High capacity thin module system and method

[0001] This application is a continuation-in-part of U.S. patent applicatio Ser. No. 11/007,551, filed Dec. 8, 2004, which application is a continuation-in-part of U.S. patent application Ser. No. 10/934,027, filed Sep. 3, 2004. U.S. patent applications Ser. No. 10/934,027 and 11/007,551 are hereby incorporated by reference herein.

[0002] The present invention relates to systems and methods for creating high density circuit modules.

[0003] The well-known DIMM (Dual In-line Memory Module) board has been used for years, in various forms, to provide memory expansion. A typical DIMM includes a conventional PCB (printed circuit board) with memory devices and supporting digital logic devices mounted on both sides. The DIMM is typically mounted in the host computer system by inserting a contact-bearing edge of the DIMM into a card edge connector. Systems that employ DIMMs provide, however, very limited profile space for such devices and conventional DIMM-based solutions have typically provided only a moderate amount of memory expansion.

[0004] As bus speeds have increased, fewer devices per channel can be reliably addressed with a DIMM-based solution. For example, 288 ICs or devices per channel may be addressed using the SDRAM-100 bus protocol with an unbuffered DIMM. Using the DDR-200 bus protocol, approximately 144 devices may be address per channel. With the DDR2-400 bus protocol, only 72 devices per channel may be addressed. This constraint has led to the development of the fully-buffered DIMM (FB-DIMM) with buffered C/A and data in which 288 devices per channel may be addressed. With the FB-DIMM, not only has capacity increased, pin count has declined to approximately 69 from the approximately 240 pins previously required.

[0005] The FB-DIMM circuit solution is expected to offer practical motherboard memory capacities of up to about 192 gigabytes with six channels and eight DIMMs per channel and two ranks per DIMM using one gigabyte DRAMs. This solution should also be adaptable to next generation technologies and should exhibit significant downward compatibility.

[0006] This great improvement has, however, come with some cost and will eventually be self-limiting. The basic principle of systems that employ FB-DIMM relies upon a point-to-point or serial addressing scheme rather than the parallel multi-drop interface that dictates non-buffered DIMM addressing. That is, one DIMM is in point-to-point relationship with the memory controller and each DIMM is in point-to-point relationship with adjacent DIMMs. Consequently, as bus speeds increase, the number of DIMMs on a bus will decline as the discontinuities caused by the chain of point to point connections from the controller to the “last” DIMM become magnified in effect as speeds increase. Consequently, methods to increase the capacity of a single DIMM find value in contemporary memory and computing systems.

[0007] There are several known methods to improve the limited capacity of a DIMM or other circuit board. In one strategy, for example, small circuit boards (daughter cards) are connected to the DIMM to provide extra mounting space. The additional connection may cause, however, flawed signal integrity for the data signals passing from the DIMM to the daughter card and the additional thickness of the daughter card(s) increases the profile of the DIMM.

[0008] Multiple die packages (MDP) are also used to increase DIMM capacity while preserving profile conformity. This scheme increases the capacity of the memory devices on the DIMM by including multiple semiconductor die in a single device package. The additional heat generated by the multiple die typically requires, however, additional cooling capabilities to operate at maximum operating speed. Further, the MDP scheme may exhibit increased costs because of increased yield loss from packaging together multiple die that are not fully pre-tested.

[0009] Stacked packages are yet another strategy used to increase circuit board capacity. This scheme increases capacity by stacking packaged integrated circuits to create a high-density circuit module for mounting on the circuit board. In some techniques, flexible conductors are used to selectively interconnect packaged integrated circuits. Staktek Group L.P. has developed numerous systems for aggregating CSP (chipscale packaged) devices in space saving topologies. The increased component height of some stacking techniques may alter, however, system requirements such as, for example, required cooling airflow or the minimum spacing around a circuit board on its host system.

[0010] Another trend to increase DIMM capacity is the use of larger capacity ICs such as, for example, 512 Mega-bit, 1 Giga-bit, and 2 Giga-bit or larger DRAM devices. The trend indicates that larger devices are forthcoming. Such larger devices may necessitate packages with larger dimensions until technological advances provide smaller feature sizes. For example, some high-capacity DRAM devices may be too big for a 30 mm DIMM.

[0011] Another problem associated with some such high-capacity is that their thickness may be greater than the specified thickness for many standard DIMM designs. For example, many JEDEC DIMM thickness specifications require a 1 mm package thickness to allow DIMMs with stacked devices to fit in specified dimensions with adequate airflow. Some new high-capacity devices may have a greater thickness than the specified 1 mm. Such thickness may lead to stacked DIMMs would exceed the maximum specified thickness.

[0012] What is needed, therefore, are methods to fit provide thin DIMM modules with high capacity. What is needed also needed are methods and structures for increasing the flexibility of FB-DIMMs.

[0013] Multiple DIMM circuits or instantiations are combined in a single module to provide on a single module circuitry that is substantially the functional equivalent of two or more DIMMs but avoids some of the drawbacks associated with having two discrete DIMMs. In one embodiment, registered DIMM circuits are used. In another, FB-DIMM circuits are used.

