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Reducing contact thermal resistance
This thermal resistance can be expressed as Rja, where:
Therefore, one way to reduce Rja is to reduce Rcs, the contact resistance between the electronic device case and ambient-cooled, finned heat sinks or liquid cold plates. There are several factors that impact Rcs, including surface flatness, surface roughness, contact force or clamping pressure, surface cleanliness and interface materials.
Surface flatness can be understood as widely spaced surface irregularities or waviness of a surface. Surface roughness is the submicron scale irregularities of a surface, usually as a result of machining, usage and/or wear.
The contact between two imperfect surfaces will result in air gaps between them. (See Fig. 1.) Most contact areas consist of more than 90% air voids, which represent a significant resistance to heat transfer since air is not a very effective thermal conductor. Table 1 shows typical surface roughness values for different manufacturing processes.
| Process | Units in µm | Units in µin |
|---|---|---|
| Polishing | 0.1 – 0.4 | 4 – 16 |
| Grinding | 0.1 – 1.6 | 4 – 64 |
| Laser cutting | 0.8 – 6.3 | 32 – 252 |
| Die casting | 0.8 – 1.6 | 32 – 64 |
| Machining | 0.8 – 1.6 | 32 – 64 |
| Extrusion | 0.8 -3.2 | 32 – 128 |
| Drilling | 1.6 – 6.3 | 64 – 252 |
A mounting surface flatness of 0.001 in/in is generally required for satisfactory contact between the electronic device and the heat sink or cold plate. The surface roughness should be equivalent to that of the electronic device, where 32-64 µin is usually adequate. Finer finishes add unnecessary cost with little or no improvement in thermal performance. Surface flatness is typically much more critical than surface finish in achieving a good thermal interface.
Another important factor in minimizing contact thermal resistance is contact force or the force with which the electronic device is pushed against the heat sink or cold plate. Electronic devices and heat sink surfaces will never be perfectly flat. Consequently, there will always be air gaps in between these surfaces. However, as the contact force pushing the two surfaces together increases, so does the number of contact points between the two surfaces, resulting in a lower case-to-sink thermal resistance, Rcs. This relationship between force and thermal resistance does not follow a linear curve. As contact force is increased, contact thermal resistance will decrease until a point where it will show diminishing returns in thermal resistance reduction and maximum force the package can handle is approached. The electronic device manufacturer should be contacted for recommended contact forces.
Mounting surface cleanliness is also important in minimizing contact thermal resistance. Mounting surfaces should be kept free of all foreign material, such as dirt, oil, oxides and films. Since most heat sinks and cold plates are stored after machining, a cleaning operation is recommended prior to mounting the device. A satisfactory cleaning technique is to lightly polish the mounting surface with fine steel wool, followed by a semiconductor cleaning solvent wipe.
Finally, in order to further improve Rcs, an appropriate thermal interface material (TIM) should be used to fill air gaps between the two surfaces. There are a number of technologies that can be used, including thermal greases and thermally conductive compounds, elastomers, adhesive tapes, etc., each with their own characteristics (operating temperatures, ease of application, curing time, pressure requirements, etc.) that can make them more or less desirable depending on the application. Contact Eaton to consult on how to select the appropriate TIM for your application. Table 2 shows typical thermal resistance and thermal conductivity values for these TIMs.
| Interface | Thickness (in.) | Thermal Conductivity, k(W/m-K) | Rcs(°C/W) |
|---|---|---|---|
| Dry Joint | N/A | N/A | 2.9 |
| Thermal Grease | 0.003 | 0.7 | 0.9 |
| Thermal Compound | 0.005 | 1.2 | 0.8 |
| Elastomer | 0.010 | 5.0 | 1.8 |
| Adhesive Tape | 0.009 | 0.7 | 2.7 |
Contact conditions encompass a number of areas including surface flatness, surface roughness, surface cleanliness, contact pressure and interface materials. There are many technologies and techniques available for optimizing the thermal path from the electronic device junction to the heat sink. It’s important to minimize the thermal resistance in order to maintain the electronic device temperature below its maximum rated value and increase the end product reliability.
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