Abstract:
The printed circuit board (PCB) header attachment station mounts a header (PCB) onto a PCB having preexisting solder joints, such as underneath a direct mount IC chip. The apparatus applies a soldering paste to PCB lands configured to receive the pins of the header; nests the header component in stacked alignment with the PCB in order to bring the header pins into contact with the respective PCB lands; heats the PCB to a temperature approaching the reflow temperature of the solder in the pre-existing PCB solder joints; and actuates a compliant heating block or bar to heat the header pins to an extent sufficient for the pins to conduct enough heat to locally reflow the solder on the corresponding PCB lands without reflowing the solder in the pre-existing solder joints. The local reflow of the solder precludes solder reflow in the surrounding pre-existing joints which may coalesce under the direct mount IC chip and render the PCB defective.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/245,125 filed Jan. 14, 1999, now U.S. Pat. No. 6,152,353. 
    
    
     FIELD OF INVENTION 
     The invention generally relates to the art of printed circuit board (PCB) manufacture and more particularly to the manufacture of a PCB having a stacked header component. 
     BACKGROUND OF INVENTION 
     Many techniques are employed in the manufacture of PCBs in order to increase component density and reduce the area and/or size of the PCB. The exploded view of FIGS. 1A and 1B show the results of one such technique in which an integrated circuit (IC) semiconductor chip  20  is directly mounted onto a PCB  22  using a “direct chip attach” process. In this process, a solder mask is placed on an etched or coined PCB and solder paste is screened onto the copper lands of the PCB. The PCB  22  is then stuffed with components, including IC chip  20 . The IC chip  20  is not housed in a chip carrier or any other kind of package and thus is able to occupy a minimal footprint on one side the PCB  22  as compared to a fully packaged IC, e.g., one encased in a common dual in-line package (DIP). After stuffing, the PCB is heated by a heat radiation means to reflow the solder paste and electrically and mechanically connect the components to the PCB. The PCB is then washed to remove excess solder flux. Thereafter components such as the IC chip  20  are wire-bonded directly to the PCB. After wire-bonding and a potential testing phase, an encapsulant  24  is applied to the IC chip  20  and possibly other components in order to hermetically seal these components from the external environment. 
     In the illustrated embodiment, the PCB  22  includes a copper backplane  25  which provides a low profile means for dissipating heat. In circumstances where the IC chip  20  produces significant operating heat it is directly attached to the backplane  25  for efficient heat transfer. The encapsulant  24  protects the chip and wirebonds from the surrounding environment. 
     The component density of the PCB  22  is also increased by stacking a header  26  onto the PCB  22  after the encapsulant  24  is applied. The header  26  may carry on its underside  28  relatively large electronic components such as inductors  30  thereby eliminating the need to reserve a relatively large footprint on the PCB  22  for these bulky components. The header  26  includes a number of friction-fitted pins  32 . A portion  32 B of the pins extend from the underside  28  of header  26  for mounting it onto respective header pin lands  34  located on the PCB  22 . Some of the pins  32  and correspondingly some of the lands  34  serve to electrically interconnect the inductors  30  to the circuitry of the PCB. A portion  32 A of the pins  32  extend from a top-side of the header and may be used to mount the PCB/header assembly  22  and  26  to a host card or mothercard (not shown) in a larger system. In this case, some of the pins  32  and correspondingly some of the lands  34  may be electrically active and function as input/output interconnections between the PCB  22  and the host card. This feature also eliminates the need to dedicate a significant footprint of the PCB  22  for card edge connectors. 
     The header  26  is relatively large and may be sized as large as the PCB  22  itself as shown in FIGS. 1A and 1B, or may be somewhat smaller. Smaller headers may also be employed. As such, the header  26  must typically be mounted to the PCB  22  after the encapsulant  24  is applied. This creates certain thermal constraints in soldering the header pins  32  to the corresponding lands  34 . The principal constraint is that solder located under the directly attached IC chip  20  should not be allowed to reflow once the encapsulant  24  is applied. This is because the chemical composition and temperature profile of solder paste changes after the first reflow. The solder underneath the chip  20  may contain a number of small voids which, when subsequently reflowed, may coalesce to produce a large void. A direct mount chip with a large solder void underneath it is unable to efficiently dissipate heat to the copper backplane  25  and thus will have a very short field life. 
