Minimum Trim Standoff:

• Only affects Gullwing Leads the A1 dimension
• Dieter Bergman said pad trimming is not an IPC-7351 Standard
• IPC J-STD-001 says that pads under low profile parts with A1 dimension is 0.00 should be trimmed for plastic body packages
• The default setting is 0.03 meaning that any A1 dimension less than 0.03 will get trimmed
• The trimming is to the Nominal Package Body dimension
• Many people turn it off by entering a value of 0.00
• Some people tone it down to 0.01 so that it only affects part with A1 being 0.00
• The main reason for customers who requested this feature was to compensate for component manufacturers to limit their ‘L’ tolerance. In most 1.27 mm pitch SOICs, the ‘L’ dimension is min 0.41 and max 1.27. The tolerance is +/-0.43.
• Today, component manufacturers have a maximum ‘L’ tolerance of +/- 0.25 and a nominal tolerance of +/- 0.15.
• See the attached TI Case Code D0008A. the ‘L’ tolerance is +/- 0.43 which is too robust so Texas Instruments Recommended Pattern trims the pad from going under the package.

Summary: if the ‘L’ tolerance is +/- 0.15 there’s no needfor trimming. Even component manufacturers don’t use their ‘L’ tolerance fortheir pad stack calculation. A ‘L’ tolerance of 0.43 is way too robust, but inthe 1980s and 1990s the ‘L’ tolerance was +/- 0.43 but today it’s +/- 0.15. Thelarge ‘L’ tolerance is the only reason for Minimum Trim Standoff Height.

I hope this explains the issue.