This discussion of board design is interesting, especially regarding the pros & cons re creating islands. I've done it but have rarely had the opportunity to compare one technique against another, at least not conclusively. Wouldn't it be great if we, as EMC engineers, had unlimited time & money to actually prove ideas right or wrong!
One factor has to be made clear when you write about islands - that is, if the ground plane is also being broken. In my experience, that's a major step which can change things drastically - for better or for worse. Creating an island in the VCC plane (eg, coupled with a ferrite) is common and beneficial if done properly (for reasons I think Tony already stated), but I *rarely* cut the ground plane, at least not on purely logic boards. In fact, I've learned the hard way to look carefully for unintentional slotting of ground planes; eg, those slots caused by a line of through-hole pins or vias with overlapping clearance holes (easy to miss). That can be bad news when there's etch passing over that slot. On the other-hand, I've seen cases where intentional slots, intelligently placed, solved problems. The bottom line is that there are a LOT of factors to consider and many are conflicting. You can focus too much on one & create even worse problems elsewhere. Creating islands CAN create problems ( as someone said) as well as solve them so you have to make a judgement call & take your best shot. That's why we get paid so much money! :-) All this goes double for the question of grounding PWB's to the chassis or frame. There's a "gotcha" at every corner. Jack Cook Xerox Corp email: [email protected]
