Land Pattern Calculation of Gull Wing Lead Parts

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...