Low temperature 1st level interconnect in Packaging and its Challenges

Materials and processes currently used for 1
^st
level interconnects are reviewed, and possible options for achieving significant reductions in temperatures to which the processor is exposed during packaging are considered. While the low melting point tin-bismuth alloys that are being introduced to board level assembly are potential candidates for 1st level interconnect, it has yet to be confirmed that other properties of these alloys are consistent with the requirements of materials needed to make fine pitch connections in close proximity to the processor. Of particular concern is the possibility of alpha emissions, microstructural stability, susceptibility to electromigration, whisker growth, and the possibility of polymorphic transformation of the tin phase at the cryogenic temperatures to which some processors might be exposed. For high frequency circuitry the inductance and the capacitance properties of the interconnect material also have to be taken into account. Consideration has to be given to whether the low melting point indium alloys that are used in the attachment of IR Focal Plane Arrays could be used for 1
^st
level interconnects. Where the mechanical properties of low melting point alloys might not be sufficient to cope with possible service conditions, there are methods for forming a bond that is substantially comprised of intermetallic compounds at temperatures lower than the process temperatures required for reflow of conventional lead-free solders.