Attachment plate for directly mating circuit boards

[0001] 1. Field of the Invention

[0002] The present invention relates generally to printed circuit boards and, more particularly, to a daughtercard attachment plate for directly mating printed circuit boards.

[0003] 2. Related Art

[0004] Computer systems have one or more printed circuit boards on which various digital and/or analog components are mounted. The principal printed circuit board in a computer system is generally referred to as a motherboard. In personal computers, the motherboard is often called the system board or mainboard. Typically, the motherboard holds many of the digital components integral to the functioning of a computer system such as the CPU, memory and basic controllers. In many instances, additional circuit boards can be attached to the motherboard to provide additional functionality. Such additional circuit boards include expansion boards and daughtercards (sometimes referred to as daughterboards). Expansion boards are circuit boards that plug into the computer's expansion slots. Expansions boards include, for example, controller boards, LAN cards and video adapters.

[0005] Daughtercards are attached directly to another printed circuit board, typically the above motherboard. In contrast to expansion boards, daughtercards access the motherboard components (memory and CPU) directly rather than through a slower expansion bus. Daughtercards typically include one or more integrated connectors, commonly referred to as a socket or header, that attach to an integrated connector on the motherboard. Once the integrated connectors are mated, the boards are typically joined by screws passing through holes in each circuit board.

[0006] A mating force generated by tightening the aforementioned screws to urge the circuit boards toward each other, serves to establish and maintain the physical and electrical connection between the boards. To maintain the circuit boards in relatively parallel planes, a number of appropriately distributed spacers are sometimes used. The height of the spacers is approximately the same as the combined height of both integrated connectors when those connectors are fully mated.

[0007] Problems can arise when directly mating circuit boards such as the noted daughtercard and motherboard. If the force applied to the connectors during mating is not parallel to the pins and sockets of the connectors; for example, if the circuit boards are not maintained in parallel planes or laterally translate during mating, damage to the connector pins and sockets can easily result. One example of such a circumstance is when one or both of the circuit boards is not planar due to manufacturing imperfections, thermal cycling or other circumstances that can cause a printed circuit board to warp or curve. In this case, rotational or other non-axial forces can be placed on connector pins and sockets during the mating. Under some circumstances, this can result in the bending of pins, incomplete electrical contact between some pins and their corresponding sockets, or other types of connection failures.

[0013] In one aspect of the invention, an apparatus for adjoining along a mating axis a first and second circuit board to directly mate a first integrated connector of the first board with a second integrated connector of the second board is disclosed. The apparatus comprises an attachment plate to be supported in a substantially horizontal plane; a plurality of extension arms extending downward from the plate to suspendingly restrain the first board away from the plate except to permit the first board to translate along the mating axis toward the plate; an pressure platform extending downward from the plate to apply to the first board a mating-axis force collocated with the first connector; and a plurality of attachment elements to secure the plate to the second circuit board without applying forces to the first circuit board. Forces applied to the plate are transferred to the first board through only the pressure platform.

[0014] In another aspect of the invention, an apparatus for adjoining along a mating axis a first and second circuit board to directly mate a first integrated connector of the first board with a second integrated connector of the second board is disclosed. The apparatus comprises an attachment plate to be supported in a substantially horizontal plane; suspension means for suspendingly restraining the first board away from the plate except to permit the first board to translate along the mating axis toward the plate; attachment means for securing the plate to the second circuit board without directly applying forces to the first circuit board; and mating means for applying to a rear side of the first board collocated with the first connector mating-axis forces applied to the attachment plate.

[0015] In a further aspect of the invention, a method for adjoining along a mating axis a first and second circuit board to directly mate a first integrated connector of the first board with a second integrated connector of the second board is disclosed. The method comprises suspending the first circuit board over the second circuit board with the first connector aligned with the second connector; and applying a mating force to a rear side of the first circuit board collocated with the first integrated connector to cause the first connector to mate with the second connector.

[0016] The present invention is directed to an apparatus for facilitating the direct electrical connection between two printed circuit boards each having an integrated electrical connector. FIGS. 1A and 1B are perspective views of two printed circuit boards, a motherboard 102 and a daughtercard 106. Motherboard 102 has an integrated electrical connector 104A. Similarly, daughtercard 106 has an integrated connector or socket 104B. Connectors 104A and 104B are corresponding halves of a conventional connector 104 such as ZIP socket, zero force insertion socket, and the like.

