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100 SMT Magazine • July 2017 shrinkage voids that had been subjected to a to- tal of 1,000 -55°C to 125°C thermal cycles in accordance with the IPC-9701 specification. Initially, there was industry concern that the shrinkage voids would be a solder joint crack initiation site, but industry testing [9-12] has dem- onstrated these concerns were unfounded. An equally contentious industry debate is the influence of large intermetallic compounds (IMCs) on solder joint integrity. The primary IMCs found in the typical tin/lead solder joint are tin/copper, usually Cu6Sn5 structures (Fig- ure 10). These large tin/copper IMC structures do not degrade solder joint integrity and are the result of a reflow profile that used excessive time/temperature parameters. The use of ade- quate thermal profiling eliminates their pres- ence in most assemblies. With the introduction of lead-free solder al- loys, it is not unusual to have tin/copper, tin/ nickel, silver/tin and even tin/gold/copper IMC structures in the solder joint microstructure. There is one camp of thought that the IMCs assist solder joint cracking and another camp of thought that the IMCs blunt solder crack grow. Figure 11 and Figure 12 are two examples of lead-free solder joints containing large IMC structures that have undergone 2000 thermal USING LEAD-FREE BGAs IN A TIN/LEAD SOLDERING PROCESS Figure 9: Shrinkage voids in a lead-free BGA solder joint. Figure 11: Large Ag3SnSilver/Tin IMC structures in a mixed metallurgy solder joint [9] . Figure 10: Large Cu6Sn5 IMC structures in a Sn63Pb37 solder joint. Figure 12: Large Cu6Sn5 IMC structures in a SAC305 solder joint.