Flexible substrate using for LED application is gaining attraction due to their flexibility for design and low cost manufacturing.Typical flexible substrate consists of a circuit layer of copper sandwiched between an insulation layer of polyimide (PI) and a reflective layer of solder mask,and beneath the PI layer,there is backside copper layer for heat dissipation.Low thermal conductivity of PI insulation layer is the limiting factor that affects the thermal resistant.Circuit routing layout design,that fulfill the electrical requirement and simultaneously provide sufficient thermal dissipation,is critical for system reliability.In this study,circuit routing scheme that fulfill the co-design requirement of thermal and electrical is investigated.Optimization of the circuit layout and the structural design are used to improve the thermal properties of the substrate was explored by finite element simulation and experimental measurement.Extension of the circuit layer can improve heat conduction which helps decrease the thermal resistance of the whole substrate.