Apparatus for attaching/detaching a land grid array component to a circuit board

A system is provided for selectively attaching or detaching a land grid array component to a surface of circuit board where the attachment is a grid array of solid conductive solder beads or balls. A chamber contains an inert liquid, and a heater heats the inert liquid to a temperature above the melting temperature of the solder beads. A fluid level adjustment means adjusts the level of the inert liquid in the chamber between a first level below the component and a second level above the component. A first mounting means supports the circuit board above the component, and a second mounting means is positioned at least partially in the container to support the component below the circuit board and to bias the component against the circuit board. An component/board assembly is positioned in the system. The beads are uniformly melted by raising the level of the inert liquid in the chamber to above the component to permit removal of the board. The beads of the grid array resolidify by lowering the level of the inert liquid in the chamber to below the component while the second mounting means biases the component against the circuit board, thereby reflowing the solder and establishing the solid conductive connections of the grid array.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for attaching or detaching a land grid 
array component to a circuit board. 
Circuit boards ordinarily include circuit patterns or traces printed on 
various layers internal to the board and/or on one or both sides. 
Components are ordinarily mounted on one side of the board and 
electrically connected to the circuit patterns. Prior to the introduction 
of surface mount and land grid array components, it was common to mount 
components to a circuit board and extend leads from the components through 
plated-through holes in the circuit board to mounting pads on the opposite 
side. The plated-through holes were electrically connected to desired 
locations on the circuit pattern. The component leads were ordinarily 
arranged in a pattern along the edges of the component. The ends of the 
component leads were soldered to the pads on the opposite side of the 
board to establish mechanical and electrical connection to the board and 
circuit pattern. The arrangement provided satisfactory mechanical and 
electrical connection of the component to the circuit board. Soldering 
apparatus was developed to simultaneously connect all of the ends of the 
leads of a component to the pads opposite the circuit pattern. These 
soldering apparatus employed a solder bath to solder the leads. Ordinary, 
a template or other guide was used to direct the molten solder to the 
desired positions on the circuit board for connecting the component leads 
that extend through the through holes to the circuit board. An example of 
such apparatus is found in U.S. Pat. No. 3,684,151 for "Solder Machine" 
issued Aug. 15, 1972 to Burman et al. 
Surface mounted components were developed for mounting and connecting to 
one side of the board. In one form of surface mounted components, leads 
extend from along the edges of the component for attachment to pads on the 
circuit board. In another form of surface mounted components, compliant 
leads were attached to pads along the edges of the component and to pads 
on the circuit board. An example of a surface mounted component employing 
compliant leads may be found in U.S. Pat. No. 4,827,611 granted May 9, 
1989 for "Compliant S-Leads for Chip Carriers" by D. K. Pal et al. and 
assigned to the same assignee as the present invention. Apparatus was 
developed that permitted reflow of solder on the leads for connection to 
the circuit board pads and to the component pads (for compliant leads). 
The reflow apparatus directed heat along the edges of the component 
between the component and the circuit board to uniformly heat the solder 
connections and reflow the solder. The apparatus heated all of the solder 
connections simultaneously, thereby substantially simultaneously reflowing 
the solder connections between the surface mounted component and the 
circuit pads. 
Land grid array components are a class of surface mounted components that 
employ a grid array of solder connections on a surface of the component. 
The grid array is not limited to the edges of the component, but instead 
are disbursed across the surface in spaced relation in an area array, 
usually uniform. Land grid array components include ball grid array 
components, solder grid array components and column grid array components. 
The surface bearing the land grid array is positioned to confront an 
identical grid array of circuit pads on a circuit board, and the solder 
beads, balls or columns are heated to reflow to attach or detach the 
component from the circuit board. These devices, like other surface 
mounted components, require simultaneous reflow of all solder connections 
to the circuit board. However, conventional solder/desolder machines were 
not capable of uniformly heating the solder connections of the component. 
More particularly, the lack of a uniform heating pattern resulted in the 
failure to uniformly melt all of the solder balls or columns of the land 
grid array substantially simultaneously. 
