Patent Publication Number: US-6705877-B1

Title: Stackable memory module with variable bandwidth

Description:
RELATED PATENT APPLICATIONS 
     This application is related to U.S. Pat. No. 6,264,476, issued to Li et al. for WIRE SEGMENT BASED INTERPOSER FOR HIGH FREQUENCY ELECTRICAL CONNECTION, to U.S. Pat. No. 6,172,895, issued to Brown et al. for HIGH CAPACITY MEMORY MODULE WITH BUILT-IN HIGH SPEED BUS TERMINATIONS, to copending U.S. patent application Ser. Nos. 09/932,525, filed Aug. 17, 2001; 09/932,654, filed Aug. 17, 2001; 10/077,057, filed Feb. 19, 2002; and 10/127,036, filed Apr. 22, 2002, all of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to high input/output (I/O), high density, low cost electronic modules and, more particularly, to the high I/O, high density, high capacity, low cost packaging of high performance, high capacity memory devices such as Double Data Rate (DDR) Synchronous Dynamic Random Access Memory (SDRAM) and having impedance-controlled buses for maintaining high electrical performance. 
     BACKGROUND OF THE INVENTION 
     In data processing and network systems, it is always a certainty that the demand in memory throughput will increase at a high rate. In recent years such increase has taken on a new dimension. While the demand for memory throughput has increased, the area available for mounted memory devices, the high quantities of I/O they require, and the height available has become increasingly restricted. 
     The Electronic Industries Association (EIA) has set up a standard for the dimensions for rack-mountable equipment. Traditionally, a piece of rack-mountable equipment has a standard width of 19 inches and a height in increments of 1.75 inches. This is also known as “1U”. However, a trend has begun to reduce the height for the servers in a server rack to dimensions appreciably lower than 1U. 
     This equipment height restriction has also placed height restrictions on other components such as memory modules. The traditional SDRAM dual inline memory modules (DIMMs) are simply too high to be able to be mounted vertically on the system board. Special sockets have been designed to allow DIMMs to be mounted either at an angle or even parallel to the system board. 
     One way to increase memory throughput is to increase the operating frequency of the memory devices. But this also requires the memory modules and connectors to support the higher speeds, which is becoming increasingly difficult to implement. Another way to increase memory throughput is to increase the bandwidth of the memory channel. 
     A 256-bit memory channel has four times the throughput of a 64-bit channel when operated at the same frequency. A higher throughput is important in many industries that run real-time applications (e.g., gaming, video graphics, speech processing, and networking applications). Increasing throughput through widening the bus is often much easier to implement and less expensive than methods such as doubling the clock frequency of the memory subsystem, reducing latency in bus cycles, and implementing complex multi-symbol modulation schemes or pulse code modulation (PCM) type approaches. 
     Increasing the throughput through widening the memory channel requires a significant increase in the quantity of I/O connections to support these memory devices while still trying to minimize the area used. This precludes the use of edge-interconnected memory modules such as traditional memory module form factors such as DIMMs and RAMBUS® Inline Memory Modules (RIMMs) and forces one to explore the use of area array interconnections. In some applications the interconnection is permanent (i.e., soldered) through a technique known as ball grid array (BGA) attachment, while others are field separable through pin grid array (PGA) and land grid array connectors. 
     BGA interconnections are viable for a quantity of up to approximately 1000 I/O. The mechanical reliability of larger BGA arrays is a concern due to the larger distance from neutral point (DNP) of the array, which is caused by coefficient of thermal expansion (CTE) mismatches. Moreover, manufacturability due to the nonplanarity of mating surfaces is also a concern. 
     PGA connectors are viable for field separable applications requiring a quantity of up to about 500 I/O. The mechanical reliability of larger surface mount PGA arrays is also a concern due to the larger distance from DNP of the array, which is caused by CTE mismatches. 
