Inter-dependence of processing and alloy composition on the reliability of Sn-based lead free solders in fine pitch FCOB interconnection

Market demands and legislation are driving the electronics manufacturing sector to move rapidly toward for a lead free future, with Pb containing electronics products are to be banned in Europe from 2006. The UK readiness for joining global actions to enable this ´green´ packaging urgently requires the replacement of SnPb with suitable lead free alloys. Although the related scientific research has been undertaken for a decade, a number of technical complications still exist, which are further exaggerated due to the concurrent developments of the new technologies needed for the miniaturisation and multi-functionality of microelectronic products. As the packaging joint geometry shrinks towards a microscopic scale, the joint fabrication and reliability become extremely sensitive to the composition and resulting microstructure generated from an essentially hybrid joining process. The current level of understanding on such issues is still in its infancy and therefore requires further fundamental study. Thermodynamic modelling is employed in this work as a major computational tool to study the sensitivity of processing ranges (e.g. reflow temperature) and the resultant reliability of the micro-joints by changing the alloying elements and their content in Sn-based lead-free systems. The work is implemented using the MTDATA program developed by the National Physical Laboratory (NPL), UK. With a newly-developed database containing critically assessed thermodynamic data appropriate for lead free solder systems, MTDATA allows the prediction of the dependence of the liquid-solid transformation and phase formation, for example, as a function of chemical composition and temperature. The paper emphasises the formation and mass fraction of intermetallic precipitates of different phases in the bulk solder joints and the modelling is also validated through carefully designed experimental work and recent literature. The results are expected to assist the optimisation of processing parameters and cost-effective production using lead free solders.