Patent Abstract:
An assembly includes a circuit board with a ball grid array device attached to a first side of the circuit board. A brace surrounding the ball grid array device has a series of mounting holes and a series of members extending between the mounting holes. The brace is removably secured to the first side of the circuit board at the mounting holes.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO MICROFICHE APPENDIX 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   Printed circuit assemblies (“PCAs”) are used in many types of electronic products. A PCA typically has several electronic components mounted on a printed circuit board (“PCB”), which is also often referred to as a printed wiring board. The types of electronic components used in PCAs include individual components, such as packaged capacitors, chip capacitors, inductors, individually packaged transistors, integrated circuits (“ICs”), and many others. 
   The electronic components are usually soldered to the PCB, which provides secure mechanical and electrical connections. Metal layers on the printed circuit board are patterned to interconnect the electronic components to each other and to interfaces, such as sockets, plugs, and cables, which are then connected to other PCAs or other devices. Some components have leads that extend into plated holes in the circuit board, which are then soldered in place. Using soldered leads is particularly desirable for components that undergo high stress loads, such as plugs, sockets, and large ICs. 
   Many large ICs are packaged in lead-frames that have leads around the perimeter of the device. The leads are soldered to corresponding solder pads or inserted into corresponding plated holes of the printed circuit board and soldered in place. The leads are relatively compliant and reduce the stress loads on the solder joints arising from shock and vibration of the PCA. However, perimeter leads limit the placement of connections on the PCB, and frequently of the IC, to the perimeter of the device. Similarly, the number of leads around the perimeter of a device increases in proportion to the perimeter, whereas the functionality of the IC, and hence the need for additional connections, often increases as the square of the device area. 
   Ball-grid array (“BGA”)-type ICs, which will be referred to as simply “BGAs,” have an array of solder balls on the bottom of an IC, which provide electrical connections to the IC. The solder balls on the bottom of BGA are aligned with corresponding solder pads or solder bumps on a PCB, and the solder balls are reflowed. BGAs are desirable because they can provide a very large number of electrical connections to the IC, and the number of connections scales with the area of the IC. BGAs are also desirable because electrical connections can be made to the interior of the IC without having to come through the perimeter of the IC, which simplifies trace routing in many IC designs. 
   However, the solder balls are relatively short, stiff, and weak compared to the perimeter leads used with other packaged ICs, such as quad packs and dual in-line packages (“DIPs”). Even though there are usually many solder balls in a BGA device, solder joint may fail in some instances. In particular, if a PCB with a BGA device is subjected to large shock loads, vibration, or distortion (such as twisting or relative contraction/expansion of the printed circuit board due to thermal or mechanical stress), a reflowed solder ball might crack or detach from the BGA or the PCB, causing an open circuit. All the reflowed solder ball connections have failed in some cases, and the BGA has detached from the PCB. 
   Various techniques have been used to try and improve the reliability of BGAs mounted on PCAs. One approach is to add adhesive under the BGA after it is attached to a PCB. The adhesive is typically applied in a fluid state and cured by heating to a temperature below the melting point of the BGA solder. While the adhesive adds strength to the BGA-PCB bond, the adhesive is not very stiff or strong. Application of the adhesive is relatively messy and subject to process variations. Rework (i.e. removing the BGA and the cured adhesive from the PCB and replacing the BGA) is time-consuming, and in many instances is not possible. 
   Another approach is to pre-load the BGA with stress in a selected direction by securing the BGA to an external brace, such as a pedestal that is cast or machined into a metal shield, or a tab or flap folded out of a sheet metal brace. A gap pad is typically installed between the pedestal and the BGA to help conduct heat away from the BGA. Most vibration loads will not exceed the preload force, and so the BGA will not lift off of the external brace in vibration. However, this approach uses a stiff structure spanning the PCA, which is itself stiff, adding weight, cost, thickness, and complexity to the overall assembly. Additionally, this method does not guard against heavy shock loads, such as an end-use type pulse (e.g. 125 G&#39;s, 2 msec, half-sine), since the amount of preload that is required to protect against this type of shock may cause unacceptable board deflection. 
