In this paper we present the analysis of delamination in light-emitting diode (LED) packages by transient thermal measurement and thermo-mechanical simulation. The LED samples were subjected to various moisture preconditioning treatments, and subsequently heat block testing. Transient thermal measurements were used to investigate the thermal behavior of the delaminated LEDs. Thermal resistance from chip to lead frame was increased from 41.7 to 70.9 K/W by a preprocessing for 24 h. Blocking of thermal path induced by delamination was regarded as being responsible for the increased thermal resistance. The maximum thermal stress for the die attach obtained from FEA simulation is 91.5 Mpa, and the thermal stress distribution is consistent with the observed micrograph for the delamination in the lead frame. Thermal stress induced in the reflow process caused delamination and moisture absorption expanded the delamination in the LED package investigated. (c) 2006 Published by Elsevier B.V.