SMTA International Conference Proceedings


Author: Gary Brist et al.
Company: Intel Corporation
Date Published: 9/22/2002   Conference: SMTA International

Abstract: The speed of information transfer is reaching a critical point in modern electronic devices. Military electronics, computing platforms, and communication systems are only the most immediate of the many segments affected by current PCB limitations. To provide for the increasing signal frequencies at which these devices operate, compute and transmit, OEM’s must design circuitry to an unprecedented level of performance, density, and predictability.

At the same time, cost and processing restraints mandate the extended use of traditional epoxy/copper PCB constructions. If today’s materials are to be used on tomorrow’s designs, the designers need to predict the exact performance of signals by detailed experimentation.

The integrity of a high-frequency signals can be affected by several factors during printed circuit board (PCB) construction. Ideally, conductors should have low resistivity and substrates should be well insulating.

More importantly, however, the properties of the materials should be extremely consistent and predictable. An experiment was conducted to provide predictability for materials used in modern PCB manufacturing.

Variations in test vehicle construction included conductor surface finish, substrate dielectric material, and copper foil roughness. Organic solderability preservative, electroless nickel immersion gold, immersion tin and immersion silver surface finishes were studied.

Substrates under investigation included high Tg epoxy/ woven glass, modified epoxy/ woven glass, and allylated polyphenylene ether/ woven e-glass. Standard ED copper foil was compared to RTF foil for affects on signal loss. Specifically designed differential pair microstrip circuits were tested within a frequency range of 100 MHz to 16.0 GHz.

Significant loss effects were observed as a function of conductor geometry. Trace shape, controlled by imaging and etch parameters had less influence than the selection of copper foil type. Surface finish selection was more important to this study employing differential pair designs than had been observed in previous studies using singleended transmission lines. The experimentation provides for a comparative method for evaluating PCB materials.

Key words: microstrip, differential pair, signal loss, surface finish, PCB interconnect.

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