   Another approach as been to reduce the span of a PCB by providing machined or cast shields. For example, a shield has walls surrounding a BGA that are attached to the PCB with screws. Alternatively, a shield on one side of the PCB is attached to a shield on the opposite side of the PCB. The shield walls reduce the span over which the PCB can deflect, decreasing stress on the solder joints of the BGA. However, the shields add mass, cost, complexity, and thickness to the total assembly, and the walls of the shield(s) take up area on one or both outside layers of the PCB. The walls of the shield generally extend along an outer surface of the PCB, which affects the electrical characteristics of surface traces (i.e. traces in the first metal layer of the PCB) that pass beneath the wall. This reduces the amount of first metal layer area available for trace routing, and complicates routing signals beneath the walls of the shield. 
   Therefore, techniques for reducing the bending stress at BGAs on PCBs that avoid the problems discussed above are desirable. 
   BRIEF SUMMARY OF THE INVENTION 
   An assembly includes a circuit board with a ball grid array device attached to a first side of the circuit board. A brace surrounding the ball grid array device has a series of mounting holes and a series of members extending between the mounting holes. The brace is removably secured to the first side of the circuit board at the mounting holes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a plan view of a brace according to an embodiment of the invention. 
       FIG. 1B  is an isometric top view of the brace of  FIG. 1A . 
       FIG. 2A  is a plan view of a brace according to another embodiment of the invention. 
       FIG. 2B  is an isometric bottom view of the brace of  FIG. 2A . 
       FIG. 2C  is a side view of the brace of  FIG. 2A . 
       FIG. 3A  is an isometric partially exploded view of an assembly according to yet another embodiment of the invention having screws extending through a brace and a PCB. 
       FIG. 3B  is an isometric partially exploded view of an assembly according to yet another embodiment of the invention having screws extending through a PCB and secured in a threaded brace. 
       FIG. 3C  is an isometric partially exploded view of an assembly according to yet another embodiment of the invention having identical braces on opposite members of a PCB. 
       FIG. 4A  is a plan view of a brace accommodating two BGAs according to an embodiment of the invention. 
       FIG. 4B  is a plan view of a brace accommodating two BGAs according to another embodiment of the invention. 
       FIG. 4C  is a plan view of a brace accommodating two BGAs according to yet another embodiment of the invention. 
       FIG. 5A  is a plan view of a brace accommodating three BGAs according to an embodiment of the invention. 
       FIG. 5B  is a plan view of a brace accommodating three BGAs according to another embodiment of the invention. 
       FIG. 6A  is a plan view of a brace with corner holes according to an embodiment of the invention. 
       FIG. 6B  is a plan view of a brace with corner holes and with side holes according to an embodiment of the invention. 
       FIG. 6C  is a plan view of a brace with corner holes and with side holes according to another embodiment of the invention. 
       FIG. 6D  is a plan view of a brace with corner holes and with side holes according to yet another embodiment. 
       FIG. 7  is an isometric exploded view of an assembly according to an embodiment of the invention. 
       FIG. 8  is an isometric exploded view of an assembly according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1A  is a plan view of a brace  100  according to an embodiment of the invention. The brace  100  has mounting holes  102 ,  104 ,  106 ,  108  in the corner portions  110 ,  112 ,  114 ,  116  of the brace  100  and raised members  118 ,  120 ,  122 ,  124  connecting the corner portions. 
   The brace  100  is stamped and coined from aluminum sheet, stainless steel sheet, or other stiff, tough material (relative to a PCB). Stamping and coining metal sheet is particularly desirable because it produces simple, low-cost braces. Additionally, deformation of the sheet metal resulting from the coining step further strengthens the metal. Coining also produces a stamped part with high dimensional and angular precision, which is particularly desirable for use on complex PCBs with high dimensional tolerances, although coining uses presses that produce greater tonnage than those used for free-bending similar material. Alternatively, braces are stamped from sheet metal and formed using free-bending or three-point bending techniques. 