[0017] When daughtercard 106 and motherboard 102 are mated, they are brought together along a mating axis 108 parallel with the longitudinal axis of the pins and receptacles of connector 104, as shown in FIG. 1B. A force substantially parallel with mating axis 108 (referred to as a mating axis force) is then applied to daughtercard 106 to urge the daughtercard toward motherboard 102. Such mating axis forces cause the electrical and physical connection of the two integrated connectors 104A, 104B. Thereafter, daughtercard 106 is directly connected to motherboard 102; that is, connectors 104A and 104B are connected to each other with no intervening cables, wires or the like.

[0018] As noted, when mating daughtercard 106 and motherboard 102, rotational forces and translational forces that are not parallel to the mating axis (collectively referred to as non-mating axis forces) can damage connector pins or fracture traces or other electrical connections on the printed circuit boards 102, 106, thereby reducing the reliability of the resulting assembly. For example, conventional daughtercards are typically mated directly to a motherboard with attachment screws located at the four comers of the daughtercard. Due to the lack of planarity of the daughtercard or the variability with which the attachment screws are adjusted, the applied mating forces can cause the printed circuit boards to bend, rotate, etc., causing stresses in the boards.

[0019] Generally, the present invention is an attachment plate system that prevents non-mating axis forces from being applied to the integrated connectors of a mating motherboard and daughtercard. The invention physically suspends the daughtercard over the motherboard to allow the proper alignment of the integrated connectors to dictate the relative position of the printed circuit boards, and, preferably, to allow at least a partial mating of the connectors to occur before securing the printed circuit boards together. Any additional forces such as mating axis forces applied to further mate the connectors and/or to join the printed circuit boards, are applied solely to the attachment plate and not the suspended daughtercard. Such forces are transferred to a localized region of the suspended daughtercard immediately behind and collocated with the daughtercard's integrated connector, preventing the daughtercard from translating and rotating in response to such applied forces, other than to translate along the mating axis. Thus, variations in daughtercard tolerances, variability in operator mating technique, etc., do not cause internal stresses in the daughtercard which can damage the daughtercard connector, printed circuit board or components and connections on the printed circuit board.

[0020] In the following description, embodiments of the attachment plate assembly will be described with reference to FIGS. 2 and 3. Then, use of the attachment plate assembly will be described with reference to the cross-sectional views of FIGS. 4A through 4C. FIGS. 2 and 3 are perspective views of one embodiment of an attachment plate assembly of the present invention. In FIG. 2, the attachment plate assembly is illustrated upside-down to show how the attachment plate system can receive a printed circuit board such as a daughtercard. FIG. 3 is a perspective view of the attachment plate shown in FIG. 2 rotated to mate the daughtercard suspended thereon to a motherboard.

[0021] Generally, attachment plate assembly 200 comprises an attachment plate 202 configured to retain a daughtercard 250 with a rear side of the daughtercard facing attachment plate 202 and a top side of the daughtercard exposed for mating to a motherboard (not shown). In the embodiment shown in FIGS. 2 and 3, attachment plate 202 has a rear side 201 and a mating side 203 with a raised edge or sidewall 222 defining a shallow bay 204. All components of attachment plate assembly 200 are coupled to attachment plate 202.

[0022] Integrated on mating surface 203 of attachment plate 202 are a plurality of extension arms 208 configured to restrain daughtercard 250 in a generally fixed position relative to attachment plate 202, and to provide limited relative movement of daughtercard 250 substantially parallel to mating axis 108. Mating axis 108 is parallel to the longitudinal axis of the pins and receptacles of connectors 254 and 302, and travels through the center of the connectors. Daughtercard 250 includes holes 252 adapted to receive extension arms 208 as described below. In the orientation of attachment plate system 200 shown in FIG. 2, daughtercard 250 is placed on extension arms 208 causing each extension arm 208 to pass through a corresponding hole 252 in daughtercard 250.