     For field separable applications requiring greater than 500 I/O, and grid array (LGA) connectors, and in particular LGA connectors as taught in some of the referenced copending U.S. patent applications, provide improved performance, increased density, lower height, and a CTE that better matches that of the surrounding structures. 
     One method being used today to solve the need to increase both memory capacity and density is to stack two, thin small outline package (TSOP) SDRAM devices on top of each other on a DIMM. An alternate approach is to stack two devices within a chip scale package (CSP). These stacking schemes, while increasing memory density, are not easily reworkable. 
     It is desirable to find a packaging solution that resolves the memory throughput capacity and density, interconnection quantity and density, and the height issues. In addition, the solution must also be low in cost, readily manufacturable, upgradeable with ample granularity, have improved electrical performance even at high frequencies, and have good reliability. Ample granularity allows the throughput on a given memory module to be increased or decreased as required (e.g., in increments of 16 bits, instead of 64 bits). 
     It is therefore an object of the invention to provide a variable bandwidth, high density, low profile SDRAM memory module for high performance memory devices. 
     It is another object of the invention to provide a variable bandwidth, high density, low profile SDRAM memory module that is readily manufacturable and upgradeable. 
     It is still another object of the invention to provide a variable bandwidth, high density, low profile SDRAM memory module that provides improved electrical performance at high frequencies and good reliability. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a family of specialized embodiments of the modules taught in the referenced copending U.S. patent applications. A memory module is desired with granularity and upgradeability of bandwidth, and a low profile using 256 MB SDRAM or DDR SDRAM memory devices in CSPs to support a memory data bus width of up to at least 512 bits. 
     Each module includes a substrate, having contact pads and memory devices on its surfaces, and impedance-controlled transmission line signal paths. The substrates may be conventional printed circuit cards preferably with CSP packaged memory devices along with other components attached directly to both sides of the substrates. 
     The inclusion of spaced, multiple area array interconnections allows a row of memory devices to be symmetrically mounted on each side of each of the area array interconnections, thereby reducing the interconnect lengths and facilitating matching of interconnect lengths. The footprints for the interconnections between the substrates and to the system board are the same to reduce part number and reliability and qualification testing. Short area array interconnections, including BGA and LGA options provide interconnections between modules and the rest of the system. The distance between the spaced multiple area array interconnections is preferably chosen to ensure that the solder joints in the BGA interconnection option are reliable. 
     Driver line terminators may be included on the substrates for maintaining high electrical performance. Thermal control structures may also be included to maintain the memory devices 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 a  is a representation of the bussed interconnection between a memory controller device and a multi-card memory arrangement of a memory subsystem of the prior art; 
     FIG. 1 b  is an enlarged, side elevational view of a vertical plated-through-hole attach connector and memory card of the prior art interconnection shown in FIG. 1 a;    
     FIG. 2 a  is an enlarged top view of a memory module in accordance with one embodiment of the present invention; 
     FIG. 2 b  is a cross-sectional view of a multi-card configuration based on the memory module of FIG. 2 a;    
     FIG. 2 c  is a cross-sectional view of the multi-card configuration in accordance with an extension of the embodiment of FIG. 2 b;    
     FIG. 3 is a cross-sectional view of the multi-card configuration of FIG. 2 b  including a termination module; 
     FIG. 4 a  is an enlarged top view of a memory module in accordance with another embodiment of the present invention; and 
     FIG. 4 b  is a cross-sectional view of a multi-card configuration based on the memory module of FIG. 4 a.    
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally speaking, the present invention relates to a family of specialized embodiments of the modules taught in the referenced copending U.S. patent applications. A memory module is desired with granularity and upgradeability of bandwidth, and a low profile using 256 MB SDRAM or DDR SDRAM memory devices in CSPs to support a memory data bus width of up to at least 512 bits. 
