Make sense of high-frequency test methods

Glenn Oliver, Senior Engineer, DuPont & DesignCon Technical Program Committee -December 22, 2015

Digital signals are fundamentally electromagnetic waves added together.  The higher the data rate required, the higher the frequency of the waves required to generate the signals.  Once the signal leaves the chip, there are a myriad of factors that act to degrade the quality of the signal as it propagates.  The “field” of signal integrity is focused on minimizing these factors.  It is often overlooked that one of the factors that has to be considered is a “field.”  That is, the electromagnetic field generated in the dielectric as the signals propagate.
The market for circuit board materials consists of “Rigid”, “RF/Microwave”, and “Flex” where each type has separate standards.  In general, only RF/Microwave materials require any sort of characterization of properties like relative permittivity (Er) and Loss Tangent (tan d) at frequencies above 1 GHz.  In addition, the characterization required for RF/Microwave materials is in a narrow band of frequencies around 10 GHz since this is where most radar systems operate which consume these materials.  For high speed digital, the existing standards are insufficient since there is a dire need to standardize measurement methods for Er and tan d over a wide band of frequencies.  Ideally this would be done over a frequency range of “DC to Light,” but for practical considerations the immediate need of evaluating methods at frequencies higher than 10 GHz is an area of focus.
The IPC-D24C Task Group is charged with assessing and developing test methods for printed circuit board materials that measure Er and tan d.  At DesignCon 2016, the Round Robin of High Frequency Test Methods panel is organized by the chair of this task group and is focused on a current project in which a round-robin of 10 different printed circuit board materials from 5 different suppliers were measured by 5 different test labs.  The material suppliers delivered thin, low-loss copper clad laminates.  Permittivity and loss tangent were measured in the way considered most valid to the test lab.  The panelists will represent two material suppliers and three test labs who are active in this project.  The results will be shared along with a discussion of the test methods used.  Attendees will come away with a better understanding of what high frequency test methods are available and how their results compare over frequencies up to 110 GHz.

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