Layout guidelines for good Return Path in PCB

Introduction

In previous article we have studied about the return path in PCB, but in many conditions we can’t avoid some return path discontinuities coming from different factors but we can solve that problems with proper layout design. in this article we will learn about ideas to overcome return path discontinuities in PCB

What will affects Return path?

The common affecting factors of return path are

  1. Slots in Plane layers
  2. Split planes
  3. giving power plane as reference plane 
  4. Routing with different Reference planes
  5. plane voids
  6. Routing Signals on Plane edges

Slots in Plane Layers:

  • Slots in Reference ground and power planes cause return path problems
  • When trace crosses a slot in the adjacent power or ground plane, the return current is diverted from underneath the trace in order to go around the slot. These forms bigger loop area in return path
  • To avoid problems, make sure that no traces cross over the slots on adjacent layers
  • In unavoidable situation, place few small stitching capacitors across the slot, one on either side of trace. This will provide high frequency continuity across the slot for return current, and minimize the problems created by a separated return path
  • See left side of below figure 1 image shows diverted return path and makes bigger loop. This will resolve by placing stitching capacitor in right side of below image

Figure 1 : Plane discontinuities

Power plane as reference plane:

  • When a signal uses power plane as reference, the return signal needs to travel over the plane. In both source and sink signals are reference to ground.to change the reference to power plane stitching capacitors are needed
  • If both sink and source uses same power rail for their supply, decoupling capacitors can act as stitching capacitors if they placed close to signal entry and exit point

Figure 2 : changing Reference plane to power

Split Planes in reference layers:

  • When a trace crosses a split in the adjacent plane the return current path interrupted
  • Left side of FIG 3 image shows the return current interrupted due to split planes in reference layer
  • In this case the return current tries to find another way to get across the split. This makes bigger loop area in return current

Figure 3 : multiple plane shapes in reference layer

  • For example, take a 4-layer stack up shown in FIG 4

Figure 4 : Return current path in 4 layer PCB Stackup

  • In above image the return current will divert to the nearest decoupling capacitor in order to cross over to the ground plane then on the other side of the split current must find another decoupling capacitor in order to return to the reference adjacent power plane
  • To resolve this return path discontinuity do not route over split planes, if unavoidable situation place a stitching capacitor between two reference planes. (see right side of FIG 3 image)
  • This capacitor allows the return current to travel from one reference plane to another plane

Routing with Different Reference planes:

  • The routing of signal changes from one layer to another layer on printed circuit boards with different reference layers, the return path will interrupt
  • So, to avoid this return path interruption, the return current must also need to change reference planes
  •  This is happening into two cases, they are
    • Two reference planes are different type
    • Two reference planes are same type

Two reference planes are different type:

  • If signal trace switches from one layer to another layer which has different reference layer (see fig 5)
  • Here the return current cannot jump one reference layer to another different reference layer (see fig 6)
  • In this case the return current search for nearest decoupling capacitor in order to change reference layer
  • This obviously increases the loop area and make many problems
  • To avoid this problem, always try to route with one reference layer, avoid switching reference planes

Figure 5 : Routing with two different type reference layers

Figure 6 : Return path interruption

  • If unavoidable situation of switching reference planes, place additional decoupling capacitor near to the signal via to avoid return path discontinuities
  • This stitching capacitor placement should be symmetrical for
    differential signals. (see fig 7 and fig 8)

Figure 7 : solved interruption with stitching capacitor

Figure 8 : Return current flow through stitching capacitor

Two reference planes are Same type:

  • If signal trace switches from one layer to another layer which has Same reference layer. (See fig 9)
  • Here the return current cannot jump one reference layer to another different reference layer (see fig 10). This causes problems
  • To avoid this problem, always try to route with one reference layer, avoid switching reference planes

Figure 9 : Routing with two same type reference layers

Figure 10 : Return path interruption

  • If unavoidable situation of switching reference planes, place via (ground to ground via or power to power via) near to the signal via to avoid return path discontinuities. (see fig 11 and fig 12)
  • This stitching via placement should be symmetrical for differential signals

Figure 11 : solved interruption with stitching vias

Figure 12 : Return current flow through stitching Vias

Plane Voids:

  • Placing Vias together closely in reference planes creates voids
  •  Aware of such voids while routing high speed signals will not disturb the return path.
  • These voids create return path discontinuities (see left side of below image)

Figure 13 : Plane voids interrupting Return current

Routing on Reference Plane Edges:

  • Do not route high speed signals on the edge of the adjacent reference layer
  •  This will affect the impedance of trace

Figure 14 : Routing on Plane Edges

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