     In the past, the pins  32  were hand-soldered to the PCB  22 . This was a labour intensive and economically undesirable method of manufacture. The problem was exacerbated due to the thermally conductive copper backplane  25  which acted as an effective heat sink making it difficult to manually solder each pin. 
     Alternatively, a heat radiation and flux dispenser apparatus was employed to reflow solder (previously applied) on lands  34  in order to create a joint with the header pins  32 . This apparatus was often unable to create successful joints. In cases where the lands  34  were very close to the site of the IC chip  20 , e.g., less than 0.25 inches, the solder on lands  34  did not receive enough heat to reflow due to the aforementioned thermal constraint. If the heat radiation time was increased to reflow the solder on lands  34 , solder would also reflow under the IC chip  20 , creating unwanted voids and defective PCBs  22 . The problem is exacerbated due to the rapid heat conduction properties of the thermal backplane  25  to which the IC chip  20  is directly attached. 
     Furthermore, in an effort to keep within the limits of the aforementioned thermal constraint, the apparatus was used to reflow only one side of the PCB  22  at a time in order to keep the temperature of the solder underneath the direct mount IC chip  20  below the solder reflow point. This uneven heating of the sides of the PCB caused header  26  to tilt and reduced the number of successfully soldered pins on the opposite side of the PCB in the following manner: One side of the PCB was heated first. Assuming that the voiding described above did not occur, the solder was reflowed on the first side and the header pins travelled downward due to gravity to touch the underlying copper-plated surface or land of the PCB on that side. However, the solder on lands on the second side of the PCB  22 , being ball-like in shape, were still solid and high, causing the header  26  to tilt somewhat, with the first side down relative to the second side. The apparatus then advanced to reflow the solder on the second side of the PCB. However, the header pins  32  were high and would not travel down to meet the copper land of the PCB, since the header  26  is constructed from a solid plastic mould and the pins  32  are friction inserted into the plastic. This caused a great failure rate in the joints on the second side of the PCB. 
     SUMMARY OF INVENTION 
     Broadly speaking, the invention overcomes various problems of the prior art by employing a heat conduction, as opposed to heat radiation, approach to creating the header-PCB solder joint. 
     One aspect of the invention relates to a method for mounting a component having one or more pins onto a printed circuit board (PCB) having one or more respective lands for receiving the component pins. The method includes: (a) applying solder and flux, preferably in paste form, onto the lands; (b) bringing the pins in contact with the lands; (c) preheating the PCB to at least a flux-activation temperature; and (d) applying additional heat only to the pins in order for the pins to conduct sufficient heat to reflow the solder on the PCB lands. 
     The method may be advantageously applied to PCBs having pre-existing solder joints, such as an un-packaged IC chip directly mounted onto a copper backplane. In this case the PCB is heated in step (c) to a temperature approaching but not reaching the reflow temperature of the solder in the pre-existing joints, and in step (d) heat is applied so that the pins conduct only enough heat to locally reflow the solder on the lands without reflowing the solder in the pre-existing solder joints. 
     In the preferred embodiment the component is a header and its pins are exposed on top and bottom sides of the header. The top portions of the pins provide contact points for a heating element and the bottom portions of the pins provide a part for assembly onto the PCB. 
     The apparatus according to the preferred embodiment includes a nest for locating the header and the PCB in stacked alignment. A top and bottom heater apply heat to the PCB. The bottom heater receives the nest and provides a general heating of the PCB to at least a flux-activation temperature but less than the reflow temperature of the preexisting solder joints. The top heater includes a top heating block connected to an actuating mechanism such as a piston for bringing the heating block into contact with the exposed header pins for a time sufficient for the pins to conduct enough heat to locally reflow the solder on the lands. 
     In the preferred embodiment the top heating block is resiliently suspended from the actuating mechanism in order to reduce the impact between the heating block and the header pins. Thermally insulative material such as a ceramic shield is disposed between the heating block and the actuating mechanism in order to reduce heat transfer. 
     The heating block preferably features a satbilizer member resiliently suspended therefrom. The stabilizer member contacts and applies a light pressure onto the header in order to stabilize it prior to the heating block contacting the header pins. The stabilizer member also assists in stabilizing the header, whose recently formed solder joints are still substantially liquid, as the top heating block is retracted. 
