Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

Land size

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Land size | 7 July, 2008

Hi All,

Can anyone advise the spec for land size of a lead 0.2mm in width? I am placing a TQFP128 0.4mm pitch, the leads seem to be wider than the actual pads they are been placed on?

I am also experiencing poor wetting on this part , it has NiPdAU finish on the leads and I am usig SAC305 - profile seems fine but it is impossible to get a good toe fillet on the joint.

Any ideas , please share.


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Land size | 7 July, 2008

Most QFP 128s have a 0.16mm wide lead. Might have to modify your pads if you'r buying some odd ball part. But generally 0.2mm should be OK for this part.

You may never get good toe wetting. Most QFPs are cut apart after plating, thus the toes are not plated to accept solder. That's why IPC has you inspect the heel fillet for such a part.

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Land size | 9 July, 2008

I have carried out some further measurements on the device and the results are as follow:

Component Lead width measures 183 microns.

Actuals PAD width measures 158 microns.

What I am thinking is that the lead is attracting the solder hence preventing the solder from wetting out properly.

Also, is there a a standard for Pad to lead widths . ie. for an 180 micron lead the pad width should be 200 micron.

any replies greatly appreciated


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Land size | 10 July, 2008

Try IPC SM 782. Also most parts manufacturers should be able to provide you a pad design.

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Land size | 22 July, 2008

It's been my experience that most flat pack land patterns have far too much exposed toe. It accomplishes nothing for the reliability of the solderjoint. And often the ends of the leads are sheared and not very solderable anyway. Anything more than a few mils over the max width is wasted real estate. 10-15 mils of toe is the result of committeeitis piling on every conveivable worst case tolerance. Consider that if the part shifted that far, the leads on the adjacent side would be misaligned out of spec.

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Land size | 23 July, 2008

Hi all,

Thanks for all the replies.

I got some XRF carried out on the boards ( ENIG) and they measure 0.03-0.04 microns of gold thickness.

Boards that we have had no problems with measure 0.07-0.1microns.

This looks like the main contributor along with the fact that the footprint is not to spec either.


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