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Land Pattern Calculation of Gull Wing Lead Parts

Printed From: PCB Libraries Forum
Category: Libraries
Forum Name: Footprints / Land Patterns
Forum Description: [General or a CAD specific issues / discussions]
URL: http://www.PCBLibraries.com/forum/forum_posts.asp?TID=1314
Printed Date: 25 Feb 2021 at 9:53am


Topic: Land Pattern Calculation of Gull Wing Lead Parts
Posted By: ragz217
Subject: Land Pattern Calculation of Gull Wing Lead Parts
Date Posted: 15 Apr 2014 at 8:58pm
Hi All,

Can some body explain me why the solder joint calculations of the following part category has negative values in it?

Inward Flat ribbon L and Gull wing leads (Ex: SMA,SMB and Tatlum caps)

The Side solder joint (Js) values in IPC7351 are given as 


Due to this the Pad size will actually be less than the lead size.

-RaGZ.





Replies:
Posted By: Tom H
Date Posted: 16 Apr 2014 at 6:52am

Good question and we like people to challenge the standard and not take it for granted.

These 7 factors are used to calculate the optimum Land Size

  1. Component Body Tolerance – ± 4 Mil (0.1) thru ± 12 (0.3)
  2. Component Terminal Tolerance – ± 8 Mil (0.2) thru ± 20 Mil (0.5)
  3. Fabrication Tolerance – ± 2 Mil (0.05 mm)
  4. Placement Tolerance – ± 1 Mil (0.025 mm)
  5. Land Size Round-off – ± 1 Mil or (0.01 mm or 0.05 mm)
  6. Land Spacing Round-off – ± 1 Mil or (0.01mm or 0.05 mm)
  7. Solder Joint Goals for Toe, Heel and Side

Sometimes the normal IPC-7351 tolerance calculations can make the Footprint pad too large.

In the Inward Flat Ribbon component lead, there is no need for a "Side Solder Joint Goal" so the values in the table compensate for the accumulation of Tolerances.

There is still a Side Pad on the Inward Flat Ribbon component lead but only because of the Tolerances that are used to calculate the pad size.

It's all in the math...



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Posted By: Tom H
Date Posted: 03 Mar 2020 at 12:06pm
New Gull Wing rule for pad stack calculation. 



See this forum post: 
https://www.pcblibraries.com/forum/ipcjstd001-gull-wing-leads-and-solder-joints_topic2587.html" rel="nofollow - https://www.pcblibraries.com/forum/ipcjstd001-gull-wing-leads-and-solder-joints_topic2587.html



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