[0014] In a preferred embodiment, integrated circuits (preferably memory CSPs) and accompanying AMBs are arranged in two ranks in two fields on each side of a flexible circuit. The flexible circuit has expansion contacts disposed along one side. The flexible circuit is disposed about a supporting substrate or board to place at least one FB-DIMM instantiation on each side of the constructed module. In alternative, but also preferred embodiments, the ICs on the side of the flexible circuit closest to the substrate are disposed, at least partially, in what are, in a preferred embodiment, windows, pockets, or cutaway areas in the substrate. Other embodiments may only populate one side of the flexible circuit or may only remove enough substrate material to reduce but not eliminate the entire substrate contribution to overall profile. Other embodiments may connect the constituent devices in a way that creates a FB-DIMM circuit or instantiation with the devices on the upper half of the module while another FB-DIMM instantiation is created with the devices on the lower half of the module. Other embodiments may, for example, combine selected circuitry from one side of the module (memory CSPs for example) with circuitry on the other side of the module (an AMB, for example) in creating one of plural FB-DIMM instantiations on a single module. Other embodiments employ stacks to provide multiple FB-DIMM circuits or instantiations on a low profile module. The flexible circuit may exhibit one or two or more conductive layers, and may have changes in the layered structure or have split layers. Other embodiments may stagger or offset the ICs or include greater numbers of ICs.

[0015] FIG. 1 is a depiction of a preferred embodiment of a module devised in accordance with the present invention.

[0016] FIG. 2 depicts a contact bearing first side of a flex circuit devised in accordance with a preferred embodiment of the present invention.

[0017] FIG. 3 depicts the second side of the exemplar populated flex circuit of FIG. 2.

[0018] FIG. 4 is a cross-sectional depiction through the devices as populated in an embodiment of the present invention.

[0019] FIG. 5 is an enlarged view of the area marked ‘A’ in FIG. 4.

[0020] FIG. 6 depicts a cross-sectional view of a module devised in accordance with an alternate preferred embodiment of the present invention.

[0021] FIG. 7 depicts the area near an end of a substrate in the embodiment shown in FIG. 6.

[0022] FIG. 8 depicts a cross-sectional view of a module assembly devised in accordance with a preferred embodiment of the present invention.

[0023] FIG. 9 is an enlarged view of a portion of one preferred embodiment.

[0024] FIG. 10 depicts one perspective of an exemplar module devised in accordance with a preferred embodiment of the present invention.

[0025] FIG. 11 is another depiction of the relationship between flex circuitry and a substrate 14 which has been patterned or windowed with cutaway areas.

[0026] FIG. 12 depicts a cross sectional view of an exemplar substrate employed in FIG. 11 before being combined with populated flex circuits.

[0027] FIG. 13 depicts another embodiment of the invention having additional ICs.

[0028] FIG. 14 is a representation of impedance discontinuities in typical FB-DIMM systems.

[0029] FIG. 15 is a representation of impedance discontinuities in an embodiment of the present invention.

[0030] FIG. 16 depicts yet another embodiment of the present invention.

[0031] FIG. 17 presents another embodiment of the present invention.

[0032] FIG. 18 depicts a low profile embodiment of the present invention.

[0033] FIG. 19 depicts one side of a flex circuit used in constructing a module according to an alternative embodiment of the present invention.

[0034] FIG. 20 is a perspective view of a module according to an alternative embodiment of the present invention.

[0035] FIG. 21 is an exploded depiction of a flex circuit cross-section according to one embodiment of the present invention.

[0036] FIG. 22 depicts a clock transmission line topology connecting to DIMM registers according to one embodiment of the present invention.

[0037] FIG. 23 depicts a clock the depicted topology connecting to SDRAM devices according to one embodiment of the present invention.

[0038] FIG. 24 depicts one perspective of an exemplar module devised in accordance with another preferred embodiment of the present invention.

[0039] FIG. 25 is another depiction of the relationship between flex circuitry and a substrate which has been patterned or windowed with cutaway areas.

[0040] FIG. 26 depicts a cross sectional view of exemplar substrate employed in FIG. 25 before being combined with populated flex circuits and as viewed along a line through windows of substrate.

[0041] FIG. 1 depicts a preferred embodiment devised in accordance with the present invention. Module 10 is depicted in FIG. 1 exhibiting ICs 18 and circuit 19.

[0042] FIG. 2 depicts a first side 8 of flex circuit 12 (“flex”, “flex circuitry”, “flexible circuit”) used in constructing a module according to an embodiment of the present invention. Flex circuit 12 is preferably made from one or more conductive layers supported by one or more flexible substrate layers as further described with reference to later Figs. The construction of flex circuitry is known in the art. The entirety of the flex circuit 12 may be flexible or, as those of skill in the art will recognize, the flexible circuit structure 12 may be made flexible in certain areas to allow conformability to required shapes or bends, and rigid in other areas to provide rigid and planar mounting surfaces. Preferred flex circuit 12 has openings 17 for use in aligning flex circuit 12 to substrate 14 during assembly.

[0043] ICs 18 on flexible circuit 12 are, in this embodiment, chip-scale packaged memory devices of small scale. For purposes of this disclosure, the term chip-scale or “CSP” shall refer to integrated circuitry of any function with an array package providing connection to one or more die through contacts (often embodied as “bumps” or “balls” for example) distributed across a major surface of the package or die. CSP does not refer to leaded devices that provide connection to an integrated circuit within the package through leads emergent from at least one side of the periphery of the package such as, for example, a TSOP.

