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The Need for Speed - Electronic Interconnection Files from SiliconPipe - No. 2
August 28, 2005 |Estimated reading time: 3 minutes
"I feel the need... the need for speed" That memorable line, many will recognize, is from the 1986 movie Top Gun. Today it could serve well as the mantra for electronic systems developers as well. Faster electronics have been the circuit designer's objective for many years. While the measure of a jet's speed is fundamentally its Mach number, there are a number of different speed designators, which have been applied over the years to electronics. Depending on the technology sector, the units of measure will vary. For processors, the measure is clock frequency which is measured today in gigahertz. For computing the measure is FLOPS (floating point operations per second), which for supercomputers are now measured in petaflops. For switches and routers the measure is data rate which is measured in bits-per-second (bps) and presently leading edge systems are operating in the gigabit per second (Gbps) range. While the measure may change, the fundamental issues remain much the same relative to achieving higher speed. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
There are a number of basic factors that determine speed. Perhaps the most basic and practical to address is distance. The closer electronic elements are together the faster they can communicate. <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Moore's Law, which addresses doubling of transistor density also indirectly addresses speed because on-chip signal interconnection distance is effectively halved with each doubling of density. There are a number of other factors related to materials and feature design and layout that moderate performance, however, so there is not a perfectly accurate relationship in the real world.
Another factor in determining speed in electronics is the materials used for both conductors and insulators. Electrons can, in theory, travel at the speed of light on bare copper through a vacuum. Unfortunately, for all practical reasons, electronics must be fabricated from physical materials and those materials degrade the performance of the transmitted signal to varying degrees. A vacuum, (air is roughly the same) with dielectric constant of 1, is obviously best, every thing else slows the signal. For example in FR4 the signal can propagate at only about 70% of the speed of light. That is plenty fast and was suitable for electronics for many years but it is not enough to meet current performance needs. Improved materials are required.
There are a number such materials that have been in play for many years. Fluropolymers such at DuPont Teflon(R) are prime examples. They have been used for many years in RF and microwave products and as digital signals move into this increasingly common high frequency range, such materials are being tapped. Loss tangent is also becoming increasingly important as it is a companion key factor in signal attenuation. Moisture uptake of insulating material is a common culprit so hydrophobic materials are looked to with great favor. This is because water, as a polar molecule, "flips" up and down with changing electro magnetic fields and saps signal energy as signals propagate. These signals levels can quickly attenuate to zero depending on frequency and moisture content.
The last item on this short list is the conductor. Conductor loss is also a concern, of course, but its effect is normally of a lower order in high speed signaling because unlike typical DC circuits, the currents at high frequency are very low. The conductor material is still important, however, because more resistive metals and metals which have magnetic properties are generally not suitable.
In summary, the pursuit of high speed is continuing without pause. Digital electronics are now well into the gigabit per second data rates and a new level of understanding is required of those wishing to move into this new realm. Next time we will look at some of the effects of common circuit design features and artifacts on the signal quality.
Joseph Fjelstad, a founder of Silicon Pipe Inc., is an international authority and innovator in the field of electronic interconnection and packaging technologies with more than 150 U.S. patents issued or pending. He is the author of "An Engineer's Guide to Flexible Circuits" and author, co-author or editor of several other books including the most recent Chip Scale Packaging for Modern Electronics. He has also authored numerous technical papers and articles. He frequently presents seminars on PCB, flex circuit and chip scale packaging technologies at industry conferences. You may contact him at 408-973-1744 x203, or by e-mail at JosephFjelstad@aol.com or J_Fjelstad@sipipe.com.