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Combining two similar footprints - absolute no, or might work - tips to improve odds?

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#71948

Combining two similar footprints - absolute no, or might work - tips to improve odds? | 14 April, 2014

I've got a small smt board I've already got working prototypes of, which uses a tactile switch. I've got another much less expensive tact switch of similar size that I had avoided due to a difference in actuators I originally had no way around. Now I think I've got a way around that issue, and would like to design the board so it could be used with either switch if possible.

Both switches have four leads. There is overlap between the manufacturer-recommended pad layout for one switch and the manufacturer-recommended layout for the other switch, but it isn't exact. (One has the leads slightly further apart and a bit differently sized.) Also, one manufacturer recommends rectangular pads, the other recommends a squat T shaped pad. I've attached an image that shows the two separate footprints, and also shows them overlaid.

I've designed a number of prototype boards in the past, but am new to design for automated SMT production. So I thought I'd get some advice here about the advisability / likelihood of success of combining the two footprints. My thought was to make a composite pad the shape of the two pads overlaid, but have separate stencils, applying solder paste only for the particular switch being used in that run. We are likely to be having them assembled in runs of 1k-2k boards. The boards are double sided with SMT components on both side. The switch and a couple of 8-pin SOICs are the largest components.

This is for a consumer application in a very benign environment, so I'm not too worried about part of the pad (corresponding to the switch not being used) being exposed without soldermask.

Thoughts?

Attachments:

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#71953

Combining two similar footprints - absolute no, or might work - tips to improve odds? | 15 April, 2014

This does not look difficult. It appears the rectangle of proper size and spacing would work for both, or even a square pad. Don't make it more difficult than it should be. But then I could be missing something?

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#71954

Combining two similar footprints - absolute no, or might work - tips to improve odds? | 15 April, 2014

Thanks Bob. I just didn't have enough experience with production SMT (vs hand-soldered prototypes) to know how finicky some things like this are. I've tried to do a lot of reading, but I haven't seen much that talks about things like this. I also haven't heard back from the manufacturer who recommends the T-shaped pad about why they suggest that, and what paste shape they recommend. I suspect it might be an attempt to get more precise centering of the component via surface tension, in which case I may keep the little extra-width section at its place in the composite pad rather than going with a straight rectangle, especially since the other pad for the other switch has a long toe.

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#71956

Combining two similar footprints - absolute no, or might work - tips to improve odds? | 15 April, 2014

Rather than using an over-lay of both pads, I would just stay with a rectangle pad shape, a little wider to accommodate both footprints. Keep it simple.

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