[0044] Embodiments of the present invention may be employed with leaded or CSP devices or other devices in both packaged and unpackaged forms but where the term CSP is used, the above definition for CSP should be adopted. Consequently, although CSP excludes leaded devices, references to CSP are to be broadly construed to include the large variety of array devices (and not to be limited to memory only) and whether die-sized or other size such as BGA and micro BGA as well as flip-chip. As those of skill will understand after appreciating this disclosure, some embodiments of the present invention may be devised to employ stacks of ICs each disposed where an IC 18 is indicated in the exemplar Figs.

[0045] Multiple integrated circuit die may be included in a package depicted as a single IC 18. While in this embodiment memory ICs are used to provide a memory expansion board or module, and various embodiments may include a variety of integrated circuits and other components. Such variety may include microprocessors, FPGA's, RF transceiver circuitry, digital logic, as a list of non-limiting examples, or other circuits or systems which may benefit from a high-density circuit board or module capability. Circuit 19 depicted between ICs 18 may be a memory buffer, or controller (“register”) as are used in common DIMMs such as, for example, registered-DIMMs. In a preferred embodiment is the well known advanced memory buffer or “AMB”.

[0046] The depiction of FIG. 2 shows flex circuit 12 as having first and second fields F1 and F2. Each of fields F1 and F2 have at least one mounting contact array for CSPs such as the one depicted by reference 11A. Contact arrays such as array 11 are disposed beneath ICs 18 and circuits 19. An exemplar contact array 11A is shown as is exemplar IC 18 to be mounted at contact array 11A as depicted. The contact arrays 11A that correspond to an IC plurality may be considered a contact array set.

[0047] Field F1 of side 8 of flex circuit 12 is shown populated with first plurality of CSPs IC_R1 and second plurality of CSPs IC_R2 while second field F2 of side 8 of flex circuit 12 is shown populated with first plurality of CSPs IC_R1 and second plurality of CSPs IC_R2. Those of skill will recognize that the identified pluralities of CSPs are, when disposed in the configurations depicted, typically described as “ranks”. Between the ranks IC_R2 of field F1 and IC_R2 of field F2, flex circuit 12 bears a plurality of module contacts allocated in this embodiment into two rows (C_R1 and C_R2) of module contacts 20. When flex circuit 12 is folded as later depicted, side 8 depicted in FIG. 2 is presented at the outside of module 10. The opposing side 9 of flex circuit 12 is on the inside in several depicted configurations of module 10 and thus side 9 is closer to the substrate 14 about which flex circuit 12 is disposed than is side 8. Other embodiments may have other numbers of ranks and combinations of plural CSPs connected to create the module of the present invention.

[0048] FIG. 3 shows side 9 of flex circuit 12 depicting the other side of the flex circuit shown in FIG. 2. Side 9 of flex circuit 12 is shown as being populated with multiple CSPs 18. Side 9 includes fields FI and F2 that each include at least one mounting contact array site for CSPs and, in the depicted case, include multiple contact arrays. Each of fields F1 and F2 include, in the depicted preferred embodiment, two pluralities of ICs identified in FIG. 3 as IC_R1 and IC_R2. Thus, each side of flex circuit 12 has, in a preferred embodiment, two fields F1 and F2 each of which fields includes two ranks of CSPs IC_R1 and IC_R2. In later FIG. 4, it will be recognized that fields F1 and F2 will be disposed on different sides of substrate 14 in a completed module 10 when ICs 18 are identified according to the organizational identification depicted in FIGS. 2 and 3 but those of skill will recognize that the groupings of ICs 18 shown in FIGS. 2 and 3 are not dictated by the invention but are provided merely as an exemplar organizational strategy to assist in understanding the present invention.

[0049] Various discrete components such as termination resistors, bypass capacitors, and bias resistors, in addition to the buffers 19 shown on side 8 of flex circuit 12, may be mounted on either or both of sides 8 and 9 of flex 12. Such discrete components are not shown to simplify the drawing. Flex circuit 12 may also depicted with reference to its perimeter edges, two of which are typically long (PE_long1 and PE_{long 2}) and two of which are typically shorter (PE_short1 and PE_short2) Other embodiments may employ flex circuits 12 that are not rectangular in shape and may be square in which case the perimeter edges would be of equal size or other convenient shape to adapt to manufacturing particulars. Other embodiments may also have fewer or greater numbers of ranks or pluralities of ICs in each field or on a side of a flex circuit.

[0050] FIG. 2 depicts an exemplar conductive trace 21 connecting row C_R2 of module contacts 20 to ICs 18. Those of skill will understand that there are many such traces in a typical embodiment. Traces 21 may also connect to vias that may transit to other conductive layers of flex 12 in certain embodiments having more than one conductive layer. In a preferred embodiment, vias connect ICs 18 on side 9 of flex 12 to module contacts 20. An example via is shown as reference 23. Traces 21 may make other connections between the ICs on either side of flex 12 and may traverse the rows of module contacts 20 to interconnect ICs. Together the various traces and vias make interconnections needed to convey data and control signals amongst the various ICs and buffer circuits. Those of skill will understand that the present invention may be implemented with only a single row of module contacts 20 and may, in other embodiments, be implemented as a module bearing ICs on only one side of flex circuit 12.

[0051] FIG. 4 is a cross section view of a module 10 devised in accordance with a preferred embodiment of the present invention. Module 10 is populated with ICs 18 having top surfaces 18^T and bottom surfaces 1 ⁸B-Substrate or support structure 14 has first and second perimeter edges 16A and 16B appearing in the depiction of FIG. 4 as ends. Substrate or support structure 14 typically has first and second lateral sides S₁ and S₂. Flex 12 is wrapped about perimeter edge 16A of substrate 14, which in the depicted embodiment, provides the basic shape of a common DIMM board form factor such as that defined by JEDEC standard MO-256.

