I have a mezzanine card, 60 by 40 mm, mounted near one end of a main PCB, about 160 by 40 mm. This card is a known source of EMI problems and, in fact, has a metal shield over it. But, I suspect that it is injecting EMI into the main PCB and it is radiating from there.
There is a single power/signal connector from the mezzanine card to the main PCB with two ground pins. These are connected to the main card ground plane via thermal reliefs. The card also has four mounting holes, plated through and continuous with its ground plane. These are used to mount the metal shield using metal hardware, but they are at present electrically isolated from the main PCB ground plane.
My question is: is it good that the main PCB ground plane connects to the mezzanine card only via the two pins in the connector or would it be better to mount the card using metal pillars and have the mounting holes on the main PCB plated through and continuous with the main PCB's ground plane?
There is a second connector between the boards, but it is just to route a signal from the mezzanine card off the board and there are no electrical connections from that connector to ground.
Troublesome emissions are in the range 25 to 300 MHz, some higher.