     The heating block preferably features a plurality of teeth resiliently suspended therefrom, with each tooth being configured for separate contact with an individual header pin. This enables the heating block to comply with variations in the heights of the header pins. 
     The apparatus according to the preferred embodiment further includes a conveyor having a moving element for transporting the nest underneath the top heater. The bottom heater is embedded in the conveyor moving element. The nest is located on a carrier tray and the conveyor moving element is keyed to locate the carrier tray thereon. Lifters are also disposed proximate to a terminating end of the conveyor for raising the carrier tray off of the hot bottom heater in order to cool the former without operator intervention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other aspects of the invention are described in greater detail below with reference to the following drawings, provided for the purpose of description and not of limitation, wherein: 
     FIG. 1A is an exploded, perspective view of an assembled PCB comprising a direct-mount IC chip and a stacked header, taken from above; 
     FIG. 1B is an exploded, perspective view of an assembled PCB comprising a direct-mount IC chip and a stacked header, taken from below; 
     FIG. 2 is an isometric view of a PCB header attachment station in accordance with the preferred embodiment; 
     FIG. 3 is a top view of the station shown in FIG. 2; 
     FIG. 4 is a cross-sectional view of the station taken along line A—A in FIG.  3  and showing a heating cell in operation in accordance with the preferred embodiment; 
     FIG. 5 is an enlarged detail view as indicated by arrow B in FIG. 4 of points of contact between a PCB, header and heating cell; 
     FIG. 6 is an isolated isometric view of a conveyor and a heating/soldering subsystem (comprising plural heating cells) of the station shown in FIG. 2 in operation; 
     FIG. 7 is an enlarged detail view as indicated by arrow C in FIG. 6 of an upper portion of the heating cell; 
     FIG. 8 is a front view of the upper portion of the heating cell shown in FIG. 7; 
     FIG. 9 is an isometric view of a carrier tray, in accordance with one embodiment of the invention, for transporting plural PCBs and header assemblies along the conveyor; 
     FIG. 10 is an isometric view of a PCB/header locator nest of the carrier taken in isolation; 
     FIG. 11 is an isometric view of a preferred embodiment of a heating block employed in the heating cell; 
     FIG. 12 is a side view of the heating block shown in FIG. 11; and 
     FIG. 13 is a functional block diagram of a control subsystem for the assembly station. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring additionally to FIGS. 2,  3  and  4  an automated PCB header attachment station  40  is shown in accordance with the preferred embodiment. The major components of station  40  include a table  42 ; a conveyor  44  comprising a moving element or pallet  46  and a track  48 ; a card carrier tray  50  (FIG.  4 ), also shown in isolation in FIG. 9, for mounting a specific combination of PCBs  22  and headers  26  onto the pallet  46 ; a heating/soldering subsystem  54 ; and a programmable logic controller (PLC)  52  for control of the conveyor and heating/soldering subsystem. The heating subsystem  54  is encased in Lexan™ or Plexiglas™ shielding  56  and thus is not visible in the isometric view of FIG. 2 but a portion, i.e., one cell  58 , of heating subsystem  54  can be seen in the cross-sectional view of FIG.  4 . In addition, the heating subsystem  54  and conveyor  44  can be seen in isometric view in FIG. 6 wherein much of the shielding  56  is removed. 
     Referring to FIGS. 4,  5  and  6 , heating subsystem  54  comprises in the preferred embodiment eight ( 8 ) heating cells  58  (FIG. 6) which operate in unison to simultaneously attach eight headers  26  to eight PCBs  22 . Each cell  58  comprises a top heater assembly  60  and a bottom heater assembly  62  (FIG.  4 ). 
     Briefly, the top heater assembly  60  comprises a frame  64  (shared by all eight cells) secured to table  42  (FIG.  6 ). An actuating mechanism such as a piston  66  is mounted (FIG. 4) to the frame  64  for raising and lowering a heating block  68  along a vertical axis Z. The bottom heater assembly  62  is mounted (FIG. 4) within conveyor pallet  46  and thus moves linearly along a horizontal axis X (FIG. 6) in conjunction with conveyor  44 . Cells  58  enter an “operative state”, i.e., ready to reflow solder paste on the header pin lands  34 , when the top and bottom heater assemblies  60  and  62  thereof are aligned with one another. 
