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34 SMT Magazine • July 2014 WHISKER GRoWTH IN TIN AlloyS oN GlASS-EPoxy lAMINATE continues fEATURE der surface. Local compressive stress in the sol- der layer due to large differences in the coef- ficient of thermal expansion (CTE) of the resin and glass fiber promotes whisker growth in the area of the alloy soldered on the Cu layer over the glass fiber (Figure 1) (i.e., in regions of grid fields of the visible grid pattern [12] ). The coef- ficient of thermal expansion is much higher for the resin (e.g., about 63 ppm/°C), than for the glass fiber (about 5 ppm/°C) [11] , while for tin and copper it is equal to 23 ppm/°C and 16.5 ppm/°C, re- spectively. This effect does not occur for an alloy soldered on Cu over a paper-phenol lami- nate, for example. The lines of the grid frame correspond to the area of the solder placed over the resin while the grid fields between the grid frames correspond to the area of the solder placed over the glass fi- ber in a cross-section [12] . Experiment The studied samples were PCB glass-epoxy laminate covered by a Cu foil (at least 17 µm thick) with a layer of tin or other, commer- cially available tin rich solder alloys: Sn100C (Sn 99.3 Cu 0.7 Ni), Sn99Cu1, Sn 97 Cu 3 , Sn 99.5 Ag 3 Cu 0.5 , Sn 99.3 Cu 0.7 AgNiGe, and Sn 99 Ag 0.3 Cu 0.7 NiGe. The alloys were applied by hand soldering with the application of a water flux. The soldering temper- ature was dependent on the alloy composition. The samples were tested in a VO¨ TSCH chamber for 1500 shocks within a cyclic tem- perature range of -45°C to +85°C, with each cycle lasting 20 min and the transition time of the lift between hot and cold zones equal to 5 s. These conditions are recommended by iNEMI (International Electronics Manufactur- ing Initiative) and JEDEC (Joint Electronic De- vice Engineering Council) as inducing whisker growth in most Sn and Sn-alloy layers [13] . The FEI Helios NanoLab 600 DualBeam FIB system was applied to the milling (cross-section- ing) of samples with a precise gallium ion beam and also for scanning ion microscopy imaging (i.e., induced by the ion beam and detected by the secondary-electron detector) for the range of energies up to 30 keV. The cross-sections were thoroughly cleaned in FIB to remove contami- nations introduced during the FIB cross-sec- tioning process [14, 15] . EDXS with a Philips XL30 SEM and EDAX spectrometer (with 130 eV spec- tral resolution) was used, with an energy range from 5–15 keV applied in the consid- ered case. Results and Discussion Scanning ion microscopy imaging and EDXS measure- ments were performed at vari- ous points on the mentioned grid to determine the correla- tions between the spatial non- uniformities of the structure of glass-epoxy laminate, of IMC distribution in the solder and of whisker growth after standard reliability tests. The grid on the solder is formed by grid fields (rectangular regions with hillocks and whiskers) sur- rounded by the grid frame (flat re- gions without surface roughness). The measurements were first performed when the samples were observed in a planar view, with the sample surface placed perpen- dicular to the electron beam. Results obtained from grid fields (with hillocks and whiskers) showed a significant Cu content, much higher than that from grid frames where this content was negligible. Afterwards, FIB was used for milling sam- ples with a gallium ion beam. These cross-sec- tions were performed at several adjacent points spaced along a line from the grid frame to the grid field, with the aim of comparing neighbor- ing points (instead of comparing random points in these two regions, because additional factors may be relevant in remote locations). Images of cross-sectioned layers were taken by use of electron and ion microscopy. The scan- ning ion microscopy performed in FIB system, where detected secondary electrons are generat- ed by the incident gallium ions, allowed for bet- ter image contrast than that observed in images obtained from standard secondary-electron (SE) The lines of the grid frame correspond to the area of the solder placed over the resin while the grid fields between the grid frames correspond to the area of the solder placed over the glass fiber in a cross-section. " "