Solder alloy selection for flip chip on board

Improvements in PCB technology and the demand for smaller form factor at lower cost have provided the primary motivation for interest in flip chip on board (FCOB) assembly. With the development of direct chip attach (DCA) technology using high-lead solder bumps joined to eutectic solder on the board, these goals have been substantially met. However, efforts to further reduce overall product cost continue to motivate a search for a better solution. A survey of the literature and the user community was performed to assess the state of contemporary wisdom with regard to the selection of an appropriate solder alloy for FCOB applications. Although there is yet no consensus, the preliminary conclusion is that the current DCA technology (lead-rich bumps mounted by tin-rich solder to the PCB) is the most appropriate solution for the near term. Longer term, a homogenous system may offer simplified assembly with higher reliability at finer pitches. This would reduce cost by eliminating the solder deposition step for the PCB and, perhaps, eliminating the need for underfill.