   The corner portions  110 ,  112 ,  114 ,  116  and the raised members  118 ,  120 ,  122 ,  124  define an aperture  126  having a shape essentially matching the form of a BGA (not shown) that the brace  100  is intended to be used with. For example, a brace for use with a square BGA has a square aperture and a brace for use with a rectangular BGA has a rectangular aperture. When the brace  100  is mounted to a PCB/PCA (see, e.g.,  FIGS. 3A–3C ,  7 , and  8 ), the corner portions  110 ,  112 ,  114 ,  116  are secured to a surface of the PCB, such as with screws extending through the PCB to nuts on the opposite, or to a threaded brace or other threaded device. The raised members  118 ,  120 ,  122 ,  124  are positioned a selected height above the surface of the PCB, which allows routing surface traces, and in some embodiments mounting components, underneath a raised member. Raising the members also improves the stiffness of an assembly (see, e.g.,  FIGS. 3A–3C ,  7 , and  8 ) incorporating the brace because it moves the member further away from the PCB, thus increasing the moment of inertia of the structural cross section. 
   In a particular embodiment the bottom of a member is 1.4 mm above the PCB to allow mounting transistors, which are typically 1.05 to 1.15 mm high, and capacitors, which are typically about 1.1 mm to 1.2 mm high, underneath the member. It is often desirable to mount capacitors, such as blocking capacitors, next to a BGA. A height of 1.4 mm underneath a member allows mounting a 1.2 mm capacitor under the member with a manufacturing tolerance of 0.2 mm to allow for variation in the machining or coining operation. 
     FIG. 1B  is an isometric top view of the brace  100  of  FIG. 1A . The raised members  118 ,  120 ,  122 ,  124  are all approximately the same height relative to the corner portions  110 ,  112 ,  114 ,  116 . Alternatively, raised members are not all the same height. In some embodiments, one or more members essentially lie on the top surface of a PCB. The term “top” surface is arbitrarily chosen for purposes of convenient discussion to refer to the surface of the PCB that a BGA is mounted on. The opposite surface of the PCB will be referred to as the “bottom” surface, although these terms do not indicate how the PCB is eventually oriented in use. 
     FIG. 2A  is a plan view of a brace  200  according to another embodiment of the invention. The brace  200  has mounting holes  202 ,  204 ,  206 ,  208  in the corners of the brace  200  and raised members  218 ,  220 ,  222 ,  224  extending between the corners. The brace defines an aperture  226  configured to surround a BGA device (not shown) when the brace is secured to a PCB (not shown) on which the BGA device is attached. The brace  200  is machined, cast, sintered, or similarly formed from stiff, tough material, such as metal, ceramic, cermet, or composite materials. Such techniques, some of which use starting material in a fluid or powered state, allow brace materials and shapes that are not obtainable using sheet metal stamping techniques. 
     FIG. 2B  is an isometric bottom view of the brace  200  of  FIG. 2A . The mounting holes  202 ,  204 ,  206 ,  208  extend through legs  228 ,  230 ,  231 ,  232  in the corners of the brace  200 . The legs  228 ,  230 ,  231 ,  232  keep the raised members  218 ,  220 ,  222 ,  224  above the top surface of the PCB. In alternative embodiments, additional mounting holes and legs are placed between the corners along a member. 
   The raised members  218 ,  220 ,  222 ,  224  have the same thickness and are raised to the same height above a PCB (not shown). Alternatively, raised members have different heights and thicknesses. In a particular embodiment, opposite raised members (e.g. member  218  and member  222 ) are thicker than the other members (e.g. member  220  and member  224 ). For example, the thicker raised members run along an axis of a PCB that is more prone to deflection, such as a long axis of a PCB or along an axis that is likely to have more curvature during loading. Reducing the stress through the BGA device during shock loading is particularly desirable because solder, which is typically used to attach the BGA to the PCB and can cold-flow in response to static stress, does not substantially flow in response to shock stress. 
     FIG. 2C  is a side view of the brace  200  of  FIG. 2A . Bottom surfaces  234 ,  236  of the legs  231 ,  232  contact a surface of a PCB (not shown) when the brace  200  is attached to the PCB in a PCA (not shown). 
