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98 SMT Magazine • July 2017 ricators to develop acceptable hot reflow pro- files that do not introduce component integri- ty concerns [5, 6] . The advantage of using a hot profile is minimal process parameters changes and low cycle time impact. The disadvantages of using a hot profile are the time/costs associ- ated with the additional due diligence testing and laborious reflow profile generation neces- sary to demonstrate component/solder joint in- tegrity acceptability. Figure 6 and Figure 7 illus- trate a typical tin lead solder joint with uniform microstructure and a lead-free solder joint with mixed metallurgy microstructure with segregat- ed regions of lead-free and tin-lead solder. Fig- ure 8 illustrates the solder joint microstructure resulting from a hot profile. A third methodology to address the mixed metallurgy concern is to use the lead-free BGA component in a tin/lead soldering process and then underfill it. The resulting BGA solder joints will have various degrees of segregated solder joint microstructure dependent upon the size and density of the BGA component popu- lation on the printed circuit assembly. Howev- er, the application of an underfill material re- duces the impact of coefficient of thermal ex- pansion (CTE) mismatch stresses on the solder joints by directly coupling the BGA component to the printed circuit board [7, 8] . The advantages of using an underfill approach are the reduced costs and better availability of standard lead- free BGA components as well as minimal sol- dering process changes. The disadvantages of using an underfill material are the costs/time associated with the additional assembly pro- cess step of underfilling components and the potential need for using a nonreworkable un- derfill material, i.e., the printed circuit assembly becomes a non-repairable item. Rockwell Col- lins uses the underfill methodology to address lead-free BGA components processed in a tin/ lead soldering process. The additional assembly process step has been demonstrated to be a low- cost impact in comparison to the procurement cost/availability of tin/lead BGA components for product designs. The Solidification Concern It is no accident that the electronics indus- try has traditionally used the Sn63Pb37 and Sn60/Pb40 solder alloys as the assembly alloy materials of choice. These two tin/lead solder alloys are eutectic in their solidification behav- ior. In a eutectic composition, the mixture of metals changes from a solid to a liquid at a sin- gle temperature. This solidification behavior is extremely beneficial in creating solder joints and processing of electronic assemblies. Most metallic alloys change from a solid to a liquid over a range of temperature, often described as having a "pasty range," which can complicate the handling/motion of in-process electronic assemblies. Since the primary lead-free solders being used today are not eutectic alloys, addi- tional processing due diligence is required. Ad- ditionally, a non- eutectic metal alloy can often form solidification shrinkage voids. Figure 9 il- lustrates a BGA solder joint, which contained USING LEAD-FREE BGAs IN A TIN/LEAD SOLDERING PROCESS Figure 6: BGA solder joint: tin/lead alloy in a tin/lead process. Figure 7: BGA solder joint: mixed metallurgy (lead-free alloy in a tin/lead process). Figure 8: BGA solder joint: hot profile, lead-free alloy in a tin/lead process.