[0023] It should be understood that extension arms 208 preferably have a diameter that allows for a close fit within daughtercard holes 252. Such a close fit restricts rotation and lateral translation of daughtercard 250 relative to attachment plate 202. After daughtercard 250 is mounted on extension arms 208, o-rings 218 are attached to the extension arms as shown in FIG. 2. When attachment plate 202 is rotated into the proper orientation to mate daughtercard 250 with a motherboard, as shown in FIG. 3, daughtercard 250 will shift away from attachment plate 202 to come to rest against o-rings 218 in a position on extension arms 208 suspended away from attachment plate 202. This is described in detail below.

[0024] A raised pressure plate 206 is provided on mating side 203 of attachment plate 202. In one preferred embodiment pressure plate 206 has approximately the same cross-sectional area and, more preferably, approximately the same cross-sectional dimensions as connector 254.

[0025] Pressure plate 206 is centered on mating axis 108 to be collocated with connector 254 when daughtercard 250 is mounted on attachment plate 202. Importantly, pressure plate 206 is the forward-most contacting surface on attachment plate 202. As such, pressure plate 250 is the only portion of attachment plate 202 that applies a force to daughtercard 250 when attachment plate 202 is secured to motherboard 300 (shown in FIG. 3). More specifically and as will be described in detail below, attachment plate 202 transfers mating forces applied to attachment plate 202 to pressure plate 206 which in turn transfers the applied mating forces to the rear side of daughtercard 250 behind connector 254. Because pressure platform 206 has approximately the same cross-sectional area as connector 254, the applied mating forces are distributed evenly across the connector.

[0026] Attachment plate 202 has a plurality of attachment screw/spring assemblies 210 for securing the attachment plate directly or indirectly to the motherboard. Attachment screw/spring assemblies 210 include an attachment screw 212 surrounded by an attachment spring 214. As is well know in the art, motherboard 300 includes corresponding threaded holes 302 for receiving screws such as attachment screws 212. Alternatively, motherboard 300 can be mounted on a plate or other support structure that includes such corresponding threaded holes 302.

[0027] Attachment screw/spring assemblies 210 are distributed around daughtercard 250 and oriented parallel to mating axis 108. When connector 254 is aligned with an integrated connector 304 on motherboard 300, attachment screws 212 are aligned with a corresponding threaded hole 302 in the motherboard. Preferably, attachment screw/spring assemblies 210 are symmetrically distributed around connector 254. In the illustrated embodiment, four attachment screw/spring assemblies 210 are shown located at each of the four corners of attachment plate 202.

[0028] As shown in FIGS. 2 and 3, attachment screw/spring assemblies 210 are positioned on rear side 201 of attachment plate 202, with each attachment screw 212 extending through a hole in the attachment plate. Springs 214 are compressed between attachment plate 202 and the head of a corresponding attachment screw 212. Spring 214 of each attachment screw/spring assembly 210 provides a biasing force on its associated screw 212 to urge screw 212 toward an unengaged position; that is, to prevent it from falling through attachment plate 202 when the plate is rotated into the mating orientation shown in FIG. 3. Each attachment screw/spring assembly 210 is coupled to attachment plate 202 by a snap ring or c-clip 213 on mating side 203 of attachment plate 202. Each screw 212 has an undercut (not shown) that operates with snap ring 213 to prevent screw 212 falling from attachment plate 202 when the plate is in the position shown in FIG. 2. In contrast to conventional attachment screws which apply mating forces directly to the daughtercard, attachment screws 212 apply a mating axis force to attachment plate 202 which translates the mating forces to connector 254 via pressure plate 206, as noted above and described in detail below. Because daughtercard 250 is not in contact with attachment plate 202 at the vicinity of attachment screws 212, and because the attachment screws are coupled to the plate rather the daughtercard, the mating forces applied by the attachment screws cannot cause any tolerance stack on any of the four comers of the daughtercard.

[0029] The operation of the illustrative embodiment of attachment plate system 200 will now be described with reference to FIGS. 4A-4C. FIGS. 4A-4C are three cross-sectional views of daughtercard 250 and motherboard 300. FIG. 4A shows the two printed circuit boards as they are initially brought into contact with each other. FIG. 4B shows the two connectors 254, 302 partially mated, for example, under the weight of daughtercard 250. FIG. 4C shows the two connectors 254, 302 fully mated in response to a mating force applied through attachment plate 202. It should be understood that in the cross-sectional views illustrated in FIGS. 4A-4C attachment plate assembly 200 is oriented as shown in FIG. 3; that is, in the “mating” orientation. It should also be appreciated that in FIGS. 4A-4C only certain portions of attachment plate assembly 200 are illustrated; primarily, pressure platform 206, extension arms 208 and attachment screw/spring assembly 210, as defined by section line “FIGS. 4A-C” shown in FIG. 2.