     FIG. 3A  is an isometric partially exploded view of an assembly  300  according to yet another embodiment of the invention having screws  302 ,  304 ,  306 ,  308  extending through a brace  310  and secured in threaded inserts  312 ,  314 ,  316 ,  318  of a PCB  320 . Alternatively, threaded inserts are omitted and the screws  302 ,  304 ,  306 ,  308  are secured with nuts (not shown) or a second brace on the opposite side of the PCB  320  (see  FIG. 3C ). A BGA device  322  is surrounded by the brace  310 . Attaching a brace that surrounds a BGA device to a PCB regionally stiffens the PCA in the vicinity of the BGA device to re-direct stress that would otherwise arise in the BGA device area (i.e. the area generally between the BGA device and the PCB, particularly in the region where the solder joints between the BGA device and PCB occur) around the BGA device area, and through the brace and other portions of the PCB. 
   Washers  324 ,  326 ,  328 ,  330 , such as plain washers, Belleville washers, or split-ring washers are optionally used. The brace  310  is removably secured to the PCB  320  and locally stiffens the assembly  300  to protect the BGA device  322  and the solder bonds (not shown) securing the BGA device  322  to the PCB  320  from shock, vibration, handling, or other stresses that bend the PCA. Attaching the brace  310  to the PCB  320  with screws provides a secure mechanical coupling of the legs  332 ,  334 ,  336 ,  338  to the PCB  320  that allows easy removal of the brace  310  for re-work of the assembly, such as removing and replacing the BGA device  322 . Attaching the brace  310  at its legs  332 ,  334 ,  336 ,  338  minimizes the area of the PCB  320  used, compared to a shield, for example, which is typically soldered to the PCB around its entire perimeter. Reducing the area needed to attach the brace to the PCB is desirable because it leaves more room on the PCB for placing components and routing traces. 
     FIG. 3B  is an isometric partially exploded view of an assembly  340  according to yet another embodiment of the invention having screws  302 ,  306 ,  308  (a fourth screw is not shown) extending through a PCB  320  and secured in a threaded brace  310 ′. The threaded brace  310 ′ has threaded mounting holes  342 ,  344 ,  346 ,  348  that cooperate with the screws to removably secure the threaded brace  310 ′ to the PCB  320 . When secured to the PCB  320 , the threaded brace  310 ′ locally stiffens the assembly  340 , protecting the BGA device  322 , and the solder bonds (not shown) securing the BGA device  322  to the PCB  320 , from shock, vibration, or handling stresses. The brace is removable to allow re-work of the assembly, such as removing and replacing the BGA device  322 . 
     FIG. 3C  is an isometric partially exploded view of an assembly  350  according to yet another embodiment of the invention having identical braces  352 ,  352 ′ on opposite members of a PCB  320 . A BGA device  322  is attached to a first (“top”) side  321  of the PCB  320  and lies within an aperture (see  FIG. 2A , ref. num.  226 ) formed by the brace  352 . The brace  352  on the first side  321  of the PCB  320  has plain holes  354 ,  356  in diagonally opposite corners that screws  358 ,  360  extend through. The brace  352  also has threaded holes  362 ,  364  in opposite corners that screws  366 ,  368  from a second (“bottom”) side  323  of the PCB  320  are screwed into. 
   The brace  352 ′ on the bottom side  323  of the PCB also has plain holes  354 ′,  356 ′ in opposite corners that the screws  366 ,  368  extend through, and threaded holes  362 ′,  364 ′ that the screws  358 ,  360  from the top side  321  are screwed into to secure the braces  352 ,  352 ′ to the PCB  320 . The symmetry of the braces  352 ,  352 ′ allow the same part (brace) to be used on both sides of the PCB  320 . Alternatively, the threaded holes are not diagonally opposite each other, but are at opposite ends of a member (i.e. along an edge of the brace), as are the through holes on the opposite edge of the brace. Such a brace also has symmetry allowing a single part to be used on both sides of the PCB. In yet another embodiment, identical braces having four through holes are used on both the top and bottom, and nuts are used to secure the screws. In alternative embodiments, the brace on the bottom side of a PCB is different from that on the top side. For example, the brace on the bottom side has thinner members that cooperate with the PCB and top-side brace to form a structure wherein the neutral plane is positioned to improve reliability of the solder joints attaching the BGA device to the PCB. 
