Patent Application
20060250780
The present invention relates to systems and methods for creating high density circuit modules that improve interconnection designs for circuit boards.
As integrated circuits (ICs) increase in capacity, there is typically an increase in the interconnection density between ICs. Often, the circuit boards upon which ICs are mounted must have multiple layers of traces devised to route electrical signals between ICs. More density of connections typically requires more layers. Such an increase in layers increases the cost and material required to manufacture circuit boards.
In some circuit boards, interconnection trace density may be a constraint that determines the area of the circuit board. For example, a system may be constrained to a two-layer circuit board because of cost or thickness requirements. The integrated circuit devices and other devices that need to be mounted on the board may fit in 15 square inches, for example. If such a system has a high density of circuit board traces, the area required to fit all the interconnect traces may be, for example, 20 square inches. In such a case, the interconnect density, and not the area needed to mount the devices, determines the area of the circuit board.
Further, even in systems without such demanding interconnect density requirements, high interconnection density requires increased design effort to produce a route design or “layout” of the circuit board. High interconnection density may also increase the electrical interference or “noise” effect that a circuit board trace has on its neighboring traces.
Another problem associated with some circuit boards is sub-optimal placement of memory devices in proximity to microcontroller devices. For example, many network processors are installed on circuit boards in systems such as, for example, switches and routers. Often, DRAM memory for the network processor is mounted on the opposite side of the circuit board from the network processor. Such double-sided mounting is often needed because of inadequate surface board space or signal trace routing constraints. However, double-sided mounting has many drawbacks.
One such drawback is that the components on the back side of the circuit board often do not get enough cooling airflow. Another drawback is that population of double-sided circuit boards is more expensive than population of singe-sided circuit boards. Yet another drawback is that the crowded electrical signal traces along and through the circuit board may have poor signal integrity, or quality of electrical signals passing through the traces, due to noise and electrical trace properties.
What is needed, therefore, is a system for improving crowded circuit board interconnections while providing enhanced signal integrity. What is also needed is a system for improving cooling airflow on many circuit boards.
In some embodiments, a high density circuit module is provided having a support frame supporting a flexible circuit. A main integrated circuit and one or more support integrated circuits are mounted to the flexible circuit. The module is preferably mounted to a circuit board. Electrical connections between the main integrated circuit and the one or more support integrated circuits are made on the flexible circuit. In some embodiments, such a connection scheme can greatly reduce the number of interconnections needed on the circuit board.
In other embodiments, a main integrated circuit such as, for example, a network processor, is mounted to a flexible circuit. Support integrated circuits, such as, for example, memory devices used by the network processor, are mounted on side portions of the flexible circuit. The side portions are folded to place the support integrated circuits higher than the main integrated circuit. Such placement may be employed to preserve circuit board space. Also, such placement may direct cooling airflow over the main integrated circuit's heat sink.
Module 10 may be a computer module, digital signal processing module, or other logic module or submodule. Such modules are typically mounted on a board 8 such as, for example, a system motherboard or expansion board. However, this is not limiting and a module 10 may be mounted in other arrangements. Such modules often include a processor or logic device such as, for example, a microprocessor, a DSP, an ASIC, or an FPGA. Such a device is preferably embodied as base element CSP 14. Base element CSP 14 may also be other devices such as, for example, a memory register or buffer such as the fully-buffered advanced memory buffer (AMB). Heat sink 8, attached to base element CSP 14, may be any type of heat sink or structure for conducting heat away from an integrated circuit.
Support element CSPs 18 are, in this embodiment, mounted along flexible circuit 12 and are connected to base element CSP 14 through conductive traces of flex circuit 12. In preferred embodiments, support element CSPs 18 are memory CSPs such as, for example, DRAM devices. Other embodiments may include other types of support element CSPs 18 such as, for example, input-output (I/O) chips, buffers, co-processors, or other devices for supporting functionality of a processor unit. Further, while CSP devices are shown for both base and support elements, leaded devices or other structures for interconnecting ICs may be used. For example, flip-chip devices may be used. CSP packaged devices are merely preferred.
