Abstract:
A method of assembling and configuring multiple mezzanine cards on a carrier card is disclosed. The method includes the establishing an I/O profile that represents the I/O configuration of a mezzanine card. The I/O of the mezzanine card is not enabled unless the I/O profile matches a known value stored on the carrier card. In this way, the electronic circuitry is protected if an incorrect mezzanine card is connected to the carrier card.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit to U.S. Provisional Application Ser. No. 60/822,571 filed on Aug. 16, 2006, the disclosure of which is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to a printed circuit (PC) card assembly. More particularly, the present invention relates to a mezzanine circuit board that is mounted to a carrier circuit board. 
   BACKGROUND 
   Requirements for input/output (I/O) pin connections between circuit cards and motherboards often exceed the available circuit card edge length and exceed the maximum available connector pin density. One solution to this problem is the use of a supplemental card portion called a “mezzanine” card (sometimes referred to as a “daughter” card) that is mounted to the main circuit card (referred to generically as the carrier card) in order to provide one or more additional connectors and additional I/O pins. Such mezzanine cards are useful to provide additional functionality to a main circuit card, typically in the form of increased I/O capability. This increased I/O capability serves a variety of applications such as data acquisition and communication just to name a few. Mezzanine cards provide a convenience for configuring a carrier card. For a given carrier card with a suitable Field Programmable Gate Array (FPGA), there can be numerous configurations of attachable mezzanine cards that can satisfy a wide application scope. Modules can be digital, analog, communication, etc. and there can be a “mix” of such modules on a single carrier card. As applications continue to demand increased data processing and communications, while retaining compact physical size, mezzanine cards are commonly used to meet these demands. 
   However, the use of mezzanine cards creates new design issues because the connectors on the main circuit card and the mezzanine card must be spaced at a carefully controlled distance so that the circuit card and mezzanine card connectors can be mated properly with the mating connectors that are positioned on the main circuit card. The spacing between the connectors on the main circuit card and the mezzanine card is relatively small. Since electronic devices on the mezzanine card give off heat during operation, the confined area between the main circuit card and the mezzanine card also makes heat dissipation an issue. Furthermore, since the applications for which the mezzanine cards are used often are harsh environments with considerable mechanical vibration, there is also an issue regarding the robustness of the mechanical mounting of the mezzanine card to the main circuit card. Therefore, it is desirable to have an improved mezzanine structure for PC assemblies. 
   SUMMARY OF THE INVENTION 
   A novel arrangement and assembly method are disclosed in order to more efficiently arrange mezzanine cards on a carrier card. The carrier card may conform to any one of a number of standards, including, but not limited to, PCI Mezzanine Card (PMC), AMC, CMC, XMC, CompactPCI, PC104, PCI, PCI Express, and VME. One preferred embodiment of the mezzanine card of the present invention has a substantially square form factor, of about 1.25 inches on each side. This form factor provides the advantages of making efficient use of board space, and allowing up to four such mezzanine cards to be placed on a PMC, and still comply with the dimensional requirements of both the IEEE 1386 and VITA 20 standards. The mezzanine card of the present invention is a “single connector” mezzanine card. A single connector is used on the mezzanine card to simplify alignment issues, thereby providing more efficient assembly. The mezzanine card is mounted on standoffs to facilitate attachment of semiconductor devices (chips) on both sides of the mezzanine card. In a preferred embodiment, the single connector is a high density 2 millimeter stacking connector. The standoffs are preferably secured in place on the carrier card with solder, and then fasteners, such as screws having an adhesive substance applied to them are used to fasten the mezzanine board to the carrier card. As an alternative, or in addition to the adhesive, lock washers may be used, so long as the desired dimensional requirements are not exceeded. 