   Having braces on both sides of a PCB is desirable because the braces form a stiff, box-like structure with the BGA solder joints within the box-like structure. The box-like structure diverts bending stress in a PCB around the BGA device and away from the plane of the BGA solder joints. Alternative embodiments omit a second (bottom-side) brace. A single, top-side brace also forms a box-like structure, in this instance with the PCB, and diverts bending stress in the PCB around the BGA and away from the plane of the BGA solder joints, which are between the top side of the PCB and the raised members of the brace. 
     FIG. 4A  is a plan view of a brace  400  accommodating two BGA devices  402 ,  404  according to an embodiment of the invention. The BGA devices  402 ,  404  are not part of the brace, but are shown for purposes of discussion. The brace  400  has four perimeter members  406 ,  408 ,  410 ,  412  and an interior member  414  between perimeter members  406 ,  410 . The interior member  414  and perimeter members define two apertures  416 ,  418 , one for a first BGA device  402 , and one for a second BGA device  404 . Alternatively, each aperture is differently shaped and/or differently sized. 
   The perimeter members  406 ,  408 ,  410 ,  412  and the interior member  414  are raised members, allowing surface traces to be routed beneath the raised members and/or allow components to be mounted on a PCB beneath the raised members. Raised members are also desirable because they move the stress through them further away from the plane of the BGA solder joints. Alternatively, one or more members are not raised. 
   The brace  400  has four corner mounting holes  401 ,  403 ,  405 ,  407  for mounting the brace  400  to a PCB (not shown, see, e.g.,  FIG. 3A , ref. num.  320 ). Legs (not shown, see  FIG. 3A , ref. nums.  332 ,  334 ,  336 ,  338 ) are associated with each corner mounting hole to raise the members  406 ,  408 ,  410 ,  412  off the surface of the PCB, improving stiffness of the brace-PCB assembly and allowing component mounting and surface trace routing on the PCB beneath the raised members. Optional side mounting holes  409 ,  411  with associated legs (not shown) further secure the members  406 ,  410  to the PCB. Side mounting holes are further discussed below, in light of  FIGS. 6B–6D . 
     FIG. 4B  is a plan view of a brace  420  accommodating two BGAs according to another embodiment of the invention. Perimeter members  406 ′,  408 ′,  410 ′,  412 ′ form a single aperture  422 . The aperture  422  accommodates two BGA devices of the type that would fit in the aperture  418  of the brace  400  shown in  FIG. 4A ; however, the side-by-side configuration of BGA devices in the aperture  422  without an intermediate stiffening structure allows closer spacing of the BGA devices. Additional electronic components, such as chip resistors, capacitors, transistors, or diodes, are optionally located within the aperture  422 . 
     FIG. 4C  is a plan view of a brace  430  accommodating two BGAs according to yet another embodiment of the invention. Perimeter members  432 ,  434 ,  436 ,  437 ,  438 ,  440 ,  442 ,  443  and an intermediate member  444  define two apertures  446 ,  448 . The two apertures  446 ,  448  lie along a diagonal in a “figure-8” configuration that is particularly desirable for protecting closely spaced BGA devices that do not provide sufficient room for two separate braces. Similarly, using one brace to protect more than one BGA device reduces the part count on a PCA, compared to using a separate brace for each BGA device. Alternatively, each aperture is differently shaped and/or differently sized. 
     FIG. 5A  is a plan view of a brace  500  accommodating three BGAs according to another embodiment of the invention. Perimeter members  502 ,  504 ,  506 ,  508 ,  510 ,  512  and intermediate members  514 ,  516  define three apertures  518 ,  520 ,  522 , each of which surrounds a BGA device when incorporated in a PCA. The perimeter members and intermediate members are raised members. Alternatively, one or more of the members is not raised. In other embodiments, each aperture is differently shaped and/or differently sized. 