     Each module includes a substrate, having contact pads and memory devices on its surfaces, and impedance-controlled transmission line signal paths to support high-speed operation. The substrates may be conventional printed circuit cards preferably with CSP packaged memory devices along with other components attached directly to both sides of the substrates. 
     The inclusion of spaced, multiple area array interconnections allows a row of memory devices to be symmetrically mounted on each side of each of the area array interconnections, thereby reducing the interconnect lengths and facilitating matching of interconnect lengths. The footprints for the interconnections between the substrates and to the system board are the same to reduce part number and reliability and qualification testing. Short area array interconnections, including BGA and LGA options, provide interconnections between modules and the rest of the system. The distance between the spaced multiple area array interconnections is preferably chosen to ensure that the solder joints in the BGA interconnection option are reliable. 
     Referring first to FIG. 1 a , there is shown a representation of a portion of a memory subsystem  10  of the prior art located on a system board  12 , including a memory controller  14  and a multi-card memory arrangement with bussed interconnection therebetween. In this embodiment, memory subsystem  10  is based on DDR SDRAM technology, although many other technologies would also be applicable. Memory controller  14  is electrically connected to memory modules  16   a - 16   d , each comprising a plurality of memory devices  28 , through a plurality of bussed interconnections  18   a - 18   d . In order to provide even higher density, memory devices  28  may be located on both sides (FIG. 1 b ) of memory modules  16   a - 16   d  and even stacked upon each other on either one or both sides; however, this type of stacking is costly, unreliable, and difficult to rework. It is also very difficult to cool such stacked devices. 
     Memory controller  14  connects to system board  12  through an array of BGA solder interconnections (not shown) located on the bottom surface of memory controller  14 . Memory modules  16   a - 16   d  are approximately 5.25 inches long and 1.38 inches tall, with the top edge about 1.50 inches above the surface of the system board  12 , and with a distance between them, “P”, of 0.5 inch. Modules  16   a - 16   d  include an array of contact pads  24  along their lower edge. Contact pads  24  provide electrical connection to system board  12  through an edge connector, which is not shown for purposes of clarity. A more detailed cross-sectional view of a single connector  20  comprising mating contacts  22  and housing  26 , and corresponding memory module  16   a  is shown in FIG. 1 b.    
     Memory modules  16   a - 16   d  typically are printed circuit structures comprising epoxy-glss-based materials (i.e., FR4) and including one or more conductive (i.e., signal, power and/or ground) layers therein. Due to stringent electrical specifications, the impedance of the signal traces must match the impedance of the corresponding traces on system board  12  within ten percent. 
     Assuming that each memory module  16   a - 16   d  has 512 megabytes of memory (a quantity that is available today), the volumetric area required for the four modules  16   a - 16   d  is 5.25 inches by 1.80 inches by 1.50 inches, or about 13.90 cubic inches, and about 9.45 square inches of area on system board  12 . Also, while the height of memory modules  16   a - 16   d  just fit in a 1U high enclosure, it is unlikely that these memory modules  16   a - 16   d  will fit vertically in sub-1U high enclosures. 
     As stated hereinabove, edge-interconnected memory modules such as modules  16   a - 16   d  are precluded from use in wider memory channel applications due to the limited quantity of I/O connections  24  available to support the memory devices  28 . 
     Referring now to FIGS. 2 a  and  2   b , there are shown a top view of a memory module  40  in accordance with one embodiment of the present invention, and a cross-sectional view of a multi-card configuration  60  based on the memory module  40  of FIG. 2 a , respectively. 
     In the embodiment of FIG. 2 a , memory module  40  includes a substrate  42 , a plurality of memory devices  48   a - 48   d , phase lock loops (PLLs)  44   a - 44   d , registers  46   a - 46   d , a configuration memory device  50 , resistors  36 , capacitors  38 , and upper contact pad arrays  52   a - 52   d . Lower contact pad arrays  54   a - 54   d  and, optionally, additional memory. devices  48   a - 48   d  are located on the opposite side (FIG. 2 b ) of substrate  42 . 
