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This paper presents a comprehensive study on the effects of critical process conditions on solder joint metallurgy and reliability of mixed-alloy solder joints. The solder joint metallurgy of mixed alloys was characterized and the lead distribution through the solder joint was analyzed, for different package types and under various process conditions. The results showed that the solder paste amount (ultimately the tin percentage (Sn%) in the alloy) and the reflow temperature play critical roles in the mixed-alloy assembly, both in terms of compositional homogeneity and voiding. The reliability of mixed-alloy solder joints was then studied at various process conditions, under different thermal and mechanical stress environments. The study revealed that the sensitivity of the reliability of the mixed-alloy solder joints to the process condition depends on the type of environmental loading.