ICs 18 on flexible circuit 12 are, in the depicted embodiment, chip-scale packaged 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) distributed across a major surface of the package or die. 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.
The depicted CSPs 18 are mounted to portions of flexible circuit 12 that are bent vertically beside two sides of base element CSP 14. Such bent portions may be referred to as “wing portions”, and may have various shapes. Further, while two wing portions are shown, wing portions may be provided beside any side of base element CSP 14.
In this embodiment, support frame 16 is disposed adjacent to flexible circuit 12. Preferably, flexible circuit 12 is attached to support frame 16 with thermally conductive adhesive. The depicted support frame 16 is disposed between flexible circuit 12 and the body of base element CSP 14. A window through support frame 16 allows attachment of the CSP contacts of base element CSP 14 to flexible circuit 12.
Flex circuit 12 (“flex”, “flex circuitry”, “flexible circuit”) is preferably made from one or more conductive layers supported by one or more flexible substrate layers as found in U.S. patent application Ser. No. 10/934,027, for example. The entirety of the flex circuit 12 may be flexible or, as those of skill in the art will recognize, the flexible circuit 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. Flex circuit 12 will be further described when referencing later Figures.
In this embodiment, support element CSPs 18 are arranged into stacks 100, interconnected with flexible circuits 30. Flexible circuits 30 have an array of module contacts for connecting to flexible circuit 12. Examples of such stacks may be found in U.S. patent application Ser. No. 10/453,398, filed Jun. 3, 2003. While two-high stacks are shown, of course other embodiments may use higher stacks or may mix stacks with other devices.
The depicted arrows show flow of air from cooling fan 52 through fins of heat sink 6. Air flow slows and disperses at the opposite side of heat sink 6 from fan 52. The depicted structure of support frame 16, flex circuit 12, and support element ICs 18 provides additional air channeling structure to direct cooling airflow over the outer fins of heat sink 6. Further, the aligned placement of ICs 18 provides direction of airflow over and along the surfaces of ICs 18 for improved cooling performance.
A similar effect may be achieved with other embodiments such as, for example, the module depicted in
Further, the placement in
Bottom side 4 of flexible circuit 12 (
While in this embodiment one flexible circuit is shown, other embodiments may use more flexible circuits to achieve similar results in the overall structure of a module 10.
In step 903, flexible circuit 12 is attached to support frame 16. Preferably, flexible circuit 12 is laid flat and a layer of adhesive is applied to it. Then support frame 16 is affixed by placing on the adhesive. In step 904, bending tools are used to shape support frame 16 and flexible circuit 12 into their desired configuration. More than one bend may be applied on each end of flexible circuit 12. In step 905, the assembled module 10 is attached to a host circuit board 8.
Selected pairs of contacts 105 may be electrically connected by a via 106 devised to interconnect a base element CSP contact 20 with a module contact 21. Some contacts 105 may not have such interconnection. For example, the left-hand depicted contacts 105 are electrically isolated from each other. Such a scheme allows the left hand contact 20 to be electrically connected to a support element CSP contact 20. Further, some module contacts 21 may be connected to support element CSPs 18 and not base element CSP 14.
Interconnecting the base element CSP 14 to the support element CSPs through flexible circuit 12 allows a reduced number of interconnections through module contacts 21. Further, such interconnection typically allows reduction in the number of layers of circuit board 8. Interconnection on flexible circuit 12 may also provide signal interconnections with higher signal integrity, compared with making such connections through traces and vias on circuit board 8. Another advantage is that the unmatched module contacts 21, such as the left-hand depicted contact 21 in
Still further, such placement of support element CSPs may provide added system design options. For example, a network processor board may have memory support devices mounted on its circuit board. To increase the memory capacity of the system with similarly-sized memory devices, the circuit board design must be changed to hold more memory support devices. With a preferred module 10 according to the present invention, the circuit board 8 design may remain the same and changes be made only to flexible circuit 12.
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.
