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July 2014 • SMT Magazine 35 or backscattered-electron (BSE) modes of SEM. Various tin-alloy, copper and IMC grains look quite different in these FIB images (Figures 2, 3), due to significantly different ion channeling in- side various grains even in the case of the same material (e.g., tin alloy [16] ). Small dotted lines drawn along boundaries of IMC layer (at the alloy/Cu interface) enhance its visibility in the Figure 2a. Due to tilting of the sample the vertical marker is different from the horizontal one. Cross-section images show three metal- lic layers above the laminate: a Cu layer at the bottom, a tin-alloy layer on the top and a thin IMC layer between them. The studies revealed a distinct difference between distinguished re- gions of grid frames and grid fields with respect to the quantity, size and location of IMC pre- cipitates within the solder layer (Figures 2 and 3). Although in the region of grid frames even long Cu 6 Sn 5 protrusions extended into the sol- der layer from the IMC layer (i.e., situated at the interface between the layers of tin-alloy and copper), but only small IMC precipitates (with a sub-micrometer diameter) were observable in- side the solder layer(i.e., without a direct con- tact with the IMC layer), as shown in Figures 2a,b. figure 2: Scanning ion microscopy image of cross-sections milled by fiB in three consecutive adjacent places along a line from the grid-frame to the grid-field region: (a, b) in the grid-frame region; (c) at the boundary between the grid-frame and the grid-field regions. from the bottom to top, the copper film, the interface iMC layer (with iMC protrusions extending upwards) and the solder layer (with small separate Cu 6 Sn 5 precipitates inside) becoming thicker towards the grid-field region, are visible in these cross-sections. figure 3: Scanning ion microscopy image of cross-sections milled by fiB in the grid field re- gion in the two consecutive adjacent places along a line from the grid-frame to the grid-field region (situated next to the three places shown in figure 2). going from the bottom to the top the copper film, the interface iMC layer and the solder layer (with large separate Cu 6 Sn 5 precipitates inside) are visible in these cross-sections. Due to tilting of the sample the vertical marker is different from the horizontal one. WHISKER GRoWTH IN TIN AlloyS oN GlASS-EPoxy lAMINATE continues fEATURE