Direct Thermal Path PCB: Advanced Thermal Management for High-Power LED Lighting

Home Blog Direct Thermal Path PCB: Advanced Thermal Management for High-Power LED Lighting

November 20, 2017
HT Global Circuits Team

Although traditional MCPCBs were originally conceived for use in the power-supply industry, these substrates are the most widely used in production of LED products. There are many names for these products; Aluminum Clad, Aluminum Base, Metal Clad (MCPCB), Insulated Metal Substrate (IMS or IMPCB) and Thermally Conductive PCBs. All these references refer to the same design. A thin layer of thermally conductive, but electrically insulating, dielectric laminated between a metal base and a copper foil. The copper foil is etched into the desired circuit pattern and the metal base draws heat away from this circuit through the thin dielectric.

LEDs require heat dissipation that standard PCB material can’t handle. The solution is a dielectric material that provides 1-2 W/mK of thermal conductivity. Standard PCB material typically has a thermal conductivity of 0.5 W/mK, which is not enough for the current high-intensity LEDs. Metal clad PCB materials will increase the life of your LEDs with advantages that include increased heat dissipation, reduced thermal expansion and increased dimensional stability.

Styles of MCPCBs & Thermal Configurations

When selecting a Metal Clad PCB (MCPCB) for high-power LED applications, the choice of thermal architecture dictates how effectively heat moves away from diode junctions:

Traditional MCPCB (Non-Direct Thermal Path): Consists of a metal base layer (typically aluminum or copper), a continuous dielectric insulation layer, and a top copper circuit layer. While cost-effective, heat must travel through the insulating dielectric (1.0 – 3.0W/mk, creating a thermal bottleneck for ultra-bright LEDs.
Direct Thermal Path PCB (DTE 3D Metal Core Pedestal): Utilizes a raised 3D metal core pedestal where the insulating dielectric layer is removed directly underneath the component’s heat slug. This places the LED thermal pad in direct metallic contact with the copper or aluminum base, achieving thermal conductivity up to 400W/mk
Direct Thermal Path PCB (Plated Copper Splash Pad): Incorporates a dedicated copper splash pad drilled and plated directly through to a solid copper heat sink. Heat transfers straight from the LED component pad down through heavy plated copper walls, bypassing dielectric thermal resistance.

Key Engineering & Design Considerations

Thermal Conductivity Ratings: Dielectric materials range from 0.5W/mK (standard FR-4) up to 8.0W/mK for premium IMS laminates. A direct thermal path PCB bypasses this dielectric limitation at the component junction.
Dielectric Thickness: Available in customizable thicknesses (typically 2.5ml to 6.0ml) to balance electrical isolation voltage requirements against heat transfer performance.
Multi-Layer Stackups: Supports up to 4 layers with plated through-holes (PTH) for complex signal and power routing.
Metal Core Substrates: Aluminum alloys offer the most cost-effective solution for standard lighting fixtures, while Copper alloys provide maximum thermal performance for high-lumen density modules.
Manufacturing Lead Times: Standard single-sided MCPCB prototypes turn around in 3 to 5 business days, while precision-milled direct thermal path PCB designs typically require 10 to 15 business days depending on CNC pedestal complexity.=

Keeping MCPCB Designs Cost Friendly

There are several important factors when designing a circuit board which will help keep your price per unit as low as possible.

Copper Weight – Stick with 1-2 oz. Cu foil. Thicker starting foil can significantly increase board spacing requirements.
Final Finish – Try to stay away from expensive surface finishes such as ENIG / Immersion Au
Board Size – The typical panel sizes are usually 18”x24” or 21”x24”. A half inch is required around the full panel for handling/manufacturing so you are left with 17”x 23” or 20”x23” respectively of manufacturable space. Try to design your MCPCB array to utilize as much of that space as posable.
High End Dielectric Prepreg – Target your design around a dielectric material that provides 1-2 W/mK of thermal conductivity. The high end materials come at a significant cost.
Complex CNC Routing – CNC routing can be one of the most challenging aspects of manufacturing MCPCB’s. If at all possible keep the routing to a minimum and utilize V-scoring as much as possible.
V-Scoring – Keep the design simple enough to for V-Scoring, CNC routing can add considerable costs.
Number of Layers – Keep the layer count as low as possible. Not only does increasing the layer count have an impact on pricing but it often reduces the thermal conductivity of the dielectric.
Metal Core – Stick with an aluminum based alloy if possible. Cu alloys will add significant cost.

When you add extra features to a design like heavy copper foil, blind/filled vias, complex machining/profiling and expensive surface finishes, these features and processes will typically add an extra 40-50% in price and will be costly for production volume