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Characterization of PCB Material & Manufacturing for High Frequency
Characterization of PCB Material & Manufacturing for High Frequency
This paper investigates the impact of different PCB manufacturing technologies and their relation to their high frequency behavior.
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Authored By:
Oliver Huber, Thomas Faseth, Holger Arthaber
University of Technologym Vienna

Erich Schlaffer
Austria Technologie & Systemtechnik AG
Leoben Austria

Summary
Today's Electronic Industry is changing at a high pace. The root causes are manifold. So world population is growing up to eight billions and gives new challenges in terms of urbanization, mobility and connectivity. Consequently, there will raise up a lot of new business models for the electronic industry. Connectivity will take a large influence on our lives. Concepts like Industry 4.0, internet of things, M2M communication, smart homes or communication in or to cars are growing up. All these applications are based on the same demanding requirement - a high amount of data and increased data transfer rate. These arguments bring up large challenges to the Printed Circuit Board (PCB) design and manufacturing.

This paper investigates the impact of different PCB manufacturing technologies and their relation to their high frequency behavior. In the course of the paper a brief overview of PCB manufacturing capabilities is be presented. Moreover, signal losses in terms of frequency, design, manufacturing processes, and substrate materials are investigated. The aim of this paper is, to develop a concept to use materials in combination with optimized PCB manufacturing processes, which allows a significant reduction of losses and increased signal quality.

First analysis demonstrate, that for increased signal frequency, demanded by growing data transfer rate, the capabilities to manufacture high frequency PCBs become a key factor in terms of losses. Base materials with particularly high speed properties like very low dielectric constants are used for efficient design of high speed data link lines. Furthermore, copper foils with very low treatment are to be used to minimize loss caused by the skin effect. In addition to the materials composition, the design of high speed circuits is optimized with the help of comprehensive simulations studies.

The work on this paper focuses on requirements and main questions arising during the PCB manufacturing process in order to improve the system in terms of losses. For that matter, there are several approaches that can be used. For example, the optimization of the structuring process, the use of efficient interconnection capabilities, and dedicated surface finishing can be used to reduce losses and preserve signal integrity.

In this study, a comparison of different PCB manufacturing processes by using measurement results of demonstrators that imitate real PCB applications will be discussed. Special attention has be drawn to the manufacturing capabilities which are optimized for high frequency requirements and focused to avoid signal loss. Different line structures like microstrip lines, coplanar waveguides, and surface integrated waveguides are used for this assessment.

This research was carried out by Austria Technologie & Systemtechnik AG (AT&S AG), in cooperation with Vienna University of Technology, Institute of Electrodynamics, Microwave and Circuit Engineering.

Conclusions
Structuring processes show different deviations of line width and trapezoidal degree of the cross-section, which also impairs the applied immersion nickel/gold top plating. In a comparison of RF-losses, the fabrication processes have demonstrated certain characteristics for the investigated transmission modes, MS, CBCPW, and SIW. MS has shown that for lines lengths up to 5 mm there is no observable difference in terms of manufacturing process. Nonetheless, for longer lines a degradation of panel plating over pattern plating occurs, which increases quite linearly for the investigated materials. CBCPW exhibits slightly larger overall losses as MS.

Due to the measurement results, the frequency dependence has shown a quite divergent behavior, as for low frequencies there is already a noticeable loss term present, however, with less overall increase over frequency than for MS. This behavior has been observed for both investigated materials. The SIW shows independent losses regarding the manufacturing process. For SIW, the losses can be assumed to be constant for frequencies above e.g. 65 GHz up to 100 GHz for material type A.

In a future work, these described observations will be extended to investigate the impact of different interconnection methods and top end surface platings. Additionally, for the CBCPW further designs will be made and measurements will be carried out for different gap widths. This is done to try to evaluate when the discussed change of loss difference starts to emerge compared to the MS line structure.

Initially Published in the IPC Proceedings

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