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Question about pad pullbacks

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#46233

Question about pad pullbacks | 18 December, 2006

I am always looking for ways to make my boards more reliable and easier to fabricate and assemble.

I have been baselining my footprints off of IPC-7351 and with the latest revision of the calculator they have started a new feature.

Normally on say a plastic SOIC the old calculator and most footprints I have seen will run the pad up underneath the plastic body of the part. Most board layout tools will just copy that pad onto the paste screen. This means when the paste is put down the plastic part of the body will actually be sitting in the paste if the component height is not enough.

The new calculator takes this into account and asks for the component body height. If the component body height is below say 5 mils (this can be set by the user) then it will trim the pads from underneath the body. To compensate it will make the toe fillet bigger and square up the pad for more paste.

I have not had a chance to run any of these footprints but was wondering. Does this seem like a non issue? Does this seem like a good idea?

Thanks

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#46240

Question about pad pullbacks | 18 December, 2006

Hi John,

Depending on your board, it could make a difference. Copper finishes might leave that part of the pad exposed, which is a no-no in most places. Where as a HASL board prolly won't care if you pulled the print back. On the other end, over printing the toe end may casue solder shorts if you doen't have resist between pads.

Depending on your paste it also could make a difference. Some SOICs may cause your paste to ball. Over printing the toe could also cause solder fines or balls.

IN running no-lead DOE on solder paste, this was one thing we did look at. It was found that touching the body of an SOIC or QFP did not cause any affect.

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