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Perforated Tabs

Mike Demos

#3770

Perforated Tabs | 19 June, 2000

I'm looking for some design paramters for perforated tabs. These are the tabs that connect the circuit board to the break-away pannel material. I've never used these before, but I've seen some that have reportedly worked well. These tabs are idented into the perimeter of the board so that when the board is broken-out, there is no ragged-material along the edge of the board.

Does anyone on this forum have some design suggestions I can pass on to our designers? Perforation pitch, hole size, tab size, etc?

Thanks!

Mike Demos.

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#3771

Re: Perforated Tabs | 19 June, 2000

Mike: Look a the Hadco DFM manual ( http://www.hadco.com/ )

Yano if you�re lookin� for uniformly smooth (not silky smooth) board edges, have you considered scoring to separate your boards? Scoring has: * Minuses (straight lines only, stress on solder joints close to score, additional operation at PCB fabrication, and edge of PCB is not smooth) * Pluses: (leaves sorta smooth surface, increase panel use, ease of separation, and better in assembly).

Look at scoring in the Hadco DFM manual also.

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#3772

Re: Perforated Tabs | 20 June, 2000

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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#3773

Re: Perforated Tabs | 20 June, 2000

Chrys you're fine.

On scoring: * Be sure spec material remaining, not what is removed. * Be sure that the v-grooves are perpendicular to each other, not off set. * Typ spec for for .062 thick boards FR-4 0.018-0.024 MC-3 0.030-0.040 * HADCO recommends a remaining material thickness of 0.012" (+/-.002") for .062" thick boards. The tolerance precludes scoring simultaneously with two blades. * Thirty degree cutters are standard and remove less material than a wider blade. * Establish a minimum keep-out zone of 0.025" adjacent to score line for copper.

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Mike Demos

#3774

Re: Perforated Tabs | 26 June, 2000

Chrys & Dave:

Thanks for your replies. Scoring won't work in this case. There are some overhanging components and some 90 degree routes as well.

My experience with scoring has been similar to Chrys'. For large panels I also had some issues with planarity of the board after double pass reflow. This resulted in inconsistent conformal coating coverage in a subsequent operation.

Thanks, Mike.

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