Screaming Circuits: Footprint Dimension Mismatch


Footprint Dimension Mismatch

Here's another simple case of part to pad dimensional mismatch. The part is an asynchronous SRAM, but that's not important. What's important is this is another case of "close, but no cigar."


The part has an 0.8mm center to center lead spacing. That's a little more than ten pie mils, or 31.496063 mils. That would be okay, except that the footprint used has 30 mil center to center spacing. That might work on a smaller part, like a 16 lead jobby, but for a 44 pin component, it's just not a good fit. Over the run of the full length of the side of the part, you end up with 22 mils misalignment. That's almost a whole lead space off.

In this case, we made it work, but - and you've heard this from me before - in a production environment, this would be bad. All of the tolerance is used up in the misalignment so there really isn't room for normal manufacturing gives and takes. You must find or make a part library component that exactly matches the part.

Duane Benson
The nearer your dimension, the more you're Slip slidin' away


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One thing that always makes me wonder is that, despite the footprint mismatch, does this still fall within IPC Class II standards? As far as I know, that's what you guys assemble to, and I heard somewhere that as long as 50% of the lead is aligned over the pad it falls within Class II spec. Is that true?

- Jered

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