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February 2015 • SMT Magazine 59 all four types of samples (i.e., on Sn100 and Sn99.3Cu0.7Ni soldered both directly on the Cu layer, as well as the Ni/Au sublayer, as shown in Figures 1 and 2). However, they were found only at the edge of the solder (where the solder thickness is significantly smaller than typical) while not at the typical thickness of the solder. Copper Content Cu content, measured directly on the whis- ker (found as mentioned earlier at the edge of solder pad) was amounted to 0.46 wt%. This value can be regarded as negligible, because less than 1 wt% can be related to the EDS fluctua- tion signal. So it means that the whisker body was free from Cu content. On the base of 70% of whiskers the cop- per content significantly increased—from 7.4 to 9.32 wt%. At the base of 30% of whiskers the copper content was at the level of 1 wt%. It does not mean, however, that at the base of the whiskers the increased copper content has not occurred. It may only mean that it was not detected by specific settings at the EDS stud- ies. Namely, if on the way of the incident beam there is a whisker, then it partially covers the base against the electron beam and the precise measurement cannot be done. No particularly distinguished copper content in the other areas of tin was noticed, regardless if the analysis was done at the edge or in the middle of the sol- dered pad. Oxygen Content For measuring the oxygen content, the method including the elimination of surface contamination was used. This method involves measuring of the carbon content and the calcu- lation of approximately constant dependence of oxygen on carbon content (which corre- sponds to their constant relation in the chemi- cal compounds or in the conglomerate forming contamination of the sample). On this basis, it is possible to determine how much oxygen is associated with sample contamination and how much with tin oxides. On the whisker body (found as mentioned earlier only at the edge of solder pad) the de- tected oxygen content was at the level of 2.3–3 wt%. The oxygen content on the whisker base reached from 14–21wt%. For comparison, oxy- gen content on the alloy surface located in the middle of the pad was from 4.6–9 wt%. Such high oxygen content in the area of whisker base is related to the formation of tin oxide. When the measured oxygen content exceeds 12 wt% it suggests the occurrence of SnO2 [5] and in less- er degree, SnO. The exemplary EDS spectra for Sn99.3Cu0.7Ni (with signals from contamina- tions removed), presenting the results described above, are shown in Figure 3. Figure 2: SeM images of the samples soldered directly on Cu layer after 3000 h exposure at 87% rH and 60°c—the edge of the solder area. Feature TIN WHISKer GrOWTH ON THe SurFaCe OF TIN-rICH Lead-Free aLLOyS continues