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In this paper, the acceleration factors for lead-free solder materials are discussed. The focus is on the analytical models proposed by Norris-Landzberg, Pan, and Salmela. As most reliability prediction models are originally created for eutectic SnPb solder material, it is expected that the models must be recalibrated in order to be useful in conjunction with lead-free solder materials. Pan et al. have reparameterized the Norris-Landzberg's model for SnAgCu already earlier. However, the data provided by Syed did not fit very well the reparameterized Norris-Landzberg model. An approach to explain this, and a benchmark of results based on alternative parameter value selections, is presented. The underlying assumptions related to the Norris-Landzberg's model and the applicability of the model in case of lead-free solders is discussed. Finally, an alternative form of Norris-Landzberg's equation is introduced and the fit of the existing experimental data to the proposed model is studied.