   Due to dimensional requirements of IEEE 1386, the standoffs are fairly low, leaving little space between the mezzanine card and the carrier card assembly. Therefore, heat dissipation is an area of concern when using mezzanine cards. The present invention optionally provides additional heat dissipation via a conformal heat conductive material that is placed between the carrier card assembly and the mezzanine card. The use of the conformal heat conductive material depends on the application. In a low-power application, air cooling may be sufficient. However, high-power applications may require the additional heat dissipation achieved with the conformal heat conductive material. 
   In an alternate embodiment, the carrier can actually be another mezzanine card, in which case, the mezzanine card of the present invention acts as a so called “sub-mezzanine” card. However, it is possible to use the mezzanine card of the present invention directly with a main circuit board configured with the proper connections without departing from the scope of the present invention. 
   The present invention provides the advantages of increasing the I/O flexibility of a carrier card such as a PMC, provides acceptable heat dissipation within a small physical form factor, and provides a secure mechanical mount of the mezzanine card to the carrier card. Another key advantage of the present invention is the ability to support multiple mezzanine cards on a single PMC. This provides the flexibility of configuring multiple functions on a single carrier card. 
   The present invention also provides the advantage of identifying the type of mezzanine card present, comprising the steps of reading an identification string with a processor or FPGA on the carrier card. The identification string includes an I/O profile as part of its data. This I/O profile contains information about the status of each I/O pin on the mezzanine card, including whether a particular I/O pin is to be used as an input, output, or bi-directional. This I/O profile is compared with a constant value (reference data) stored in the FPGA or memory on the carrier card. Only if the I/O profile matches the constant value, will the I/O pins be enabled (transitioned from tri-stated to active) on the mezzanine card. If the I/O profile does not match, the I/O pins on the mezzanine card are tri-stated, thereby protecting the mezzanine card circuitry. These advantages, and others, will be apparent from the detailed description and drawings that follow. 
   ASPECTS OF THE INVENTION 
   1. This aspect of the invention is a circuit card assembly comprising: 
   a carrier card ( 300 ); 
   at least one single connector mezzanine card ( 100 ); 
   wherein the single connector mezzanine card makes electrical contact with the carrier card via a connector ( 108 ) on the mezzanine card that is in electrical contact with a corresponding connector ( 304 ) on the carrier card, the carrier card having a plurality of mounting holes ( 312 ), and a plurality of standoffs ( 308 ), each mounting hole ( 312 ) having a corresponding standoff ( 308 ) aligned with it, the single connector mezzanine card ( 100 ) having a plurality of mounting holes ( 104 ,  106 ) aligned with the standoffs ( 308 ), and having screws ( 306 ) having a threaded shaft ( 602 ) wherein the lower portion of the threaded shaft ( 602 ) has adhesive ( 604 ) applied to it, thereby securely fastening the mezzanine card ( 100 ) to the carrier card ( 300 ). 
   2. This aspect is the circuit card assembly of aspect 1, further comprising a layer of conformal heat conductive material ( 702 ) is applied on the carrier card  300 , underneath mezzanine card  100 . 
   3. This aspect is a method for assembling a single connector mezzanine card to a carrier card, comprising the steps of; 
   Aligning standoffs ( 308 ) with mounting holes ( 312 ) of a carrier card ( 300 ), and soldering the standoffs to the carrier card, placing a single connector mezzanine card ( 100 ) onto the carrier card such that mounting holes ( 104 , 106 ) of the mezzanine card are aligned with the standoffs, and applying adhesive ( 604 ) to the lower portion of the threaded shaft ( 602 ) of screws ( 306 ), and fastening the mezzanine card to the carrier card with the screws. 
   4. This aspect is a method for the carrier card to identify the type of mezzanine card present, comprising the steps of reading an identification string transmitted by the mezzanine card, and receiving the identification string with a processor or FPGA on the carrier card, the processor or FPGA using the identification string as a means of establishing signal directionality between the FPGA on the carrier card and the mezzanine card I/O. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a plan view of the connector side of a preferred embodiment of the mezzanine card of the present invention. 
       FIG. 2  shows a plan view of the non-connector side of the embodiment shown in  FIG. 1 . 
