Authored By:
Russell Dudek
Compunetics, Inc.,
John Kuhn and Patricia Goldman
Dielectric Solutions, LLC.
Pittsburgh, PA
Summary
The signal channels that link high speed processors to memory and various other peripherals, are limited by the inherent characteristics of the printed circuit board. These are what ultimately connect information to the outside world. One limiting factor is the effect of non-uniformity of the glass fiber distribution in the printed circuit substrate material, also known as fiber weave effect (FWE). FWE introduces signal skew and timing errors which place an upper limit on bit rate and trace length.
Using unique fabrication techniques and a proprietary low dielectric constant glass composition, a revolutionary glass fabric is presented that is essentially free of fiber weave effect while demonstrating inherently improved resistance to conductive anodic filament (CAF) formation. Improved laminate performance is demonstrated with finite element modeling and HyperLynx simulations, and corroborated with dielectric property measurements on prototype substrates.
A printed circuit board using this material demonstrates superior signal integrity performance over the traditional glass-based solution. By uniformly distributing glass fibers the maximum surface area becomes available to bond with the resin, which is enhanced by direct application of a finish to provide a high quality interface between glass and resin. Two high profile performance issues, fiber weave effect and CAF, are addressed by a unique laminate reinforcement
Conclusions
The question has been asked for the industry to provide a solution to Fiber Weave Effect. Many solutions have been proposed to mitigate FWE.3,7 However, we propose that to eliminate FWE it must be addressed at the earliest stages of manufacture, at glass fabric manufacture. By solving the two contributing factors to FWE, ADk and homogeneous fiber distribution, this new technology is the complete and unique solution to FWE.
Based on finite element modeling, HyperLynx simulations and subsequent verification by measurement, a new level of high-speed design is anticipated using existing FR-4 processable resin systems. Bogatin8 alluded to increasing difficulties meeting tighter skew specifications in systems over 3 Gbps, due to glass weave / signal line interactions. The use of new spread fiber glass fabrics with low loss resins have demonstrated the capability to reduce skew by nearly an order of magnitude and permit designs up to 10 Gbps and beyond.
The proposed solution of a spread fiber glass fabric using direct finish technology has also demonstrated significant performance advantages beyond elimination of FWE. It appears that, with spread fiber technology, elimination of FWE and CAF resistance are integrally connected. The use of advanced composite technology offers an improved resin-glass interface and demonstrates superior CAF resistance. Two performance issues are addressed with one unique product.
Test results to date have verified the modeling and prediction of FWE elimination, as well as material characteristics such as CAF resistance, laser drilling, and other requisites of a performance material solution. Further work is in progress to empirically define performance capability, particularly in the area beyond 10 Gbps.
Initially Published in the IPC Proceedings
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