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36 SMT Magazine • July 2014 In contrast, in the region of grid fields much bigger IMC precipitates (with diameters even in the range 5-8 µm) were observed inside the sol- der layer, without a direct contact with the IMC layer and close to the solder top surface (Figures 3a,b). In many cases densely distributed round- shaped Cu 6 Sn 5 precipitates with diameters from one to several micrometers even reached the top surface of the tin alloy and these were ob- servable at close distances from each other (Fig- ure 4). The occurrence of IMC precipitates on solder surfaces has been observed previously [17] . Numerous previous observations have shown and discussed IMC protrusions extend- ing from the IMC layer into the solder layer as well as have shown IMC precipitates located within the solder layer and positioned along grain boundaries [18, 19] . However, in our inves- tigations (as well as observations in some other studies [20] ) Cu 6 Sn 5 precipitates were found not only along grain boundaries but also inside the solder grains, i.e., without a visible contact with grain boundaries (Figures 2, 3 and 5). Separate Cu 6 Sn 5 precipitates were also observed even in- side some whisker crystals (Figure 5), confirm- ing that during their upward migration IMCs can penetrate into various grains. It can also be seen in this figure that the density of IMC precipitates in the solder increases closer to the whisker. Generally, during the soldering operation, material from the metal sublayer dissolves and mixes with the solder, allowing the formation of IMCs. In alloys consisting of Sn and Cu, two different phases of IMCs may be present: Cu 6 Sn 5 and Cu 3 Sn. Especially the formation of a Cu 6 Sn 5 phase (because of its large-volume expansion in comparison with the pure tin) is a strong fac- tor promoting whisker growth in the neighbor- ing alloy [3] . Our EDXS measurements revealed a content of about 39 wt.% Cu in the IMC layer at the interface between copper and tin-alloy layers, i.e. measurably equal to the Cu content in the Cu 6 Sn 5 layer. The same content of about 39 wt.% Cu was found for all measured IMC precipitates observed within the tin-alloy layer, confirming that they also constituted a Cu 6 Sn 5 phase. In contrast, the content of Cu in another possible Cu-Sn IMC phase (i.e., Cu 3 Sn) would fEATURE figure 4: SEM image of a single whisker. Densely distributed round-shaped Cu 6 Sn 5 precipitates at the top surface of the tin alloy at the grid field are visible close to the whisker apparent in the center of the image. EDXS spectrum (obtained at elec- tron energy 15 kev) of an example precipitate is shown below. figure 5: Scanning ion microscopy image of a cross-section with a whisker grown in the grid- frame region. An increased density of Cu 6 Sn 5 pre- cipitates in the tin-alloy solder close to the whis- ker base and a small separate Cu 6 Sn 5 precipitate inside the whisker are seen. WHISKER GRoWTH IN TIN AlloyS oN GlASS-EPoxy lAMINATE continues