     FIG. 5B  is a plan view of a brace  530  accommodating three BGAs according to an embodiment of the invention. Perimeter members  502 ′,  504 ′,  506 ′,  508 ′,  510 ′,  512 ′ define an aperture  532  that can accommodate three BGA devices such as would be used in the apertures  518 ,  520 ,  522  in the brace  530  of  FIG. 5A . The BGA devices may be spaced closer together in the aperture  532  of the brace  530  of  FIG. 5B  because no intermediate members take up area within the aperture  532 . Alternatively the “legs” of the brace  530  are not of equal length and/or width so as to accommodate BGA devices of various sizes. 
     FIG. 6A  is a plan view of a PCA  600  with a brace  601  having corner holes  602 ,  604 ,  606 ,  608  according to an embodiment of the invention. The brace  601  and a BGA device  610  are mounted on a PCB  612 . The screws attaching the brace  601  to the PCB  612  are omitted for simplicity of illustration. The BGA device  610  is within an aperture  611  formed by the brace  601 . An edge  614  of the BGA device  610  is set back a selected distance from an inner edge  616  of the brace  601 . 
   The distance is selected so as to insure that stresses in the PCA  600 , such as stresses arising from vibration, shock, or handling, do not extend under the brace  601  into the aperture  611  so as to cause solder joints between the BGA device  610  and PCB  612  to fail. In a particular embodiment, a BGA device is 25 mm by 25 mm and the aperture formed by a brace is 27 mm by 27 mm. The set-back between the BGA device and the inner edges of the brace is 1 mm on all sides. Alternatively, the set-back is not equal on all sides. The desired set back is selected according to the span of members, stiffness of the members, thickness of the PCB, and the nature and number of the solder joints between the BGA device and PCB, among other factors. 
   Generally, the set back is selected so as to provide enhanced reliability of the solder joints between a BGA device and a PCB when a PCA is mechanically stressed under expected (design) loading. Particular embodiments have set-backs ranging from about 1 mm to about 9 mm. The appropriate set-back depends on several factors, such as board thickness, brace size, brace material, brace thickness, and the size of the BGA device. A set-back of between about 5 mm and about 7 mm is particularly desirable in some embodiments, such as with relatively large PCB assemblies, and most particularly with PCBs about 1.6 mm thick. 
     FIG. 6B  is a plan view of a brace  620  with corner holes  622 ,  624 ,  626 ,  628  and with side mounting holes  630 ,  632  according to an embodiment of the invention. Thus, two opposite members  634 ,  636  have three holes  622 ,  630 ,  624 ;  626 ,  632 ,  628 . The side mounting holes  630 ,  632  have been added along the members  634 ,  636  perpendicular to the axis of the PCA having the most curvature when stressed during shock, vibration, or handling of the PCA. The axis having the most curvature when stressed is frequently the long axis of a PCA, which is represented by a double-ended arrow  638 . The side mounting holes  630 ,  632  secure the centers of the sides  634 ,  636  of the brace  620  to a PCB (not shown), further stiffening the brace-PCB assembly. A PCA having a brace and BGA device with a 5 mm set back was modeled. Adding a side mounting hole in a member of the brace perpendicular to the long axis of the PCB reduced the maximum stress to about half the maximum stress of the PCA modeled with a side hole in a member of the brace parallel to the long axis of the PCB. The difference in maximum stress between braces with side mounting holes in members perpendicular to the long axis of a PCB and braces with side mounting holes in members parallel to the long axis of a PCB becomes more pronounced with less offset. Thus, it is particularly desirable to include side mounting holes in members of a brace perpendicular with the axis of the PCA likely to have the most curvature when stressed when the offset is less than 5 mm. 
     FIG. 6C  is a plan view of a brace  640  with corner holes  642 ,  644 ,  646 ,  648  and with side mounting holes  650 ,  652 ,  654 ,  656  according to another embodiment of the invention. Additional mounting holes are desirable to improve stiffness and redirect stress, but the number of side mounting holes is balanced with the need to use surface area on the PCB (not shown) for other purposes, such as trace routing and electronic component mounting. 
     FIG. 6D  is a plan view of a brace  660  with corner holes  662 ,  664 ,  666 ,  668  and with side mounting holes  670 ,  672 ,  674 ,  676 ,  678 ,  680 ,  680 ,  682 ,  684  according to yet another embodiment. The members  686 ,  688  having the most mounting holes are perpendicular to the axis of a PCA likely to have the most curvature when stressed, represented by the double-ended arrow  638 . 