       FIG. 3  shows a plan view of an exemplary carrier card of the present invention. 
       FIGS. 4A and 4B  show plan views of exemplary carrier cards utilizing multiple mezzanine cards of the present invention. 
       FIGS. 5A and 5B  show side views of a mezzanine card mounted to a carrier card. 
       FIG. 6  is an exploded view of a mezzanine card mounted to a carrier card. 
       FIG. 7  shows a side view of a mezzanine card mounted to a carrier card with a conformal heat conductive material. 
       FIG. 8  shows another embodiment of a carrier card of the present invention. 
       FIG. 9  shows a logical view of a preferred embodiment of the present invention. 
       FIG. 10  shows a sequence of steps to generate an identification string. 
       FIG. 11  shows the flowchart of steps performed during module identification. 
       FIG. 12  shows an exemplary ID string  1200  containing various data fields. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a plan view of the connector side of a preferred embodiment of the single connector mezzanine card  100  of the present invention. Mezzanine card  100  comprises printed circuit card  102 , which has mezzanine connector  108  attached thereto. In a preferred embodiment, connector  108  is a high density 2 millimeter stacking 80-contact connector. 
   Using a single connector  108  provides an advantage over previous designs that have employed multiple connectors. When multiple connectors are used, there is increased chance for tolerance build-up that causes alignment issues, and prevents mounting of the mezzanine card. The present invention overcomes these problems by using a single high density connector. In one embodiment, the single high density connector is not keyed. Using a non-keyed connector reduces the risk of damage due to someone inadvertently trying to force the two parts of the keyed connector together in the wrong position. Therefore, in order to reduce the risk of incorrect attachment, the single high density connector  108  is purposely placed off-center within the mezzanine card  100 . By having an offset mezzanine connector, it becomes noticeable when the mezzanine card  100  is inserted incorrectly, since the mezzanine mounting holes  104  and  106  of the mezzanine card  100  will not line up with mounting holes  312  of the carrier card  300 . 
   Mezzanine card  100  has a plurality of mounting holes. In a preferred embodiment, two mezzanine mounting holes are positioned at opposite corners, indicated as  104  and  106 . Mezzanine card  100  accommodates multiple integrated circuit devices (hereinafter referred to as “chips”). In the embodiment shown, eight chips, referenced as U 2  through U 9  are shown. 
     FIG. 2  shows a plan view of the non-connector side of the mezzanine card  100  shown in  FIG. 1 . Additional devices may be placed on this side. In the embodiment shown, two devices, indicated as U 1  and U 10  are shown. In a preferred embodiment, one of the devices (chips) present on the mezzanine card  100  is a microcontroller. The microcontroller is used for generating and transmitting an identification string. This identification string can be used by the carrier card to identify an attached mezzanine card. More particularly, the identification string contains an I/O profile for the mezzanine card  100 . This I/O profile contains data representative of the I/O status (e.g. input, output, or bi-directional) for each I/O signal on the mezzanine card  100 . 
     FIG. 3  and  FIG. 4A  show plan views of an exemplary carrier card  300  of the present invention. The carrier card  300  comprises a printed circuit card  302 , with a plurality of devices mounted thereon. The embodiment shown in  FIG. 3  is a PCI Mezzanine Card (PMC). PMC cards are generally known in the art. However, the PMC card of the present invention has a unique arrangement of four carrier card connectors, each indicated as  304 . Each carrier card connector  304  mates with mezzanine connector  108  on a mezzanine card  100 , forming a connector pair. The arrangement of connectors  304  allow up to four mezzanine cards  100  to be mounted on PMC carrier card  300 , as is shown in  FIG. 4  Note that in this case, the carrier card  300  is itself a mezzanine card, making mezzanine card  100  a sub-mezzanine card. However, for consistent terminology, carrier card refers to the circuit card on which mezzanine card  100  is mounted. The term “mezzanine” card is used to refer to the smaller “daughter” card, regardless of whether the carrier card is another mezzanine card, or a main circuit card. 
