Heterogeneous multicore processors have been suggested as alternative microarchitectural designs to enhance performance and energy efficiency. Using Amdahl's Law, heterogeneous models were primarily analyzed in performance and energy efficiency aspects to demonstrate its advantage over conventional homogeneous systems. In this paper, we further extend the study to understand the lifetime reliability consequences of heterogeneous multicore processors, as reliability becomes an increasingly important constraint. We present the lifetime reliability models of multicore processors based on Amdahl's Law, including compact thermal estimation that has strong correlation with device aging. Lifetime reliability is analyzed by varying i) core utilization (Amdahl's scaling factor), ii) processor composition (number of big and small cores), and iii) thread scheduling method. The study shows that the heterogeneous processor may have a serious reliability challenge. If the processor is comprised of only one big core and many small cores, stresses can be biased to the big core especially when workloads spend more time on sequential operations. Our study reveals that incorporating multiple big cores can mitigate reliability bottleneck in big cores and enhance processor lifetime, but adding too many big cores will have an adverse impact on lifetime reliability as well as performance.