Pb-free solders and other joining materials for potential replacement of high-Pb hierarchical solders

High-temperature Pb-free solder joints that do not fail during subsequent reflow assembly with low temperature solders are needed for multi-chip modules, die attachments, and other heat resistant applications with high reliability. It is well known the future RoHS rules will expand soon to include Pb-(5-10)Sn (wt.%) solders, which drives accelerated development of replacements for these Pb-based high temperature solders. Options include improvements to Pb-free joining alternatives, including Au-Sn, Zn-Al, Zn-Sn, Sn-Sb, Bi-based systems, and Pb-free composite solder pastes, i.e., with high melting metallic reinforcement particles that are blended with low melting Pb-free solder powders, along with sintered nano-particulate Ag and conductive adhesives. It is preferred if a solder alloy candidate is capable of forming conventional BGA spheres for direct drop-in replacement of the Pb-Sn solders, but solder “bumping” with paste may substitute for applying an array of solder spheres for BGA applications. This paper will review examples of the competing high-temperature interconnect methods and their potential for practical application in the electronics industry.