[0052] FIG. 5 is an enlarged view of the area marked ‘A’ in FIG. 4. Edge 16A of substrate 14 is shaped like a male side edge of an edge card connector. While a particular oval-like configuration is shown, edge 16A may take on other shapes devised to mate with various connectors or sockets. The form and function of various edge card connectors are well know in the art. In many preferred embodiments, flex 12 is wrapped around edge 16A of substrate 14 and may be laminated or adhesively connected to substrate 14 with adhesive 30. The depicted adhesive 30 and flex 12 may vary in thickness and are not drawn to scale to simplify the drawing. The depicted substrate 14 has a thickness such that when assembled with the flex 12 and adhesive 30, the thickness measured between module contacts 20 falls in the range specified for the mating connector. In some other embodiments, flex circuit 12 may be wrapped about perimeter edge 16B or both perimeter edges 16A and 16B of substrate 14. In other instances, multiple flex circuits may be employed or a single flex circuit may connect one or both sets of contacts 20 to the resident ICs.

[0053] FIG. 6 depicts a cross-sectional view of a module 10 devised in accordance with an alternate preferred embodiment of the present invention with the view taken along a line through two AMBs and selected ICs 18 from IC_R1. The module 10 depicted in FIG. 6 differs from that shown in earlier embodiments in that rather than a single flex circuit 12, the depicted exemplar module 10 employs two flex circuits 12A and 12B with 12A being disposed on one lateral side S1 of substrate 14 while flex circuit 12B is employed on lateral side S2 of substrate 14.

[0054] FIG. 7 depicts the area near end 16A of substrate 14 in the embodiment shown in FIG. 6 that employs two flex circuits identified as 12A and 12B to implement a module in accordance with an alternate preferred embodiment of the present invention. Each of flex circuits 12A and 12B are populated with ICs 18 on one or both of their respective sides 8 and 9 and each of flex circuits 12A and 12B employ a buffer circuit 19 such as, for example, an advanced buffer circuit or AMB to implement, along with the resident CSPs, multiple FB-DIMM circuits mounted on a single module 10. The area on side 9 of each of flex circuits 12A and 12B opposite the disposition of buffer circuit 19 disposed along side 8 of flex circuits 12A and 12B is, in the depicted module, filled with a conformal material 31 to provide support along the length of module 10 where structure is not provided by the bodies of circuits such as ICs 18 or buffers 19.

[0055] FIG. 8 depicts a cross-sectional view of a module 10 devised with a substrate 14 that has cutaway areas into which ICs 18 are disposed to reduce the profile of module 10. Corresponding ICs 18 from each of fields F1 and F2 pass through windows 250 in substrate 14 as shown in later Figs. in further detail and the inner ICs 18 are preferably attached to each other's upper surfaces 18_T with a thermally conductive adhesive 30. While in this embodiment, the depicted ICs are attached to flex circuit 12 in opposing pairs, fewer or greater numbers of ICs may be connected in other arrangements such as, for example, staggered or offset arrangements in which they may exhibit preferred thermal characteristics. In a preferred embodiment, ICs 18 will be CSPs and typically, memory CSPs. To simplify the drawing, discrete components such as resistors and capacitors typically found on embodiments of module 10 are not shown.

[0056] In this embodiment, flex circuit 12 has module contacts 20 positioned in a manner devised to fit in a circuit board card edge connector or socket and connect to corresponding contacts in the connector (not shown). While module contacts 20 are shown protruding from the surface of flex circuit 12, other embodiments may have flush contacts or contacts below the surface level of flex 12. Substrate 14 supports module contacts 20 from behind flex circuit 12 in a manner devised to provide the mechanical form required for insertion into a socket. In other embodiments, the thickness or shape of substrate 14 in the vicinity of perimeter edge 16A may differ from that in the vicinity of perimeter edge 16B. Substrate 14 in the depicted embodiment is preferably made of a metal such as aluminum or copper, as non-limiting examples, or where thermal management is less of an issue, materials such as FR4 (flame retardant type 4) epoxy laminate, PTFE (poly-tetra-fluoro-ethylene) or plastic. In another embodiment, advantageous features from multiple technologies may be combined with use of FR4 having a layer of copper on both sides to provide a substrate 14 devised from familiar materials which may provide heat conduction or a ground plane.

[0057] FIG. 9 is an enlarged view of a portion of one preferred embodiment showing lower IC 18₁ and upper IC 18₂ and substrate 14 having cutaway areas into which ICs 18 are disposed. In this embodiment, conductive layer 66 of flex circuit 12 contains conductive traces connecting module contacts 20 to BGA contacts 63 on ICs 18₁ and 18₂. The number of layers may be devised in a manner to achieve the bend radius required in those embodiments that bend flex circuit 12 around edge 16A or 16B, for example. The number of layers in any particular portion of flex circuit 12 may also be devised to achieve the necessary connection density given a particular minimum trace width associated with the flex circuit technology used. Some flex circuits 12 may have three or four or more conductive layers. Such layers may be beneficial to route signals in a FB-DIMM which may have fewer DIMM input/output signals than a registered DIMM, but may have more interconnect traces required among devices on the DIMM, such as, for example, the C/A copy A and C/A copy B (command/address) signals produced by an FB-DIMM AMB.

