Composite core circuit module system and method

[0001] The present invention relates to systems and methods for creating high density circuit modules.

[0002] 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 interface edge of the DIMM into an edge connector socket. Systems that employ DIMMs provide limited space for such devices and conventional DIMM-based solutions have typically provided only a moderate amount of memory expansion.

[0003] As die sizes increase, the limited surface area available on conventional DIMMs limits the number of devices that may be carried on a memory expansion module devised according to conventional DIMM techniques. Further, 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 addressed 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 signal pins from the approximately 240 pins previously required.

[0004] This improvement has, however, come with some cost. 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.

[0005] A variety of techniques and systems for enhancing the capacity of DIMMs and similar modules are known. For example, multiple die may be packaged in a single IC package. A DIMM module may then be populated with such multi-die devices. However, multi-die fabrication and testing is complicated and few memory and other circuit designs are available in multi-die packages.

[0006] Other techniques have populated FR4 circuit boards with stacks comprised of packaged integrated circuits. Others techniques have employed daughter cards to increase module capacities. Typically, however, as more circuitry is aggregated on a circuit module, thermal issues become more prominent.

[0007] Thermally efficient solutions have been offered by Staktek Group L.P., the assignee of the present invention. For example, Staktek has devised a number of circuit module designs that dispose IC-populated flexible circuitry about a thermally-conductive core to provide a thin and thermally efficient circuit module that may supplant traditional DIMMs such as, for example, registered DIMMs as well as FB-DIMMs to name a few modules that may employ such technologies.

[0008] Alternatives to traditional FR4 cored circuit modules have found profitable employment in applications where thermal performance is a significant consideration. Consequently, new designs that provide the advantages of thermal performance with readily understood and inexpensive materials are welcome in the field.

[0009] A circuit module is provided in which at least one secondary substrate and preferably two such secondary substrates are populated with integrated circuits (ICs). A rigid core substrate for the circuit module is comprised of a structural member and a connective member. In a preferred embodiment, the structural member is comprised of thermally conductive material while the connective member is comprised of conventional PWB material. The secondary substrate(s) are connected to the connective member with a variety of techniques and materials while, in a preferred embodiment, the connective member exhibits, in a preferred embodiment, traditional module contacts which provide an edge connector capability to allow the module to supplant traditional DIMMs.

[0010] FIG. 1 is a depiction of a module devised in accordance with a preferred embodiment of the present invention.

[0011] FIG. 2 is an enlarged depiction of the area designated by an “A” in FIG. 1.

[0012] FIG. 3 is an enlarged depiction of a connective member of a substrate prepared for attachment of secondary substrates.

[0013] FIG. 4 is a perspective view of a circuit module in accordance with a preferred embodiment of the present invention.

[0014] FIG. 5 is an enlarged depiction of the area of FIG. 4 designated with a “B”.

[0015] FIG. 6 is an enlarged depiction of a portion of a rigid core composite substrate before attachment of a secondary substrate.

[0016] FIG. 7 is a plan view of a connective member of a core composite substrate devised in accordance with a preferred embodiment of the present invention.

[0017] FIG. 8 is an enlarged depiction of the area of FIG. 7 designated by a “C”.

[0018] FIG. 9 is a perspective and enlarged view of a portion of a side of a connective member of a composite substrate devised in accordance with a preferred embodiment of the present invention.

[0019] FIG. 10 is a cross-sectional depiction of a circuit module devised in accordance with an alternative embodiment of the present invention.

[0020] FIG. 1 depicts module 10 devised in accordance with a preferred embodiment of the present invention. On each of sides 10A and 10B of module 10 are disposed secondary substrates, 21A and 21B, respectively, each of which is populated with ICs 18 that are, in the depicted embodiment, chip-scale packaged (“CSP”) memory devices. 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 such as the depicted IC contacts 28) distributed along 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.

[0021] 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.

[0022] Multiple integrated circuit die may be included in a package depicted as a single IC 18. In the depicted embodiment, memory ICs are represented to provide a memory expansion board or module. Various other embodiments may, however, employ a variety of integrated circuits and other components. Such variety may include microprocessors, FPGA's, RF transceiver circuitry, and digital logic, as a list of non-limiting examples, or other circuits or systems which may benefit from enhanced high-density circuit board or module capability. Thus, the depicted multiple instances of IC 18 may be devices of a first primary function or type such as, for example, memory, while other devices may be devices of a second primary function or type such as, for example, signal buffers, one example of which is the Advanced Memory Buffer (“AMB”) in the fully-buffered circuitry design for modules.

[0023] A rigid core composite substrate that will be identified as 14-15 provides structure for module 10 and will be said to be “composite,” meaning that it is comprised of a structural member 14 and connective member 15 and thus will be identified as rigid core substrate 14-15 or substrate 14-15. Structural member 14 of substrate 14-15 has lateral sides 14A and 14B along which are respectively disposed, secondary substrates 21A and 21B. Those of skill will recognize that module 10 may be devised to exhibit only one secondary substrate. Structural member 14 is preferably thermally conductive with a metallic material such as aluminum, for example, being preferred, but other materials are suitable as well. For example, certain carbon based materials can be devised as structural member 14 and certain plastics are known to provide thermal conductivity while providing fabrication advantages over carbon materials. Aluminum and other metallic materials are, however, typically cost effective and readily malleable materials from which structural member 14 may be preferentially comprised. Optional extension 16 of structural member 14 is shown as part of structural member 14 and encourages extraction of thermal energy from module 10. Extension 16 may also be appended to structural member 14.