     FIG. 4B  shows an alternative embodiment of the carrier card  400  of the present invention. In this embodiment, the carrier card  400  adheres to the VME standard, and can support up to twelve mezzanine cards  100 . 
     FIG. 5A  shows a side view of a mezzanine card  100  mounted to a carrier card  300  as viewed from position A of  FIG. 1 . The mezzanine card  100  has a plurality of chips installed on it, referred to generally as UX for the chips on the non-connector side of the mezzanine card  100 , and UY for the chips on the connector side of the mezzanine card  100 . For clarity of the drawing, not all chips are marked with a reference. Carrier card  300  has a plurality of carrier card mounting holes  312  aligned with holes  104  and  106  of mezzanine card  100 . In a preferred embodiment, standoffs  308  are placed on carrier card  300  during the board population process, and then soldered on to permanently mount them. In a preferred embodiment, standoff  308  is a 2 millimeter standoff, such as that manufactured by PennEngineering of Danboro, Pa., USA. Standoffs  308  are positioned on carrier card  300  aligned with mounting holes  312 . Mezzanine card  100  is then placed on to carrier card  300  such that the connector side of mezzanine card  100  (shown in  FIG. 1 ) faces the carrier card  300 . Mezzanine card connector  108  makes electrical contact with carrier card connector  304 . A plurality of screws, indicated as  306 , mechanically fasten mezzanine card  100  to carrier card  300 . 
   The type and number of chips shown are dependent on the specific application. It is preferable that low profile chips, such as Small Outline Integrated Circuits (SOICs) are used, to remain within the acceptable physical size limits. For example, in the case of the IEEE 1386 specification, the cumulative height of the mezzanine board (including mounted components) must be less than 4.7 millimeters from the surface of the carrier card. 
     FIG. 5B  shows the key dimensions that are considered to be within limits of the cumulative height Hc. To remain within the cumulative height limit, the mezzanine card thickness Tm of the mezzanine card  100  must be small enough such that Hc, which is the sum of standoff height Hs (Hs is approximately 2 millimeters in a preferred embodiment, thereby allowing enough space to handle standard height SOIC devices), mezzanine card thickness Tm, and device height Hd (in a preferred embodiment, Hd has a maximum value of 1.9 millimeters) is less than the cumulative height limit, which is 4.7 millimeters in the case of the IEEE 1386 specification. In a preferred embodiment, the mezzanine card thickness Tm is approximately 0.8 millimeters. 
     FIG. 6  is an exploded view of a mezzanine card  100  mounted to a carrier card  300  as viewed from position A of  FIG. 1 . In this view, threaded shaft  602  of screw  306  is shown. As part of the assembly process, a layer of adhesive  604  is applied to the lower portion of shaft  602 . The screw  306  is then fastened onto carrier card  302  shortly thereafter, before the adhesive sets. After the screws  306  are in place, the adhesive then sets, securing the screws  306 , and thus mezzanine card  100 , to the carrier card  300 . Using this method provides increased robustness in a harsh environment, such as in an industrial application, where the carrier card may be subject to considerable vibration. While it is possible to use lock washers to prevent the screws  306  from loosening after assembly, the adhesive provides an advantage over using a lock washer in that the overall height of the mezzanine card does not increase when an adhesive is used, whereas lock washers do increase the height. In an exemplary embodiment, the adhesive  604  is LOCTITE 222MS, manufactured by Henkel Consumer Adhesives, of Avon, Ohio, USA. LOCTITE 222MS is a non-permanent adhesive. It serves to secure screws  306  and prevent them from loosening due to mechanical vibration and the like. However, since adhesive  604  is non-permanent, the screws  306  may be loosened with a screwdriver, so that the mezzanine card  100  can be removed and replaced as necessary. 