     More specifically, the pallet  46  carries PCB  22  and header  26  in a stacked alignment for assembly to one another during solder paste reflow. The bottom heater assembly  62  is used to preheat the PCB  22  and solder paste to at least a solder paste reflux activation temperature and less than a reflow temperature of solder attaching previously mounted components, including IC chip  20 , to the PCB. When the stacked/header assembly on pallet  46  is docked underneath (FIG. 4) the top heater assembly  60  its piston  66  is actuated to bring heating block  68  into contact with the top portions  32 A of header pins  32  (seen best in the detail view of FIG.  5 ). The heating block  68  provides sufficient heat for conduction along the pins  32  to locally reflow solder paste on the corresponding header pin lands  34  without reflowing solder in the pre-existing joints, especially underneath the thermally constrained direct attach IC chip  20 . 
     Referring additionally to FIGS. 7 and 8, the construction of the top heater assembly  60  for a given cell  58  is described in greater detail. The piston  66  is pneumatically actuated by valves (not shown) controlled by the PLC  52 . The piston  66  includes output shafts  70  which are rigidly connected to a load plate  72  which moves linearly along vertical axis Z. The load plate  72 , in turn, is rigidly connected to a press plate  74 . A top cover  80  is slidingly secured to the press plate  74  by guide pins  86  which extend upwardly therefrom. The top cover  80  includes bores  83  featuring embedded bearings  84  for accepting the guide pins  86 . 
     The press plate  74  also features a number of bores  76  (seen best in FIG. 8) through which spring guide rods  78  are permitted to slide. The rods  78  pass through rebates  82  (seen best in FIG. 7) in the top cover  80  and are capped with collars  79  at their upper ends. The collars  79  prevent the guide rods  78  from slipping through the press plate  74  when it is raised or retracted by the piston  66 . The bottom ends of rods  78  are rigidly connected to a ceramic slab  92  and the heating block  68 . The rods  78  carry springs  94  between the press plate  74  and the ceramic slab  92 . Thus when the heating block  68  encounters an immovable object and piston  66  is actuated, the press plate  74  compresses the spring  94  which urges the heating block downward, with the rods  78  remaining stable. In this way, the heating block  68  is resiliently suspended from the piston  66  to lessen the impact between the heating block  68  and header pins  32 . 
     The heating block  68  features legs  102  having a profile designed to contact the upper portions  32 A of header pins  32 , as seen best in the detail view of FIG.  5 . The heating block  68  can in the preferred embodiment reach operating temperatures of over 350 degrees Celsius and thus is preferably formed from nickel plated copper to resist surface corrosion at such temperatures. Heating block  68  is rigidly connected to the ceramic slab  92  by ceramic spacers  96  (seen best in FIG. 8) to create an air gap  98 . The ceramic slab  92  in conjunction with air gap  98  function as an inner heat shield to reduce heat transfer from block  68  to the piston  66 , frame  64  (FIG. 6) and table  42 . 
     A ceramic stabilizer block  104  depends from a shaft  106  slidingly mounted within a recess  108  in the ceramic slab  92  (shown only in FIG.  8 ). A spring  110  is fitted on the shaft  106  between heating block  68  and stabilizer block  104  in order to resiliently suspend block  104  from block  68 . The stabilizer block  104  functions to press against and stabilize the body of header  26  (see FIG. 5) prior to the heating block  68  contact with header pins  32 . The resilient suspension lessens the impact between stabilizer block  104  and header  26 . The foregoing structure also reduces the possibility of the heating block  68  perturbing or tilting the carefully positioned header  26  due to the very thin line of contact between these elements, and on retraction provides a gentle disengagement as the heating block  68  retreats from the header  26  whose recently formed joints are still in a substantially liquid state. 