[0058] In this embodiment, there are three layers of flex circuit 12 between the two depicted ICs 18₁ and 18₂. Conductive layers 64 and 66 express conductive traces that connect to the ICs and may further connect to other discrete components (not shown). Preferably, the conductive layers are metal such as, for example, copper or alloy 110. Vias such as the exemplar vias 23 connect the two conductive layers 64 and 66 and thereby enable connection between conductive layer 64 and module contacts 20. In this preferred embodiment having a three-layer portion of flex circuit 12, the two conductive layers 64 and 66 may be devised in a manner so that one of them has substantial area employed as a ground plane. The other layer may employ substantial area as a voltage reference plane. The use of plural conductive layers provides advantages and the creation of a distributed capacitance intended to reduce noise or bounce effects that can, particularly at higher frequencies, degrade signal integrity, as those of skill in the art will recognize. If more than two conductive layers are employed, additional conductive layers may be added with insulating layers separating conductive layers. Portions of flex circuit 12 may in some embodiments be rigid portions (rigid-flex). Construction of rigid-flex circuitry is known in the art.

[0059] With the construction of an embodiment such as that shown in FIG. 9, thermal energy will be urged to move between the respective ICs 18₁. Thus, the ICs become a thermal mass sharing the thermal load. Flex circuit 12 may be particularly devised to operate as a heat spreader or sink adding to the thermal conduction out of ICs 18₁ and 18₂.

[0060] FIG. 10 depicts one perspective of an exemplar module 10 devised in accordance with a preferred embodiment of the present invention. As those of skill will understand, the depiction of FIG. 10 is simplified to show more clearly the principles of the invention but depicts fewer ICs 18 than would typically be presented in embodiments of the present invention.

[0061] The principles of the present invention may, however, be employed where only one IC 18 is resident on a side of a flex circuit 12 or where multiple ranks or pluralities of ICS are resident on a side of flex circuit 12, or, as will be later shown, where multiple ICs 18 are disposed one atop the other to give a single module 10 materially greater.

[0062] FIG. 10 depicts a cross sectional view of an exemplar module showing a reduced number of ICs 18 to allow a clearer exposition of the principles of the present invention as illustrated by this depicted embodiment. The module shown in FIG. 10 is formed with an exemplar flex circuit such as that depicted in FIGS. 2 and 3. The second side 9 of flex circuit 12 shown in FIG. 3 is folded about substrate 14 shown in FIG. 10 to place ICs 18 into the windows 250 arrayed along substrate 14. This results in ICs of ranks IC_R1 and IC _R2 being disposed back to back within windows 250. Preferably, a thermally conductive adhesive or glue is used on the upper sides of ICs 18 to encourage thermal energy flow as well as provide some mechanical advantages. Those of skill will recognize that in this embodiment, where FIG. 10 depicts the first or, in this case, the outer side of the flex circuit once combined with substrate 14, the flex circuit itself will have staggered mounting arrays 11A on side 8 of flex circuit 12 relative to side 9 of flex circuit 12. This is merely one relative arrangement between ICs 18 on respective sides of substrate 14.

[0063] As shown in FIG. 10, ICs 18 which are on second side 9 (which in this depiction is the inner side with respect to the module 10) of populated flex circuit 12 are disposed in windows 250 so that the upper surfaces 18_T of ICs 18 of row ICR1 of F1 are in close proximity with the upper surfaces 18_T of ICs 18 of rank ICR1 of F2. Thus, a first group of ICs (CSPs in the depiction) may be considered to be comprised of the ICs of IC _R1 from both fields F1 and F2 while a second group of ICs may be considered to be comprised of the ICs of IC_R2 from both fields F1 and F2. The ICs 18 that are populated along side 9 of flex circuit 12 are positioned in the cutaway areas of the first and second lateral sides, respectively, of substrate 14. In this case, the cutaway areas on each lateral side of substrate 14 are in spatial coincidence to create windows 250. Those of skill will recognize that the depiction is not to scale but representative of the interrelationships between the elements and the arrangement results in a profile “P” for module 10 that is significantly smaller than it would have been without fitting ICs 18 along inner side 9 of flex circuit 12 into windows 250. Profile P in this case is approximately the sum of the distances between the upper and lower surfaces of IC 18 plus 4× the diameter of the BGA contacts 63 plus 2× the thickness of flex circuit 12 in addition to any adhesive layers 30 employed to adhere one IC 18 to another. This profile dimension will vary depending upon whether BGA contacts 63 are disposed below the surface of flex circuit 12 to reach an appropriate conductive layer or contacts which typically are a part of flex circuit 12.

[0064] FIG. 11 is another depiction of the relationship between flex circuitry and a substrate 14 which has been patterned or windowed with cutaway areas. The view of FIG. 11 is taken along a line that would intersect the bodies of ICs 18. In FIG. 11, two flex circuits 12A and 12B are shown populated along their respective sides 9 with ICs 18 (i.e., CSPs in the depiction). The ICs 18 along the inner side 9 of flex circuit 12A are staggered relative to those that are along inner side 9 of flex circuit 12B when module 10 is assembled and flex circuits 12A and 12B are combined with substrate 14. This staggering may result in some construction benefits providing a mechanical “step” for ICs 18 as they are fitted into substrate 14 and may further provide some thermal advantages increasing the contact area between substrate 14 and the plurality of ICs 18. Those of skill will recognize that flex circuits 12A and 12B even though depicted as being populated on only one side, may be populated on either or both sides 8 and 9 just as in those embodiments that employ a single flex circuit 12 may be populated one or both sides of flex circuit 12 and may have populated one or both fields or ranks within fields on one or both sides with CSPs or other circuits.