Air-Vac of Seymour, Conn. developed apparatus intended to perform 
simultaneous reflow of the solder connections between a land grid array 
component and a circuit board. This apparatus, known as the DRS22 reflow 
system, is partially described in U.S. Pat. No. 5,419,418 and is partially 
diagrammatically illustrated in FIG. 1. The Air-Vac system includes a 
housing 10 having a lower surface 12 arranged to abut a surface 14 of 
printed wiring board 16. Block 18 in housing 10 includes passages 20 
between an upper chamber 22 and a lower chamber 24, the lower chamber 
receiving land grid array component 26. Hot gas, such as air, is forced 
into chamber 22 and through passages 20 on three sides of block 18. The 
hot air enters the space of the land grid array between the printed wiring 
board 16 and component 26. The hot air enters on three sides and exits on 
the fourth side through opening 28 in housing 10. Printed wiring board 16 
is supported on a platform that may itself be heated. The hot air forced 
through the land grid array melts the solder, thereby allowing ready 
attachment or detachment of the component from the printed wiring board. 
The Air-Vac system also includes a microscope and camera to permit the 
operator to view the progress of the operation within chamber 24, the 
camera permitting display of the operation on a display unit. 
One problem with the Air-Vac system is that the flowing gas does not 
rapidly heat the solder of the land grid array. More particularly, the 
heat transfer between a gas and a solid is not as rapid as, say, between a 
more dense fluid and the same solid. As a result, the process employing 
the Air-Vac apparatus takes a considerable amount of time, up to three to 
four minutes. The circuit board is heated to about 100.degree. C., which 
is difficult to maintain over a large board surface. Moreover, in the 
Air-Vac apparatus, the heat transfer is concentrated at three of the edges 
of the land grid array, resulting in some of the connections joints 
becoming overheated. Overheating of the solder joints results in an 
alteration of the grain size of the solder joints, and the solder joint 
grain size is not uniform across the entire grid array. Moreover, heat 
concentration at the three edges of the land grid array causes warpage in 
the circuit board due to uneven heating of the board in the region of the 
component. 
SUMMARY OF THE INVENTION 
The present invention is a system for selectively attaching or detaching a 
land grid array component to a first surface of circuit board. The 
component is arranged for surface mounting to the first surface by a land 
grid array of solid conductive connections, such as solder beads, balls or 
columns, that establish electrical conduction between the component and 
the circuit board and provide mechanical support for the component on the 
circuit board. The solder beads have a predetermined melting temperature 
so that upon melting and resolidifying the beads form the solid conductive 
connections that establish mechanical and electrical connection between 
the component and the circuit board. A chamber contains an inert liquid. A 
heater heats the inert liquid to a temperature above the melting 
temperature of the solder beads, and a fluid level adjustment means 
adjusts the level of the inert liquid in the chamber between a first level 
below the component and a second level above the component. A first 
mounting means supports the circuit board above the liquid, and a second 
mounting means is positioned at least partially in the container to 
support the component below the circuit board and to bias the component 
against the circuit board. 
To detach a component from a circuit board, the first mounting means 
supports the circuit board so that the component attached to the circuit 
board is below the circuit board and in the container. The solder beads of 
the land grid array are uniformly melted by raising the level of the inert 
liquid in the chamber from its first level to its second level. The 
circuit board may be removed from the first mounting means leaving the 
component supported by the second mounting means. 
To attach a component to a circuit board, the first and second mounting 
means are arranged so that component is supported in the chamber with the 
solder beads of the land grid array sandwiched between the component and 
the circuit board. The beads are uniformly melted by raising the level of 
the inert liquid in the chamber from its first level to its second level. 
The beads of the land grid array resolidify by lowering the level of the 
inert liquid in the chamber from its second level to its first level while 
the second mounting means biases the component against the circuit board, 
thereby establishing the solid conductive connections of the land grid 
array. 
One feature of the system resides in the use of spring mounts for the 
second mounting means. These mounts support the component parallel to 
level of the inert liquid in the chamber and are compliant to adjust for 
variations in heights of the pads on the component and board, as well as 
for changes due to melting of the solder during reflow. Additionally, the 
spring mount supports the component after it is removed from the circuit 
board when the system is operated to detach a component from a circuit 
board, and biases the component against the circuit board when the system 
is operated to attach a component to a circuit board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 is perspective view of apparatus for attaching/detaching a land grid 
array component to a circuit board. The apparatus includes a housing 30 
having support points 32 for supporting a printed circuit board 60 (FIG. 