     A nitrogen gas manifold  88  is secured to the top cover  80 . The manifold  88  features passageways  90  (seen best in FIG. 8) for the delivery of the gas to the PCB soldering site. (The gas supply hoses and manifold connectors are not shown.) The top cover  80  and manifold  88  are preferably formed from a heat resistant material and thus additionally function as heat shielding. As the top cover  80  is slidingly secured to the press plate  74 , the nitrogen manifold  88  is able to translate vertically relative to the piston  66  and press plate  74  whilst still being secured thereto. An exhaust  126  (FIG. 3) is provided in the Lexan™ or Plexiglas™ shielding  56  for suction of spent nitrogen gas. 
     The heating block  68  is shown in isolation and in greater detail in FIGS. 11 and 12. Each pin-contacting leg  102  of heating block  68  preferably comprises a series of individual teeth  116  configured so that each tooth contacts a separate header pin  32 . A linking bore  118  is formed through the aggregate of teeth and a pin  120  is loosely fitted into the bore  118  to secure the teeth  116  to the block  68  yet allow some vertical play to each tooth. An elastically compressible thermally conductive pad  122 , such as a Thermagon™ brand pad available from Thermagon Inc. of Cleveland, Ohio, is sandwiched between teeth  116  and heating block  68 . In this way the teeth  116  are resiliently suspended from the block  68  to further cushion the impact between the teeth and header pins  32 . The foregoing structure also enables the pin-contacting legs  102  of heating block  68  to comply to small irregularities in the level or height of the header pins  32  in order to ensure good heat transfer contact between these members. 
     The heating block  68  includes bores  100  for the insertion of a heater core, such as part no. E1J42-G36C38H from the Watlow Electric Manufacturing Company of St. Louis, Mo. The heater core wires  112  are connected to the PLC  52  which controls the current therethrough. In addition, a thermocouple  114  is mounted to the heating block  68  via a sandwich plate  117 . The thermocouple  114  is also connected to the PLC  52  to establish a thermal control circuit. 
     The conveyor  44  and embedded bottom heater assembly  62  are described in greater detail with reference to FIGS. 4,  6 ,  9  and  10 . The conveyor pallet  46  comprises an undercarriage  130  having runners  132  (FIG. 4) which include heat insulative bearings  134  for gliding the pallet  46  along the track  48 . A drive means (not shown) is connected to the PLC  52  for the linear translation of the carriage  130 . The drive means may include any of a number of known drive mechanisms, including a motor controlled rack and pinion mechanism; a belt drive; or a linear motor. The drive means in the preferred embodiment comprises a pneumatically actuated piston disposed in a cutout  136  (FIG. 3) of table  42 . A magnet is provided on a moving end of the piston and a complimentarily polarized spaced-apart magnet is provided on the undercarriage  130  in order to magnetically interlock these members and impart translational motion to the undercarriage. This contactless structure minimizes heat transfer to the drive means. 
     The pallet  46  includes a heat insulative pan  138  mounted (FIG. 6) atop undercarriage  130 . The bottom heater assembly  62  is mounted to the pan  138  via ceramic spacers  139  (FIG.  4 ). The bottom heater assembly  62  comprises a nickel plated copper heating block  140  (FIG. 5) having bores  142  for the insertion of the aforementioned heater cores. A series of such blocks are provided to serve as heaters for each cell  58 . Thermocouples (not shown) are also connected to the heating block  140  and PLC  52 . 
     As mentioned, the card carrier tray  50  transports PCBs  22  and headers  26  in stacked alignment on the pallet  46 . The tray  50  comprises in the preferred embodiment eight PCB/header locator nests  150 . Each nest  150  comprises a cutout  152  having flat wall portions  154  (FIG. 10) configured to locate the PCB  22  and header  26  in stacked alignment, i.e., first the PCB is dropped in the nest and then the header is dropped in the nest such that the header pins  32  rest atop the corresponding header pin lands  34 . Oversized curved wall portions  156  are provided for the dextrous insertion or removal of the PCB/header from the nest. Rebates  158  and  160  are respectively provided in the nest and its cutout to vent the nitrogen gas provided by manifold  88  to the surface of the PCB. 
