Burn-in is an effective means for screening out defects that contribute to infant mortality failures. Various studies have been conducted for determining: the optimum hum-in time for a device. However, most of them assume cost models, which are difficult to quantify. In this article, we take a more realistic approach for determining the effective brim-in time for a PBA(Printed Board Assembly) to satisfy the customer requirement that the failure rate after burr-in be less than a specified value. The proposed method is applied to two types of PBAs used in a certain electronic switching system.