In part 1 of our 2 part series, “How to increase a heat sinks surface area without increasing its size using microscopic texturing” we covered what this technology is, options to apply, and under what conditions it is effective. In part 2 we’ll cover accurate surface texture characterization.
Accurate surface texture characterization is often needed to control and fine tune the texturing process. Surface texture is not a measurable quantity but it is possible to measure some of its intrinsic characteristics and parameters. These parameters have been developed based on data obtained using a stylus-based instrument. This device, known as 3D Surface Profilers, uses a diamond-tipped stylus to detect minute surface variations and topology. The stylus is mechanically couples to an LVDT (Linear Variable Differential Transformer). The sample is precisely guided underneath the stylus while the stylus rides over the surface, detecting roughness variations as small as ten angstroms in height [7]. The LVDT produces an analog signal in response to the stylus movement. This signal is then sent to a computer data acquisition system where the signal is amplified, digitized and analyzed.
The main parameters of a texture surface are waviness and roughness. These parameters are repetitive and random irregularities from the normal surface that forms the three dimensional topography of the surface. The spacing of these irregularities is what differentiates waviness from roughness. Two or more spacings between irregularities will cause a wavy surface. However, as the space between them decreases excessively, the resulting surface would become flat but rough. A typical surface exhibits roughness superimposed over waviness [7]. A sophisticated Profiler implements advanced high and low pass filters in order to measure each parameter separately while filtering out the other.
The one parameter that is standardized all over the world and is specified and measured far more frequently than any other is the arithmetic average roughness height, or roughness average, designated by Ra. It is defined as the arithmetic mean of the departures of the profile from the mean line. An approximation of the average roughness, Ra is obtained by adding the Y values without regard to sign and dividing the sum by the number of the samples. Ra is used to detect general deviation in overall profile height but it is incapable of detecting differences in spacing and its distribution [7].
Roughness Average, Ra Is Sum Total Of The Maximum Valley
And Maximum Peak Of Roughness [7].
Another parameter that is often used in analysis of a texture surface is the root-mean-square average of the departures of the roughness profile from the mean line. This is known as RMS and designated by Rq. RMS or Rq has statistical significance because it represents the standard deviation of the profile heights and it is used in the more complex analysis of skewness, the measure of symmetry of a profile about the mean line [7].
RMS Value of Roughness [7].
A well equipped Profiler system includes both the Ra and Rq standard analytical functions as well as other widely used parameters to analyze surface roughness and waviness. Contrary to general beliefs radiation heat transfer could be as important as convection heat transfer in electronics cooling, especially in natural convection and low-airflow applications. To further enhance radiation, surface treatments such as surface anodize and surface texturing is a viable option that could increase effective surface area and increases surface emissivity. Surface Anodize also has the advantage of corrosion and wear resistance and it also electrically isolates the cooling components from the electrically charged electronics.
References:
- Radiation Heat Transfer and Surface Area Treatment ,Qpedia Thermal eMagazine, June 2008.
- Edwards, J., Coating and Surface Treatment Systems for Metals, Finishing Publications Ltd. and ASM International, 1997.
- Aluminum Anodizer Council Web Forum, http://www.anodizing.org/index.html.
- Gustavsen, A., Berdahl, P. Spectral Emissivity Of Anodized Aluminum And Thermal Transmittance Of Aluminum Windows Frames, Nordic Jounnal Of Building Physics, Vol.3, 2003.
- Ozisik Necati, M., Heat Transfer A Basic Approach, McGraw Hill, 1985.
- Highly Emissive Ion Beam Textured Surfaces For Improved Cooling Of Electronic Devices, Electronics Cooling Magazine, September 1997.
- Chi, T. , Ballinger, R., Olds, R., Zecchino, M., Surface Texture analysis using Dektak Stylus Profilers, Veeco Instrument Inc.http://www.veeco.com/pdfs/appnotes/an525%20_dektak_surface_97.pdf