Some notes about thermal management for LED applications

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The design of LED modules can be conditionally divided into three parts:
1. light engineering (selection of lenses and LEDs),
2. circuit engineering (development of PCB topology and power supply selection),
3. thermal management (selection of PCB material, TIM and heat sink).

Development is carried out in parallel in all directions because some of the issues overlap. Let’s look at the thermal characteristics of PCB for LED applications in detail.  IMS laminate is used as a material for PCB (MCPCB) in the vast majority of cases. Key parameter of IMS laminate is thermal conductivity. Thermal conductivity is used to calculate thermal resistance of MCPCB.

Consider basics of thermal management. The Three Pillars of thermal management for LED applications are used to building an analytical model: thermal conductivity, thermal impedance, thermal resistance, table 1 [1, 2].

Basics of thermal management. LED module
λ – thermal conductivity; Z – thermal impedance; R – thermal resistance; Q – heat flux; h – thickness; A – area; ΔT – temperature gradient; D – thermal diffusivity; cp – specific heat capacity; ρ – density.

It’s necessary considering the heat spreading in the model to get more accurate results. For a quick and simple analysis of the spreading resistance the thermal engineer has to resort to rules of thumb such as the 45° spreading angle [3] or other formulas e.g. base on the ratio of thermal conductivities of the layers [4], Figure 1.

Heat spreading_LED module
Figure 1. Heat spreading.

The analytical thermal model gives approximate results that determine the direction of LED module design and the luminaire design in general. That needs to be confirmed by finite element simulation and/or thermal measurement of the prototype.

The important challenge is estimating the thermal conductivity of MCPCB and thermal resistance of heat sink. It would seem a simple task, but there is a heated debate among engineers about this. And not only LEDs need cooling, Figure 2.

Thermal conductivity_MCPCB
Figure 2. Thermal conductivity of MCPCB

Ice cube is problem-solving, but we recommend using PREMALSYS online calculator for preliminary thermal analysis of the LED-MCPCB-heatsink system.

PREMALSYS Team

REFERENCES
[1] LEDiL STRADA-IP-2X6-T2-B. Product datasheet. (Available for download at: http://www.dkthermal.co.uk/wp-content/uploads/2016/04/DK-Thermal-White-Paper-3.pdf)
[2] Clemens J.M. Lasance , «Basics of (PCB) thermal management for LED applications»  (Available for download at: https://www.researchgate.net/publication/260349109_Basics_of_PCB_Thermal_Management
_for_LED_Applications_Basics_of_PCB_thermal_management_for_LED_applications)
[3] «Effective Heat Spreading Angle», Electronics Cooling, No. 2, September 2015.
[4] Ceramic Interconnect Technology Handbook. Fred D. Barlow, IIIAicha Elshabini / CRC Press