Semi-Automatic Copper Foil Surface Roughness Detection from PCB Microsection Images Soumya De #1 , Aleksandr Gafarov #2 , Marina Y. Koledintseva #3 , R. Joe Stanley #4 , James L. Drewniak #5 , Scott Hinaga *6 # Center for Electromagnetic Compatibility, Missouri University of Science & Technology (MS&T) 4000 Enterprise Dr., HyPoint, Rolla, MO, 65401, USA 1 sdwt2@mst.edu ; 2 agtgb@mst.edu ; 3 marinak@mst.edu ; 4 stanleyj@mst.edu ; 5 drewniak@mst.edu * CISCO Systems Inc. San Jose, CA 95134, USA 6 shinaga@cisco.com Abstract— Characterization of surface roughness of printed circuit board (PCB) conductors is an important task as a part of signal-integrity analysis on high-speed multi-GHz designs. However, there are no methods to adequately quantify roughness of a signal trace or a power/reference plane layer within finished PCBs. Foil roughness characterization techniques currently available can only be applied to the base foil, prior to its incorporation into a finished board. In a finished PCB, a foil surface is not directly accessible, as it is embedded in the dielectric of the board, and attempting to expose the surface will damage the board and the surface of interest. In this paper, a method of surface roughness quantification from microsectioned samples of PCBs is presented. A small, non-functional area, e.g., a corner of the PCB, can be removed, and the surface roughness of the circuit layers can be assessed without impairing the function of the PCB. In the proposed method, a conductor (a trace or a plane) in the microsectioned sample is first digitally photographed at high magnification. The digital photo obtained is then used as an input to a signal- and image-processing algorithm within a graphical user interface. The GUI-based tool automatically computes and returns the surface roughness values of the layer photographed. The tool enables the user to examine the surface textures of the two sides of the conductor independently. In the case of a trace, the composite value of roughness, based on the entire perimeter of the trace cross- section, can be calculated. I. INTRODUCTION Signal attenuation in transmission lines due to skin effect loss and surface roughness in copper conductors on printed circuit boards (PCBs) is a well-documented issue, confronting in particular designers of high-speed (>10 Gb/s) circuits [1]- [4]. Knowledge of copper roughness on PCBs is important for high-speed electronics, where accurate separating and modeling of conductor and dielectric losses at the design stage determine quality of the performance of designs [5], [6]. To minimize the variation in channel loss within a large population of PCBs built by multiple board shops over an extended period of time and on a variety of different laminate materials, it has become a standard practice at many Original Equipment Manufacturers (OEMs), to attempt to control the surface roughness of copper foils through specification of the roughness grade on the fabrication drawing. Maximum roughness values for PCB circuit foils are governed by the appropriate industry specification for metal foils, IPC-4562A [7]. However, as is demonstrated in [8] and [9], the roughness profile of an inner-layer trace is influenced not only by the grade of copper foil used on the laminate core material, but also by the oxide or alternative-oxide inner-layer treatment process applied by the PCB fabricator. (a) (b) Fig. 1. Typical photos of cross-sections: (a) optical photo, (b) SEM picture. Existing measurement methods for conductor surface roughness may be applied to raw copper foil or a sheet of copper-clad core material, either in its original form or following circuitization (including inner-layer treatment). The traditional measurement method used in the PCB industry for over thirty years has been stylus profilometry, in which the movements of a mechanical stylus dragged in a straight line across the sample are used to calculate values of roughness [10]. Modern non-contact commercially available techniques 978-1-4673-2060-3/12/$31.00 ©2012 IEEE 132