3). Supports 32 are preferably adjustable to permit alteration of the 
orientation of the circuit board with respect to the housing. Lower 
portion 34 of housing 30 defines a chamber 36 in which platform 38 is 
mounted. As shown particularly in FIG. 3, platform 38 is mounted to 
bracket 42 by a resilient mechanism such as springs 40. Bracket 42 is 
mounted to portion 34 of housing 30. Chamber 36 contains an inert liquid 
44 which is heated by heater 46. One suitable inert liquid that is used 
for solder reflow and exhibits a boiling point in excess of 300.degree. C. 
is "Fluoroinert," commercially available from Minnesota Mining and 
Manufacturing Company of St. Paul, Minn., Preferably, heater 46 is 
electrically operated and is controlled by control knob 48 (FIG. 2) 
connected to a rheostat control in a manner well known in the art. 
Indicator 50 provides an display to the operator of the temperature of 
liquid 44. Indicator 50 is connected to a thermostat (not shown) within 
chamber 36. Float 52 is connected to indicator 54 to provide a display to 
the operator indicative of the level of liquid 44 within chamber 36. 
Conduit 56 provides fluid communication to a reservoir (not shown) of 
liquid. The reservoir includes a displacement piston (not shown) to 
controllably transfer a controlled volume of liquid from the reservoir to 
chamber 36. The displacement piston is controllable to accurately control 
the volume of liquid 44 within chamber 36, and hence the height of the top 
surface 58 of the liquid. 
In operation, printed wiring board 60 is positioned on supports 32 so that 
surface 62, to which component 64 is to be attached, is parallel to the 
surface 58 of liquid 44 within chamber 36. More particularly, supports 32 
are adjusted to assure that surface 62 of the board is parallel to surface 
58 of the liquid. The inert liquid in chamber 31 is heated, preferably to 
about 215.degree. C. To detach component 64 from circuit board 62, the 
assembly is placed in the apparatus so that board 60 is supported on 
supports 32 and component 64 bears against support 38. Springs 40 bias 
component 64 toward circuit board 62. Preferably, the springs forming the 
bias force are independent of each other so that the position of the 
component conforms to varying levels of connection pads on the component 
and circuit board. Weights or other counter forces may be applied to board 
60 to counter the bias force of springs 40, as well as any buoyancy 
effects of the liquid to be raised within chamber 36. The displacement 
piston within the reservoir attached to conduit 56 is operated to raise 
the liquid level 58 within chamber 36 so that the hot inert liquid 
contacts the solder connections of land grid array 66 to thereby melt the 
solder forming the land grid array. Circuit board 60 is then be lifted off 
and separated from component 64, leaving component 64 resting on support 
38. The piston within the reservoir is then be operated to withdraw liquid 
from chamber 36 through conduit 56 there by lowering the level 58 of 
liquid within chamber 36 to below the component, to permit removal of the 
component. 
To attach a component to a circuit board or to reflow solder connections 
between a component and a circuit board, the component is placed on 
support 38 within chamber 36 such that the land grid array 66 is facing 
upwardly. Primed wiring board 60 is positioned on support 32 so that 
surface 62 is parallel to the surface 58 of the liquid within chamber 36 
and so that the contacts on the circuit board align with the land grid 
array. As in the case of detachment, a counter force may be necessary to 
counter the bias effects of springs 40 and any buoyancy effects of the 
liquid on the component. The level of heated inert liquid within chamber 
36 is raised to the land grid array 66 causing the solder of the land grid 
array to reflow, thereby establishing electrical and mechanical connection 
between component 64 and the contacts on surface 62 of printed wiring 
board 60. Noteworthy, the internal printed wiring, as well as the printed 
wiring 68 on board 66 are unaffected by the process. Also noteworthy, the 
inert liquid permits an even application of heat to the land grid array, 
thereby assuring that the solder of the land grid array does not form 
uneven grain joints and assuring that circuit board 60 is evenly heated to 
avoid warpage. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize that 
changes may be made in form and detail without departing from the spirit 
and scope of the invention.