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July 2017 • SMT Magazine 101 cycles per the IPC-9701 specification using a -55°C to 125°C temperature range. In both cas- es the large IMC structures did not initiate sol- der joint cracking and failure of the solder joint. Large IMC structures are an indication that the reflow profile can potentially be improved to reduce or eliminate their presence. Howev- er, the presence of large IMC structures in sol- der joints does not necessarily indicate a need for their rework. The Tin Whisker Concern The topic of tin whiskers has been the single focus of entire technical conferences and con- tinues today to be a topic of significant discus- sion. In general, there is industry consensus that tin whiskers do not initiate and grow from sol- der surfaces under standard conditions. A num- ber of industry investigations have document- ed solder joints with high ionic content and/ or corrosion situations [13, 14, 15] that initiated tin whisker growth. These tin whiskers have been shown to originate at the edges of component pads or solder joint fillet/component lead inter- faces where the solder is very thin and behaves like tin plating rather than a bulk solder alloy. Figure 13 illustrates tin whiskers in this thin sol- der region for a surface mount component [16] . However, industry investigations have doc- umented how the addition of rare earth ele- ments (REEs) as constituent additions to lead- free solder alloys does result in the initiation and growth of tin whiskers [17-23] . The REEs com- bine with tin to form REE/Sn intermetallic phas- es that segregate to the solderball surface. As these REE/Sn intermetallic phases oxidize, the oxide increases the volume, which creates com- pressive stresses that lead to tin whisker forma- tion. P. Snugovsky and S. Meschter [21] conduct- ed 85C/85%RH humidity testing of BGAs with SAC105 solderballs that included 0.01% cerium and were soldered using a SAC305 solder paste. These solderballs produced tin whiskers up to 70 microns in length after 4,000 hours. Figure 14 illustrates an example of the tin whiskers re- sulting from their testing. Many high-performance electronic industry segments such as avionic, defense, and space applications have restricted the use of BGA components that have solderballs with REE ad- ditions to avoid the introduction of BGA tin whisker issues. Conclusions The impact of having BGA components transitioning from tin/lead solderball alloys to lead-free solderball alloys has created signifi- cant challenges for product design teams. Issues such as mixed metallurgy, alloy solidification and tin whiskers require the implementation of USING LEAD-FREE BGAs IN A TIN/LEAD SOLDERING PROCESS Figure 13: Tin whiskers on solder fillet/ component lead interface. (Courtesy of P. Snugovsky, Celestica) Figure 14: SEM image of tin whiskers produced using a REE/SAC105 solderball soldered with SAC305 solder paste. (Courtesy of P. Snugovsky, Celestica)