The process of selecting thermal interface material can be quite challenging when conductivity measurements do not correlate with actual cooling efficiency.
Since thermal conductivity and heat transfer coefficient are two different phenomena, it is hard to determine their impact on the choice of TIM. This article explains how both parameters affect TIM choice under various heat loads, cooling methods, and interface conditions.
Thermal Conductivity vs. Heat Transfer: A Quick Comparison
Thermal conductivity and heat transfer coefficient are two different parameters that characterize different stages of the heat transfer process. Knowing their distinctions is critical for the assessment of TIM performance. The table below will help you distinguish between these parameters and understand their significance for the proper selection of thermal interface material (TIM).
| Key Variables to Compare | Thermal Conductivity | Heat Transfer Coefficient |
| What is measures | · Heat conduction through material | · Heat transfer between a surface and its surrounding environment |
| Unit of measurement | · W/m·K | · W/m²·K |
| Primary influencing factors | · Material composition and temperature | · Cooling medium, flow conditions and surface characteristics |
| Why it matters | · It determines how efficiently TIM conducts heat | · It determines how efficiently the cooling system removes heat |
| Role in thermal interface material selection | · It helps select a TIM with suitable heat-conducting capability | · Helps match the TIM to the application’s cooling conditions |
How Thermal Conductivity and Heat Transfer Coefficient Influence TIM Selection
· Match Thermal Conductivity to Application Heat Load
Higher thermal conductivity suits components that produce a lot of heat within a confined space. For instance, power electronics, EV battery modules, and high-output LEDs require good heat spreading capabilities. Thermal interface materials (TIMs) have their conductivities measured in W/m·K, and high values correspond to better conduction performance.
TIM materials with moderate conductivity may be considered for applications with low heat generation and/or large cooling area. Choosing highly conductive materials is costly and doesn’t always improve overall thermal path efficiency. Thermal budget, component power, interface area, and the maximum temperature rise determine your conductivity needs.
· Evaluate Cooling Environment before Selecting TIM
The cooling environment has an effect on the effectiveness with which heat is extracted from the assembly after passing through the TIM. In passive systems, the process is carried out using natural convection; in forced air systems, increased flow increases the heat dissipation efficiency. Liquid-cooling systems may offer better heat dissipation performance if the conditions are appropriate.
Heat transfer coefficient describes thermal transfer between the fluid and the surface but not conduction across the TIM. The value of the heat transfer coefficient is dependent on various aspects, including the fluid properties, fluid speed, geometry, and surface condition. Thus, your TIM needs have to correspond to the cooling architecture rather than just thermal conductivity.
· Consider Thermal Contact Resistance Alongside Material Conductivity
TIM may possess excellent thermal conductivity but fail to perform adequately if the interface is not smooth or well-fitted. Microscopically uneven surfaces introduce air gaps into the interface while inadequate pressure results in high interface resistance. These factors increase interface resistance between heat source, TIM material and heat sink.
Contact resistance becomes a concern in a situation where the TIM layer is very thin. ASTM D5470 takes into account the effect of contact pressure and the sample surface characteristics on thermal transmission properties measurements. Thus, you should consider interface resistance during your selection together with thermal conductivity.
· Balance Thermal Performance with Mechanical Compliance
A TIM that is mechanically compliant can deform under compression to accommodate surface irregularities and provide good contact within an assembly. Soft gap pads can compress against uneven surfaces, while a harder TIM gives greater dimensional stability. As such, compressibility, hardness, and recovery properties are just as important as thermal conductivity.
Thermal cycling can cause adjoining parts to expand and contract. An appropriate TIM maintains contact without undergoing much permanent deformation or creating mechanical stresses. Therefore, your specification should include details of compressibility and gap filling along with thermal specifications.
· Select an Appropriate TIM Thickness
Thicker TIM layers increase thermal resistance since they provide longer paths for heat to flow. In uniform TIM layers, the resistance is determined by thickness, thermal conductivity, and contact area. A thin TIM layer decreases bulk resistance provided that there are small gaps between the contact surfaces.
Larger gaps need thick TIM layers to ensure that contact remains at the interface. Thick TIM layers increase resistance and reduce heat conduction through the TIM layer. You need to specify the gap sizes in your design rather than choosing the TIM layer based on thermal conductivity alone.
· Choose TIM that Matches Operating Conditions
The operating temperature can influence thermal conductivity, mechanical properties, and long-term stability of the material. In addition, thermal cycling can affect compression, adhesion, and dimensional stability, depending on the TIM structure. Mechanical load becomes another factor when the assemblies are subjected to vibrations, clamping loads and thermal expansions.
Long-term reliability depends on how the entire TIM behaves under real conditions. Therefore, testing needs to include the actual operating temperatures, TIM thicknesses, pressures, and interface configurations. Your selection should be based on application-specific data, not a single conductivity value from a catalog.
FAQs
How can you test thermal conductivity for thermal interface material?
Thermal conductivity can be evaluated through standardized tests like ASTM D5470. The test method has to correspond to the type and use of TIM.
Which are the common mistakes when choosing thermal interface material?
Among the most common errors include; concentrating only on conductivity but ignoring other factors like thickness, resistance, gap, pressure, and operating conditions. Selecting a TIM without looking at the total interface may degrade the thermal performance.
What standards are used to test thermal interface material?
There is a universal standard for measuring thermal transmission characteristics of TIMs which is ASTM D5470. Different standards can be used for other types of TIM and their properties.
Can TIMs be supplied in custom die-cut shapes?
Yes, many types of TIMs can be delivered in custom die-cut form. These specifications may include dimensions, thicknesses, tolerances, adhesive layers, and geometrical requirements.
Can one thermal interface material specification be used across different product models?
No, because each product line has its own gap, heat loads, pressure, and working temperature. Every single design needs to confirm the chosen TIM specification with its own requirements.
What factors affect the cost of custom thermal interface material?
The cost depends on thermal conductivity, material type, thickness, dimensions, tolerances, die-cut complexity, and order quantity. Also, the adhesive layer, liners, packing, and other requirements can affect pricing.
Final Thoughts
Thermal conductivity is a key parameter when choosing TIM, but it is not the only parameter that you need to take into account. Besides that, your choice needs to be based on the thickness, contact resistance, mechanical characteristics, cooling conditions, and operating conditions. Taking into consideration all these factors will help your team select the proper TIM that best matches your thermal management needs.
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