Thermal Management Tips and FAQs
Thermal Management Tips and Frequently Asked Questions
Thermal interface material (TIM) that provides the lowest thermal resistance is critical for the performance and reliability of the high performance power devices. The lowest thermal resistance interface is generally not directly correlatable to the highest bulk thermal conductive interface materials. Please CLICK HERE FOR A VIDEO from AI Technology, Inc. for a detailed theoretical and practical considerations in using different thermal interface materials in thermal management of high performance electronic devices.
1. What are the most important factors that effect cost effective thermal management?
The following components comprise an effective thermal management solution:
- A good heat sink or heat pipe to remove heat from the power component.
- A high thermally conductive interface material.
- As thin as possible thermal interface material.
- Void free interface material.
- Void free interface between the power device, interface material and the heat sink (or heat pipe).
The order of the contributing factors of these components for a fixed power or heat generation device are:
- Heat sink design and proper airflow is the first and most critical factor in any thermal management design. The component should be optimized for any physical space limitations Airflow and cost consideration are also main factors.
- The thermal interfacial resistance between the heat sink, interface material and the power device is at least 2-3 times more important than the thermal conductivity of the interface material when the thickness is less than 3 mils.
- The most important factor of thermal resistance is the elimination of voids along the interfaces. Voids along the interfaces contribute to more than twice the interfacial thermal resistance than voids inside the thermal interface material. It is critical that there is adequate interface material and thickness to accommodate any warpage of the surfaces.
- Only through adequate flow of the interface material will one be able to eliminate the trapped air between the interface material, power device surface and heat sink surface.
- Higher bulk thermal conductivity of the thermal interface material is only an indication that having a lower thermal resistance is a possibility. This does not always directly correlate to high performance. Testing is the only way to confirm the performance.
- The thermal interface resistance is related to phonon scattering and thus not completely predictable by the thermal conductivity of each part of the components. The compatibility factor in heat transfer is a very complex phenomenon.
2. Is the initial performance of thermal resistance a good indicator for long-term reliability and life of the power device?
- Not really.
- Sometimes interfacial stress due to thermal expansion over thermal cycling may change the initial thermal resistance performance.
- When interfacial mechanical stress induces voids over time along the interfaces,
the reliability and life of the device will be shortened with the increase in thermal interface resistance and junction temperature of the power devices. Problems occur when the interfacial stresses are high due to using high bond strength and high modulus adhesives.
- Sometimes thermal greases will be “pumped” out of the interfacial areas and thus create voids. This will induce higher junction temperatures and shorten the life of the power device. Accordingly thermal gels with the same initial performance are always more reliable than grease. A flexible, highly molecular weight pad that properly flows and eliminates interfacial trapped air will perform the same as gel.
- Thermal interfacial materials that remain flexible during all phases of the operating conditions from cold to hot are more reliable than materials that are hard and solid during cold conditions and become more fluid during hot conditions.
- In general, a stress-free interface between the thermal interface material, the power device surface and the heat sink surface is critical for long-term reliability.
3. When one thermal interface material has a measured higher bulk thermal conductivity than another, will it also have a lower thermal resistance?
- Not always, please refer to question #1.
- Only a direct thermal resistance measurement can confirm this.
- Some thermal interfacial materials are more compatible in heat wave transfer (phonon and electronic) than others in relation to the heat sink and power device materials.
- If the thermal interface material does the following (A and B), the thermal interfacial resistance be reduced.
- A. The thermal interfacial material must be induced to flow to remove any trapped air along the interfaces and within the interface materials.
- B. The thermal interface material must be flexible and compliant from the lowest temperature to the highest temperature to ensure there is no interfacial stress that can generate voids.
4. Is heat spreading or heat removal more important?
- Heat spreading is more important since it is the first part of thermal management.
- Heat removal becomes critical when the power generation is much faster than heat transfer to the air.
- A physical space that allows for larger heat spreading is a better solution than increased air circulation.
5. Is it good to spread the thermal interface material-compound over the complete area of the power device?
- No. Placing the device over the complete area does not form a good thermal interface with low thermal resistance.
- No matter what kind of shape or size of the interface compound being used, the key is to ensure FLOW of the compound that will force out any trapped air when two surfaces are being mated together.
- The best proven method is to dispense a “star” shape pattern of material. The thickness or quantity of the heat transfer compound should be enough that you can observe compound oozing out of the perimeter. Remove excess compound whenever possible.
- This flow should be observed either in the placement of the heat sink over the power device or in the case of a phase-change pad, during the first power-up cycle of the device.
6. What if I have a large area power module, how can I compensate for the height variations over the area containing multiple components?
- AIT offers a patented solution, its compressible phase change thermal interface pad placed between the module and heat sink.
- The compressibility of the thermal interface pad material allows for the first critical part of accommodating the difference in height tolerance so there are no air gaps existing anywhere on the module’s components.
- Another property of the phase-change thermal interface pad is that it is engineered for most applications at 50°C to ensure elimination of trapped air between each of the components and the heat sink surface.
- The flow aspect is critical to ensure the best thermal interface performance that cannot be achieved by just being compressible.
The failure rate of an electronic device doubles with every 10°C increase in chip junction temperature!
Comparison of Thermal Interface Materials Performance: Lower temperature rise represents a measurement of the efficiency of the thermal interface material in transferring heat generated by the power electronic device to the heatsink (with fan) that dissipates heat to the environment by forced circulating air. The thickness of the thermal interface material also contributes significantly to the efficiency of heat dissipation and should be minimized as much as the physical configuration or construction permits. The effectiveness of a specific thermal interface material cannot be easily predicted by the claimed or even measured bulk thermal conductivity data or value: the thermal resistance of the thermal interface material to the substrates in their respective interfaces for a thin bondline or interface thickness are significant and represents the “coupling efficiency” that cannot be predicted and must be measured.
The data comparing thermal interface materials is collected by using an Intel CPU and forced air heatsink as represented in the above configuration. A thermocouple is drilled and embedded at the heatsink junction that interfaces with the thermal interface materials to provide direct temperature data at the interface for measuring the “coupling” of the specific thermal interface materials to the heatsink.
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