A　noninvasive　approach　is　used　to　study　the　die　attach　layer　of　chip-on-board　(COB)　packaged　light-emitting-diodes　(LEDs)　after　temperature　cycling.　Failure　analysis　of　the　die　attach　layer　is　performed　by　monitoring　the　changes　in　the　thermal　resistances　in　differential　structure　function　curves　of　the　COB.　The　results　suggest　that　delamination　occurs　at　the　interface　between　the　chip　and　die　attach　and　fatigue　voids　appear　in　the　die　attach　layer,　which　is　consistent　with　the　findings　of　X-ray,　scanning　electron　microscopy　and　C-mode　scanning　acoustic　microscopy　analyses.　Various　temperature　cycling　experiments　are　carried　out　to　apply　cycling　temperature　stress　to　the　LEDs　to　examine　the　influences　of　junction　temperature　and　power　on　device　failure.