If possible, for high speed traces don’t use vias. Vias appear as a small series inductor and shunt capacitor and they create a discontinuity in the line which causes reflections. Depending on the frequency and the electrical distance between the discontinuity and the driver or between two or more vias (other discontinuities) on the same line greater reflections and distortions of the signal will result.

For example, at 500 MHz, the wavelength of the signal is approximately 1 foot in FR4 or a signal travels 12” in 2ns. The concern is about reflections. If the distance to the discontinuity or between discontinuities exceeds approximately a quarter of the wavelength, reflections will be generated that distort the signal. A stub at a quarter of the wave length, around 3”, for 500MHz will cause problems. Depending if the stub is open or shorted, the signal propagates down the stub, and reflects from the discontinuity (stub) at the other end. It then comes back to the line and propagates down the main line in both directions, toward the driver and toward the load.

Depending on the length of the stub, the reflection can come back to become a double pulse, causing two edges at the receiver end. On the source end, if a driver is sending the signal across, the second reflected pulse will add to the voltage on the driver so that the next pulse that propagates down the line will have a higher amplitude, which will follow the previous signal and could double clock the receiver. The end of the stub (open circuit or terminated with a matched/unmatched resistor) and the length of the stub will determine the type of interference of the reflected signal with the original signal.

Take for example, a 500 MHz signal, where one period has a 2 ns pulse width. Forgetting the frequency components, if the stub flight time (the time it takes the signal to travel down the stub, reflect and come back to the original trace) is half a nanosecond, then another pulse hits the receiver half a nanosecond behind the real pulse. The second pulse is latched in and a setup and hold violation is created. The receiver may not clock it or may double clock. The amplitude decreases roughly by 50%.

Nuvation finished an optical interface with 10GHz differential signal traveling on a pair of traces. The traces had to go between 2 layers in a 26 layer board, with a thickness of 110 mils. To minimize the discontinuity, we back drilled the vias from the other side of the PCB and provided ground vias close to the signal via to create a vertical transmission line. The back drilling eliminated the copper from the via beyond the connection point to prevent a stub from forming. Back drilling ads expense to the board but significantly reduces the discontinuity and dramatically improves the signal integrity. Alternatively, a blind via could have been used but this ads even more expense to a board and has some severe limitations on the maximum length of the via (blind vias are vias that pass only through part of the board and connects an outer layer to an inner layer)

A design that uses differential signals may require vias to allow the signals to cross each other or to jump layers. However, to ensure that both the true and complement signals experience the same discontinuity, it is a good idea to have the same number of vias on each member of the pair and have each via in approximately the same position on the trace.

If vias are designed to look like a transmission line and have the same impedance as the traces they are connecting, then the only problems left are at the connection points between the via and the trace. These take the form of a right angle bend into the board and the transition usually creates a discontinuity.

In general abrupt right-angle bends in high speed traces should be avoided. At the bend, the effective transmission line width increases, that results in a variation of impedance at the bend from the remaining trace impedance. The larger trace area increases the capacitive component of the line and effectively decreases the impedance. The impedance of a LC ladder network is Zo= , and for a lossless transmission line Zo= where ΔL and ΔC represent the incremental capacitance and inductance per unit length of the line, when the trace widens going around an abrupt right angle bend the incremental capacitance increases while the incremental inductance remains constant. For frequencies exceeding a 100MHz it is strongly recommended to use curved or continuous width bends instead of sharp 90° bends. Mitered 45° bends reduce reflections on the signal by minimizing impedance discontinuities while continuous curved bends eliminates them.