[0030] Referring now to FIG. 4A, the relevant dimensions of these particular components is first described. In operation, after daughtercard 250 is mounted on attachment plate 202, the attachment plate is turned over as shown in FIG. 3, and daughtercard connector 254 is aligned with motherboard connector 304. The position of daughtercard 250 on attachment plate 202 when oriented for mating is shown in FIG. 4A. As shown there, when rotated, daughtercard 250 travels downward on extension arms 208 to rest on compliant stops 216. In this position, there is a gap between pressure platform 206 and daughtercard 250.

[0031] In one embodiment, extension arms 208 are formed with a standoff 404 defined by a larger diameter than an upper post 406 of the extension arms. Upper post 406 has a recessed 205 for receiving compliant stop 216. Standoff 404 has a height 408 that is less than height 410 of pressure plate 206. Upper post 406 is comprises of post regions 406A and 406B with recess 205 interposed between the two post regions 406. Post regions 406A and 406B have a diameter that is no larger than the diameter of daughtercard hole 252. Preferably, at least lower post region 406A has a diameter that allows for a close fit within hole 252 to provide positional stability of daughtercard 250 with respect to attachment plate 200 to prevent rotation and lateral translation of the daughtercard when it is engaged with lower post 406A.

[0032] Recessed 205 accepts an correspondingly configured o-ring 216. Preferably, the diameter of o-ring 216, upper post 406B and recess 205 have relative dimensions that requires o-ring 216 to be slightly expanded to slide or roll over post region 406B until o-ring 216 reaches recess 205. Recess 205 allows o-ring 216 to contract to a smaller diameter to securely fit within the recess. The outside diameter of o-ring 216 is greater than the diameter of hole 252, enabling o-ring 216 to provide a support surface for the suspended daughtercard 250. It should be appreciated that other compliant and non-compliant stops other than o-ring 216 can be used. Preferably, such stops do not require the use of additional tools. However, a person of ordinarily skill in the art will recognize that a variety of different retaining hardware such as a c-clip or snap ring can be used to facilitate the function of stop 216.

[0033] The height 412 of lower post region 406A is greater than thickness 410 of daughtercard 250. When daughtercard 250 rests against o-ring 216, there is a gap 414 between the daughtercard and the post standoff 404. Gap 414 is greater than gap 416 between pressure plate 206 and daughtercard 250, as shown in FIG. 4A. When suspended in this position, daughtercard 250 is held in a generally fixed position relative to attachment plate 202 with limited movement parallel to mating axis 108. That is, daughtercard 250 is free to move along mating axis 108 on extension arms 208, while movement along other axes and rotation with respect to attachment plate 202 is restricted. Distance 416 defines the amount of limited axial movement of daughtercard 250. In one embodiment, distance 416 is approximately 0.4 mm. In the illustrative embodiment, the combination of distance 416 and the resilience of o-ring 216 provides daughtercard 250 with the capability of traveling up to 0.8 mm along mating axis 108. It should be apparent to those of ordinary skill in the art that other distances can be implemented in other embodiments.

[0034] The operation of mating daughtercard 250 and motherboard 300 will now be described with reference to FIGS. 4A-4C. As noted, FIG. 4A shows the two printed circuit boards 250, 300 as they are initially brought into contact with each other. As shown in FIG. 4A, connector halves 254, 304 are the first surfaces to contact each other. Height 418 of extension arm 208 and height 420 of attachment screw housing 226 are such that neither element contacts motherboard 300 prior to connector 254 contacting connector 304. Thus, connectors 254, 304 are first aligned with each other in the absence of an influence of other elements of attachment plate 202 and daughtercard 250 coming into contact with motherboard 300.