     FIG. 7  shows a side view of a mezzanine card  100  mounted to a carrier card  300  as viewed from position B of  FIG. 1 . In this figure, optional conformal heat conductive material  702  is shown. A layer of conformal heat conductive material  702  is applied on the carrier card  300 , underneath mezzanine card  100 . The conformal heat conductive material  702  conforms to the chips (referenced as UY) on the connector side of mezzanine card  100 . The conformal heat conductive material  702  is most typically used on VITA 20 Conduction Cooled PMC carrier cards. In a low-power application, convection or forced air cooling may be sufficient. However, when the devices give off sufficient heat, conformal heat conductive material  702  can optionally be used to improve heat dissipation. The conformal heat conductive material is an electrically isolating material, preferably having a thermal conductivity of at least 1.0 W/mK (Watt per meter Kelvin), a dielectric breakdown voltage greater than about 6,000 volts AC, and a dielectric constant greater than about 5.4, as measured by ASTM D150. The conformal heat conductive material typically is in sheet form. In one embodiment, the Young&#39;s Modulus of the conformal heat conductive material is preferably about 55 kPA, and the density (g/cc) is about 1.6. In an exemplary embodiment, conformal heat conductive material  702  is from the Gap Pad VO Ultra Soft product family, which is manufactured by Bergquist Company of Chanhassen Minn., USA. In an exemplary embodiment, part number GPVOUS-0.100-AC-0816 is used. However, when practicing the present invention, there may be some variation in the part number due to different thickness requirements from one application to the next. 
     FIG. 8  shows another embodiment of a carrier card of the present invention. In this case, copper ground plane  808  is optionally present on the surface of PMC carrier card  800  in the proximal area of each carrier card connector  304 . For the sake of clarity, not all ground planes are indicated with reference numbers in this figure. The presence of the copper ground plane  808  is essential when the optional conformal heat conductive method of the present invention is used. This is shown in  FIG. 7 . If the conformal heat conductive method of the present invention is used, conformal heat conductive material ( 702  in  FIG. 7 ) makes contact with the copper ground plane  808 . The heat is transferred to copper ground plane  808 , and away from the electronic circuitry. Preferably, side rails (not shown) divert the heat from the copper ground plane  808 . 
     FIG. 9  shows a logical view of a preferred embodiment of the present invention. PCI bus  900  is connected to PCI bus interface  902 . PCI bus interface  902  provides the necessary circuitry to communicate with FPGA (Field Programmable Gate Array)  904 . The PCI bus interface is well known in the art. The FPGA  904  communicates with one or more mezzanine cards, indicated here as  906 A- 906 D. Note that while an FPGA is used to interface with the PCI bus interface in this embodiment, it is possible to use other technologies, such as a microcontroller, to perform this function, without departing from the scope of the present invention. Each mezzanine card  906 A- 906 D is mechanically similar to mezzanine card  100 . The mezzanine cards  906 A- 906 D are electrically connected to I/O (input/output) connector  909  through I/O Signals  907 . The references  906 A- 906 D refer to specific instances of a mezzanine card. Each mezzanine card can have different electronics to perform a different function. For example, mezzanine card  906 A may provide signal conditioning for serial communications (e.g. RS-232) and/or parallel communications (e.g. IEEE-1284), mezzanine card  906 B may provide signal conditioning for analog signal acquisition, mezzanine card  906 C may provide signal conditioning for digital I/O, and mezzanine card  906 D may provide for memory storage, provide for on-board sensors such as temperature sensors, accelerometers, or other transducers, or perform yet another function. While four mezzanine cards are shown in this embodiment, it is possible to have more or less without departing from the scope of the present invention. In general, FPGA  904  provides the logic operations necessary for a particular function, and the signal conditioning is performed on the mezzanine cards  906 A- 906 D. Not all mezzanine cards need be present during use. For example, if the user desired to configure a carrier card with only two functions, then only two mezzanine cards would be used. 
