Printed Circuit Design & Fab - October 2008 - (Page 32)

ModElinG Modeling Conductor Surface Roughness Stripline models can be used to determine the impact of copper surface roughness on transmission properties. by AnTOnIO CICCOMAnCInI SCOGnA increases as a function of frequency. It therefore must be In high speed digital interconnects, signal attenuation is a taken into account of stripline surface roughness One result of both dielectric losses and conductor losses. ManyPerformance analysiswhen designing stripline structures. models of the best ways to estimate the incidence on the electrical articles have shown in detail the characterization and modelA. Ciccomancini Scogna, PhD performance of a stripline due to surface roughness is to coning efforts related to the impact of dielectric loss in PCBs and 1. Never# sider different profile models. the differences between various dielectric materials CST of America Inc, 492 Old Connecticut Path, Suite 505, Framingham, MA, 01701, phone: +1-508-665-4400 Fax: +1-508-665-4401, e-mail: antonio.ciccomancini@cst.com theless, high-speed characterization modeling efforts do not often take into account the variations in conductor losses due Test Vehicle to copper foil roughness or treatments made to the copper A cross-sectional view of the test vehicle structure, along with foil to promote adhesion. the geometric dimensions, is illustrated in FiGurE 1. It is a 400 Two types of interconnect structures are mainly used in μm (L) long copper stripline (σ = 5.8e7 S/m), with a width of order to propagate signals in integrated circuits (ICs) and 10 μm and a height of 3.5 μm. The dielectric material is FR-4 printed circuit boards (PCBs): microstripline and stripline. (εr = 4.9) and is considered loss free with the specific purpose Stripline is constructed by sandwiching a metallic strip within of concentrating the attention on the conductor loss only. a dielectric material. The benefit of the closed structure, CST Microwave Studio (CST MWS)7 is used to perform compared to the microstripline, is better protection against external signals and unwanted radiation. 25µm 3.5µm Stripline is one of the most popular methods to transfer signals in PCBs and ICs. In an ideal world, the conductor 10µm would be perfectly smooth and have a perfectly rectangular FiGurE 1a cross-section. In reality, this is not the case because the conductors have microscopic dips and grooves as a result of the foil manufacturing and PCB fabrication process. Due to the importance of these transmission lines, significant research has been done in the last few years to accurately analyze the transmission properties2-6. A critical aspect of this analysis is the focus on the characterization of the conductor losses due to surface anomalies on the copper traces. Copper foils are roughened to improve adhesion between the copper and dielectric resin of the laminate material. The adhesion at the interface between the conductor and the insulator must be robust to survive the fabrication, assembly and standard usage conditions. When the wavelength of the signal has a length comFiGurE 1a and 1b. Cross-sectional view of the test vehicle parable to the amplitude of the roughness, the attenuation 1 – Test vehicle: stripline cross section view and field mode pattern distribution with Figure and geometric dimensions. impedance, wave impedance, beta and alpha values. 32 printEd CirCuit dESign & fAB OCTOBER 2008

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - October 2008

Printed Circuit Design & Fab - October 2008 Contents Our Line Market Watch Around the World Happenings Test and Inspection ROI Tip Jar Interconnect Strategies Final Finish Forum The New Wave in High-Speed Modeling The PCB Design Library Mixed Signal Design Considerations Modeling Conductor Surface Roughness Copper Erosion: The Influence of Metallurgy on Copper Dissolution The Wave of the Future Ad Index Building a Profitable Niche Marketplace Off the Shelf BGA Bulletin

Printed Circuit Design & Fab - October 2008

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