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Land pattern design



Land pattern design | 30 October, 2001


Our PCB designers are beginning to send up designs to manufacturing that have increasingly difficult footprints to work with. The footprints have been generated by the IPC-SM-782 calculator but just look wrong to me. The pitch of the devices in question is 16 and 20 thou. The problem is that the pad width is way too wide and therefore the gap between the pads is tiny, so when we run through the first prototypes they are covered in S/C's. At present I'm looking at all new footprints as they are generated and using judgement/guess work to OK the new footprints. This is obviously no good long term. The problem as I see it is the manufacturers tolerances for the lead widths...If you put in a midrange value the footprint looks OK, but if the maximun lead width data is put in, the pad is massive. Surely it has to be the maximum lead width data must be put in??? (Apart from that I think the tolerance on lead width is way too big anyway, but hey, what do I know). Side fillet info in the calculator has been set to almost zero so that's not the problem either. Anybody experienced this before?

Any help much appreciated.


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Land pattern design | 30 October, 2001

Jim Blankenhorn [] has the best understanding of designing pads for real life.

That�s because he designs them like he wants to design them and doesn�t negotiate with a committee of thousands for an IPC standard. Consider doing this: * Make the pad width 50% of lead pitch for small parts. * Make the pad width 60% of lead pitch for large parts. * Do not make the pad narrower than the lead width. * Pad length: add 20 thou to the toe and 20 thou to the heel.

Get this: Blankenhorn thinks pad sizes and component spacings should be different, depending on the product density. Scary, eh?

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Land pattern design | 31 October, 2001

Hi Dave,

Thanks for the suggestions.

To clarify a couple of the points... - What do you qualify as "small" & "large" parts? - Do not make the pad narrower than the lead width. The nominal or the maximum? If I use the maximum width I'll be back to square one.

Cheers, Dougie.

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Land pattern design | 1 November, 2001

I am just starting to get into sub 50mil pitch, but from what I can see you are correct. But I can help with solder stencil. I cut the width only half of what the pad is, but the keep the length the same. This has helped but does not fix always. Still experimenting. Good luck. dave W

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Land pattern design | 1 November, 2001

"small" versus "LARGE" parts? Jeez, what do I know? We don�t have a formal definition. They�re the ones that make you say: �Now, that�s a BIG-ass part!!!�

�nominal� versus �maximum�? Yes, yer correct.

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