Screaming Circuits: PCB Edge Clearance


PCB Edge Clearance

In my continuing saga of answering the question "what are the real limits?", I'll spend a little time giving my thoughts on edge clearance. That makes the question for the day: "how close can I put parts to the edge of a PCB?" It's a good question but, regardless of what IPC says, surprisingly difficult to nail down.

The problem is that there are a number of valid answers all governed by the phrase "it depends." I'm not going to leave it at that though. I could. But I won't. Not today anyway. It really does depend on a number of factors, but I think it can be nailed down a little tighter than that. You've got to start by considering a few things:

  1. First, our old friend IPC-7351A does specify a keep-out area (AKA Courtyard Manufacturing Zone) on the part footprint. Keep stuff out of that area. This includes the board edge. Don't have any part of the component's keep-out area hanging over the edge of the PCB.
  2. Second, look at your PCB mounting arrangement. Make sure the keep-out area does not interfere with any mounting screws, and that includes any washers you may be needing. If your PCB mounts with slots or rails, make sure the keep-out area doesn't interfere with any of the rails or slots or case edges.
  3. Check with your manufacturer about their specific line limits. Many manufacturers have specific edge clearance limits based on their assembly line.

Now, take your application and consider those three items. Whichever gives you the biggest number is the Eagle_corner edge clearance you need to follow. Based on your mounting scheme, you may have different clearance requirements on different parts of the board. Still, make sure that for each section of the board, you use the biggest number from above.

Now, in the world of prototypes, things are a little different. At Screaming Circuits, we sometimes build up boards with parts right up to, and in some cases, over the pc board edge. You just have to ask yourself two questions for your prototype:

  1. Is there enough copper to give a good electrical and mechanical connection to the PCB?
  2. Am I likely to knock the part off the board with my handling?

If the answer the #1 is yes and #2 is no, then go for it. Of course, in the prototype world, you can always accept the risk yourself and go for it whatever the answers are. That's up to you.

Duane Benson
No worries. The green patches will burn off in the reflow oven.

TrackBack

TrackBack URL for this entry:
http://www.typepad.com/services/trackback/6a00d8341c008a53ef0133ed8bc360970b

Listed below are links to weblogs that reference PCB Edge Clearance:

Comments

I never thought this way, I had been using PCB for quite some time already but don't have an idea on this anyways thanks for the insight

I think connectors still reasonably fit within the three questions I outlined (and the two for prototypes). Of course, some connectors even hang over the board edge. Some manufacturers will build those without concern and some will require panel rails.

What about connectors? Obviously it is often desirable for a connector to be flush with the PCB edge to facilitate access (usually through cutouts in a panel). I suppose this is an exception?

Very interesting article, thanks for posting.

Post a comment

If you have a TypeKey or TypePad account, please Sign In.

« IPC Says What? | Main | Weekend Wondering - is anything really new? »