I once went on a blind date with a woman who, unbeknownst to me, was always late. I got to the restaurant right on time and waited for 20 minutes before I figured I had been stood up. As I was about to leave, my date arrived. If she’d been 5 minutes later we would’ve missed each other altogether. The same kind of thing can happen on high-speed PCBs when differential pairs aren’t routed correctly. One signal will arrive where it’s supposed to be, find its partner missing, then go home. On PCBs, though, a missed date doesn’t meen hurt feelings. It means the circuit will have poor signal integrity or won’t work at all. Be a good matchmaker for your high-speed signals and route them so that they both arrive on time.
This is your high-speed signal with bad routing.
Differential Routing Tips and Tricks
On later dates, I used a few tricks to try and get my date to arrive on time. The ethics of tricking my date into arriving on time are debatable, but tricking differential pair signals into punctuality will ensure signal integrity. Use these tips to make sure differential pairs are routed with timing in mind.
Trace Length Matching: Trace length matching should be a top priority when routing differential pairs. Don’t make one signal go all the way across the PCB while the other one just has to go next door. When differential pair trace lengths are mismatched the timing difference will cause destructive interference and degrade signal integrity. Similar to how my height preferences for a date may vary from yours, different circuits have different trace length mismatch tolerances. Make sure your differential pairs see eye to eye, check their mismatch tolerances before beginning design.
Parallel Routing: Whenever routing a differential pair, try your best to keep their traces parallel. Routing a differential pair in parallel helps cancel out any radiated EMI, and assists with trace length matching.
Electrical Clearance and Creepage: Like current and ex-girlfriends, separate differential pairs should always be kept as far from each other as possible. When multiple differential pairs are routed in close proximity they will always interact with each other in negative ways. Keep fights for dominance and EMI to a minimum with lots of distance.
Differential pairs will also need to be routed far from components susceptible to EMI. This distance is measured in both clearance and creepage. There are many different approaches for meeting your circuits’ clearance and creepage requirements.
Be smart, don’t route your differential pairs like this.
No Sharp Turns: It is best to route a differential pair straight, with no turns at all. However, your PCB layout may require turns. Some women like smooth men, but differential pairs always like smooth curves. Sharp edges on turns will radiate much more EMI than a smooth curve. Any changes in direction in a differential pair should not deviate more than 45 degrees. This is important at both the inside and outside edges, as both can radiate EMI.
Vias: Just like it’s not a good idea to have lots of girlfriends, it’s not a good idea to use lots of vias. Via geometry guarantees at least a small amount of signal degradation. When used too often vias can significantly degrade signal integrity and cause destructive reflections in the differential pair.
If you do end up using vias on your PCB make sure to either shorten via stub length or back drill the stubs. A via stub will act as an open-ended transmission line, which means lots of signal reflection. Depending on the length of the stub, the signal can even be reflected back into the differential pair at 180 degrees and cancel out the useful signal. The best way to reduce the negative effects of a stub is to minimize the stub’s length. Stub length can be minimized by using blind or buried vias, or by back drilling via stubs. All of those options can increase manufacturing cost, so if you’re on a tight budget, you can simply make the via connections on distant board layers. In an 8 layer board, a 1-7 connection will have a shorter unused stub than a 1-2 connection.
It is also important to match any amount of signal delay caused by a via. This can be done by either using the same number of vias in both legs of the differential pair or by adding some serpentine routing on the leg without a via. No one likes to be the third wheel on a date, so make sure everything is matched evenly.
Look at those beautiful parallels.
How to Make your Computer Do It
In order to optimize brain power, let your computer do some of the work. Your PCB design software should be able to automatically check some of these rules, such as electrical clearance. More advanced software will also help you with the actual routing of differential pairs. Altium Designer may not have solutions for your love life, but it does have features to help users with differential pair routing.
Despite our timing differences, I continued going out with my blind date. The timing issues persist, but our relationship has retained its integrity. If you have problems with differential pair routing on your PCB, your board may not be as lucky. Make sure to follow the above guidelines to preserve signal integrity on your high-speed PCB.
Want more advice on differential pair routing? Talk to an expert at Altium.