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BGA Footprint

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=2430
Printed Date: 22 Nov 2024 at 6:59pm


Topic: BGA Footprint
Posted By: gcary
Subject: BGA Footprint
Date Posted: 20 Jan 2019 at 11:45am
I need to make a footprint for a BGA, and I'd like to understand why there is a difference between the Library Expert output and the manufacturer's recommended pattern.  Here is the datasheet for the BGA:

https://www.nxp.com/docs/en/package-information/SOT1968-1.pdf" rel="nofollow - https://www.nxp.com/docs/en/package-information/SOT1968-1.pdf

I am attaching the fpx file of the component I created, so you can confirm I entered the numbers properly.   uploads/30/NXP_SOT1968-1.fpx" rel="nofollow - uploads/30/NXP_SOT1968-1.fpx

I would like to understand the geometric dimensioning and tolerancing notation that NXP used in the datasheet.  Did I enter the proper values for the D & E numbers?  I used plus and minus 0.15/2 as the range (rounded up to +/- 0.08).

The ball dimensions range from 0.38 to 0.48 mm, yielding a nominal diameter of 0.43mm.

Page 5 shows a recommended pad size of 0.34mm.  Page 6 says that the paste opening should be the same (1:1).

I did a search on the web for info on BGA land patterns and I found this page from TI:

http://processors.wiki.ti.com/index.php/General_hardware_design/BGA_PCB_design" rel="nofollow - http://processors.wiki.ti.com/index.php/General_hardware_design/BGA_PCB_design

Pad size is the first topic discussed.  They show a chart from the IPC showing the reduction should be 20% of the nominal ball diameter.  According to NXP's recommended footprint, the reduced size would be 0.34/0.43 = 79%, which is very close to a 20% reduction, and matches the chart very well.  Library Expert is suggesting a pad size of 0.39mm, which would yield a reduced size of 0.39/0.43 = 90.6%, or about a 10% reduction.

Is TI's data old?  Has the specification evolved?

Thanks for the help!

Greg


Replies:
Posted By: Tom H
Date Posted: 20 Jan 2019 at 8:43pm
IPC-7351C is being written right now.

The document has been downgrading to a Guideline.

Library Expert allows the user to change every default value.

You must figure out what's best for you. IPC is changing the mathematical model for BGA pad calculation and it won't be public for several months.

IPC-7351C is being slow walked though the committee.

You have 2 options -

- Use the Mfr. Recommended Pattern
- Change the BGA Default vales to values in Preferences that are best for you





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Posted By: gcary
Date Posted: 21 Jan 2019 at 6:49pm
Thanks Tom.  I just noticed that TI's website references IPC-7351A.  Since you say they're working on IPC-7351C, that must mean B is the current rev.  Does that explain the difference between Library Expert's answer and the IPC-7351A chart?  Does rev B have a different chart than rev A?

I'm looking for an explanation of why Library Expert is providing a different answer than the IPC spec.  I would expect the BGA Default Values in Library Expert to yield the same answer as IPC.

Thanks,

Greg

Posted By: Tom H
Date Posted: 22 Jan 2019 at 8:27am
The BGA pad size calculations are the same in the original 2005 release of IPC-7351 and the 2007 IPC-7351A and 2010 IPC-7351B.

I heard that the Land Pattern committee is brainstorming a new method of calculating BGA pad sizes for IPC-7351C, but I need a current copy of the working draft as it was updated during the IPC APEX conference in San Diego this week.

Any new method must be approved by the 80 member sub-committee.



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