Jack, You also create return paths in the "Z" direction for high speed signals by placing GND vias around the PCB. So If a high speed signal uses a via to traverse to a different layer, and you have placed a GND via adjacent to it, you make it a happy signal :-) Otherwise the loop area is increased for that trace that is trying to find its return path in the "Z" direction. Karl From: "Jack C. Olson" <[log in to unmask]> Reply-To: "(Designers Council Forum)" <[log in to unmask]>, "Jack C. Olson" <[log in to unmask]> To: [log in to unmask] Subject: Re: [DC] Walls of Air? or Pillars of Metal? Date: Thu, 16 Mar 2006 15:09:08 -0600 Greetings, Maybe the reason for the wide variety of responses I got (mostly private. Is this a touchy subject?) was that the question was too general. Let me just give you the scenario and then ask one more question, ok? This is a farily simple example: Let's say I am designing a small 6-layer board about the size of a pack of cigarettes, to be mounted in a plastic box for an automotive application. Besides the two connectors (one for input power and to send the "result" back, and the other for a bank of LEDs and switches) the board contains a small low-power switching power supply and a microprocessor. The part that seems easy to me is the request to pour copper top and bottom and stitch the perimeter, because in my overly-simplistic mind that creates a kind of "metal box" for the circuitry both for immunity and radiating reasons (If I am already making an invalid assumption let me know, ok?) but the other question is: Should I do something between the power supply and the microprocessor? I'm not really asking anyone to design the board for me; this is just an interesting subject that I don't know enough about. Jack (aka "the new guy") At 07:45 PM 3/15/2006, Jack C. Olson wrote: I have never thought about this until now, but I'm confused about something... Many times I have been asked to put a slot in planes to isolate different areas of a PCB from each other, almost as part of the floor- planning. So in other words a power supply area might be isolated from a digital processor area by putting an air gap between them, or maybe between primary and secondary of a transfomer, or maybe the 3mm clearance you put under an opto-isolator, right? on the other hand... Recently I have been asked to start building "via fences" between areas, like stitching ground planes together, and even been asked to start putting a line of vias all around the perimeter of the board, too. And flooding ALL layers with copper and adding MORE vias to stitch THEM together. In the past I have used via stitching a lot, but it was for things like guard traces along RF lines (1.67GHz cellular signals) and occasionally along the edges of shields. but not like this. so... Today a discussion came up whether to use air gaps or via stitching, and I had a total brain freeze. In my mind they both had the same reason for existence, keeping things from interfering with each other. But of course in reality they are the exact opposite of each other; one method adds conductors and the other removes them. Now I can't seem to grasp what I am missing about this. There's noise in my head. So can someone please help me understand when to choose one over the other, if at all? (and hopefully the answer will be shorter than the question?) Jack --------------------------------------------------------------------------------- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 --------------------------------------------------------------------------------- --------------------------------------------------------------------------------- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ---------------------------------------------------------------------------------