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I'm trying to get some analytical feeling for the use of ground pours on bottom and top layer in multilayer boards with proper stackups such as (for example).

Top

Gnd

Sig1

Power

Power

Sig2

Gnd

Bottom

Every signal net has an adjacent ground plane and the internal signal layers are shielded if the power planes are carefully laid out.

I'd like to take an example. Every now and then you see DDR routing areas poured with bottom and top ground pours on multilayer boards but the benefit seems to be marginal from an EMC perspective and maybe even degenerative with regard impedance matching for the top and bottom signals.

The pours in this situation won't add any shielding to traces bellow since there is already a ground plane just bellow. These pours also add to the asymmetry of the copper distribution which increases the odds of the board bending during heat treatments. They also make rework harder due to higher thermal conductivity.

A upside for these pours though in some cases is increased thermal conductivity around dissipative electronics.

So overall is there any analytical evidence for pouring on top/bottom layers in multilayered boards with proper ground planes for other reasons then distributing heat from power hungry packages ?

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1 Answer

up vote 2 down vote accepted

In high frequency microstrip components, the "cavity" (that is, the area to the side of and above the trace) can lead to a lot of radiation in certain circumstances. The ground to the side of the trace can act as a "wall" and help mitigate the radiation (especially in situations where you may want to solder a cover over a sensitive area of the circuit. You can pour ground on the top/bottom with provisions for later soldering on a shield.)

As for the impedance matching, it shouldn't really impact the signal circuit if the pour is more that 2 to 3 times the tracewidth away from the signal trace.

And, a bit of an anecdote, but I've found that the top/bottom pour helps make the copper distribution a bit more symmetrical about the middle layer which helps with flexing over a large operating temperature range (even though the risk of bending at assembly is increased, as you pointed out).

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