     The card carrier tray  50  also features side rails  162  (FIG. 9) which are situated to engage pneumatically actuated lifters  164  (FIG. 4) located at opposite ends of the conveyor  44 . When the pallet  46  is in an initial position the lifters  164  are in an extended state for placement of the tray  50  thereon by the operator. The operator then actuates a “start cycle” push-button on the PLC  52  and it actuates the lifters to lower the tray  50  onto the heating blocks  140  of undercarriage  130 . Key holes  166  (FIG. 9) are provided for this purpose. Upon completion of a PCB/header assembly cycle the conveyor moves the pallet  46  to a track terminating position whereupon the PLC  52  automatically actuates the lifters  164  to raise the hot tray  50  off of the heating blocks  140  and engage cooling fans  141  (FIG. 6) in order to enable the tray to cool somewhat before being removed by the operator. 
     FIG. 13 is a functional block diagram of a control subsystem for station  40 . As shown, the PLC  52  controls thermal loops  170  and  172  for the heating of the top and bottom heaters, respectively, of each cell  58 . The PLC  52  also controls the pneumatics of the conveyor  44  as described above to move the pallet  46  from the initial, working to terminating positions and back. If desired, a position feedback means (not shown) may be incorporated to provide a position feedback signal  174  in order to increase the positional accuracy of the conveyor  44 . The pistons  66  and nitrogen gas supply valve are likewise controlled by the PLC  52 . 
     The PCB/header assembly process operates as follows: First, the operator loads the card carrier tray  50  with PCBs  22 . The carrier tray is then brought to an automated dispenser (not shown) as known in the art per se which dispenses a prescribed amount of flux-containing solder paste onto the header pin lands  34  located on PCBs  22 . 
     After the dispenser has applied the paste, the operator manually loads the headers  26  into the card carrier tray  50  such that each set of PCBs  22  and headers  26  are in stacked alignment with one another. 
     Next the operator places the loaded tray  50  on the lifters  164  and engages the PLC “start cycle” button. The loaded tray is automatically lowered onto the pallet  46  as described above and the PCB preheating phase begins. During this phase the heating block  68  of the top heating assembly  60  is preferably heated to about 300° C., or about 120° C. above the reflow temperature of the solder paste on lands  34 . In addition, the PCBs  22  are heated by the bottom heater assembly  62  to a temperature preferably 10-40° C., and most preferably 20-30° C., below the reflow temperature of solder attaching previously mounted components, including IC chip  20 , to the PCB. This phase typically lasts about 10 to 80 seconds, depending on heat transfer characteristics, during which the solder paste flux activation temperature is achieved. The PLC  52  then positions the loaded tray  50  underneath the top heating assembly  60 . The piston  66  of each cell  58  is then actuated. The ceramic stabilizer block  104  is the first element of the top heater assembly  60  of each cell  58  to touch and apply a light stabilizing pressure on the header  26  mounted in tray  50 . The heater block  68  of assembly  60  then touches the upper portions  32 A of header pins  32  and a light pressure is exerted thereon by the press plate  74  via springs  94 . In the preferred embodiment the top heater block  68  is applied to the header pins  32  for approximately 100 to 180 seconds. The header pins  32  conduct sufficient heat to locally reflow solder paste on the header pin lands  34  and provide a good joint without reflowing the solder in the surrounding pre-existing solder joints. In addition, heat is applied to all of the header pins  32  simultaneously such that solder paste reflows on both sides of the card and the problem of tilting is eliminated. Upon retraction of the piston  66  the stabilizing block  104  gently disengages last from the header  26  in order to avoid disturbing the recently formed hot joints. 
     Thereafter the conveyor  44  moves the tray  50  to the terminating position and the lifters  164  raise the tray off of the heated surface of the pallet  46 . The fans  141  are then engaged and the operator may then remove the tray  50  from the assembly station  40 . 
     Those skilled in this art will appreciate that the temperatures and heating times described herein have been provided for illustrative purposes only and will readily recognize that the heat transfer characteristic of different solder paste types will differ in each application, thereby requiring different temperature and time profiles. In addition, the sizes of the PCB and its components will vary in each application, requiring different time and temperature profiles. 
     The preferred embodiment has also described the application of a solder paste onto the header pin lands  34 . Those skilled in the art will understand that in an alternative embodiment the lands  34  may be HASL (hot air solder level) finished, in which case either solder paste or a flux in and of itself can be applied to achieve the results of the invention. Similarly, those skilled in the art will appreciate that numerous modifications and variations may be made to the preferred embodiment without departing from the spirit and scope of the invention.