Socket for semi-permanently connecting a solder ball grid array device using a dendrite interposer

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CROSS REFERENCE TO RELATED APPLICATIONS

The invention in the present application is related to the subject matter in copending U.S. patent application Ser. No. 08/352,301, filed Dec. 7, 1994. The applications are commonly assigned.

FIELD OF THE INVENTION

The present invention relates generally to the connection of the electronic devices. More particularly, the invention is directed to a socket which reliably connects a ball grid array configured electronic device to a printed circuit board substrate having a corresponding connection pattern, yet facilitates easy removal of the electronic device for replacement or the like.

BACKGROUND OF THE INVENTION

Shrinking integrated circuit dimensions and increasing functionality have dramatically increased the input/output connections of die packages while decreasing the contact pitch. These trends have led to die packaging technologies which utilize ball grid array input/output connections, in contrast to the peripherally located pin technologies characterizing conventional designs. A particularly efficient input/output connection technique for integrated circuit die is the flip chip technology, where the ball grid array is formed onto the die body. Integrated circuit die packages using arrays of pins instead of balls do exist, but they are very expensive, susceptible to damage, and require complex mating sockets.

Multi-chip module technology represents another packaging application where fine pitch ball grid array connections are finding increased use. Again, connection of the module ball grid array to the electronic system printed circuit board has proven to be a challenge if removal of the module with reasonable effort is contemplated.

Ball grid array modules or flip chip die are conventionally attached to correspondingly arranged arrays of copper lands on printed circuit boards through solder reflow techniques. Typically, low temperature solder or low temperature solder paste predeposited on the printed circuit board is reflowed to make the connections to high temperature solder balls formed on the ball grid array module or flip chip die.

The use of a ball grid array to connect modules or die to a printed circuit board materially increases the relative density of connections attainable when compared to peripheral pin or pin grid array packages. Unfortunately, ball grid array configured modules and die suffer in two respects. First, it is difficult to make reliable temporary connections to the balls of the die or modules for purposes such as testing or burn-in. Secondly, if a defective connection occurs between a solder ball of the grid and a printed circuit board land during reflow attachment it is difficult to locate and correct the defective solder connection. In most cases, the complete module or flip chip die must be desoldered.

Developers of the assignee recently discovered that palladium dendrites formed by plating metal vias or pads can be used to make reliable connections between the vias or pads and solder balls of modules or die. Connections between dendrite covered pads or via ends and solder balls were found to be particularly effective in that the dendrites penetrate the solder ball surface oxides, for reliable electrical connections, and become imbedded into the relatively softer solder sufficiently to produce a mechanical bond and seal between the dendrite covered via or pad and the solder ball. The related concepts are described in U.S. Pat. No. 5,137,461 and IBM Technical Disclosure Bulletin Volume 36, No. 7, July 1993, pages 137 and 138, the subject matter of both references being incorporated herein by reference.

Though the use of dendrites, and the connection of a solder ball grid array devices to dendrite plated vias or pads has been known, attempts to apply the concept have proven less than successful. When attempts were made to connect a ball grid array module, nominally having 0.75 millimeter balls with a spacing of 1.25 millimeters, or a flip chip die, nominally having 0.125 balls with a spacing of 0.25 millimeters, array alignment and reliable connection between the array of solder balls and an array of solder covered lands on a printed circuit board using a dendritic interposer proved to be a significant challenge.

Therefore, there arose a need for a ball grid array device socket which allowed for a temporary but reliable connection between the solder balls of a ball grid device and a corresponding pattern of lands on a circuit board while allowing removal or replacement of the module or die as needed.

SUMMARY OF THE INVENTION

The present invention relates to an electronic device socket, comprising a planar interposer of flexible dielectric material having a multiplicity of vias or pads covered with dendrites, which vias or pads are distributed in a pattern substantially conforming to an electronic ball grid array device, a rigid cap with a planar surface alignable to be substantially coplanar with the surface of the ball grid array device, means for aligning a pattern of balls on a surface of the ball grid array device with the planar interposer and with a pattern contact regions on a board, and means for translating the rigid cap to compress the ball grid array device, the interposer, and the contact regions of the board adequately to cause dendrite penetration into the ball grid array device balls and the contact regions on the board.

In a particularized form of the invention, the ball grid array socket comprises a rigid planar stiffener, with alignment pins for extending through a printed circuit board, a planar interposer having a multiplicity of dendrite coated vias or pads alignable by holes to the pins of the stiffener, an alignment planar having holes for alignment with the pins of the stiffener and with select balls of a ball grid array device, a rigid planar heatsink capable cap, and an arch connectable to the alignment pins of the stiffener. A screw in the arch applies a force to the cap to compress connect the balls of the ball grid array device to the top side of the interposer and the bottom side of the interposer to solder deposits on lands of the printed circuit board. In another form, the flexible interposer is itself composed of multiple layers with electrical routing to pro