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Due to environmental consciousness and legislative regulations, Pb-free solders have been increasingly used for replacement of conventional SnPb solders in the microelectronic industry. However, during the transition period from SnPb to Pb-free solders, some products will potentially consist of mixtures of Pb-free with SnPb solders. This paper presents the results of microstructural analysis of mixed solder joints which are formed by attaching different alloys of solder balls and pastes on the OSP pad finish. The effect of thermal aging on the strength and fracture energy of the mixed solder balls during high speed ball shear/pull tests was also investigated in the study. The test specimens were aged at 125degC for durations of 200, 500 and 1000 hours. The correlations between the solder ball fracture force/energy, failure modes and intermetallic compound (IMC) thickness of the mixed alloy solder joints are also established.