[0035] In addition, as noted above, attachment springs 214 are compressed and interposed between the head of screws 212 and attachment plate 202 to bias attachment screws 212 toward an unengaged position. By biasing attachment screws 212 away from their attachment position, the attachment screws are prevented from extending toward and contacting motherboard 300 before the mating of connectors 254, 304. Thus, the alignment between the printed circuit boards 250, 300 and connectors 254, 304 is dictated solely by the connectors, not the initial engagement of attachment screws 212.

[0036] Referring now to FIG. 4B, attachment plate 202 has been lowered further from the position shown in FIG. 4A. In the particular embodiment shown, connectors 254, 302 have partially mated, for example, under the weight of daughtercard 250. It should be appreciated that connectors 254, 304 may or may not partially or completely mate under the weight of daughtercard 250 depending on the weight of the card and the type of connectors.

[0037] As attachment plate assembly 200 is lowered, extension arm 208 travels through opening 252 until pressure plate 206 encounters the rear side of daughtercard 250.

[0038] Subsequent to some initial potential mating, daughtercard 250 is no longer supported by stops 216; rather, the daughtercard is supported by the resistance of connectors 254, 304 to further mating. In the illustration of FIG. 4B, attachment plate 202 has been lowered until pressure platform 206 has made initial contact with the rear side of printed circuit board 250.

[0039] Further mating of daughtercard 250 and motherboard 300 occurs in response to securing attachment plate 202 to motherboard 300. In FIG. 4C, attachment plate 202 is secured to motherboard 300 via attachment screws 212. As noted, attachment screws 212 are one component of attachment screw/spring assemblies 210. To cause attachment screws 212 to engage threaded holes 302 in motherboard 300, a manual force parallel to mating axis 108 is applied to the head of attachment screw 212, such as with a screwdriver. The applied force causes springs 214 to compress and attachment screw 212 to extend toward motherboard 300. Rotation of attachment screw 212 causes the screw to further engage threaded hole 302, causing the further mating of connectors 254, 304. As attachment screws 212 are each rotated, they apply a mating force to attachment plate 202. Since the only portion of attachment plate 200 that is in contact with daughtercard 250 is pressure platform 206 collocated with connector 254, and not extension arms 208 at the corners of daughtercard 250, all such applied mating forces are applied to daughtercard 250 through pressure platform 206. This internal attachment of daughtercard 250 to pressure platform 206 prevents variations in the threading of attachment screws 212 and variations in the planarity of daughtercard 250 from causing rotational or other non-mating axis forces to be applied to connectors 254, 304.

[0040] Once daughtercard 250 and motherboard 300 are mated, and attachment plate 202 is attached to motherboard 300, the two circuit cards are maintained in a stable relative position by attachment screw/spring assembly 210. Compressed springs 214 and the threaded mating of screws 212 and motherboard 300 maintain daughtercard 250 and motherboard 300 in a fixed relative position. This maintains the integrity of the electrical connection between connectors 254, 304 when the circuit cards 250, 300 undergo thermal expansion and compression cycles during the operational life of the mated components. Oftentimes the components and connectors expand and contract at different rates and to different extents. Such thermal cycles can cause unintended fracturing or separation adversely affecting the reliability of the electrical connection between such components.

[0041] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. For example, pressure platform 206 can be fabricated as part of attachment plate 202 or it may be subsequently attached to the plate 202 after the attachment plate is fabricated. The ability to attach pressure platform 206 to attachment plate 202 after the attachment plate is fabricated allows the user to interchange various size pressure platforms to correspond to various size connectors 254. As another example, in the embodiment of attachment plate assembly 200 described above, attachment plate 202 includes a sidewall 222 that defines a shallow bay 204. It should be appreciated, however, that such a sidewall is not necessary. In alternative embodiments, for example, attachment plate 200 does not include such a sidewall. As a further example, in the embodiment illustrated in FIG. 2, three extension arms 208 and two o-rings 218 are used to suspendingly secure daughtercard 250 to attachment plate 202. However as one of ordinary skill in the art will find apparent, in alternative embodiments different quantities and combinations of extension arms 208 and o-rings 218 can be implemented. As a further example, in an alternative embodiment, apertures 228 are threaded to retain screws 212 in attachment plate 202, eliminating the need for springs 214 and clips 216 while requiring rotations of screws 212 to bring screws 212 into engagement with threaded holes 302 on motherboard 300. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.