Solder joint reliability is one of the most important factors in the lifetime of electronic products. Typically solder joint lifetimes are expressed in numbers of cycles to failure, which are derived from accelerated temperature cycle tests. The extrapolation of test results from the test temperature range to the temperature range in operation is well known. In contrast, the effect of the time scale differences between the test and the operational condition is much more complicated and is often not taken into account. The present work addresses the effect of cycle-time on the solder fatigue. A 1D analytical model is made of a shear loaded solder joint, and a spreadsheet is used to calculate time dependent stress relaxation, including the highly nonlinear material behavior of lead free solder. The results are used to estimate safe engineering limits for lead free solder joints of LEDs driven in short time cycles typical for video signals in in a TV product. It is shown that above and below a certain threshold value the cycle time has no influence on the number of cycles to failure. When cycle times are very short, no solder creep occurs, and such cycles have no impact on the total number of cycles to failure. When cycles times are very long, solder creep occurs up to the stress free situation, and a maximum damage per cycle is incurred, corresponding to a minimum number of cycles to failure. Longer cycle times again do not impact this minimum number of cycles to failure. However when the cycle time allows for partial solder creep, the damage incurred in each cycle is less than the maximum and consequently the number of cycles to failure will be correspondingly higher, dependant on the cycle time.
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