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94 SMT Magazine • July 2017 by David Hillman ROCKWELL COLLINS Several electronic market segments remain exempt from lead-free material restrictions such as Restriction of Hazardous Substances (RoHS) legislation. These market segments tend to be electronics used in harsh environments and/ or that have life/system critical functionality. While these market segments, which include avionics, can use solder containing lead, it is in- creasingly difficult to procure advanced compo- nents in non-ROHS compliant configurations. Product design teams that continue to use tin/ lead soldering processes are faced with the di- lemma of how to use ball grid array (BGA) com- ponents that are only available with lead-free solder spheres in their assembly processes. This paper discusses several aspects of having lead- free BGA components in a tin/lead soldering process: (1) mixed metallurgy; (2) solidification; (3) tin whiskers. The possible methodologies of utilization without compromising product in- tegrity will be discussed. The Mixed Metallurgy Concern Environmental legislation has caused signif- icant impact to the electronic industry in terms of the material sets used in electronic products. The prohibition of materials such as cadmi- um, hexavalent chromium, mercury and lead has eliminated a number of electronic compo- nents and material processes that were histor- ically used in producing electronic products. The removal of lead for solder alloys has argu- ably the greatest impact on electronic products due to its role as the primary mechanical and electrical functionality material. Figure 1 illus- trates how global solder usage trends show the replacement of tin/lead solder alloys with lead- free solder alloys since 2004 as tracked by the IPC Global Solder Statistical Program [1] . Product design teams that continue to use tin/lead soldering processes are faced with the dilemma of how to use BGA components that are only available with lead-free solder balls. The combination of a tin/lead solder paste al- loy with a BGA component with lead-free sol- der balls results in a "mixed metallurgy" solder FEATURE

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