Solder Joint Characteristics and Reliability of Lead-Free Area Array Packages Assembled Under Various Tin-Lead Soldering Process Conditions

The compatibility of lead-free area array packages with tin-lead soldering processes is a critical issue, as several product categories may take advantage of the exemptions under the European Union (EU) RoHS legislation and continue to be built with tin-lead solder for some time to come, whereas the components have become predominantly lead-free. The issue of "backward compatibility" arises because for BGA packages, the contribution of solder balls to the resultant solder joint material is very high (typically of 70% to 80%), and the assembly of lead-free BGA packages with tin-lead paste becomes a major concern from the perspective of solder joint metallurgical uniformity and reliability. 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.