IPC-J-STD-001 Gull Wing Leads and Solder Joints

The Gull Wing lead is the second most popular terminal lead form on most PCB’s.

There are two things that are important here:

1. IPC-J-STD-001 does not mention any minimum Toe Goal solder fillet.
2. The only comment for the Toe is that the Terminal Lead does not hang over the pad to violate the minimum electrical clearance. The B dimension.

IPC-J-STD-001 does not put any emphasis on the Toe solder joint goal. It’s as though it’s insignificant. The Gull Wing Terminal Lead can extend to the end of the pad and even slightly beyond the pad and still pass J-STD-001 acceptance criteria.

However, IPC-J-STD-001 does have several comments on the Gull Wing Terminal Heel solder joint. The F dimension.

• The F dimension for Heel Fillet for Class 1 Fabrication is: Wetting is evident
• The F dimension for Heel Fillet for Class 2 Fabrication is: (G) + 50% (T)
• The F dimension for Heel Fillet for Class 3 Fabrication is: (G) + (T)

Here is an image from a Vishay recommended land pattern. The Terminal Lead Tip to Tip dimension = 6.20 mm and the recommended land pattern dimension for the outside pads = 6.248 mm. 

This meets the IPC-J-STD-001 solder joint criteria, but not the IPC-7351 solder joint goal criteria. 

http://www.vishay.com/docs/68969/si4925dd.pdf

Here are the IPC-7351B Solder Joint Goal tables 3-2 & 3-3 for the Gull Wing Terminal Lead. Notice that the Toe and Heel solder joint goals are the same value for every Gull Wing lead. Also notice that the difference between Most, Nominal and Least Density Levels are in 0.20 mm increments. PCB Libraries, Inc. believes that this margin between density levels is too steep. The Most density level is too large and has too much solder and the least density level is too small and does not have enough solder.

Gull Wing affects the component families SOP, QFP, SOT, SOD and DPAK. 

This is where PCB Libraries and IPC go our own separate way. IPC-7351B Nominal Density Level recommends a 0.35 mm Toe and Heel regardless of the component family, lead pitch and Terminal Lead Frame Thickness. The Lead-Frame Thickness on a common DPAK is 0.65 mm but the Lead Thickness on a 0.50 mm pitch QFP or SOP is 0.15 mm. They do not require the same Toe value

PCB Libraries, Inc. believes that the Toe value should equal the Lead-Frame Thickness to show visible wetting in the Toe solder joint. Here is a simple image of this philosophy. 

PCB Libraries created an Incremental pad stack concept for Gull Wing Leads based on Terminal Lead Thickness, which is primarily based on the Terminal Pin Pitch.

PCB Libraries, Inc. believes that this margin between density levels should be more subtle and swing 0.10 mm from the Nominal density level to the Most and Least density level. If you compare IPC-7351 with the original IPC-SM-782 land pattern standard that existed for 18 years, the SM-782 standard was about 0.10 mm larger than the 7351 Nominal.

PCB Libraries, Inc. in conjunction with our top tier customers, created an Incremental pad stack table based on the Terminal Lead Frame Thickness which is derived from the pin pitch.

Download the Library Expert Excel spreadsheet Solder Joint Goal tables here – www.pcblibraries.com/downloads

Also, if you are a Library Expert user, the Solder Joint Goal table Excel spreadsheet is already on your computer in this folder: 

C:\Program Files (x86)\PCB Libraries\Library Expert 2019\Documents\Library Expert Solder Joint Goal Tables.xlsx

Here is an image of a real Gull Wing package with an adequate Toe Solder Joint. Not too big and not too small.