Plastic encapsulated microcircuits (PEMs) have been used tremendously in commercial and industrial fields due to their cost, weight, availability and performance advantages. However, the inherent structures and materials of PEMs introduce high risks of failures and limit their use in high reliability field. Screening test for package-level plays an important part in improving reliability of PEMs by eliminating marginal components and early time failures. Failure analysis of the microcircuits failed in screening test contributes to find out failure mechanisms and helps improve design for package reliability. However, during screening test, after screening out failed components, failure analysis is not conducted to figure out the root cause of failure and does not contribute to the conclusion of the screening test or to the advice for design. In this paper, dynamic burn-in of screening test on plastic encapsulated components is presented. An integrated circuit OPA134UA (single operational amplifier) fails in the electrical test after 24hrs. Failure analysis is carried out on the failed components to discover the root causes. It turns out that OPA134UA fails because of the delamination between the mold compound/die pad interface. The result of failure analysis proves that thermal mismatch in PEMs causes delamination, which suggests that other components also have delamination and that the design for packaging reliability require lower difference of coefficients of thermal expansion (CTE) of different materials in PEMs.