Hi Mike,
The Hadco design guidelines are really good - I was just there.
To expand on what Dave said, scoring is an option, but I've never had good luck with it, myself. The scores are either too shallow, and I can't break the boards easily enough, or they're too deep, and the boards fall apart in conveyors, reflow ovens, waves, etc. But you do get a nice edge, and sometimes, better utilization of the vendor panel.
On breakaway tabs, a few things to keep in mind when you are designing: - You can only break one axis at a time. So whether its edge rails, or extra material between boards, you can only break them in one direction at a time. Don't let the layout guys create breakaways that go around corners or in arcs. When looking at the layout, think about the order and direction you will be breaking in. - Proximity of components to breakaways. Minimum (absolute) is 20 mils, for thngs like discretes. Larger spacing for finer pitch. Routing should be at least 15 mils. When breaking, the board material has a tendency to flex and tear; you don't want to crack joints or expose copper. - Mask relief all around the B/A area. Don't need soldermask there;the tendncy is to relieve it around the holes like any other non-PTH. But, if you relieve the whole area, and you do have problems breaking, it's easier to see the fracture in question. - Spacing from break patterns to break pattern depends on board thickness. Thinner boards need more, thicker need less (duh). Also depends on weight of components and types of processing (power supplies going over the wave need more B/A's than pager cards in a reflow oven). At that point, I'd consult my board fab house for input.
Hope I didn't restate anything too obvious,
Chrys
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