Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
1999-07-13
2001-08-21
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S049500, C228S047100
Reexamination Certificate
active
06276598
ABSTRACT:
This invention relates to a method and apparatus for the placement of small balls of material in an array. The invention is particularly useful, but not exclusively intended, for the placement of solder balls on a ball pick head for subsequent transfer to a ball grid array (BGA).
BGA techniques have become more common in recent years for connecting high density IC components onto circuit boards. A regular array of fluxed solder balls is deposited on the circuit board at points where the leads of an IC component may be desired to be connected. An IC component may then be mounted on the board and connections are made between the leads of the component and the solder balls contacted by those leads.
To achieve this end, in a ball grid array technique droplets of flux and solder balls must be transferred to a substrate where they are deposited in a predetermined array. One way of achieving this is to locate a stencil over the circuit board. The stencil is formed with a plurality of apertures in the same configuration as the desired array of solder balls on the circuit board. The stencil is located over the circuit board in such a position that the apertures are located over the intended locations of the solder ball on the circuit board. A supply of solder balls is then introduced over the stencil such that solder balls pass through the apertures in the stencil onto the circuit board in the proper configuration. Examples of such stencil based techniques are to be found in U.S. Pat. Nos. 5,431,332, 5,655,704, 5,620,927, 5,839,641 and 5,704,536. A major disadvantage with this technique, however, is that it requires very precise location of the stencil above the circuit board which may be difficult to achieve when the pitch between solder balls is very small as will normally be the case.
An alternative technique is to use a ball pick head. The ball pick head is used for transferring the solder balls to the substrate and is designed to carry the balls in the same array configuration as is required on the substrate and then subsequently to deposit the balls on the substrate. It is important, indeed essential, that all locations on the circuit board intended to be provided with a solder ball are so provided, since otherwise if a solder ball is missing a IC component lead may not be properly connected to the board and the entire circuit board may be useless.
Conventionally the ball pick head is formed with a plurality of locations for receiving solder balls, these locations being disposed in the same array configuration as the desired configuration of solder balls on the circuit board. The ball pick head must therefore be provided with solder balls such that each location is provided with one (and only one) solder ball. If any location is missing its solder ball, the corresponding location on the circuit board will not be provided with a solder ball. Matters are not much better if a ball receiving location receives two solder balls which is undesirable. Achieving these aims is difficult and expensive. Often means must be provided to check that each ball receiving location is provided with a solder ball before the supply of solder balls to the board, or the board itself must be checked after the supply of balls and before an IC component is mounted thereto.
However, the solder balls must firstly be transferred to the ball pick head before they can be deposited on the substrate. In the prior art, the ball pick head is formed with ball receiving locations that hold solder balls in place on the ball pick head by suction. In such prior art, one conventional technique is for the ball pick head to be simply lowered into a container having a supply of solder balls such that the solder balls are drawn to the ball receiving locations by suction and held therein. Examples of such a technique are to be found in U.S. Pat. Nos. 5,695,667, 5,753,904, 5,615,823, 5,657,528, 5,601,229, 5,680,984, 5,750,199, 5,831,247, and 5,768,775. While relatively simple, this method has a number of drawbacks. The most important of these is that it is very difficult to ensure that every single ball locating means receives one, and only one, solder ball. When the ball pick head is lowered into the supply of solder balls, it may happen that if by the random distribution of the balls in the container there is no solder ball close to one particular ball locating means, the suction may be insufficient to attract a solder ball thereto. Conversely if other ball locating means have a number of solder balls close thereto, more than one solder ball may be drawn to that ball locating means.
According to the present invention there is provided apparatus for supplying balls in a predetermined array, comprising: supply means for holding a supply of balls, means for transferring balls from said supply means to an intermediate position in which said balls are disposed in said predetermined array, and means for transferring said balls from said intermediate position to a final position while remaining disposed in said predetermined array.
By means of this arrangement the balls are placed in the desired predetermined array before the ball pick head is moved towards the balls to pick them for transfer to a final position. In this way the problems associated with the ball pick head collecting balls from a large supply reservoir containing a large number of balls are substantially eliminated or at least mitigated.
In a preferred embodiment of the invention the intermediate position comprises a template having a plurality of ball receiving locations disposed in the predetermined array. Preferably each ball receiving location is formed with means for locating a ball therein, which means may comprise each ball receiving location being formed as a recess sized to receive a substantial part of a ball therein. Preferably each ball receiving location is connected to a source of suction to hold a ball received therein.
The ball supply means may be formed adjacent the intermediate position, and it is particularly preferred that the ball supply means and the intermediate position are formed integrally as part of a single unit. This single unit may be a container, the ball supply means comprising a recessed portion thereof, and the intermediate position may be provided on a planar surface of the container. In this embodiment balls may be transferred from the supply means to the intermediate position by motion of the single unit. This motion may be a reciprocating swinging motion about an axis extending parallel to long axes of the supply means and the intermediate position.
The present invention is in its preferred forms particularly well adapted for supplying solder balls to a ball grid array package.
According to the present invention therefore there is provided apparatus for supplying solder balls to a ball grid array, comprising: supply means for holding a supply of solder balls, means for transferring solder balls from said supply means to an intermediate position in which said solder balls are disposed in a predetermined array corresponding to said ball grid array, and means for transferring said solder balls from said intermediate position to a ball grid array while remaining disposed in said predetermined array.
According to a further aspect of the present invention there is provided a method of supplying balls in a predetermined array, comprising: providing supply means for holding a supply of balls, transferring said balls from said supply means to an intermediate position in which said balls are disposed in said predetermined array, and transferring said balls from said intermediate position to a final position while remaining disposed in said predetermined array.
According to a still further aspect of the present invention there is provided a method of supplying solder balls to a ball grid array, comprising: providing supply means for holding a supply of solder balls, transferring said solder balls from said supply means to an intermediate position in which said solder balls are disposed in a predetermined array corresponding to said ball g
Cheng Chi Wah
Wong Chiu Fai
Yue Alfred Ka On
ASM Assembly Automation Ltd.
Burns Doane Swecker & Mathis L.L.P.
Dunn Tom
Johnson Jonathan
LandOfFree
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