM card  54  having termination on the card as shown in FIG.  4 . Memory devices  28  are arranged in two horizontal rows of eight devices each; however, other physical arrangements could also be used. RAMBUS channel  20  (FIG. 4) enters card  54  at a bus entry region  56  and is then connected to memory devices  28 . Terminations  52  at the end of the RAMBUS channel  20  (FIG. 4) are also disposed on card  54 . 
     FIG.  5   b  is a top view of the RIMM card of FIG.  5   a  showing memory devices  28  disposed on both sides of RIMM card  54 . 
     FIG.  5   c  is a side view of the RIMM card of FIG.  5   a  also showing memory devices  28  disposed on RIMM card  54 . 
     The advantages of the inventive configuration are many. Any reduction of bus length eases the driver requirements for devices on the bus, both reducing cost and improving reliability. Comparing FIG.  1  and FIG. 3, it may be seen that signal degradation in the prior art associated with 2½ connector sockets, signal paths between the slots and the termination are eliminated. Generally, higher memory access speeds may be obtained by improving the quality of the RAMBUS channel (i.e., reducing its length, channel delay, crosstalk, etc.). The reduced path length as well as the elimination of at least one socket interface (usually two and one-half sockets may be eliminated) between the RIMM card  54  and the terminations  44  (FIG. 1) both help to reduce crosstalk. Additional mother board real estate is freed by eliminating the traces between socket  36  (FIG. 1) and terminations  44  (FIG.  1 ). 
     These changes improve electromagnetic interference (EMI) susceptibility and also reduce radiated radio frequency (RF) emissions from the mother board  12  and RIMM card  54  combination. Another advantage of placing terminations  44  on the memory card  54  is that less noise is coupled into mother board  12 , potentially enhancing the performance of the entire system. 
     Referring now to FIG. 6, there is shown a schematic, representation of an alternate embodiment of the present invention using daughter cards  80  connected to a memory module board  82 , typically with pin/hole connection technology (not shown) to improve memory density on memory module board. This embodiment is described in more detail in co-pending patent application Ser. No. 09/461,065 filed concurrently herewith. All external features are identical to those described in detail hereinabove (FIG.  4 ). 
     Referring now also to FIGS.  7   a,  and  7   b,  there are shown a front schematic and side schematic view, respectively, of daughter cards  80  connected to memory module board  82 . Referring also to FIG.  7   c,  there is shown a front, schematic view of one of the daughter cards  80 . Memory devices  28  are disposed on daughter cards  80 . A thermal management structure (not shown) is connected to memory devices  28  to dissipate heat. These thermal control structures are described in detail in co-pending patent application Ser. No. 09/461,065 filed concurrently herewith and included herein by reference. 
     Pins (not shown) on daughter cards  80  are plugged into receptacles (not shown) on memory module board  82 . The inventive pin/hole connectors are also described in detail in co-pending patent application, Ser. No. 09/461,064 also filed concurrently herewith, as well as in issued U.S. Pat. No. 5,928,005, also included herein by reference. 
     Referring now to FIG. 8, there is shown a two channel RAMBUS module design utilizing a one channel socket of the prior art which further illustrates the advantage of the present invention. Two independant RAMBUS channels  56 ,  57  enter memory module  90 . A plurality of memory devices  28  are shown connected to RAMBUS channels  56 ,  57 . Independant terminations  52 , located on card  90  properly terminate each of the RAMBUS channels  56 ,  57 . The expanded memory module may have chips attached to both sides, and may be inserted into a socket of the conventional one channel design of the prior art. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, this invention is not considered limited to the example chosen for purposes of this disclosure, and covers all changes and modifications which does not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.