Thermal Management in LED Assembly: Can You Take the Heat?
Thermal management is key to ensuring that luminaire performance is as advertised. Consumers seldom see the thermal solutions in an LED device, but they do know when their light fails – and improper selection of thermal interface materials (TIM) during LED assembly can lead to a shorter-than-advertised bulb life.
TIM products work by removing air gaps (air is a great insulator) between heat-generating components and efficiently transferring generated heat to a “cooler” area.
There are a multitude of TIM solutions available to design engineers today; when to use which option depends on end use application and luminaire design.
Key Thermal Interface Materials (TIM) Solutions
Thermal Potting: Typically these are two-component, silicone- or epoxy-based products with modest thermal conductivity (1-1.5 W/mK). They are often used in the potting of drivers. Higher-performing products (>1.5 W/mK) are available, but tradeoffs come at the expense of higher viscosity, mix ratios and cost.
Thermal Grease: Thermal greases are most typically found between LED light engines and heat sinks. They are non-curing compounds and are available with thermal conductivities over a large range. As with potting compounds, you pay for performance. The higher the conductivity, the higher the cost.
Here are couple examples of Thermal Grease, Thermal Potting and Driver Potting:
Thermal Grease (light gray) and Thermal Potting (dark gray) materials work together to manage the heat output of the device.
Driver Potting moves heat away from critical components on the circuit board and helps to protect the driver from shock and vibration forces.
Thermal Adhesives: These products are true “bonders” and work to unitize heat sources to heat sinks. They are available with a variety of performance properties and in a variety of adhesive chemistries, allowing LED design engineers to tailor a thermal solution to the substrates involved and the desired thermal performance. Thermal adhesives can also be found with integrated “shimming” technologies to ensure a uniform gap. For high heat/performance applications, silver-filled adhesives can be used. With the cost of silver escalating over the last few years, these products are the most expensive option, and most suppliers of silver-filled materials have gone to “spot pricing” to account for fluctuations in the precious metals market.
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Films/Pads/Tapes: This family of adhesives allows for LED design flexibility and ease of use. (What’s easier than sticking two things together with tape?) Thermal performance runs the gamut of modest (1 W/mK) up to silver-filled higher performance (>7 W/mK). They are available in a variety of thickness and can be produced/die cut to any shape. Because of this design flexibility, tooling costs can raise the overall cost of implementing this type of solution.
It is also important to take into account the phase change these materials undergo when they see heat. They go from a solid to a liquid phase, and they fill the air gaps between heat sources and most typically heat sinks. (They revert back to a solid when heat is removed.) In order to work properly, these products require a clamping force between the source and the sink. Thermal conductivity of the products can exceed 3 W/mK and they can be applied in a variety of fashions.
Many other criteria go into designing a thermal solution; the thermal interface material is only one part. Working closely with your TIM supplier will ensure an optimal thermal solution that is easy to apply and implement in manufacturing.
Was this helpful? Want more on LED assembly and adhesives? Check out my blog post 14 Questions to Ask When Using Potting Adhesives for LED Assembly. Or visit Henkel’s LED assembly page for more info.