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38 SMT Magazine • July 2014 be equal to above 60 wt.% Cu, much higher than revealed in our measurements. In contrast, no distinguishable difference in the shape of the interface IMC layer was observed between the two regions (of the grid frame and of grid fields, see Figures 2 and 3). This means that the spatial non-uniformity caused by the regular structure of glass fi- bers and resin regions in the top layer of a glass-epoxy laminate also promotes spatial non-uni- formity of the IMC-precipitate distribution within the solder layer, often close to its top sur- face. This effect seems to con- stitute an additional factor for the apparent non-uniformity of whisker growth. Thick solder layers (signifi- cantly more than 10 µm) were applied in samples to ensure their resistance to whisker growth. Cross- sections performed in regions adjacent to grid frames and grid fields show that the thickness of tin alloy in the frame regions is significantly lower than in the field regions, e.g., equals about 13 µm at the grid frame (Figure 2a) while about 18 µm in the adjacent regions of grid fields (Figures 3a,b). The calculation of alloy thickness takes into ac - count that the studied samples were tilted during observations in the microscope chamber. Thus markers shown in Figures 2, 3 and 5 represent horizontal dimensions, while the vertical dimen- sions are underestimated and therefore their mul- tiplication by the tilt angle secant is required (i.e., by 1.24, because the tilt angle equals 36° there). Whisker growth depends on many factors with an impact of one factor being distinguish- able only when other important factors are fixed. Although a thinner (if not very thin) solder is considered to be more prone to whisker growth [7-10] , the mentioned additional factors related to the compressive stress have forced an absence of whiskers in grid frame regions (with thinner sol- der), and their presence in regions of grid fields (with thicker solder). Much higher thermal expansion coefficient of the resin than of the glass fiber causes the resin to expand and shrink more than the glass fiber during tests performed with a cyclic temperature range of -45°C to +85°C. A nonuniform expansion of the top area of the glass-epoxy laminate occurs in its longisection, as well as in its cross-section. It may be expected that alongside differences in compressive stress in adjacent regions, the lay - ers in various regions will be differently affected mechanically, shifted and pushed up during temperature cycles. The observed non-uniform spatial distribution of IMC pre- cipitates seems to occur due to differences in mechanical interactions and compressive stress in various regions, while the presence of IMCs is a driv- ing force for whisker growth. Therefore, it may be expected that CTE differences in various regions of PCBs with tin-alloy on glass-epoxy laminate facili- tate spatially non-uniform whisker growth in two ways, directly by the implemented compressive stress and indirectly by the generation of non-uniform IMC distribution. Summary Whisker growth on the surface of tin alloys soldered above a glass-epoxy laminate was stud- ied (i.e., for the most widely used PCB substrate material). The structure of the glass-epoxy lami- nate surface has a spatial non-uniformity caused by the regular structure of glass fibers and resin regions in the top layer of the laminate. There- fore, the local compressive stress in the solder layer due to differences in the thermal expan- sion of the resin and glass fiber promotes whis- ker growth in the area of the alloy soldered on the Cu layer over the glass fiber. Scanning ion microscopy using the FIB sys- tem and EDXS performed after standard reliabil- ity tests determined a strong correlation between the structure of glass-epoxy laminate, the spatial distribution of IMCs and whisker growth. In the region of grid frames only small IMC precipitates were observable inside the solder layer (i.e., with- out a direct contact with the IMC layer situated at the interface between the tin-alloy and Cu lay- ers). In contrast, in the region of grid fields much bigger IMC precipitates were observed inside the solder layer, without a contact with the IMC layer WHISKER GRoWTH IN TIN AlloyS oN GlASS-EPoxy lAMINATE continues fEATURE Whisker growth depends on many factors with an impact of one factor being distinguishable only when other important factors are fixed. " "