     In this embodiment, memory devices  48   a - 48   d  are 256 MB SDRAM or DDR SDRAM memory devices in CSPs, although other packages, such as bare chip, TSOP, and chip on board (COB) may be used. The preferred use of packaged devices  48   a - 48   d  eliminates the issues associated with known good die (KGD). Although 256 MB devices are the largest devices available today, it should be understood that memory device capacity is certain to increase in the future, and the use of higher as well as lower capacity memory devices is well within the scope of this invention. 
     Examples of substrate  42  suitable for interconnection include printed circuit boards, circuit modules, etc. The term “printed circuit board” is meant to include but not be limited to a multilayered circuit structure including one or more conductive (i.e., signal, power and/or ground) layers therein. Such printed circuit boards, also known as printed wiring boards, are well known in the art and further description is not believed necessary. The term “circuit module” is meant to include a substrate or like member having various electrical components (e.g., semiconductor chips, conductive circuitry, etc.), which may form part thereof. Such modules are also well known in the art and further description is not believed necessary. 
     Substrate  42  may comprise a wide variety of dielectric materials. In one example it is made of epoxy-glass-based materials typically used in printed circuit board fabrication (e.g., FR4) and also includes one or more conductive layers therein. Due to stringent electrical specifications, the signal traces typically match the system impedance within a certain tolerance (e.g., ten percent). These materials are preferred because their CTE substantially matches the CTE of the surrounding structures, especially for applications including LGA connectors, and because of their relatively low cost. Other possible materials include polyimide and RO2800 (a trademark of Rogers Corporation). It should be understood by those skilled in the art that other materials may also be used without departing from the spirit of the invention. 
     PLLs  44   a - 44   d  are used to control and synchronize the timing against a known system clock for memory devices  48   a - 48   d . Registers  46   a - 46   d  are used to buffer and latch the state of the address and control buses against a known system clock. Configuration memory device  50  is used to store configuration information about the module  40  for use by the system. In this embodiment device  50  is an electrically erasable programmable read-only memory (EEPROM) device. Resistors  36  may be placed in series in the various electrical nets to dampen reflections. Capacitors  38  are strategically located, especially near memory devices  48   a - 48   d , and function as decoupling capacitors. Both resistors  36  and capacitors  38  are implemented as surface mount devices in this embodiment but may be implemented in other form factors such as embedded components. 
     A significant contribution to the advantages of the present invention is derived from the locations of the footprint of upper contact pad arrays  52   a - 52   d  and mating lower contact pad arrays  54   a - 54   d  (FIG. 2 b ), which are interconnected by area array interconnections  53   a - 53   d  (FIG.  2   b ). The specific implementation of area array interconnections  53   a - 53   d  for interconnecting contact pad arrays  52   a - 52   d  and mating pad arrays  54   a - 54   d  is design dependent and may vary depending on a specific set of requirements. In one example, if lowest cost and height are most desirable, the BGA option may be preferred. In another example, the LGA option is demountable and is therefore useful for applications requiring field upgradeability. The LGA option may require an alignment and clamping mechanism. Implementations of these items are covered in one of the referenced copending U.S. patent applications. 
     The separated contact pad arrays  52   a - 52   d  on the top surface and  54   a - 54   d  on the bottom surface allow a row of memory devices  48   a - 48   d  to be symmetrically mounted on each side of each respective contact pad cluster, which provides the shortest possible electrical path from the memory devices  48   a - 48   d  to system board  12  (FIG. 2 b ) and facilitates the matching of interconnect length. From a mechanical point of view, the distance between the contact pad arrays  52   a - 52   d  and  54   d - 54   d  is chosen to be wide enough to support the required quantity of memory devices  48   a - 48   d , but narrow enough to ensure that the solder joints in the BGA interconnection option are reliable. 
