When designing large computer systems it is desirable to establish connections between various printed circuit boards (PCBs) within the computer system. Historically, specialized connectors have been designed to accomplish the purpose of connecting various circuit boards together. Such connectors are generally designed to conform to the particular electrical and geometric requirements of a particular computer system layout. A problem with this approach is that considerable design time may be expended designing such connectors in such a way as to not disturb the signals being transmitted along high speed connections. Such connectors have been increasingly used for debugging operations for various functions, such as, for example, connecting a system under test to a logic analyzer for diagnostic purposes. However, since the connections in question are generally performed primarily during diagnostic operations, there is generally no space on the PCBs dedicated for such connections. This lack of space arises because the boards are designed with their ultimate operation in mind at which time diagnostic operations are usually not conducted. Moreover, connectors added for diagnostic purposes may degrade signal integrity by introducing inductances and capacitances into various signal paths.
As a result, later prior art solutions used temporary connections between PCBs known as flex circuits. FIG. 1 depicts deployment of flex circuit 102 in order to establish connections to a circuit board 101 for diagnostic purposes. Signal transmission generally proceeds from PCB 101 through interposer 105, which generally includes an array of conductive or resistive contacts, to flex circuit 102 and presented at connector 103 for connection to other circuit boards in a computer system. Although flex circuit 102 appears as a line in the cutaway view of FIG. 1, flex circuit 102 generally has a planar shape substantially matching the two dimensional footprint of interposer 105 and its associated array of conductive elements. The two dimensional shape of flex circuit 102 would generally be visible when viewed from above.
Flex circuit 102 is generally held in place with back plate 104 which is secured employing bolts 106. The portion of flex circuit 102 which extends beyond the back plate 104 is generally flexed upward to avoid contact with surface mount components 107 and generally terminates at connector 103 which may be employed to transmit signals originating in PCB 101 to other circuit boards in a computer system.
A problem which generally arises with the use of flex circuits is that the flexibility of the circuit generally requires that the material be kept quite thin. The thinness of the material tends to limit the number of signal layers, and thus the number of signals which may be transmitted along the flex circuit and tends to compromise the resulting signal quality. Accordingly, it is a limitation of the use of flex circuits that the number of signals which may be transmitted from circuit board to other parts of a computer system is limited. Moreover, the construction of the flex circuit precludes the use of a sufficient amount of protective material on either side of the conductive material, thereby generally compromising the signal integrity of the flex circuit transmission. Flex circuits may be also be difficult to obtain due to their specialized designs and difficulty of manufacture.
Another approach to the connection of printed circuit boards was to directly attach two circuits boards back to back employing an interposer disposed between the two boards. This was possible many years ago because the socket technologies were thicker than they are today, and the distance between the boards was therefore sufficient to avoid interference between surface mount components on the two boards. However, the thinness of socket technologies in use today makes such direct back to back PCB connection impractical.
Therefore, it is a problem in the art that space is usually not provided on a printed circuit board for connectors dedicated to diagnostic operations.
It is a further problem in the art that dedicated diagnostic connectors generally compromise signal integrity by introducing additional inductance and capacitance along the signal paths.
It is a still further problem in the art that flex circuits generally do not provide enough signal density to transmit a sufficient number of signals from a modem printed circuit board.
It is a still further problem in the art that flex circuits generally cause signal degradation because of a lack of sufficient shielding and dielectric of the conductive portion of the flex circuit structure.
These and other objects, features and technical advantages are achieved by a system and method which disposes a pedestal between two PCBs to provide for secure mechanical attachment of, and robust electrical connection between, the two PCBs. The pedestal is preferably dimensioned so as to space the two boards sufficiently far apart that even the tallest surface mount components on each board which point toward the attached board will not contact one another.
In a preferred embodiment, the pedestal has a generally rectangular cross section for convenience of manufacturing and is made of materials typically used for the production of PCBs themselves, such as, for instance, FR4, TEFLON(copyright), polyimide, Gore-Tex(copyright), epoxy impregnated expanded PTFE (TEFLON(copyright)), GETEK(copyright), RU-DUROID(copyright), and fiberglass-epoxy.