[0065] FIG. 12 depicts a cross sectional view of exemplar substrate 14 employed in FIG. 11 before being combined with populated flex circuits 12A and 12B as viewed along a line through windows 250 of substrate 14. As depicted in FIG. 12, a number of cutaway areas or pockets are delineated with dotted lines and identified with references 250B3 and 250B4, respectively. Those areas identified as 250B3 correspond, in this example, to the pockets, sites, or cutaway areas on one side of substrate 14 into which ICs 18 from flex circuit 12A will be disposed when substrate 14 and flex circuit 12A are combined. Those pocket, sites, or cutaway areas identified as references 250B4 correspond to the sites into which ICs 18 from flex circuit 12B will be disposed.

[0066] For purposes herein, the term window may refer to an opening all the way through substrate 14 across span “S” which corresponds to the width: or height dimension of packaged IC 18 or, it may also refer to that opening where cutaway areas on each of the two sides of substrate 14 overlap.

[0067] Where cutaway areas 250B3 and 250B4 overlap, there are, as depicted, windows all the way through substrate 14. In some embodiments, cutaway areas 250B3 and 250B4 may not overlap or in other embodiments, there may be pockets or cutaway areas only on one side of substrate 14. Those of skill will recognize that cutaway areas such as those identified with references 250B3 and 250B4 may be formed in a variety of ways depending on the material of substrate 14 and need not literally be “cut” away but may be formed by a variety of molding, milling and cutting processes as is understood by those in the field.

[0068] FIG. 13 depicts another embodiment of the invention having additional ICs 18. In this embodiment, four flex level transitions 26 connect to four mounting portions 28 of flex circuits 12A₁, 12A₂, 12B₁, and 12B₂. In this embodiment, each mounting portion 28 has ICs 18 on both sides. Flex circuitry 12 may also be provided in this configuration by, for example, having a split flex with layers interconnected with vias. As those of skill will recognize, the possibilities for large capacity iterations of module 10 are magnified by such strategies and the same principles may be employed where the ICs 18 on one side of substrate 14 are staggered relative to those ICs 18 on the other side of substrate 14 or, substrates such as those shown in FIG. 4 that have no cutaway areas may be employed.

[0069] Four flex circuits are employed in module 10 as depicted in FIG. 13 and, although those embodiments that wrap flex circuit 12 about end 16A of substrate 14 present manufacturing efficiencies, in some environments having flex circuitry separate from each other may be desirable.

[0070] In a typical FB-DIMM system employing multiple FB-DIMM circuits, the respective AMB's from one FB-DIMM circuit to another FB-DIMM circuit are separated by what can be conceived of as three impedance discontinuities as represented in the system depicted in FIG. 14 as D1, D2, and D3. FIG. 14 includes two modules 10 and includes a representation of the connection between the two modules. Discontinuity D1 represents the impedance discontinuity effectuated by the connector—socket combination associated with the first module 10F. Discontinuity D2 represents the impedance perturbation effectuated by the connection between the connector-socket of first module 10F and the connector-socket of second module 10S while discontinuity D3 represents the discontinuity effectuated by the connector-socket combination associated with the second module 10S. The AMB is the new buffer technology particularly for server memory and typically includes a number of features including pass-through logic for reading and writing data and commands and internal serialization capability, a data bus interface, a deserialing and decode logic capability and clocking functions. The functioning of an AMB is the principal distinguishing hard feature of a FB-DIMM module. Those of skill will understand how to implement the connections between ICs 18 and AMB 19 in FB-DIMM circuits implemented by embodiments of the present invention and will recognize that the present invention provides advantages in capacity as well as reduced impedance discontinuity that can hinder larger implementations of FB-DIMM systems. Further, those of skill will recognize that various principles of the present invention can be employed to multiple FB-DIMM circuits on a single substrate or module.

[0071] In contrast to the system represented by FIG. 14, FIG. 15 is a schematic representation of the single impedance perturbation DX effectuated by the connection between a first AMB 19 of a first FB-DIMM “FB1” of a first module 10F and a second AMB 19 of a second FB-DIMM “FB2” of the same first module 10F.

[0072] FIG. 16 depicts another embodiment of the present invention in which a module 10 is devised using stacks to create a module 10 presenting two FB-DIMM circuits. Those of skill will appreciate that using stacks such as depicted stacks 40 owned by Staktek Group L.P. allows creating modules that have multiple FB-DIMM circuits on a single module. Stacks 40 are just one of several stack designs that may be employed with the present invention. Stacks 40 are devised with mandrels 42 and stack flex circuits 44 as described in U.S. patent application Ser. No. 10/453,398, filed Jun. 6, 2003 which is owned by Staktek Group L.P. and which is hereby incorporated by reference and stacks 40 and AMB 19 are mounted on flex circuit 12 which is disposed about substrate 14.

[0073] FIG. 17 depicts use of stacks in an embodiment of the present invention that exhibits a low profile with use of stacks. Such an embodiment presents at least two FB-DIMM circuits at its contacts 20.