     Component positioning on memory module  40  provides another benefit over the prior art. Components are positioned on memory module  40  to allow the module  40  to support multiple (four, in this case) channels  40   a - 40   d  of memory. This capability allows a single memory module  40 , with memory devices  48   a - 48   d  on both sides (assuming memory devices  48   a - 48   d  each have a capacity of 256 MB) to support up to 2 GB of 64-bit wide memory (512 MB per channel), with a granularity of 256 MB per channel, or 512 MB of 256-bit wide memory. 
     Depending on how contact pad arrays  54   a - 54   d  are wired on the system board  12  (FIG. 2 b ), the memory devices  48   a - 48   d  may be configured to operate in either a single channel or as multiple independent channels. In one example, this option allows the operation of four 64-bit memory channels ( 40   a - 40   d ) or a single 256-bit memory channel. A 256-bit memory channel has four times the throughput of a 64-bit channel when operated at the same frequency. A higher throughput is important in many industries required to run real-time applications (e.g., gaming, video graphics, speech processing, and networking applications). Increasing throughput through widening the bus is often much easier to implement and less expensive compared to methods such as doubling the clock frequency of the memory subsystem, reducing latency in bus cycles, implementing complex multi-symbol modulation schemes or pulse code modulation (PCM) type approaches. 
     Using the memory devices  48   a - 48   d  on memory channels  40   a - 40   d  as shown in this embodiment, module  40  can support a single memory channel with a bus width of 256 bits. It should be understood that while a 256-bit memory channel is used for purposes of disclosure, the contact pad arrays  52   a - 52   d  and  54   a - 54   d  and area array interconnections  53   a - 53   d  used support a memory bus width of at least 512 bits. 
     For applications requiring less memory bus width, fewer memory channels  40   a - 40   d  can be populated and therefore implemented. For this type of application, since fewer area array interconnections  53   a - 53   d  (FIG. 2 b ) are needed but mechanical stability of the overall memory module  40  is desired, to reduce costs the unused interconnection locations may be selectively depopulated or replaced by spacers of similar dimensions as area array interconnections  53   a - 53   d . For applications requiring less memory, half of the full quantity of memory devices  48   a - 48   d  on a given channel  40   a - 40   d  can be populated. 
     System electrical performance can be further enhanced by including additional functionality, such as termination components to the module  40 , without significantly increasing the cost and size of the module  40 . This is taught in one of the referenced copending U.S. patent applications. Also, heatspreaders or equivalent thermal conduction devices  72  may be placed in contact with memory devices  48   a - 48   d  to provide improved thermal management if required. This is shown in FIG. 2 c.    
     Another example of additional functionality is the inclusion of decoders (not shown) that may be used to perform functions such as generating extra chip selects for referencing additional memory channels on module  40 . 
     A third example of additional functionality is the inclusion of field programmable components (not shown), which may be used to perform functions such as changing the values of the termination components. The field programmable components may include a field programmable gate array (FPGA), whose outputs control solid state switches to switch in resistive, capacitive or inductive blocks to establish a termination scheme that provides optimized performance. Some connections on the FPGA may be dedicated to a standard PC bus interface such as I2C, to make the terminations soft programmable. 
     A field programmable component may alternatively be employed to switch the module operation type from DDR to SDR, for example. Field programmable switches may also be used to deactivate the inverting net of all differential clocks that are not used in SDR operation, as well as to switch in extra components as needed. Other components that may be added include clock synthesizers, skew control blocks, FIFOs, and thermal shutdown or thermal monitoring integrated circuits, which may be installed at strategic hot points on module  40 . A thermal shutdown device may be used to disable a power supply until conditions improve. This improves the reliability of memory devices  48   a - 48   d  on module  40 . 
