Burn-in is a widely used engineering method to screen out defects befo
re the product is shipped to the customer. We investigate the two-stag
e burn-in procedure as a means of improving the outgoing product quali
ty. The first stage involves a high stress level burn-in to screen out
material defects, and the second stage involves a low stress level bu
rn-in to detect manufacturing defects. Repairable and nonrepairable sy
stem burn-ins are discussed. A cost model is formulated to derive the
optimal burn-in plan.