   In this embodiment, each mezzanine card  906 A- 906 D has a microcontroller (not shown) installed therein to transmit an identification string. In an exemplary embodiment, the microcontroller is a PIC10F200 or similar, manufactured by Microchip Technology Inc., of Chandler, Ariz., USA. 
     FIG. 10  shows a sequence of steps performed by the PIC10F200 or equivalent to generate an identification string, hereinafter referred to as an ID string. The ID string is a sequence of data that is periodically retransmitted. The FPGA reads the ID string, and can identify the type of mezzanine card that has been inserted into the carrier card. In a preferred embodiment, the data is pulse width modulated, wherein a zero bit is one pulse width unit, a one bit is two pulse width units, and a sync pulse is three pulse width units. 
   In step  1002 , a sync pulse is sent to the FPGA. This indicates the start of the data sequence. In step  1004 , the part number data is sent. In step  1006  a serial number is sent. In step  1008 , a revision date is sent. In step  1010  a manufacturing date is sent. In step  1011  an I/O profile is sent. The I/O profile is representative of the configuration of each I/O signal. In step  1012  optional data is sent. In step  1014  a checksum of the previous data is sent. The checksum is optionally used by the FPGA to verify the integrity of the received data. After a periodic delay, the process proceeds to step  1002 , and the ID string is retransmitted at a predetermined interval (e.g. every 250 milliseconds). 
     FIG. 11  shows the flowchart of steps performed by the FPGA during module identification. In general, when a mezzanine card is inserted into the socket on the carrier card, the FPGA  904  provides power to the mezzanine cards  906 A- 906 D, but does not enable the I/O signals  907  of the mezzanine cards  906 A- 906 D. The FPGA  904  reads the ID string and can determine if a particular mezzanine card is designed to work with that carrier card. In step  1102 , the ID string is received by the FPGA  904 . In step  1104 , the FPGA  904  compares the received ID string to an internally stored table of ID strings (not shown). The I/O profile, which contains the configuration data for the I/O signals on the mezzanine cards  906 A- 906 D, is compared to a value stored internally in the FPGA on the carrier card  300 . If the I/O profile for a given mezzanine card matches the value internally stored by the FPGA  904 , then the I/O signals  907  of the mezzanine card are enabled in step  1106 . Various other parameters, such as serial number, revision date, and manufacturing date may optionally be compared. If the compared data does not match, then the I/O signals  907  remain disabled (tri-stated). This provides protection of the electronics if an incorrect mezzanine card is inadvertently placed in the wrong carrier card. 
     FIG. 12  shows an exemplary ID string  1200  containing various data fields. In an exemplary embodiment, the module part number  1202 , serial number  1204 , revision date  1206 , and manufacturing date  1208  are stored as 3 byte BCD encoded data. The I/O profile  1210  is stored as seven bytes of data. Two bits of data are used to represent the configuration setting of each I/O pin. In this embodiment, a two bit value of 00 denotes an output from the mezzanine card, a two bit value of 01 denotes an input to the mezzanine card, and a two bit value of 10 denotes a bi-directional signal. Those skilled in the art will recognize that other values may be used to represent the various I/O states without departing from the scope of the present invention. Optional data  1212  may contain additional information about the module, referred to as “Module Specific Data.” The checksum  1214  is optionally used to verify the integrity of the received data. 
   As can be understood by one of ordinary skill in this art, the present invention provides increased I/O flexibility, acceptable heat dissipation within a small physical form factor, and provides a secure mechanical mounting. Furthermore, a method of modular mezzanine cards is disclosed. The mezzanine cards identify themselves to a processor on the carrier card, and identify their I/O profile to the carrier card which ensures the mezzanine cards are the proper type for the carrier card before enabling the I/O signals of the mezzanine card, thereby minimizing the risk of damage due to human error. Those of ordinary skill in the art will recognize that the above description was simply using exemplary embodiments to illustrate the making and using of the invention and, that other combinations are possible without departing from the scope of the present invention.