Signal components under switching node of an SMPS?

Hi,
Ive always avoided putting signal components on the bottomside of a PCB under the switching nodes of an SMPS. ...For obvious reasons, as you know it can be a disaster.
However, due to space constraints, we now need to put signal components in this place. Do you think it will be OK if we put the signal components on another PCB, 4mm below the one with the SMPS on it? (this PCB "in plane parallel" with the one with the switching nodes on
it)…….we can also add a shielding layer of ground copper between the signal components and the switching nodes of the SMPS. Do you think it will now be OK?

This is a question like "If I stand next to a loudspeaker will it be too loud?". Without knowing the nature and quantity of interference and how susceptible the other circuit is, nobody can advise you.

If the signal circuit isn't particularly sensitive and its impedances are fairly low and you are not talking about KV voltages at the switching node, there is no reason to use a second PCB at all. There are no hard rules, you have to use your judgement and experimental test results to see if interference is a problem. 4mm separation is quite a long way in electrical terms and should be more than adequate to stop capacitive coupling but depending on track layout may still pick up inductively. A screen between them might help but a cheaper solution if you must use a second board is to make it double sided with the ground plane side facing the switching node.

Brian.