Carrier Board Design Guide for SOM-9X35 Module
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2.2.6 Trace Stubs
Long stub traces can act as antennas and therefore increase problems complying with EMC standards. Stub
traces can also produce reflections which negatively impacts signal integrity. Common sources for stubs are
pull-up or pull-down resistors on high-speed signals. If such resistors are required, route the signals as a daisy
chain.
Figure 16: Avoid stub traces by daisy chain routing
Vias can also act as stubs. For example, in a 6-layer board, when a signal changes from layer 1 to 3 by using a
via, the via creates a stub which reaches layer 6. Back-drilling the vias in order to avoid such stubs is a quite
expensive technology and one which is not supported by most PCB manufacturers. The only practical solution
is to reduce the number of vias in high-speed traces.
2.2.7 Ground Planes Under Pads
The impedance of a trace depends on its width and the distance between trace and reference plane. A wide
trace has lower impedance than a thin one with the same distance. The same effect also exists for connector
and component pads. A large pad has significantly lower impedance than the trace which is connected to
the pad. This impedance discontinuity can cause reflections reduces signal integrity. Therefore, under large
connector and component pads, a plane obstruct should be placed. The area of the plane should be decided by
means of simulation. In this case, an active reference plane should be placed on another layer. This reference
plane's net should be same as the normal reference plane. This reference plane needs to be stitched with vias
to the normal reference plane.
Reference Plane
(Layer 2)
Reference Plane
(Layer 2)
Signal Trace
Signal Trace
Signal Trace
L4
L3
L2
L1
Ac�ve Reverence for
Connector Pad on Layer 1
Reference Plane for Layer 1
Signal Trace
Signal Trace
Signal Trace
Signal Trace
L4
L3
L2
L1
Ac�ve Reverence for
Connector Pad on Layer 1
Reference Plane for Layer 1
Signal Trace
Reference Plane for Layer 4
Reference Plane for Layer 4
Plane Obstruct
Figure 17: Remove ground plane under large pads