Design007 Magazine


Issue link:

Contents of this Issue


Page 35 of 111

36 DESIGN007 MAGAZINE I NOVEMBER 2020 topic of vias now, which is where we wanted to be all along. There was a time early in my career as a PCB designer, where we didn't give much thought to the vias that we used. The signal speed of the designs we were working with was very slow compared to today's standards, and ex- cept for fabrication costs, it didn't really seem to matter how many holes got punched on the board. But as you know, that level of design technology is a long way behind us now. Vias are now much more than just a hole in the board to connect a signal from one layer to an- other; they are an integral part of the overall signal integrity of the design. Take high-speed transmission lines, for ex- ample. For the best signal integrity, the traces of a transmission line should be contained on a layer that is sandwiched between two ground plane layers in a stripline configuration. This gives the best impedance control and shielding for the signal, plus the signal return path on the adjacent reference plane will be coupled directly to the signal. The problem, of course, is that you can rare- ly contain a trace on just one internal layer. Even if the routing can be done on that one layer, you still need to transition to a surface layer to connect to the components through a via. Back in the day, we would have sim- ply dropped a through-hole via and been done with it, but the high-speed routing require- ments of today's designs need a via solution with a little more finesse. A through-hole via can act as an antenna and radiate energy, so we've come up with some ways to guard against that. Back-drilling the via is one of those ways while using blind vias is another. For high-density interconnect routing on boards with large pin-count ICs, microvias are often the best option. All of this is done to keep the signal path as short as pos- sible through the via, with the added benefit of opening up some more routing channels where the through-hole via used to be. But using vias on a high-speed transmission line opens up yet another can of worms that, inter- estingly enough, requires the use of even more vias to resolve. A signal needs a return path, and this is usu- ally accomplished through a ground plane. For a high-speed transmission line, however, the return path becomes even more important and must be clearly defined in order to avoid problems that can disrupt the function of the circuit. The reference planes in a design are looked at very carefully now to make sure that there aren't any plane splits, slots, and oth- er congested areas that can clog up the signal return paths. Without a clear return path for the signal, the circuit may develop unwanted noise resulting in crosstalk, interference, and even false triggering of the signal. Therefore, to maintain the best signal integri- ty, the transmission line needs to be directly ad- jacent to the ground plane that it is using for its signal return path, and the plane must supply a clear path. When the signal transitions to another layer through a via, however, the return path will be severed unless there are ground stitching vias nearby for the return path to hitch a ride on. These ground vias provide the means for the signal return to continue its path back to the source in what is known as layer paired routing. There's still more to how we choose which vias to use and where they should be placed, as well as the need for transmission line sig- nal integrity. For instance, power circuits will need a larger via to handle the current that they are carrying. Without an adequate size, things can get a little toasty for the smaller vias, which is why power pins are usually rout- ed with wider lines and larger diameter vias. Vias are now much more than just a hole in the board to connect a signal from one layer to another; they are an integral part of the overall signal integrity of the design.

Articles in this issue

Archives of this issue

view archives of Design007 Magazine - Design007-Nov2020