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Pad Width Calculation for Flat, No Lead Terminals

Printed From: PCB Libraries Forum
Category: Libraries
Forum Name: Footprints / Land Patterns
Forum Description: [General or a CAD specific issues / discussions]
Printed Date: 29 May 2024 at 5:30am

Topic: Pad Width Calculation for Flat, No Lead Terminals
Posted By: ArtCym
Subject: Pad Width Calculation for Flat, No Lead Terminals
Date Posted: 27 May 2022 at 12:42am

I'm working on a footprint for a QFN and I have a question regarding the calculation of pad size. 

In Footprint Expert I see there is -0.04mm used for calculating the pad width for flat, no lead terminals (terminal type used in QFNs, right?).

Why is a negative number used for calculating the pad width?

Posted By: Tom H
Date Posted: 27 May 2022 at 6:46am
Many years ago, IPC introduced Fabrication and Assembly tolerances which added 0.05 mm to the pad size length and width. 

Then IPC created solder joint goal tables and discovered that the F & P tolerances added too much copper to the pad size for the "Side" goals. 

The Side Goals affect the pad to pad spacing in SOP, QFP, QFN, SON component families. The minimum space between pads is set to 0.15 mm. 

In order to control the excess copper pad side goal, IPC in their infinite wisdom compensated for the F & P tolerances by adding negative solder joint goals. 

All the solder joint values in Footprint Expert are user definable (editable). If you don't like a value, then change it. 

The unreleased IPC-7351C removes the F & P tolerances (but you can change the F & P tolerance to 0.00 in Options) and manually change all negative solder joint values to 0.00. 

There is an Option .opt file on your computer that you can load that does all off this for you automatically. Just open it and save it under a different name to edit it. Otherwise, IPC-7351C.opt will be overwritten with every new installation. 

The Fabrication and Assembly tolerances used in Pad Stack calculations for many years is obsolete and needs to be turned off. 

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Posted By: sot23
Date Posted: 19 Jul 2022 at 6:52am

I am a relatively young PCB designer. To design footprint, I use PADS LP Calculator (my company is switching to PCB Footprint Expert). LP Calculator add -0.05mm to the Side value of the padstacks, making them smaller than the max tolerance value for the pad size when I am designing IPC C (Least) footprints.

I have found the "side_goal" parameter in the preference file. Now I am wondering : what should I replace the negative values for ? Do I just put them to 0 ? Is it ok to have a pad that is the same width as the pin on a IPC C footprint ?
Sorry if the question seems silly, but wondering around the internet I read so much different stuff about footprint design that it gets confusing. Although the resources I have found here seems to be the most coherent and reliable.

Posted By: Tom H
Date Posted: 19 Jul 2022 at 8:54am
We created the PADS LP Calculator and sold it to Valor and Mentor acquisitioned Valor in 2010. 

We sold the PADS LP Calculator because it followed IPC-7351B and we noticed many flaws in that guideline. 

The PADS LP Calculator was the first software program we created using Visual Basic, and most programmers look at the code and call it "Spaghetti Code". That made the program impossible to update, so Mentor never updated it for the past 12 years. 

Then we created PCB Library Expert in 2012 from scratch using Visual Basic. 100% rewrite and it was much better than PADS LP Calculator. 

Then in 2018 we decided to completely rewrite Library Expert using C# and we discovered how to really create a great library tool called Footprint Expert. 

I've been on the IPC Land Pattern Committee since 1999 and was there when they introduced a 3-density land pattern solution. However, they used information based on 1980's technology. 

We eventually had to develop Footprint Expert using IEC 61188-7 Zero Component Orientation, IPC-J-STD-001 for Solder Joint Goals and IPC-7351 for the mathematical model for pad size calculation. 

Contact me if you want an in-depth webcast training on Footprint Expert and I will answer all your questions. 

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Posted By: Tom H
Date Posted: 19 Jul 2022 at 9:18am
In the IPC-7351B they include Fabrication and Assembly Tolerances in the mathematical model. 

That adds at least 0.05 mm to the pad width. The negative side goals are to compensate for the manufacturing tolerances. 

In Footprint Expert there is a .opt Option file called IPC-7352 which turns off the manufacturing tolerances and converts all negative solder joint goals to 0.00. 

Try it, you'll like it. 

Note: IPC-7352 is the replacement for IPC-7351B. It's currently being voted on by the IPC land pattern committee. 

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Posted By: sot23
Date Posted: 21 Jul 2022 at 11:39pm
Thank you for the historical background behind LP Calculator ! Here in France, this software is used A LOT. Every PCB designer I know use or has used the software. So thank you for creating it ! 

My company is in the process of installing Footprint Expert and I should be able to use it in a few weeks. I'll be sure to contact you at that moment.

In the meantime I will just remove that negative side goal from the user preference file. 

Posted By: Tom H
Date Posted: 23 Jul 2022 at 4:29pm
V2022.10 will be released by Monday 7/25/22. 

All known bugs fixed. 

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