High-power　light-emitting　diode　(LED)　chip-scale　packages　(CSPs)　prepared　by　the　flip-chip　technology　have　become　one　of　the　most　promising　light　sources.　The　die　attach　solder　layer　always　plays　an　important　role　in　heat　dissipation,　mechanical　support,　and　electronic　conductivity.　Among　different　types　of　solder　materials,　Sn-3.0Ag-0.5Cu　(SAC305)　solder　alloy　shows　its　great　competitiveness　on　solderability　and　mechanical　properties　for　the　interconnection　of　high-power　LED　CSPs.　However,　reliability　problems　caused　by　voids　in　the　SAC305　solder　limit　its　wide　application　in　the　high-power　LED　chip-scale　packaging　process.　Existence　of　the　voids　has　been　considered　as　one　of　the　major　issues　causing　chip-on-substrate　level　reliability　problems　in　microelectronic　and　optoelectronic　devices.　In　this　paper,　mechanical　and　thermal　properties　of　SAC305　solder　layers　with　arbitrary　voids　used　in　high-power　LED　CSPs　are　studied　with　both　finite-element　simulations　and　experiments.　The　results　show　that　void　size　and　void　position　within　the　solder　layer　are　the　two　most　critical　issues　on　the　shear　strength　of　interconnection　and　the　chip-on-substrate　level　thermal　distribution　in　high-power　LED　CSPs.