[0074] FIG. 18 illustrates a low profile embodiment of the present invention. The depicted module 10 has at least two AMBs and associated circuitry such as ICs 18 which in the preferred mode and as illustrated are CSPs and needed support circuitry to create at least two FB-DIMM circuits or instantiations on a single module with a low profile. It should be understood that the second AMB in addition to the one literally shown can be disposed on either side of module 10 but preferably will be disposed closer to lateral side S2 of substrate 14 than is the depicted AMB 19 but like AMB 19 will be disposed on side 8 of flex circuit 12. In this embodiment, contacts 20 are along side 8 of flex circuit 12 and proximal to edge E of flex circuit 12. The principal circuits that constitute the first FB-DIMM circuitry or instantiation (i.e., the CSPs and AMB) may be disposed in single rank file as shown. They may be allocated to first and second mounting fields of the first and second sides of flex circuit 12 as earlier described with reference to earlier Figs. Those of skill will recognize that contacts 20 may appear on one or both sides of module 10 depending on the mechanical contact interface particulars of the application.

[0075] The present invention may be employed to advantage in a variety of applications and environment such as, for example, in computers such as servers and notebook computers by being placed in motherboard expansion slots to provide enhanced memory capacity while utilizing fewer sockets. The two high rank embodiments or the single rank high embodiments may both be employed to such advantage as those of skill will recognize after appreciating this specification.

[0076] One advantageous methodology for efficiently assembling a circuit module 10 such as described and depicted herein is as follows. In a preferred method of assembling a preferred module assembly 10, flex circuit 12 is placed flat and both sides populated according to circuit board assembly techniques known in the art. Flex circuit 12 is then folded about end 16A of substrate 14. Flex 12 may be laminated or otherwise attached to substrate 14.

[0077] FIG. 19 depicts a first side 8 of flex circuit 12 (“flex”, “flex circuitry”, “flexible circuit”) used in constructing a module according to an embodiment of the present invention. ICs 18 on flexible circuit 12 are, in this embodiment, chip-scale packaged memory devices of small scale. Circuit 19 depicted between ICs 18 may be a memory buffer or controller such as, for example, an AMB, but in this embodiment is a memory controller or register for a registered DIMM. This embodiment will preferably have further IC's 18 on an opposite side 9, which is not depicted here. Flex circuit 12 is, in this embodiment, made from 4 conductive layers supported by flexible substrate layers as further described with reference to later Figs. The construction of flex circuitry is known in the art.

[0078] In this embodiment, flex circuit 12 is provided with holes 13, which are devised to allow greater flexibility for bending flex circuit 12 to achieve a desired bend radius for curve 25 (FIG. 20). Holes 13 (“holes”, “voids”, “partial voids”) preferably pass entirely through flex circuit 12, but in other embodiments may be only partial holes or voids that may be expressed by one or more of the conductive layers of flex circuit 12, and/or one or more of the flexible substrate layers of flex circuit 12. Such partial voids may be devised to allow flexibility while still providing sufficient conductive material to allow the desired connections to contacts 20 and between the depicted ICs in field F1 and field F2.

[0079] Holes 13 in this embodiment are spaced to allow traces 21 to pass between them at the level of conductive layers of flex 13. While some preferred embodiments have a dielectric solder mask layer partially covering side 8, traces 21 are depicted along side 8 for simplicity. Traces 21 may, of course, be at interior conductive layers of flex circuit 12, as will be described further with regard to later referenced Figures.

[0080] In this embodiment, flex circuit 12 is further provided with holes 15. Holes 15 are devised to allow flexibility for bending flex circuit 12 to achieve a desired bend radius for around edge 16A or 16B of substrate 14, for example. Holes 15 may be expressed as voids or partial voids in the various conductive and non-conductive layers of flex circuit 12. Further, a desired bend radius at the portions of flex circuit 12 provided with holes 13 or holes 15 may also be achieved by providing a portion of flex circuit 12 having fewer layers, as described above with reference to FIG. 9.

[0081] This embodiment of flex circuit 12 is also provided with mounting pads 191 along side 18 of flex circuit 12. Such pads 191 are used for mounting components such as, for example, surface mount resistors 192.

[0082] FIG. 20 is a perspective view of a module 10 according to an embodiment of the present invention. Depicted are holes 13 along curve 25. Further, parts of holes 15 can be seen along the lower depicted edge of module 10. Holes 13 may have an extent such that they are present along the entirety of curve 25. Holes 15 may also be sized such that they span the entire bend around the edge of substrate 14. Holes 13 and 15 may have a span greater than the length of their respective curves, or less that such length. For example, holes 13 and 15 may be sized such that they provide an adjusted bend radius for flex circuit 12 only in portions of the bend having a desired bend radius smaller that the radius possible with an unmodified flex circuit 12.

[0083] When flex circuit 12 is folded as depicted, side 8 depicted in FIG. 2 is presented at the outside of module 10. The opposing side 9 of flex circuit 12 is on the inside in several depicted configurations of module 10 and thus side 9 is closer to the substrate 14 about which flex circuit 12 is disposed than is side 8.

[0084] The depicted topology and arrangement of flexible circuitry may be used to advantage to create high capacity and thin-profile circuit modules. Such modules include, for example, registered DIMMS and FB-DIMMs. For example, a DIMM may be constructed having double device-mounting surface area for a given DIMM height. Such doubling may allow doubling of the number memory devices or enable larger devices that would not fit on traditional DIMMs.

