Even though a via may be less than 1 percent the total length of a net, it can absolutely swamp the performance, even in a very lossy interconnect. It’s all due to the quarter wave resonator created by the dangling, unused stub. If a signal transitions not from the top of the board to the bottom, but from one layer to another, the rest of the through hole via acts as a resonating stub. The longer the stub length, the lower the maximum bit rate the interconnect can support.
The most common solution is to back drill the residual stub from the board. Sanmina has introduced automated back drilling capability for both sides of the board, with up to 12 different controlled depths. More than 1000 vias are routinely back drilled in a single backplane. Backdrilling in backplanes is pretty straightforward due to the larger via geometries.
Figure 1. Back drilled via stub in a 26 layer board, courtesy of Sanmina-SCI
The game changes when backdrilling is required on high speed line cards. The challenge is when there are 1500 vias under a single BGA on 1mm( 40 mil) or less centers, and many of the 6 mil diameter vias need to be back drilled to controlled depth. Dudnikov’s team has introduced an alternative option that addresses the stub resonance by reducing the reflections from the end of the stub. The Matched Termination Stub (MTS-via) uses a discrete SMT mounted or preferably embedded screen printed resistor between the two differential vias or between the signal via and return to terminate some or all of the incident wave at the end of the via stub.
Figure 2. Screen printed termination resistor at the bottom of a through hole via stub, courtesy of Sanmina-SCI
Of course this loads the line and reduces the signal left for the receiver, but at some data rates the terminated via enables more open eyes with less jitter than the unterminated via stub. An undistorted signal with reduced amplitude is a lot easier to recover than a full strength distorted one. Standard screen printing technology can easily meet the shrinking feature sizes and higher densities of vias used in BGAs and connectors, so the technology may even afford a cost savings over backdrilling in these cases.