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Undersized Land Width from PCB Fab - Help!

Dreamsniper

#31276

Undersized Land Width from PCB Fab - Help! | 9 November, 2004

Problem: PQFP208 Pad Width specified as 11.8 thou, PCB came and they are 9.5 thou. Component Lead Width is 10 thou to 10.6 thou. Operators have difficulty aligning and inspecting.

Issue: Fed back to PCB Fab House and they said IPC accepts +/- 20% for the conductor width so pads are within tolerance. 80% of 11.8 thous is 9.44 thou...9.5 thou is within the tolerance.

Would you agree? Does pad width or land width fall under conductor width category ? Is the above argument acceptable? Help me...what do i need to tell them, how can I argue. Our designer seemed to accept the idea.

thanks,

Dreamy

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RDR

#31278

Undersized Land Width from PCB Fab - Help! | 9 November, 2004

If I remember correctly since I cannot find my copy of IPC 600, I believe that the 20% reduction applies to "defects" and not the total etch process. By the way what is being used to measure down to .0005"? For this measurement to be correct it would require laser type measurement. The requirement of 9.44 mils = .00944" you measure 9.5mils = .00950. I seriously doubt if the inspection equipment used can measure that accurately at those numbers You have a .00006" discrepency (6 100ths of a thousanth of an inch is really small). I would have them remeasured and I am sure that they can be made to either fail or pass depending on who is checking. Calipers are good to at best .001", Reticles are operator dependent, and so on. Ask your supplier about the Gage R&R, the resolution, and accuracy specs of the equipment used to measure.

Anyway, without total clarification of the IPC spec. I would worry about this supplier using all of the tolerance. I would find it hard to beleive that there is no conductor that will fall below the "9.44 mil" min spec. if they are at 9.5 right now. I would ask them what is wrong with the etch process and why it was allowed to go as long as it did. What do the .005" traces look like? are they down to .004"

Out of curiosity why is it difficult for operators to align and inspect with these pads?

My thoughts Russ

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Dreamsniper

#31279

Undersized Land Width from PCB Fab - Help! | 9 November, 2004

Thanks Russ. When solder paste is applied, you can't see the pads. Because my aperture reduction is 5% - 10% of the pads. Pads are reduced by almost and up to 20% by the PWB Fab and the paste covers the pad. We cal align and inspect but very hard as we do IPC Class 3.

regards,

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RDR

#31280

Undersized Land Width from PCB Fab - Help! | 9 November, 2004

Can class 3 be met without a side fillet?

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Dreamsniper

#31281

Undersized Land Width from PCB Fab - Help! | 9 November, 2004

IPC Class 1, 2 & 3 do not require side fillets. I think this statement is correct. But they define what is acceptable end joint widt and side over hang.

regards,

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#31282

Undersized Land Width from PCB Fab - Help! | 9 November, 2004

What is the pitch of this PQFP?

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#31284

Undersized Land Width from PCB Fab - Help! | 9 November, 2004

Your board fabricator is: * Correct that +-20% is the tolerance of the IPC specification * A moroon for not correcting the artwork to compensate for etch factors

Find a new supplier. Baring that, if the SMT pad width is critical, you can specify that the top of the pad must meet a specified width regardless of the IPC specified conductor width.

We agree, with whatzisname, the earlier poster that wondered aloud about how in the world a fab would go about measuring this pad width. At 30X or 60X, they can measure at best in 1 mil increments. They cannot accurately measure these widths without going to 100X or greater. Most shops do not have such measuring equipment available for routine measurements, if at all.

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#31285

Undersized Land Width from PCB Fab - Help! | 9 November, 2004

A PQFP208 is 20 thou [0.5 mm] pitch.

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