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:
- First, our old friend IPC-7351A does specify a keep-out area 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.
- 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.
- 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 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:
- Is there enough copper to give a good electrical and mechanical connection to the PCB?
- 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.
No worries. The green patches will burn off in the reflow oven.