[0085] For example, one preferred embodiment provides a 30 mm 4-GByte RDIMM using 512 Mbit parts. Another embodiment provides a 50 mm 8-GByte RDIMM using 1 Gbit parts. Yet another embodiment provides a 2-GByte SO-DIMM using 512 Mbit parts. DIMM modules may be provided having multiple instantiations of DIMM or FB-DIMM circuits, as further described herein. Also, DIMMs having the usual single instantiation of DIMM circuitry may be provided where the devices employed are too large to fit in the surface area provided by a typical industry DIMM module. Such high-capacity capability may be used to advantage to provide high capacity memory for computer systems having a limited number of motherboard DIMM slots.

[0086] FIG. 21 is an exploded depiction of a flex circuit 12 cross-section according to one embodiment of the present invention. The depicted flex circuit 12 has four conducive layers 2101-2104 and seven insulative layers 2105-2111. The numbers of layers described are merely those of one preferred embodiment, and other numbers and layer arrangements may be used.

[0087] Top conductive layer 2101 and the other conductive layers are preferably made of a conductive metal such as, for example, copper or alloy 110. In this arrangement, conductive layers 2101, 2102, and 2104 express signal traces 2112 that make various connections on flex circuit 12. These layers may also express conductive planes for ground, power, or reference voltage. For example, top conductive layer 2101 may also be provided with a flood, or plane, of to provide the VDD to ICs mounted to flex circuit 12.

[0088] In this embodiment, inner conductive layer 2102 expresses traces connecting to and among the various devices mounted along the sides of flex circuit 12. The function of any of the depicted conductive layers may, of course, be interchanged with others of the conductive layers. Inner conductive layer 2103 expresses a ground plane, which may be split to provide VDD return for pre-register address signals. Inner conductive layer 2103 may further express other planes and traces. In this embodiment, floods, or planes, at bottom conductive layer 2104 provides VREF and ground in addition to the depicted traces.

[0089] Insulative layers 2105 and 2111 are, in this embodiment, dielectric solder mask layers which may be deposited on the adjacent conductive layers. Insulative layers 2107 and 2109 are made of adhesive dielectric. Other embodiments may not have such adhesive dielectric layers. Insulative layers 2106, 2108, and 2110 are preferably flexible dielectric substrate layers made of polyamide. Any other suitable flexible circuit substrate material may be used.

[0090] FIG. 22 depicts a clock transmission line topology connecting to DIMM registers according to one embodiment of the present invention. In this embodiment, four DIMM circuit instantiations are used on one module, one being on each of the four fields available on a flex circuit 12 such as, for example, the one in FIG. 2. The depicted transmission line topology shows the distribution of a clock input signal to each of four registers associated respectively with the four DIMM circuit instantiations. The transmission lines are expressed by the various conductive layers of the flex circuit, and may include vias passing between layers.

[0091] In this embodiment, clock and inverted clock signals CK and CK# enter the depicted topology from a phase-locked-loop (PLL) or delay-locked loop (DLL) output. Construction of PLLs and DLLs is known in the art. The PLL in this embodiment is preferably mounted along one side of flex circuit 12, and the depicted topology routes clock signal CK to DIMM registers 2201 on the same side of flex circuit 12 as the PLL circuitry, as well as to DIMM registers 2201 on the opposing side. Transmission line TL0 branches to two transmission lines TL1, which may be, in some embodiments, disposed at opposite sides of a substrate 14. Each transmission line TL1 branches into two TL4 lines and a TL2 line. Each transmission line TL4 has a termination resistor R1 or a bypass capacitor C1. Transmission line TL2 branches into two TL3 lines. A via through flex circuit 12 may be used at the branchpoint from TL2 to TL3. Preferably, TL2 is relatively short so as to place bypass capacitors C1 relatively close to the branchpoint of TL2 and TL3.

[0092] FIG. 23 depicts a clock the depicted topology connecting to SDRAM devices according to one embodiment of the present invention. The depicted transmission line topology routes clock signal CK to devices on both sides of flex circuit 12. Transmission line TL230 branches to a transmission line TL231 terminated with a resistor R231, and two transmission lines TL232, which may be, in some embodiments, disposed at opposite sides of a substrate 14. Each transmission line TL232 branches into two TL233 lines. A via through flex circuit 12 may be used at the branchpoint from TL232 to TL233.

[0093] FIG. 24 depicts one perspective of an exemplar module 10 devised in accordance with another preferred embodiment of the present invention. As those of skill will understand, the depiction of FIG. 24 is simplified to show more clearly the principles of the invention but depicts fewer ICs 18 than would typically be presented in embodiments of the present invention. The module shown in FIG. 24 is formed similarly to that in FIG. 10, but has a thinner substrate 14.

[0094] FIG. 25 is another depiction of the relationship between flex circuitry and a substrate 14 which has been patterned or windowed with cutaway areas. The view of FIG. 25 is taken along a line that would intersect the bodies of ICs 18. Assembly of the depicted arrangement is similar to that described with reference to FIG. 11. In this embodiment, however, substrate 14 does not have variations in thickness along different portions of module 10.

[0095] FIG. 26 depicts a cross sectional view of exemplar substrate 14 employed in FIG. 25 before being combined with populated flex circuits 12A and 12B as viewed along a line through windows 250 of substrate 14. As depicted in FIG. 26, a number of cutaway areas or pockets are delineated with dotted lines and identified with references 250B3 and 250B4, respectively.

[0096] Although the present invention has been described in detail, it will be apparent to those skilled in the art that many embodiments taking a variety of specific forms and reflecting changes, substitutions and alterations can be made without departing from the spirit and scope of the invention. Therefore, the described embodiments illustrate but do not restrict the scope of the claims.