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PQFN Pad Size

Printed From: PCB Libraries Forum
Category: PCB Footprint Expert
Forum Name: Questions & Answers
Forum Description: issues and technical support
URL: https://www.PCBLibraries.com/forum/forum_posts.asp?TID=2253
Printed Date: 26 Nov 2024 at 6:47pm


Topic: PQFN Pad Size
Posted By: ransonjd
Subject: PQFN Pad Size
Date Posted: 20 Nov 2017 at 2:04pm
I reported this via email when the website was down, but since the website is back up I figured I should post this for reference.

I created a footprint for a PQFN. The terminals on the part are .25mm x .475mm, +/-.05mm. When I generate a nominal footprint, I get pads that are .35mm wide. By my reading of IPC-7351, I expected pads that are .3mm wide. Is this a bug?

--John


Replies:
Posted By: Tom H
Date Posted: 20 Nov 2017 at 2:15pm
Library Expert now follows rules provided in IPC-J-STD-001 for solder joint acceptability. 

IPC-7351 which was created in 2003 - 2004 is 14 years old and many things have improved in the electronics industry and IPC-7351 has not kept up with the latest technology. 

IPC-7351 has been downgraded from a Land Pattern Standard to a Guideline. 

All the Toe, Heel and Side solder joint goals are 100% user editable in Library Expert "Preferences" to allow the end user to achieve the results that are best for them. If you don't like the solder joint values, change them. Do not rely 100% on IPC-7351 as you will be disappointed if you do. 

Select the IPC tab on this page -  http://www.pcblibraries.com/Products/FPX/Librarian.asp " rel="nofollow - http://www.pcblibraries.com/Products/FPX/Librarian.asp  ;



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Posted By: ransonjd
Date Posted: 20 Nov 2017 at 2:45pm
I admit that this comes as a bit of a surprise for me. I thought that PCB Libraries defaulted to the guidelines of the unpublished IPC-7351C, especially given the solder joint goal tables shipped with it.

However, I still don't see why I'm getting a .35mm wide pad. The terminal on the part is .2mm to .3mm, and in the software, I see "Periphery" set to 0mm.

--John

Posted By: Tom H
Date Posted: 20 Nov 2017 at 3:14pm
The Periphery is zero "0" but there are Fabrication and Assembly Tolerances that are still being applied to the mathematical model. Turn them off and you'll get better results. In 2017, there is absolutely no need for these tolerances as fabrication and assembly manufacturing is very accurate today. 

IPC has told PCB Libraries on numerous occasions to stop promoting IPC-7351C because it does not exist (yet) and may be years before it's released and we should discontinue the entire discussion. So we are abiding by their request. 

PCB Libraries, Inc. cannot wait for the IPC-7351C Land Pattern Committee to make up their minds on what they want to do or not do. We offer guidance to the 7351C committee that is mostly provided by our customers. 

Now that the 7351 creator (Dieter Bergman) has passed away 3 years ago and the vice chairman Gary Ferrari has been MIA for the past 3 years, the Land Pattern committee seems slightly lost in their mission to keep the 7351 standard current and up-to-date. 



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Posted By: ransonjd
Date Posted: 20 Nov 2017 at 3:46pm
Hi Tom,

Thanks. Exactly the info I needed!

--John


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