Does the copper conductor determine the maximum signal speed on a pc-board interconnect?
A discussion such as this one, which focuses on low-frequency phenomena, should consider the speed of the current flow through the bulk-copper conductor. This current flow represents the flow of electron (and “hole”) charges that occurs when you apply an electric potential to a conductor. The formula in Figure 4 shows the speed of this charge flow. At extremely low frequencies, there is time for the charge-based current to flow through the bulk of the conductor. However, the speed of approximately 6 cm/sec is wholly inadequate for a high-speed electronic system. You should now be able to appreciate why engineering textbooks call this charge flow “drift current.” So, how do the high-speed signals travel on a pc board? High-speed signals travel on a pc board as electromagnetic waves at about half the speed of light (15 cm/nsec, or 6 in./nsec) on the surface of the conductors and in the pc-board dielectric. Light, which is also an electromagnetic wave, travels in free space at 30 cm/nsec