     Compared to the prior art memory modules  16   a - 16   d  of memory subsystem  10  (FIG. 1 a ), the inventive memory modules  40  offer improved signal integrity, due to the fact that the modules  40  have a reduced stub effect. Each electrical net in the prior art memory modules  16   a - 16   d  has a stub length up to 1.5 inches long. A stub is any net connecting parallel to the net, or controlled transmission line, of interest. It may include components. Unterminated stubs often are the result of used connectivity pathways for one or more components that are not populated in a given assembly, and can result in composite reflections that are twice the level of the initial signal. A stub degrades performance due to factors such as the timing relationships of the reflections exiting the stub, how that compares with the propagation delays to the other components on the bus, and the length of duration of the bus cycle. In short the design performance degradation associated with stubs tends to worsen with increasing frequency, longer stub lengths, more stubs, and greater spacing between stubs. 
     It should be understood by those skilled in the art that the various components of the invention may consist of alternate materials, instead of or in addition to the particular ones described in the disclosed embodiments, without departing from the spirit of the invention. 
     Referring now again to FIG. 2 b , there is shown a cross-sectional view of a multi-card configuration  60  based on the memory module  40  of FIG. 2 a . In one example of this embodiment, multi-card configuration  60 , which includes two memory modules  41   a  and  41   b , has a capacity of four gigabytes of 64-bit memory, or one gigabyte of 256-bit memory in a volume of just 4.54 inches by 5.66 inches by 0.36 inch, or about 9.25 cubic inches using BGA-based interconnections  53   a - 53   d . In another example using field separable LGA-based area array interconnections  53   a - 53   d  as taught in some of the referenced copending U.S. patent applications, multi-card configuration  60 , which again includes two memory modules  41   a  and  41   b , has a capacity of four gigabytes of 64-bit memory, or one gigabyte of 256-bit memory in a volume of just 4.54 inches by 5.66 inches by 0.29 inch, or about 7.45 cubic inches. 
     The additional amount of system board  12  real estate required for printed circuit traces to wire memory controller  14  to memory modules  16   a - 16   d  in FIG. 1 a is significantly greater than for wiring memory controller  14  to memory modules  41   a  and  41   b  of multi-card configuration  60  in the present invention, for additional system board  12  real estate savings. 
     Lower contact pad arrays  54   a - 54   d  on the lower module  41   a  are provided to allow electrical interconnection to a memory controller  14  on system board  12  through area array interconnections  53   a - 53   d . Upper contact pad arrays  52   a - 52   d  on the lower module  41   a  mate with lower contact pad arrays  54   a - 54   d  on the upper module  41   b  through area array interconnections  53   a - 53   d  to extend the address and control buses from the memory controller  14 . Upper contact pad arrays  52   a - 52   d  on the upper module  41   b  provides for the stacking of additional memory modules  40  (FIG. 2 a ) in the future. Maintaining uniform footprints for the interconnection between memory modules as well as to system board  12  reduces the proliferation of different memory module  40  (FIG. 2 a ) part numbers, and minimizes reliability and qualification testing. The substrates  42  are designed so that the modules  41   a  and  41   b  are positionally independent within the stack. In other words, the lower module  41   a  and upper module  41   b  may be interchanged within multi-card configuration  60  and still function properly. 
     The positioning of memory devices  48   a - 48   d  on stackable modules  41   a  and  41   b  relative to memory controller  14  provides several improvements compared to the prior art of FIG. 1 a . This is taught in one of the referenced copending U.S. patent applications. The improvements include: reduced propagation delays and hence potentially higher operating frequencies; reduced settling times and periods for the ringing cycle established and associated with impedance mismatches between the bus and termination in a given net, which can also reduce inter-symbol interference (ISI); potentially less intra-bus skew variation, which leads to better timing margins; and simplified and reduced system board  12  routing. Reducing the spacing between modules  41   a  and  41   b  compared to the prior art examples shown herein above provides a performance improvement since reflections off stubs will exist longer on the bus if the spacing between stubs is longer. 
