Thermal Resistance and Heat Flow in LED PCB Design: Understanding LED PCB Layout for Better Heat Dissipation

In the previous LED PCB Layout article, we discussed several practical considerations for LED PCB layout and routing. Proper placement and routing are essential for achieving reliable LED performance. In this article, our design team will take a deeper look at the thermal aspects of LED PCB design, including thermal resistance, heat flow paths, and how PCB layout affects heat dissipation. Understanding these fundamentals helps designers create more efficient and reliable LED PCB layouts.

Why Do LEDs Generate Heat?

Limits of electro-optical conversion efficiency:

Although LEDs are often called “cold light sources,” their electro-optical conversion efficiency is not 100%. In most commercial LEDs, the efficiency is typically 20%–30%, which means that about 70% of the electrical energy is converted into heat rather than light. This unavoidable energy loss is the fundamental reason why LEDs generate heat. Even with improvements in luminous efficiency, a portion of the energy will always be dissipated as heat.

Risks of excessive temperature:

When the PN junction temperature becomes too high, it can directly affect LED performance and reliability:

• Brightness degradation: Reduced quantum efficiency leads to lower light output.
• Color shift: The phosphor conversion efficiency changes with temperature, causing color instability.
• Shortened lifetime: High temperatures accelerate aging of the LED chip and packaging materials, which is one of the main causes of LED lumen depreciation and failure.

For this reason, thermal management is a critical part of LED PCB design and LED PCB layout.

Thermal Resistance Basics

Definition: Thermal resistance (θ) is a physical quantity that describes a material’s resistance to heat transfer.

• Formula: θ = ΔT / P (Unit: °C/W or K/W)
• Meaning: The temperature difference generated when 1 watt of power is dissipated through the material.

Physical meaning:

A lower thermal resistance (θ) indicates better heat conduction and improved heat dissipation performance.

Electrical analogy:

The thermal path can be understood using an electrical circuit analogy:

• Thermal resistance θ ↔ Electrical resistance R
• Temperature difference ΔT ↔ Voltage difference ΔV
• Heat flow P (power) ↔ Current I

Series rule:

Heat must travel through multiple materials from the LED junction to the ambient air. The total thermal resistance is therefore the sum of each layer:

 θtotal = θ1 + θ2 + ... + θn。

​Understanding this concept is essential for optimizing LED PCB layout and thermal design.

LED PCB Heat Flow Path

In most LED PCB designs, heat flows through a multi-stage thermal conduction path:

1. Junction → Package (θJC): Heat first travels from the LED PN junction to the thermal pad or package base. This internal thermal resistance is defined by the LED manufacturer as θJC (junction-to-case thermal resistance).

2. Package → PCB Pad: Heat then transfers through the solder layer from the LED package to the PCB pad. The solder quality and contact area directly affect the thermal resistance at this interface.

3. Heat spreading inside the PCB: The PCB acts as the main path for heat spreading. In LED PCB layout, factors such as copper area, copper thickness, and thermal via distribution determine how efficiently heat can spread across the board.

4. PCB → Ambient Air (θSA): Finally, heat dissipates from the PCB surface to the surrounding air through convection and radiation. This stage is known as θSA (sink-to-ambient thermal resistance).

How PCB Design Affects Thermal Resistance

Proper LED PCB design and layout can significantly reduce thermal resistance and improve reliability.

• Increasing copper area reduces spreading thermal resistance: Large copper planes allow heat to spread quickly across the board instead of accumulating near the LED.
• Thermal vias transfer heat to inner or bottom layers: Dense plated vias create a vertical thermal path that conducts heat through the insulating FR-4 layers into internal or bottom copper planes.
• Metal-core PCBs significantly reduce overall thermal resistance: Metal-core PCBs replace thick FR-4 with a thin high-thermal-conductivity dielectric layer, allowing heat to transfer directly to the metal base.
• Thermal paths should be short and continuous: Efficient LED PCB layout should maintain a direct and continuous thermal path, minimizing material interfaces and abrupt geometry changes that may trap heat.

（Image from internet）

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