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August 2016 • SMT Magazine 87 tailed metallography is performed that defects are observed in both configurations. It is essen- tial that any micro-sectioning of solder joints assemblies, after completion of environmental testing, includes the removal of all flux resi- dues followed by mounting the samples in a room temperature hardening epoxy (under low vacuum). When a fluorescent agent is added to the molding resin fine cracks can be observed during optical inspection under UV light. SnPb solders degrade by thermal fatigue within the solder alloy, however it was surprising to find that the SAC soldered assemblies can also de- grade in two distinct failure modes. Some SAC joints become completely recrystallized, and comprise of single grains. These are very strong in the Z direction and cause extensive pad lift- ing and fracture in the resin below the pad. Be- neath the same AGA some SAC joints are seen to be multi-grained and these were noted to fail 7TH ELECTRONIC MATERIALS AND PROCESSES FOR SPACE WORKSHOP at the solder to pad interface. Figure 3 shows images of SAC solder joints taken with Electron Back Scatter Diffraction (EBSD) after thermal cy- cling—the left-hand image shows a single grain of random orientation (other single-grained balls had different orientations) and there is no crack within this grain, only a lifted pad. The right hand ball is multi-grained and here the failure is within the solder. Dr. Tegehall concluded that cracks forming in the PCB laminate might improve the fatigue life of SAC joints, but they can lead to an over- estimation of the fatigue life of such joints by at least a factor of three. Jussi Hokka outlined the roadmap identi- fied by ESA-Estec, the Netherlands for advanc- ing technologies related to PCBs and several assembly technologies until 2020. He detailed the activities of several ESA-Industry working groups actively engaged with SMT and PCBs Figure 3: AGA ball joints made from SAC alloy but having various grain sizes, orientation and two distinct thermal-fatigue failure modes. Courtesy of SWERA-IVF.