     Referring now to FIG. 2 c , there is shown a cross-sectional view of a multi-card configuration  70  in accordance with an extension of the embodiment of FIG. 2 b , further including thermal management structures  72 . 
     The natural cooling efficiency of a module  40  is low due to the lack of an effective thermal transfer medium from the die or package of memory devices  48   a - 48   d  to the air, and the lack of a short air channel in the direction of air flow (i.e., parallel to system board  12 ). The thermal problem is exacerbated by the relatively large size of today&#39;s memory devices  48   a - 48   d  and the proximity to other heat generating devices  48   a - 48   d  in such a dense module  40 . The thermal management structures  72  of the inventive modules  40  are designed to optimize both thermal conduction and radiation, thus allowing maximum circuit density without heat build-up, which could degrade memory device  48   a - 48   d  performance and reliability. 
     Thermal management structures  72  are intended to sink heat away from memory devices  48   a - 48   d . Such structures  72  may be stand alone elements (e.g., heatsinks) or they may provide a low resistance thermal path to another surface such as the outer enclosure of a device (e.g., a laptop computer), which may include thermally conductive material. 
     Thermal management structures  72  may be implemented in many ways. Structures  72  may be as simple as a layer of thermally conductive material, such as aluminum, attached or retained to memory devices  48   a - 48   d  by thermally enhanced compounds or clamps. Structures  72  may be more complex and include elements such as fins (not shown) to augment cooling. Other methods may include the use of conformal pouches of liquid thermal transfer material, thin heat pipes, and thermoelectric devices. Even other methods of solving thermal issues will be obvious to those skilled in the art. 
     System electrical performance can be further enhanced, and significant system board real estate can be saved, by including additional functionality through the inclusion of additional stacked modules. The much lower profile of memory modules  40  allows the stacking of these modules with additional functionality even in sub-1U high applications. 
     Referring now to FIG. 3, there is shown a cross-sectional view of a multi-card configuration  80  based on the memory module  40  of FIG. 2 a . In this embodiment, multi-card configuration  80 , which includes two memory modules  40 , further includes a termination module  82 . Termination module  82  comprises a plurality of components  84 , which typically requires a large number of bulk capacitors, ferrite bead inductors, switching regulators, decoupling capacitors and termination components. The termination components may be passive components such as resistors and/or capacitors, but they may also include active filter-type components. 
     In another embodiment, it may be desirable to cluster the module interconnections. The concept of clustered area array connections also allows different quantities of modules containing a varied number of devices to share the same area array connection from module to module within the stack. Similarly, the same area array connection can be shared with a system board. These benefits are taught in one of the referenced copending U.S. patent applications. 
     For soldered interconnections such as BGA, column grid array (CGA), and surface mount soldered PGA connectors, clustering also provides improved mechanical reliability. This is due to reducing the distance from neutral point (DNP), which minimizes the mechanical reliability issues caused by coefficient of thermal expansion (CTE) mismatches. Moreover, manufacturability is improved, since non-planarity over a smaller area is less of a concern. These benefits apply to LGA connectors as well. The overall quantity of I/O and the area required also help to determine whether a particular interconnection technology is appropriate for use in a specific application. 
     Referring now to FIG. 4 a , there is shown a top view of a memory module  90  in accordance with another embodiment of the present invention. FIG. 4 b  depicts a cross-sectional view of a multi-card configuration  100  based on the memory module  90  of FIG. 4 a.    
     In the embodiment of FIG. 4 a , memory module  90  includes a substrate  42 , a plurality of memory devices  48   a , PLL  44   a , register  46   a , a configuration memory device  50 , resistors  36 , capacitors  38 , and upper contact pad arrays  52   a . Optionally and not shown in this drawing, a plurality of memory devices  48   b , PLL  44   b , register  46   b , a configuration memory device  50 , resistors  36 , capacitors  38 , and lower contact pad arrays  54   a  are located on the opposite side of substrate  42 . The embodiment in FIG. 4 b  shows components on both sides of substrate  42 . 
