An Introduction to the Sdr Parameter and its Applications in Characterizing the Functionality of Surface Data

Are you familiar with the Sdr parameter? It is possible it could be the perfect indicator for distinguishing the functionality of your surface data.

In this article, a number of Mountains® users discuss the advantages of examining the correlation between the developed surface area and surface functionalities, including wettability, adhesion and conductivity.

Initially, a definition of Sdr must be provided. The Sdr parameter can be thought of as the ratio between the area of the “real” developed surface and the area of the “projected” surface. This ratio is, at times, described as the surface’s “complexity.”

The ISO 25178 standard defines Sdr through the formula:

Case Study 1: Adhesion & Conductivity in the Electronics Industry

Within the production of electronics, the Sdr parameter plays a vital part in managing surface properties of materials.

An application specialist at Carl Zeiss (Shanghai), Roy Li, stated, “Customers in the printed circuit board (PCB) industry are using the developed interfacial area ratio as one of the key parameters for measuring the performance of brown oxide treatment.”

“The purpose of brown oxide treatment on PCBs is to create a uniform layer of organic metal complex on copper surfaces to increase roughness and adhesion.”

Sdr calculated on a surface treated with brown oxide

Sdr calculated on a surface treated with brown oxide

A further functionality in the production of electronics which necessitates additional study is high frequency electrical conductivity.

We manufacture electro-deposited copper foil which has a wide range of applications in electronics. At higher frequencies electrical signals are only conductive at the surface and no longer inside the copper tracks. For such applications, our customers often need to obtain lower surface roughness. However they also have to maintain high adhesion between the copper and the substrate materials (e.g. low-loss resins) in order to minimize the so-called conductor loss. The Sdr parameter, in combination with Sa and Sq, is especially useful for describing the surface in this case.

Bodo Gemsleben, Circuit Foil Luxembourg

Case Study 2: Industrial Coatings

Coating adhesion on mechanical workpieces can be studied using the Sdr parameter.

Coating adhesion on mechanical workpieces can be studied using the Sdr parameter.

The tool manufacturing industry offers further illustration of the functionality of the Sdr parameter.

One of our specialties is oil-free compressors, the rotors and casing of which must be coated. To ensure coating adhesion, the surface must be made rougher (this is done with sand-blasting). The forms of our workpieces are quite complex and the dimensions are rather big. We decided to use the Sdr parameter after performing optical measurement of these, as it is the areal variant of the Plo parameter (developed length).

Daan Boden, PhD student in Industrial Engineering Working, Atlas Copco

Atlas Copco is an air compressor manufacturing company.

Case Study 3: Biofilm Wettability

Top: rose petal. Bottom: hydrophobic biofilm. The Sdr values of the two surfaces are very close.

Top: rose petal. Bottom: hydrophobic biofilm. The Sdr values of the two surfaces are very close.

In order to examine the surface properties of biofilms, Carolina Falcón García, Professor in Biomechanics at the Technical University of Munich (Institute of Medical Engineering), is using µsoft software from Nanofocus, based on Mountains® technology.

The colonization of surfaces by bacterial biofilms constitutes a huge problem in healthcare and industry. When attempting biofilm inactivation or removal, it is crucial to sufficiently wet the biofilm surface with antibacterial agents. However, certain biofilms efficiently resist wetting and the origin of this behavior remains to date unclear. We discovered that the Sq value, which is generally accepted as a measure of the roughness of a surface, was not sufficient in describing the complexity of these biosurfaces. We found that Sdr was the actually the most accurate parameter for doing this. Using Sdr in combination with contact angle measurements, we were able to easily distinguish between a hydrophilic surface and two types of hydrophobic biofilm surfaces. In a recent study, we found that the Sdr values we obtained for the peripheral region of one biofilm were very close to the values we obtain for rose petals. This further underscored the analogy drawn between the wetting resistance of rose petals and that of one of the hydrophobic biofilms.

Carolina Falcón García, Professor in Biomechanics, Institute of Medical Engineering, Technical University of Munich

How to Get the Most Out of the Sdr Parameter

The characterization of the Sdr value in the ISO 25178 standard dictates that it should be determined on the scale-limited surface, i.e. the S-F or S-L surface. This would indicate that both the nominal form and the microroughness should be eliminated.

The spatial resolution of the instrument, which is dependent on the x,y spacing between measured points and on the variety and metrological features of the sensor (including numerical aperture and tip radius), has a strong influence on the Sdr parameter.

MountainsMap® Premium software features the Sdr parameter, including the value of the developed and projected areas. It can also be found in the 3D Advanced Surface Texture and the new Scale-sensitive analysis modules.

This information has been sourced, reviewed and adapted from materials provided by Digital Surf.

For more information on this source, please visit Digital Surf.

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