To avoid the problem with thermo-mechanical stress induced fatigue using conventional flip-chip mounting of bare chips, an elastic chip socket has been developed.
The socket is made by casting silicone elastomer into micro structured silicon molds to form micro bump arrays. After the elastomer is cured and released from the mold, a metal layer is deposited and patterned.
A chip is placed in the socket utilizing guiding structures for chip self alignment. The chip is then held in place by a spring loaded back-plate which can also serve as a heat sink for highly effective chip cooling. Since no adhesives, underfills or solders are used, the rework process becomes very simple and it can also be repeated many times for-the same socket.
Initial contact resistance and thermo-mechanical robustness measurements indicates that this type of sockets could work as a superior replacement for conventional flip-chip technologies in many applications. The particular design of the contact bumps results in metal structures that resemble (although up side down) and are scalable as those in the Chip-First technology. Preliminary thermal shock experiments from room temperature to liquid nitrogen and back show good survival. Thus, this new chip interconnect method indicates the possibility of getting the advantages of the Chip-First technology while eliminating the demand of placing the chip first. The concept will work for chips with rim positioned pads as well as for high density area arrays.
2003. Vol. 26, no 1, 33-40 p.
chip-first; chip socket; flip chip; multi-chip modules; packaging; silicone elastomer; thermal fatigue; thermal stress