A Typical Power Device Thermal Challenge and the TIM Solution
The objective of thermally conductive materials is to transfer heat away from the heat-generating device. Reducing resistance by filling air gaps (voids) at the interface between the component and the heat sink provides an uninterrupted pathway for the most efficient heat-dissipating route. Heat transfer is improved significantly by reducing or eliminating thermal resistance.
In a typical power electronics thermal stack up, there are several thermal resistances in a series, and the thermal resistance of the interface material can account for a significant percentage of the total and end-of-line thermal resistance.
In addition, the thermal stack up (substrate, component, thermal interface material, heat sink) includes materials that all have different coefficients of thermal expansion (CTEs) and, from these differences, thermal stress and warpage can result if not properly managed. As the system goes through power and thermal cycling, there is mechanical stress on the interface. To account for the varying tolerances, while reducing stress and efficiently transferring heat, the thermal interface material should be selected carefully in order to maximize lifetime thermal performance, align with manufacturing dynamics and account for environmental conditions.
Some of the issues caused by ineffective heat management include:
- Component failure
- Device overheating
- Device degradation
- Wiring erosion and melting
- Reduced performance
- Increased periods of cooling-off downtime required
- Potential for increased fire risk
Power Electronics Design and Thermal Material Considerations
To ensure efficiency in operation - which is often 24/7/365 for power electronics - and long-term reliability, the TIM selection is based on several criteria. These criteria can be broadly categorized as:
- Performance – Thermal impedance, adhesive strength, etc.
- Reliability – Change in performance under thermal stress, humidity, power cycling, etc.
- Manufacturing – Working time, shelf life, safety and compliance, and costs, etc.
The building blocks of any TIM are its physical form, resin system and filler content. These properties are interdependent and cannot be manipulated in isolation. Modifications to any of them impact various values such as adhesion, working time, durability and shelf life, among others. And, since optimal performance depends on thermal conductivity and rheology, the TIM must be carefully aligned to the application requirements and manufacturing environment realities.
One Objective, Multiple Solutions
While the overarching goal is securing maximum function through the most efficient removal of heat from a power device, there are many different types of TIMs from which to choose. These include:
- Thermal greases – Greases are lower viscosity materials that will deform continuously under external stress. Though effective for some applications, greases can lose performance over time as they have a tendency to migrate - or ‘pump out’.
Thermal conductivity range: 1.0 – 5.0 W/m-K; Bond line thickness range: 50 – 100 µm
- Phase change TIMs – Phase change materials are solid at room temperature and melt/flow into the interface gaps when they reach their respective phase change temperature. In contrast to greases, phase change TIMs are easy to handle, can be automated in a label application-type process for film phase changes TIMs, or dispensed and/or stencil printed for paste-based materials. These TIMs also provide electrical isolation.
Thermal conductivity range: 1.0 – 5.0 W/m-K; Bond line thickness range: 50 – 200 µm
- Thermal pads – Soft, conforming thermal pads are generally reinforced with film or fiberglass for electrical isolation. Pads perform well under pressure, protecting devices from assembly and operational stress, and provide superb reliability.
Thermal conductivity range: 1.0 – 12.0 W/m-K; Bond line thickness range: 150 – 250 µm
- Liquid gap fillers – Two-part gap filling liquid TIMs offer high volume, automated dispensing capabilities in combination with excellent thermal and mechanical performance. They induce almost no stress on components during assembly and allow effective wet-out even in complex, multi-level designs.
Thermal conductivity range: 1.0 – 7.0 W/m-K; Bond line thickness range: 100 – 500 µm
- Thermal gels – One-part liquid formable and liquid curable thermal gels hold their shape once dispensed. They provide low component stress and are usually reworkable. Offering manufacturing flexibility, thermal gels are highly-conformable, shear-thinning and do not require mixing, curing or refrigeration. Some gels have exceptional vertical stability, making them ideal for high-power applications in demanding environments, such as telecom infrastructure systems.
Thermal conductivity range: 1.0 – 6.0 W/m-K; Bond line thickness range: 0.5 – 4.0 mm
The growing demand for power-efficient electronics is well-documented and spans every geographic region and industry, especially as the world becomes increasing digital, mobile and connected. Ensuring optimal performance of these systems and power components is dependent on many factors, not the least of which is robust thermal control. Where high-powered technology is in use, the need for dissipating heat to maintain functional durability is essential.
Browse the full range of LOCTITE® Bergquist® thermal management solutions to find products designed for optimal performance in a variety of environments. Or get in touch with our thermal management experts below to find the right solution for any application.
Find out more about effective heat management in data centers and other high-power devices with insights across the Henkel blog.
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