     FIG. 7  is an isometric exploded view of an assembly  700  according to an embodiment of the invention. The assembly  700  is a portion of a PCA that includes a PCB  702  with a BGA device  704  attached (e.g. soldered) to the PCB  702 . A brace  706  stiffens the assembly  700  when the brace  706  is secured to the assembly with screws  708 ,  710 ,  712 ,  714 . The screws  708 ,  710 ,  712 ,  714  extend through springs  716 ,  718 ,  720 ,  722 , a heat sink  724 , the PCB  702  to a second brace  726  threaded to accept the screws so as to removably secure the brace  706  to a first (“top”) side  703  of the PCB  702  and to secure the second brace  726  to a second (“bottom”) side  705  of the PCB  702 . Alternatively, the second brace  726  is omitted and separate nuts (not shown) cooperate with the screws  708 ,  710 ,  712 ,  714  to secure the brace  706  and heat sink  724  to the PCB  702 . In another embodiment, the PCB has threaded inserts (not shown) that the screws  708 ,  710 ,  712 ,  714  cooperate with to secure the brace  706  and heat sink  724  to the PCB. 
   The brace  706  includes pedestals  728 ,  730 ,  732 ,  734  that limit the travel of the screws  708 ,  710 ,  712 ,  714  and springs  716 ,  718 ,  720 ,  722  that press the heat sink  724  against the BGA device  704  through a gap pad  736 . The gap pad is a pad of thermally conductive compliant material that improves heat transfer between the BGA device  704  and the heat sink  724  in addition to evenly distributing the pressure. Alternatively, pedestals are not integrated into the brace, and separate tubular standoffs are used with a brace, such as one in accordance with  FIG. 2B  or  FIG. 1B . 
   In other words, when assembled, the screws  708 ,  710 ,  712 ,  714  stop against the pedestals  728 ,  730 ,  732 ,  734 , and the springs  716 ,  718 ,  720 ,  722  push the heat sink  724  against the gap pad  736  and BGA device  704 . The springs allow for assembly variations in the height of the BGA device above the PCB, and the dimensions of the brace and the heat sink. The brace keeps the PCB underneath the BGA flat, which limits solder ball stress. In an alternative embodiment, thermally conductive grease is applied between the BGA device and the heat sink. Thermally conductive grease provides good thermal coupling between the BGA device and the heat sink while mechanically decoupling the BGA device from the heat sink. 
     FIG. 8  is an isometric exploded view of an assembly  800  according to another embodiment of the invention. The assembly  800  is a portion of a PCA that includes a PCB  802  with a BGA device  804  attached (e.g. soldered) to the PCB  802 . A brace  806  stiffens the assembly  800  when the brace  806  is secured to the assembly with shoulder screws  808 ,  810 ,  814  (a fourth shoulder screw is not shown in this view). Shoulder screws have a plain (unthreaded) region of larger diameter than the threads. The shoulder screws  808 ,  810 ,  814  extend through springs  816 ,  818 ,  822  (a fourth spring is present but not shown in this view), which are compressed by the shoulder screws against a heat sink  824 . In one embodiment, the shoulder screws are captivated in heat sink  824  and the heat sink  824 , springs  816 ,  818 ,  822 , and captivated shoulder screws  808 ,  810 ,  814  form a sub-assembly that is conveniently installed onto the PCB. A second brace  826  is threaded to accept the screws. Alternatively, the second brace  826  is omitted and nuts (not shown) cooperate with the shoulder screws  808 ,  810 ,  814  to secure the brace  806  and heat sink  824  to the PCB  802 . In another embodiment, the PCB has threaded inserts (not shown) that the shoulder screws  808 ,  810 ,  814  cooperate with to secure the brace  806  and heat sink  824  to the PCB  802 . 