     Still referring to FIG. 4 a , in this particular embodiment, memory module  90  has 32 memory devices  48   a  located on the top surface and 32 memory devices  48   b  on the bottom surface of substrate  42 . Each memory device  48   a - 48   b  has a bus width of 8 bits, thereby allowing memory module  90  to have a memory data bus width of 256 bits per surface, or a total of 512 bits. Approximately 1200 I/O are required to support a 512-bit data bus and are shown in I/O area  92   a . The memory devices  48   a - 48   b  on memory module  90  use only one half of total I/O area  92 . Memory module  90  allows enough I/O to pass through I/O area  92   b to support another  512- bit data bus on a second memory module (FIG. 4 b ) thereby providing an overall memory data bus 1024 bits wide. If desired, the granularity of the width of the data path can be limited to 256 bits by populating the memory devices  48   a - 48   b  on only one side of the memory module  90 . 
     Memory module  90  may be implemented with other types of memory devices  48   a - 48   b , such as RAMBUS devices, which may require high-speed impedance matching. 
     One or more additional devices  94  may be added to module  90  to increase the overall functionality. For example, devices  94  may be implemented as a plurality of low-cost microprocessors. 
     Referring now again to FIG. 4 b , there is shown a cross-sectional view of a multi-card configuration  100  based on the memory module  90  of FIG. 4 a . In one example of this embodiment, multi-card configuration  100  includes two memory modules  102   a  and  102   b , each with 32 memory devices  48   a  located on each top surface and 32 memory devices  48   b  on each bottom surface of substrates  42 . Each memory device  48   a - 48   b  has a bus width of 8 bits, thereby allowing each memory module  102   a  and  102   b  to have a memory data bus width of 512 bits. As stated hereinabove, approximately 1200 I/O are required to support a 512-bit data bus and are shown in I/O area  104   a . The memory devices  48   a - 48   b  on memory module  102   b  use only one half of total I/O area  104 . Memory module  102   b  allows enough I/O to pass through I/O area  104   b  to support another 512-bit data bus on a second memory module  102   a  thereby providing multi-card configuration  100  with an overall memory data bus 1024 bits wide. The electrical connection through I/O area  104   b  is provided in this example by a plurality of vias  108 . 
     As stated hereinabove, a significant advantage of the present embodiment is derived from the clustering of the upper contact pad arrays  52   a  and mating lower contact pad arrays  54   a , which are interconnected by area array interconnections  53   a . Specific implementation of area array interconnections  53   a  for interconnecting contact pad arrays  52   a  and mating pad arrays  54   a  is design dependent and may vary depending on a specific set of requirements. In this example, due to the high quantity (approximately 2400) and density of I/O area  104 , an LGA connector is the only known option that will provide reliable interconnections. 
     Lower contact pad arrays  54   a  on the lower module  102   b  are provided to allow electrical interconnection to a memory controller  14  on system board  12  through area array interconnections  53   a . Upper contact pad arrays  52   a  on the lower module  102   b  mate with lower contact pad arrays  54   a  on the upper module  102   a  through area array interconnections  53   a  to extend the address and control buses from the memory controller  14 . Maintaining uniform footprints for the interconnection between memory modules as well as to system board  12  reduces the proliferation of different memory module  90  (FIG. 4 a ) part numbers, and minimizes reliability and qualification testing. The substrates  42  may be designed so that the modules  102   a  and  102   b  are positionally independent within the stack by rotating one of the two modules  102   a  or  102   b  180 degrees. 
     Spacer  106  may be implemented just as are area array interconnections  53   a  (i.e., an LGA connector). To reduce cost, spacer  106  may be implemented as a block of dielectric material that has approximately the same vertical dimension as interconnection  53   a  when compressed, since only half of the area array interconnections  53   a  must reach upper module  102   a.    
     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.