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March 2014 • The PCB Magazine 33 SPHERICAL BEND TESTING continues Failure distributions were best modeled us- ing Weibull distributions for two reasons. First: The failure rate is expected to be constantly in- creasing as the stressing factor is increased. In fact, there is a point at which no further survivors will be detected. This is the boundary between Zone 2 and Zone 3. The data includes both failures and survivors. In statistical terms the data is right censored. As expected the scat- ter in the results increases as the testing strain rate is incr eased. In general the distributions of all data at the various strain rates are con- sistent with the behavior currently accepted by the industry. Th e survivable peak strain de- creases as rising strain rate increases. Distributions are combined for individual board thicknesses. Data and calculations of safe limits were plotted in the common strain / strain rate format on log linear graphs. Basic re- gression techniques were used to generate safe working limits over the range strain rates as- sociated with manufacturing processes. Figure 8 is a typical output from this process and dis- plays individual data points, and the two limit curves based on diagonal and principal strains for a single board thickness. This process was repeated for all board thicknesses under study. This data was compared against the current es- timate of the board thickness relationship and seems to correlate well. There is some evidence that the strain rate dependency for this fracture mode is different than the relationship current- ly accepted by the industry for SnPb assembly. Results: DOE2 The two process streams provided very different thermal histories for the lots. The "primary attach" lot was exposed to approxi- mately 6000 degree seconds above T g for the laminate whereas the "forced rework" lot was exposed to approximately 14000 degree sec- onds above the T g point. Figure 8: typical strain/strain rate behavior in FcBga devices.