   Braces according to embodiments of the invention locally stiffen a PCA around a BGA device, protecting the solder joints securing the BGA device to the PCA, and also protecting the BGA device from cracking in some applications. Curvature of the PCB within the brace when stress is applied is reduced, compared to the curvature that the PCB in this region would exhibit if the brace were omitted. Analytical simulations show that the stress inside a brace is a small percentage of that which would occur without the brace. 
   Various PCA models were evaluated using M ECHANICA ™ software, available from P ARAMETRIC  T ECHNOLOGY  C ORPORATION  of Needham, Mass. All models assumed a standard PCB and a brace made of 6061 aluminum alloy having members 1.59 mm thick with legs extending an additional 1.59 mm to the PCB, substantially in accordance with the brace  200  shown in  FIG. 2A . The PCB is also about 1.59 mm thick. In other words, the top of the brace was 3.18 mm from the surface of the PCB, and the legs provided 1.59 mm of clearance between the surface of the PCB and the underside of the members of the brace. In alternative embodiments, the top of the brace is raised further from the surface of the PCB, increasing stiffness of the PCA in the vicinity of the BGA device. Alternatively, the thickness of the members of the brace is increased. The BGA device was assumed to be 25 mm by 25 mm in all cases. 
   In one set of simulations, the aperture formed by the brace was 27 mm by 27 mm, and the outer dimensions of the brace were 42 mm by 42 mm. Load in these simulations was 60 N, applied normal to and distributed uniformly on the face of the BGA. A PCA without any brace has a maximum stress of 45 mega-Pascals (“MPa”) through a portion of the BGA device under this load. The same PCA with a brace on the side of the PCA having the BGA device has a maximum stress of 15 MPa through the BGA device. The same PCA with identical braces on both members of the PCA (see, e.g.,  FIG. 5C ) has a maximum of 8.3 MPa stress, which is about 18% of the stress without the braces. By increasing the size of the aperture to 35 mm by 35 mm (i.e. a 5 mm set back), the maximum stress through the BGA device was reduced to 6% of the un-braced PCA. 
   Additional simulations were performed with this load, and are summarized in the following table: 
                                                         TABLE 1                           Relative   Relative           Peak Stress   Typ. Stress   Peak   Typical       Type   (MPa)   (MPa)   Stress   Stress                                No Brace   82   44   1.0   1.0       Single Brace (FIG. 2A)   30   15   .37   .34       27 mm × 39 mm       Double Brace FIG. 2A   26   8.3   .32   .19       27 mm × 39 mm       Double Brace FIG. 2A   21   10.0   .26   .23       35 mm × 47 mm       Double Brace FIG. 1A   17.4   9.0   .21   .20       27 × 42       1.6 mm thick Al       Double Brace FIG. 2A   10.4   6.6   .13   .15       35 × 47       six mounting holes       Double Brace FIG. 2A   11   5.5   .13   .13       35 × 47       eight mounting holes                    
All of the modeled examples have four mounting holes, one in each corner, except the last two samples (rows), which have six mounting holes (see  FIG. 8B ) and eight mounting holes (see  FIG. 8C ).
 
   The same examples modeled in Table 1 were evaluated for a gravity load of 125 G&#39;s, simulating an “end-use” type shock. The results are summarized in the following table: 
                                                         TABLE 2                           Relative   Relative           Peak Stress   Typ. Stress   Peak   Typical       Type   (MPa)   (MPa)   Stress   Stress                                No Brace   316   185   1.0   1.0       Single Brace (FIG. 2A)   139   51   .44   .28       27 mm × 39 mm       Double Brace FIG. 2A   114   23   .36   .12       27 mm × 39 mm       Double Brace FIG. 2A   82   25   .26   .14       35 mm × 47 mm       Double Brace FIG. 1A   62   21   .20   .11       27 × 42       1.6 mm thick Al       Double Brace FIG. 2A   35   12   .11   .06       35 × 47       six mounting holes       Double Brace FIG. 2A   41   10.5   .13   .06       35 × 47       eight mounting holes                    
Tables 1 and 2 show that handling and shock loads in the BGA device mounting area can be significantly reduced by using embodiments of the invention. The values in Tables 1 and 2 are provided for purposes of illustration, and are merely exemplary.
 
   While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to these embodiments might occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.

Technology Classification (CPC): 7