Abstract--As an alternative to traditional life testing, degradation tests can be effective in assessing product reliability when measurements of degradation leading to failure can be observed. This article proposes a new model to describe the nonlinear degradation paths caused by nano-contamination in plasma display panels (PDP): a bi-exponential model with random coefficients. A likelihood ratio test was sequentially executed to select random effects in the nonlinear model. Analysis results indicate that the reliability estimation can be improved substantially by using the nonlinear random-coefficients model to incorporate both inherent degradation characteristics, and contamination effects of impurities for PDP degradation paths.