[0024] Conductive member 15 is, preferably, PWB comprised of FR4 fabricated with plural layers to provide appropriate connections to module contacts 20 and conductive assembly 30 and thus, to secondary substrates 21A and 21B and their resident ICs. A variety of types of PWB or epoxy or fiber based materials and boards may be employed to implement conductive member 15 which is devised under principles that are well known to those in the industry. The use of familiar and traditional materials for connective member 14 provides advantages of economy and design familiarity that allow beneficial features of traditional DIMM manufacturing to be employed in a new setting that provides thermal and capacity advantages.

[0025] FIG. 2 is an enlarged depiction of the area identified with an “A” in FIG. 1. As shown in FIG. 2, connective member 15 and structural member 14 are joined together with adhesive 34 between parts 15E of connective member 15 and 14E, of structural member 14 to form composite substrate 14-15. Other techniques may be employed to connect structural member 14 and connective member 15 to form composite substrate 14-15. For example, later FIG. 10 illustrates a bolt assembly to connect structural member 14 and connective member 15.

[0026] Conductive assembly 30 is, in the depicted embodiment, comprised of plural pins 31 held together by matrix 32 which, in practice, pass through connective member 15 to provide a connection path between secondary substrates 21A and 21B, respectively, and module contacts 20 (e.g., edge connector contacts) of connective member 15. Conductive assembly 30 implements a connection path therefore, between ICs 18 and edge connector contacts 20 of module 10.

[0027] FIG. 3 depicts connective member 15 of composite substrate 14-15 and illustrates extension part 15E of connective member 15 and its surface 15A which provides an adhesive surface for connection of connective member 15 to structural member 14 of rigid core composite substrate 14-15. As earlier described, conductive assembly 30 is, this particular depicted embodiment, a pin assembly that comprises plural pins 31 held together by matrix 32.

[0028] FIG. 4 illustrates a perspective view of a circuit module 10 devised in accordance with a preferred embodiment of the present invention. Pins 31 are visible along the lower edge area of module 10 just above module contacts 20 which in this embodiment are edge connector contacts while the composite nature of core substrate 14-15 is illustrated.

[0029] FIG. 5 is an enlarged depiction of the area marked “B” in FIG. 4. Secondary substrate 21B is shown penetrated by and connected to pins 31.

[0030] FIG. 6 is an enlarged illustration of a portion of composite substrate 14-15 corresponding to the area designated with a “B” in FIG. 4 before attachment of secondary substrates. The two members 14 and 15 of composite substrate 14-15 are shown attached and conductive assembly 30 which, in this embodiment, is comprised of plural pins 31 is illustrated. Matrix 32 holds the plural pins together and, most of those embodiments that employ such a pin strategy, a conductive assembly will be comprised of multiple sub-assemblies each of which is comprised of plural pins 31 held together by a matrix or similar attachment 32.

[0031] FIG. 7 illustrates a connective member 15 in accordance with a preferred embodiment of the invention. Extension part 15E of connective member 15 is shown along the lateral extent of connective member 15 opposite the lateral portion adjacent to which plural module contacts 20 reside.

[0032] FIG. 8 is an enlarged depiction of a side 15B of connective member 15 of composite substrate 14-15 illustrating the use of more than one conductive assembly 30, in this case implemented with assemblies of pins 31. Connective member 15 has two major sides with side 15B being shown in FIG. 8. Shown in larger detail is extension part 15E that, in this embodiment, is employed to implement the attachment together of connective member 15 and structural member 14 of composite substrate 14-15. Other methods to attach together structural member 14 and connective member 15 will be apparent to those of skill in the art after appreciating this disclosure.

[0033] FIG. 9 is an enlarged illustration of side 15A of connective member 15, Pins 31 of conductive assembly 30 are shown emergent from penetrations 35 through connective member 15 that exhibit, in the depicted embodiment, conductive surfaces 36 which, in this embodiment, are sleeves that provide connection between the conductive network of connective member 15 and conductive member 30. Those of skill will understand that being typically comprised as a multi-layer PWB, connective member 15 will typically have conductive network articulated by traces and vias of the plural layers of the member.

[0034] FIG. 10 depicts an alternative embodiment in accordance with the present invention. As earlier stated, there are a variety of techniques that may occur to those of skill in the art to combine the structural member 14 and connective member 15. This embodiment illustrates another general technique in addition to the adhesive technique earlier shown. The depicted module 10 includes a rigid composite core substrate 14-15 comprised from structure member 14 and connective member 15 which are, in this embodiment, attached to each other through a structural connector 40 which, in this instance, is depicted as a bolt and nut combination although many other structural connectors are known in the art. Certainly both adhesive and structural connector(s) may be employed on the same module. There are also two conductive assemblies 30 shown. In the depicted instance, one conductive assembly may be impressed through connective portion 15 from the 15A side while another of the conductive assemblies may be impressed through connective portion 15 from the 15B side. References 38 identify optional discrete devices typically found on circuit modules.