One misconception of LED lighting is it can last for a considerably long time, such as 10 years or beyond. While long lifecycle is certainly an intrinsic advantage, there are failure points. Most of the degradation and failure mechanisms that rule the performance and lifetime of an LED luminaire are caused by inefficient thermal management. LEDs convert only a small part of electrical energy into light and the rest is converted into heat. As a byproduct of operation, heat is produced as a result of non-radiative recombination in the LED junction and Stokes shift in the phosphor layer. At high power, higher current density operation which is typical in high mast lighting, a substantial amount of heat is generated. If this heat is not dissipated properly, thermal buildup within the LED causes lumen depreciation due to degradation and quenching of phosphors, die cracking, bond wire fracture, solder joint fatigue, carbonization of the encapsulant, etc.
Thermal management is a critical part of the design and engineering of LED luminaires. Thermal equilibrium in an LED system is broken by conditions that reduce an efficiency of heat dissipation. The goal of thermal management is to build a thermal path along which the thermal resistance of the components is minimized to a required level, while maximizing the effective area of the thermal path and minimizing the length of the thermal path. SSL thermal management consists of two sections: thermal conduction and convection. Thermal conduction copes with maximizing thermal conduction capacity of the heat sink, thermal interface material (TIM), metal core printed circuit board (MCPCB), and interconnects between the LED packages and MCPCB. This part of thermal management also includes minimizing difference in coefficient of thermal expansion (CTE) between the components along the thermal path. This is extremely important as outdoor luminaires can undergo repeated temperature cycling which can compromise the integrity of the thermal path.
The removal of waste heat by thermal convection is dependent upon the flow rate of the ambient air and the surface area around which air is circulating. Since there is an abundant availability of air flow around the luminaire in outdoor environments, high mast luminaires utilize natural convection to dissipate heat into the air. As a general rule of thumb, the heat sink is designed with a large surface area and an aerodynamic geometry to ensure effective air circulation.
