| A switch to lead-free solder alloys raises concerns with respect to the compatibilities of materials, solder joint properties, and the quality of soldering that is achievable. It is well established that Pb-free alloys tend to solder less readily than Sn/Pb. It is not clear whether this is always a major concern. However, two factors combine to make it more of a concern for flip chip than for other applications. Not surprisingly, wetting and spreading appears to become less effective as the solder volume is reduced and a larger fraction of it is near the surface. At the same time, assembly yields tend to become more sensitive to the reduced wetting and spreading of the solder alloy.; It is possible to reasonably define wide fluxing and reflow process windows for eutectic Sn/Pb based flip chip assembly. However, most no-Pb solders call for higher reflow temperatures, often approaching peak temperatures that damage conventional organic substrates. This tends to narrow the reflow process window considerably. The same often seems to be the case for the fluxing process windows. The results achieved for larger devices such as Ball Grid Arrays (BGAs) and Chip Scale Packages (CSPs) are not readily applicable to the assembly of flip chips.; The global objective of this research endeavor was to address issues that relate explicitly to the assembly and reliability of Sn-Ag-Cu bumped flip chips. This research entailed a detailed study into the sensitivity of the 95.5Sn-3.5Ag-1.0Cu & 95.5Sn-4.0Ag-0.5Cu solder bumped flip chips to the various assembly parameters such as flux types, fluxing methods, reflow profiles, reflow atmosphere, and substrate pad finishes. The parameters that are principally likely to impact the assembly yields were identified and studied further. A systematic study of the combinations of the various capillary flow underfills and no-clean fluxes was undertaken in terms of the effects of underfill properties on solder joint reliability. In addition, the effect of parameters such as pad finish, fluxes, and reflow parameters on solder joint reliability was also studied. Failure analysis was carried out to establish a comprehensive picture of the various damage mechanisms that contribute to eventual failure in thermal shock and thermal cycling. |
