Solder transfer technique for flip-chip and electronic assembly applications

There are numerous applications that could benefit from a capability of providing solder at a stage when it is inconvenient or not possible to be deposited by conventional flip-chip or packaging technology methods. For example, flip-chip solder bumps are usually fabricated as wafers by evaporation or plating, whereas the solder necessary for component attachment is achieved by extruding or stenciling solder paste onto a card. These methods, although well suited for initial fabrication, are not practical at subsequent stages of assembly. However, decals, which consist of solder bumps of any desired configuration and volume deposited onto a nonwettable carrier (e.g., glass, scrap silicon wafers, etc.), can often be utilized as a solder source in these situations. The paper focuses on decal fabrication, solder transfer, potential problem areas, and solutions. Applications of the technology are discussed as well. Among these are restoration and altering composition of flip-chip solder bumps, chip carrier pretinning, wettability testing, and deposition to enable direct chip attach (DCA).