8.4 Rule 4: Reduction of the trace height d above the ground plane reduces EMI

(a) Placement of Trace A and Trace B.

(b) Radiated power. Width of Trace B: b=0.49mm.

(c) Radiated power. Width of Trace B: b=2mm.

Figure 8.1:
The trace width is 0.2mm, the trace height above the ground plane is 0.65mm. A
10mV voltage source with an impedance equal to the characteristic impedance of the trace
drives the trace which is terminated with a 10pF capacitance.

With reduction of the trace height above the PCB ground plane, the radiated emission is
reduced in the whole frequency band by the factor

(8.12)

where d_{1} denotes the initial trace height above the ground plane and d_{2} denotes the
new, reduced trace height above the ground plane. This reduction can only be realized,
when the trace width is also reduced, to conserve the characteristic impedance of the
trace Z_{w}. Figure 8.7(b) shows the emission reduction of about 12dB when
the trace width is correctly reduced. A reduction of the trace height above the ground
plane without reduction of the trace width leads to a reduction of Z_{w} through the
increased capacitance. This will increase the trace currents, which almost compensates
the emission reduction effect from the height reduction, as illustrated in
Figure 8.7(c). The reduction of the trace height above the ground plane can
be realized by using thinner dielectric layers for the PCB.
A PCB layer thickness change has has an influence on every circuit net, which is routed
on the copper layers around the changed dielectric layer. Thus, a dielectric layer
thickness change must be carried out very carefully. A correct characteristic impedance
of a trace, which matches the termination impedances is crucial for all high speed signal
traces which are not electrically short, in order to avoid reflections and ensure signal
integrity. Therefore, the design of these signal nets has to be checked and eventually
adapted at any thickness change of PCB layers. The power distribution network on the PCB
also has to be checked and eventually changed to ensure power integrity.