Effectively, the pedestal may operate as a special purpose printed circuit board, which instead of supporting a variety of different surface mount components, is primarily dedicated toward providing a mechanically and electrically robust connection between two general purpose circuit boards. However, small parts, very fast parts (rapidly responsive parts), and passive components including passive resistors could be disposed either on, or within the pedestal and all such variations are included within the scope of the present invention.
In a preferred embodiment, the pedestal is designed with sufficient thickness to allow for a far greater number of signal layer paths therein than was available with flex circuits or with connectors of the prior art. Moreover, the thickness of the pedestal enables ample protection of the conductive paths to be installed, thereby providing for much improved signal integrity over the flex circuits of the prior art. The pedestal may generally be manufactured separately from the other PCBs and then affixed to them at a convenient point in the production process. Connection between the general purpose PCBs and the pedestal may be made employing land grid arrays. Alternatively, the inventive mechanism could deploy ball grid arrays, or other form of conductive interposer or conductive interface. In a preferred embodiment, in view of the sturdiness, signal efficiency, and signal integrity of the inventive pedestal, the pedestal may be appropriate for both diagnostic purposes as well for permanent installation between the two circuit boards. In a preferred embodiment, additional benefit may be obtained from deployment of the inventive pedestal by disposing it on a general purpose PCB directly opposite an ASIC (Application Specific Integrated Circuit) installed on the opposite side of the same general purpose PCB. Generally, general purpose circuit boards incorporate particularly high density signal routing in the vicinity of ASIC connections because of the number of signals traveling in both directions near an ASIC attachment. Substantial economy of design and resources may be obtained by disposing the inventive pedestal and an ASIC on opposite sides of the same region of high density signal routing. Since many signals with destinations outside a PCB originate from ASICs, the conductive path and signal integrity between an ASIC and its destination may be improved by locating the ASIC opposite a pedestal, which is in turn preferably disposed in close proximity to another printed circuit board.
Furthermore, due to the improved quality of PCB to PCB signal connection provided by the inventive pedestal in comparison with the prior art, circuit connections which previously had to be entirely disposed on a single PCB may potentially be spread out between two or more PCBs when using the inventive pedestal, thereby providing greater flexibility in PCB circuit design.
An alternative approach to providing appropriate spacing between circuit boards while providing sufficient signal transmission quality is to generate a three dimensional circuit board design which provides a protrusion on one or more surfaces of the circuit board through milling or other process which rises substantially above the rest of the circuit board surface which protrusion is used for attachment to another circuit board. The protrusion is preferably thick enough that surface mount components on the board do make contact with a board attached to the surface of the protrusion. A problem in providing the above protrusion is that the cost producing boards having such protrusions are generally prohibitive because of the low production yield. Another problem is that holes drilled for vias in the region of the board having the protrusion generally need to be wider in order to sustain an acceptable yield rate with a thicker board. Finally, the described milling process generally experiences a fairly low yield rate due to the difficulty of the process involved.
Accordingly, it is an advantage of a preferred embodiment of the present invention that a robust mechanical attachment between two circuit boards is provided by the inventive pedestal.
It is a further advantage of a preferred embodiment of the present invention that the inventive pedestal provides greater signal density as well as greater signal integrity than the flex circuits and other approaches of the prior art.
It is a still further advantage of a preferred embodiment of the present invention that the inventive pedestal may be suitable for permanent placement between two circuit boards as well as for diagnostic purposes.
It is a still further advantage of a preferred embodiment of the present invention that economy of effort in PCB design may be obtained by placing the inventive pedestal opposite a location of installation of an ASIC attached to the same PCB.
It is a still further advantage of a preferred embodiment of the present invention that the quality of signal transmission through the inventive pedestal preferably enables greater flexibility in the distribution of circuit components among a plurality of circuit boards.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.